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Mating in yeast is stimulated by the presence of a pheromone which binds to either the Ste2 receptor (in a-cells) or the Ste3 receptor (in α-cells). The binding of this pheromone then leads to the activation of a heterotrimeric G protein. The dimeric portion of this G-protein recruits Ste5 (and its related MAPK cascade components) to the membrane, and ultimately results in the phosphorylation of Fus3.
The switching mechanism arises as a result of competition between the Fus3 protein (a MAPK protein) and the phosphatase Ptc1. These proteins both attempt to control the 4 phosphorylation sites of Ste5, a scaffold protein with Fus3 attempting to phosphorylate the phosphosites, and Ptc1 attempting to dephosphorylate them.
Presence of α-factor induces recruitment of Ptc1 to Ste5 via a 4 amino acid motif located within the Ste5 phosphosites. Ptc1 then dephosphorylates Ste5, ultimately resulting in the dissociation of the Fus3-Ste5 complex. Fus3 dissociates in a switch-like manner, dependent on the phosphorylation state of the 4 phosphosites. All 4 phosphosites must be dephosphorylated in order for Fus3 to dissociate. Fus3's ability to compete with Ptc1 decreases as Ptc1 is recruited, and thus the rate of dephosphorylation increases with the presence of pheromone.
Kss1, a homologue of Fus3, does not affect shmooing, and does not contribute to the switch-like mating decision.
In yeast, mating as well as the production of shmoos occur via an all-or-none, switch-like mechanism. This switch-like mechanism allows yeast cells to avoid making an unwise commitment to a highly demanding procedure. However, not only does the mating decision need to be conservative (in order to avoid wasting energy), but it must also be fast to avoid losing the potential mate.
The decision to mate is extremely sensitive. There are 3 ways in which this ultrasensitivity is maintained:
#Multi-site phosphorylation – Fus3 only dissociates from Ste5 and becomes fully active when all 4 of the phosphosites are dephosphorylated. Even one phosphorylated site will result in immunity to α-factor.
#Two-stage binding – Fus3 and Ptc1 bind to separate docking sites on Ste5. Only after docking can they bind to, and act on, the phosphosites.
#Steric hindrance – competition between Fus3 and Ptc1 to control the 4 phosphosites on Ste3
[a and α yeast share the same mating response pathway, with the only difference being the type of receptor each mating type possesses. Thus the above description, given for a-type yeast stimulated with α-factor, works equally well for α-type yeast stimulated with a-factor.] | 1 | Biochemistry |
The DNA methylation level estimations can be confounded by varying densities of methylated CpG sites across the genome when observing data generated by MeDIP. This can be problematic for analyzing CpG-poor (lower density) regions. One reason for this density issue is its effect on the efficiency of immunoprecipitation. In their study, Down et al. developed a tool to estimate absolute methylation levels from data generated by MeDIP by modeling the density of methylated CpG sites. This tool is called Bayesian tool for methylation analysis (Batman). The study reports the coverage of ~90% of all CpG sites in promoters, gene-coding regions, islands, and regulatory elements where methylation levels can be estimated; this is almost 20 times better coverage than any previous methods.
Studies using MeDIP-seq or MeDIP-chip are both genome-wide approaches that have the common aim of obtaining the functional mapping of the methylome. Once regions of DNA methylation are identified, a number of bioinformatics analyses can be applied to answer certain biological questions. One obvious step is to investigate genes contained in these regions and investigate the functional significance of their repression. For example, silencing of tumour-suppressor genes in cancer can be attributed to DNA methylation. By identifying mutational events leading to hypermethylation and subsequent repression of known tumour-suppressor genes, one can more specifically characterize the contributing factors to the cause of the disease. Alternatively, one can identify genes that are known to be normally methylated but, as a result of some mutation event, is no longer silenced.
Also, one can try and investigate and identify whether some epigenetic regulator has been affected such as DNA methyltransferase (DNMT); in these cases, enrichment may be more limited.
Gene-set analysis (for example using tools like DAVID and GoSeq) has been shown to be severely biased when applied to high-throughput methylation data (e.g. MeDIP-seq and MeDIP-ChIP); it has been suggested that this can be corrected using sample label permutations or using a statistical model to control for differences in the numberes of CpG probes / CpG sites that target each gene. | 1 | Biochemistry |
Adhesion is an essential process to epithelial cells so that epithelium can be formed and cells can be in permanent contact with extracellular matrix and other cells. Several pathways exist to accomplish this communication and adhesion with environment. But the main signalling pathways are the cadherin and integrin pathways.
The cadherin pathway is present in adhesion junctions or in desmosomes and it is responsible for epithelial adhesion and communication with adjacent cells. Cadherin is a transmembrane glycoprotein receptor that establishes contact with another cadherin present in the surface of a neighbour cell forming an adhesion complex. This adhesion complex is formed by β-catenin and α-catenin, and p120 is essential for its stabilization and regulation. This complex then binds to actin, leading to polymerization. For actin polymerization through the cadherin pathway, proteins of the Rho GTPases family are also involved. This complex is regulated by phosphorylation, which leads to downregulation of adhesion. Several factors can induce the phosphorylation, like EGF, HGF or v-Src. The cadherin pathway also has an important function in survival and proliferation because it regulates the concentration of cytoplasmic β-catenin. When β-catenin is free in the cytoplasm, normally it is degraded, however if the Wnt signalling is activated, β-catenin degradation is inhibited and it is translocated to the nucleus where it forms a complex with transcription factors. This leads to activation of genes responsible for cell proliferation and survival. So the cadherin-catenin complex is essential for cell fate regulation.
Integrins are heterodimeric glycoprotein receptors that recognize proteins present in the extracellular matrix, like fibronectin and laminin. In order to function, integrins have to form complexes with ILK and Fak proteins. For adhesion to the extracellular matrix, ILK activate the Rac and Cdc42 proteins and leading to actin polymerization. ERK also leads to actin polymerization through activation of cPLA2. Recruitment of FAK by integrin leads to Akt activation and this inhibits pro-apoptotic factors like BAD and Bax. When adhesion through integrins do not occur the pro-apoptotic factors are not inhibited and resulting in apoptosis. | 7 | Physical Chemistry |
Photo-oxidation is the combined action of UV-light and oxygen and is the most significant factor in the weathering of plastics. Although many polymers do not absorb UV-light, they often contain impurities like hydroperoxide and carbonyl groups introduced during thermal processing, which do. These act as photoinitiators to give complex free radical chain reactions where the mechanisms of autoxidation and photodegradation combine. Photo-oxidation can be held back by light stabilizers such as hindered amine light stabilizers (HALS). | 7 | Physical Chemistry |
Pseudouridine (Ψ, 5-ribosyluracil) is the most abundant RNA modification; in fact, at one time it was considered the "fifth nucleotide". This isomer of uridine is found in various types of RNA, such as snRNA, tRNA, small nucleolar RNA (snoRNA) and many others. Pseudouridine increases the stability of the modified RNA by making the sugar-phosphate backbone more rigid and by facilitating base stacking interactions (pseudouridine contains an extra hydrogen bond donor). When it comes to Watson-Crick base pair interactions, the pseudouridine-adenosine base pair is more stable than the uridine-adenosine base pair; therefore, pseudouridine increases stability. Apart from increasing RNA stability, this modification is also involved in regulation of translation. All eukaryotic stop codons contain one uridine (UAA, UGA and UGA); conversion of this uridine to pseudouridine results in suppression of translational termination and generation of unexpected sense codons. The artificial process of pseudouridylation has an effect on the function of mRNA: it changes the genetic code by making non-canonical base pairing possible in the ribosome decoding center.
Pseudouridylation reactions are catalyzed by enzymes that contain the pseudouridine synthase domain; 13 such enzymes have been identified in humans, which are called pseudouridine syntheses (PUS). These enzymes can be either RNA-dependent or RNA-independent depending on whether a small RNA is needed to guide the enzyme to its target or not. Additionally, different PUS enzymes work in different cell compartments. For instance, PUS4 (also known as TruB pseudouridine synthase family member 1, TRUB1) and PUS7, which are responsible for most of the mRNA pseudouridylation, are located in the nucleus or the cytoplasm. On the other hand, several PUS enzymes, such as PUS1 and TRUB2 are located in the mitochondria, modifying a number of mitochondrial mRNAs (mt-mRNAs). In tRNA, PUS1 and PUS7 modify the second uridine in the UGUAR consensus sequence, as long as this sequence is located in a very structured region of the tRNA.
To date, no pseudouridine erasers or readers have been identified. It is thought that pseudouridylation is most probably an irreversible process.
Pseudouridine is most commonly found in tRNAs, with almost all tRNA molecules having at least one pseudouridine. Therefore, because the addition of pseudouridine happens during the normal processing of tRNA, it is not considered an epitranscriptomic mark. However, pseudouridine acts as an epigenetic mark in mRNAs and ncRNAs of the brain, since pseudouridylation in these two RNAs responds dynamically to stress and differentiation in the cell, giving reason to believe that pseudouridylation may act as an important regulatory mechanism for RNA function. Pseudouridylation in mRNA can be conserved, tissue-specific or inducible, which reflects plasticity and regulatory function. Furthermore, expression of TRBU1, which is mostly expressed in the brain, goes up due to fear conditioning. In addition, expression of the ncRNAs needed to guide RNA-dependent PUS enzymes also goes up in response to fear. | 1 | Biochemistry |
In the 17th century, copper miners in Saxony, Germany, began to experience irritation caused by a "dark red ore". Since the substance, which would later be called nickel, led to many ailments, they believed it to be protected by "goblins", and called it "Goblin's Copper". Josef Jadassohn described the first case of metal contact dermatitis in 1895, to a mercury-based therapeutic cream, and confirmed the cause by epi-cutaneous patch testing. In the next century nickel began to be mass-produced for jewelry worldwide due to its cheap cost, resistance to corrosion and high supply.
In 1979 a large comprehensive study of healthy US volunteers found that 9% had been unknowingly sensitized to nickel. , that number has tripled. Most importantly, nickel allergy among children is increasing, with an estimated 250,000 children sensitized to nickel.
Published literature shows an exponential increase in reported nickel allergy cases. The North American Contact Dermatitis Group (NACDG) patch tested 5,085 adults, presenting with eczema-like symptoms, showing 19.5% had a positive reaction to nickel. Nickel allergy is also more prevalent in women (17.1%) than men (3%), possibly due to cultural norms related to jewelry and ear piercings and therefore increased exposure to nickel. In order to investigate the current prevalence of nickel, Loma Linda University, Nickel Allergy Alliance, and Dermatitis Academy, are conducting a self-reporting nickel allergy-dermatitis survey. | 1 | Biochemistry |
Raymond Urgel Lemieux, CC, AOE, FRS (June 16, 1920 – July 22, 2000) was a Canadian organic chemist, who pioneered many discoveries in the field of chemistry, his first and most famous being the synthesis of sucrose. His contributions include the discovery of the anomeric effect and the development of general methodologies for the synthesis of saccharides still employed in the area of carbohydrate chemistry. He was a fellow of the Royal Society of Canada and the Royal Society (England), and a recipient of the prestigious Albert Einstein World Award of Science and Wolf Prize in Chemistry. | 0 | Organic Chemistry |
The electrons are often fired at gold foil because gold has a high atomic number (Z), is non-reactive (does not form an oxide layer), and can be easily made into a thin film (reducing multiple scattering). The presence of a spin-orbit term in the scattering potential introduces a spin dependence in the scattering cross section. Two detectors at exactly the same scattering angle to the left and right of the foil count the number of scattered electrons. The asymmetry A, given by:
is proportional to the degree of spin polarization P according to A = SP, where S is the Sherman function.
The Mott cross section formula is the mathematical description of the scattering of a high energy electron beam from an atomic nucleus-sized positively charged point in space. The Mott scattering is the theoretical diffraction pattern produced by such a mathematical model. It is used as the beginning point in calculations in electron scattering diffraction studies.
The equation for the Mott cross section includes an inelastic scattering term to take into account the recoil of the target proton or nucleus. It also can be corrected for relativistic effects of high energy electrons, and for their magnetic moment.
When an experimentally found diffraction pattern deviates from the mathematically derived Mott scattering, it gives clues as to the size and shape of an atomic nucleus The reason is that the Mott cross section assumes only point-particle Coulombic and magnetic interactions between the incoming electrons and the target. When the target is a charged sphere rather than a point, additions to the Mott cross section equation (form factor terms) can be used to probe the distribution of the charge inside the sphere.
The Born approximation of the diffraction of a beam of electrons by atomic nuclei is an extension of Mott scattering. | 7 | Physical Chemistry |
Many properties of materials are affected by their crystal structure. This structure can be investigated using a range of crystallographic techniques, including X-ray crystallography, neutron diffraction and electron diffraction.
The sizes of the individual crystals in a crystalline solid material vary depending on the material involved and the conditions when it was formed. Most crystalline materials encountered in everyday life are polycrystalline, with the individual crystals being microscopic in scale, but macroscopic single crystals can be produced either naturally (e.g. diamonds) or artificially.
Real crystals feature defects or irregularities in the ideal arrangements, and it is these defects that critically determine many of the electrical and mechanical properties of real materials. | 8 | Metallurgy |
The following content uses protein primary structure single-letter location. A "[n]" prefix indicates the N-terminus and a "[c]" suffix indicates the C-terminus; sequences lacking either are found in the middle of the protein. | 1 | Biochemistry |
As hyrax middens have been developed as palaeoenvironmental archives, there has been increasing emphasis on the application of stable isotope analyses to midden sequences. Initially this focussed on the use of bulk C data, with an emphasis on identifying changes in the relative abundance of C/C/CAM vegetation and associated palaeoecological/palaeoenvironmental inferences. This is useful in climatic transition zones, such as the Western Cape Province of South Africa, where modern rainfall seasonality has a strong impact on C/C grass distributions. δC records can also be used in some ecoregions, such as the dry savannah at Spitzkoppe in Namibia, as an indicator of the reliability of grass cover. As hyraxes will preferentially graze (grasses are C in the region), more depleted δC values from hyrax middens have been interpreted as evidence that the animals were forced to obtain a greater proportion of their diet from trees and shrubs, which are less susceptible to extended periods of drought. However, these data do not necessarily provide a direct and unambiguous indicator of past arid/humid shifts. As such, other studies have focussed on the use of δN data as a potential proxy for water availability in the environment | 9 | Geochemistry |
Ligands containing atomic chirality centers such asymmetric carbon, which usually do not have C-symmetry, remain important in catalysis. Examples include cinchona alkaloids and certain phosphoramidites. P-chiral monophosphines have also been investigated. | 4 | Stereochemistry |
Triple-decker complexes are known to obey a 30-valence electron (VE) rule. Subtracting 6 pairs of nonbonding electrons from the two metal atoms brings the number of SEPs to 9 pairs. For a triple-decker complex with cyclopentadienyl| as the decks, m + n + o + p − q = 3 + 17 + 2 + 2 − 0 = 24. Subtracting the 15 pairs corresponding to C–C sigma bonds, it becomes 9 pairs. For example, consider : 15 C–CH groups provide pairs. Each ruthenium atom provides one pair. Removing the electron corresponding to the positive charge of the complex leads to a total of + 2 − = 24 pairs. | 7 | Physical Chemistry |
Niobium-germanium (NbGe) is an intermetallic chemical compound of niobium (Nb) and germanium (Ge). It has A15 phase structure.
It is a superconductor with a critical temperature of 23.2 K.
Sputtered films have been reported to have an upper critical field of 37 teslas at 4.2 K. | 8 | Metallurgy |
During the 19th century, the demand for nitrates and ammonia for use as fertilizers and industrial feedstocks rapidly increased. The main source was mining niter deposits and guano from tropical islands. At the beginning of the 20th century these reserves were thought insufficient to satisfy future demands, and research into new potential sources of ammonia increased. Although atmospheric nitrogen (N) is abundant, comprising ~78% of the air, it is exceptionally stable and does not readily react with other chemicals.
Haber, with his assistant Robert Le Rossignol, developed the high-pressure devices and catalysts needed to demonstrate the Haber process at a laboratory scale. They demonstrated their process in the summer of 1909 by producing ammonia from the air, drop by drop, at the rate of about per hour. The process was purchased by the German chemical company BASF, which assigned Carl Bosch the task of scaling up Haber's tabletop machine to industrial scale. He succeeded in 1910. Haber and Bosch were later awarded Nobel Prizes, in 1918 and 1931 respectively, for their work in overcoming the chemical and engineering problems of large-scale, continuous-flow, high-pressure technology.
Ammonia was first manufactured using the Haber process on an industrial scale in 1913 in BASF's Oppau plant in Germany, reaching 20 tonnes/day in 1914. During World War I, the production of munitions required large amounts of nitrate. The Allies had access to large deposits of sodium nitrate in Chile (Chile saltpetre) controlled by British companies. India had large supplies too, but it was also controlled by the British. Germany had no such resources, so the Haber process proved essential to the German war effort. Synthetic ammonia from the Haber process was used for the production of nitric acid, a precursor to the nitrates used in explosives.
The original Haber–Bosch reaction chambers used osmium as the catalyst, but it was available in extremely small quantities. Haber noted uranium was almost as effective and easier to obtain than osmium. In 1909, BASF researcher Alwin Mittasch discovered a much less expensive iron-based catalyst that is still used. A major contributor to the elucidation of this catalysis was Gerhard Ertl. The most popular catalysts are based on iron promoted with KO, CaO, SiO, and AlO.
During the interwar years, alternative processes were developed, most notably the Casale process, Claude process, and the Mont-Cenis process developed by Friedrich Uhde Ingenieurbüro. Luigi Casale and Georges Claude proposed to increase the pressure of the synthesis loop to , thereby increasing the single-pass ammonia conversion and making nearly complete liquefaction at ambient temperature feasible. Claude proposed to have three or four converters with liquefaction steps in series, thereby avoiding recycling. Most plants continue to use the original Haber process ( and ), albeit with improved single-pass conversion and lower energy consumption due to process and catalyst optimization. | 7 | Physical Chemistry |
A microbatch usually involves immersing a very small volume of protein droplets in oil (as little as 1 µl). The reason that oil is required is because such low volume of protein solution is used and therefore evaporation must be inhibited to carry out the experiment aqueously. Although there are various oils that can be used, the two most common sealing agent are paraffin oils (described by Chayen et al.) and silicon oils (described by D’Arcy). There are also other methods for microbatching that don't use a liquid sealing agent and instead require a scientist to quickly place a film or some tape on a welled plate after placing the drop in the well.
Besides the very limited amounts of sample needed, this method also has as a further advantage that the samples are protected from airborne contamination, as they are never exposed to the air during the experiment. | 3 | Analytical Chemistry |
The worlds largest scale implementation of Fischer–Tropsch technology is a series of plants operated by Sasol in South Africa, a country with large coal reserves, but little oil. With a capacity of 165000 Bpd at its Secunda plant. The first commercial plant opened in 1952. Sasol uses coal and natural gas as feedstocks and produces a variety of synthetic petroleum products, including most of the countrys diesel fuel. | 0 | Organic Chemistry |
When in 1981 Rob Aalberse from the University of Amsterdam noticed the enormous cross-reactivity of some patients´ sera against virtually any plant and even insects, notably, insect venoms, it took ten years to arrive at a possible structural explanation of this phenomenon. 1991, Japanese researchers determined the structure of the epitope common to horseradish peroxidase and Drosophila neurons as being an asparagine-linked oligosaccharide (N-glycan) containing a xylose and a core-linked α1,3-linked fucose residue. These structural features are not present in humans and animals. Core α1,3-fucose was then found to be relevant for the binding of patients´ IgE to honeybee venom allergens, which contain N-glycans with structural similarities to plant N-glycans. Ever since then, core α1,3-fucose emerged as the structural element most relevant as a CCD in plants and insect allergens. Much later, both xylose and core α1,3-fucose were revealed as heart pieces of two independent glycan epitopes for rabbit IgG. The occurrence of human anti-xylose IgE, however, has not been verified so far. Still, because of the two possible epitopes and the different carrier structures, the plural CCDs is in frequent use even though core α1,3-fucose appears to be the single culprit. | 0 | Organic Chemistry |
Induced-charge electrokinetics in physics is the electrically driven fluid flow and particle motion in a liquid electrolyte. Consider a metal particle (which is neutrally charged but electrically conducting) in contact with an aqueous solution in a chamber/channel. If different voltages apply to the end of this chamber/channel, electric field will generate in this chamber/channel. This applied electric field passes through this metal particle and causes the free charges inside the particle migrate under the skin of particle. As a result of this migration, the negative charges move to the side which is close to the positive (or higher) voltage while the positive charges move to the opposite side of the particle. These charges under the skin of the conducting particle attract the counter-ions of the aqueous solution; thus, the electric double layer (EDL) forms around the particle. The EDL sign on the surface of the conducting particle changes from positive to negative and the distribution of the charges varies along the particle geometry. Due to these variations, the EDL is non-uniform and has different signs. Thus, the induced zeta potential around the particle, and consequently slip velocity on the surface of the particle, vary as a function of the local electric field. Differences in magnitude and direction of slip velocity on the surface of the conducting particle effects the flow pattern around this particle and causes micro vortices. Yasaman Daghighi and Dongqing Li, for the first time, experimentally illustrated these induced vortices around a 1.2mm diameter carbon-steel sphere under the 40V/cm direct current (DC) external electric filed.
Chenhui Peng et al. also experimentally showed the patterns of electro-osmotic flow around an Au sphere when alternating current (AC) is involved (E=10mV/μm, f=1 kHz).
Electrokinetics here refers to a branch of science related to the motion and reaction of charged particles to the applied electric filed and its effects on its environment. It is sometimes referred as non-linear electrokinetic phenomena as well. | 7 | Physical Chemistry |
Pd(PPh) is widely used as a catalyst for palladium-catalyzed coupling reactions. Prominent applications include the Heck reaction, Suzuki coupling, Stille coupling, Sonogashira coupling, and Negishi coupling. These processes begin with two successive ligand dissociations followed by the oxidative addition of an aryl halide to the Pd(0) center:
:Pd(PPh) + ArBr → PdBr(Ar)(PPh) + 2 PPh | 0 | Organic Chemistry |
The plasmalysis of wastewater and liquid manure enables hydrogen to be recovered from pollutants contained in the wastewater (ammonium (NH4) or hydrocarbon compounds (COD)). The plasma-catalytic decomposition of ammonia takes place as shown in the following reaction equation:
The treated wastewater is purified in the process. The energy requirement for the production of green hydrogen is approx. 12 kWh/kg.
This technology can also be used as ammonia cracking (chemistry) technology for splitting the hydrogen carrier ammonia. | 7 | Physical Chemistry |
The bond length between two atoms in a molecule depends not only on the atoms but also on such factors as the orbital hybridization and the electronic and steric nature of the substituents. The carbon–carbon (C–C) bond length in diamond is 154 pm. It is generally considered the average length for a carbon–carbon single bond, but is also the largest bond length that exists for ordinary carbon covalent bonds. Since one atomic unit of length (i.e., a Bohr radius) is 52.9177 pm, the C–C bond length is 2.91 atomic units, or approximately three Bohr radii long.
Unusually long bond lengths do exist. Current record holder for the longest C-C bond with a length of 186.2 pm is 1,8-Bis(5-hydroxydibenzo[a,d]cycloheptatrien-5-yl)naphthalene, one of many molecules within a category of hexaaryl ethanes, which are derivatives based on hexaphenylethane skeleton. Bond is located between carbons C1 and C2 as depicted in a picture below.
Another notable compound with an extraordinary C-C bond length is tricyclobutabenzene, in which a bond length of 160 pm is reported. Longest C-C bond within the cyclobutabenzene category is 174 pm based on X-ray crystallography. In this type of compound the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.
The existence of a very long C–C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylene dianions, although this concerns a 2-electron-4-center bond. This type of bonding has also been observed in neutral phenalenyl dimers. The bond lengths of these so-called "pancake bonds" are up to 305 pm.
Shorter than average C–C bond distances are also possible: alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond. In benzene all bonds have the same length: 139 pm. Carbon–carbon single bonds increased s-character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm).
In propionitrile the cyano group withdraws electrons, also resulting in a reduced bond length (144 pm). Squeezing a C–C bond is also possible by application of strain. An unusual organic compound exists called In-methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a triptycene and a phenyl group. In an in silico experiment a bond distance of 136 pm was estimated for neopentane locked up in fullerene. The smallest theoretical C–C single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative.
The same study also estimated that stretching or squeezing the C–C bond in an ethane molecule by 5 pm required 2.8 or 3.5 kJ/mol, respectively. Stretching or squeezing the same bond by 15 pm required an estimated 21.9 or 37.7 kJ/mol. | 4 | Stereochemistry |
Phenolphthalein ( ) is a chemical compound with the formula CHO and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.
Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols in experiments. It is a weak acid, which can lose H ions in solution. The nonionized phenolphthalein molecule is colorless and the double deprotonated phenolphthalein ion is fuchsia. Further proton loss in higher pH occurs slowly and leads to a colorless form. Phenolphthalein ion in concentrated sulfuric acid is orange red due to sulfonation. | 3 | Analytical Chemistry |
Usually the time-of-flight tube used in mass spectrometry is praised for simplicity, but for precision measurements of charged low energy particles the electric and the magnetic field in the tube has to be controlled within 10 mV and 1 nT respectively.
The work function homogeneity of the tube can be controlled by a Kelvin probe. The magnetic field can be measured by a fluxgate compass. High frequencies are passively shielded and damped by radar absorbent material. To generate arbitrary low frequencies field the screen is parted into plates (overlapping and connected by capacitors) with bias voltage on each plate and a bias current on coil behind plate whose flux is closed by an outer core. In this way the tube can be configured to act as a weak achromatic quadrupole lens with an aperture with a grid and a delay line detector in the diffraction plane to do angle resolved measurements. Changing the field the angle of the field of view can be changed and a deflecting bias can be superimposed to scan through all angles.
When no delay line detector is used focusing the ions onto a detector can be accomplished through the use of two or three einzel lenses placed in the vacuum tube located between the ion source and the detector.
The sample should be immersed into the tube with holes and apertures for and against stray light to do magnetic experiments and to control the electrons from their start. | 7 | Physical Chemistry |
Several groups have applied atomic manipulation techniques for artistic purposes to demonstrate control over the adatom positions. These include various institutional logos and a movie called “A Boy and His Atom” composed of individual STM scans by IBM researchers.
Several notable condensed matter physics experiments have been realized with atomic manipulation techniques. These include the demonstration of electron confinement in so-called quantum corrals by Michael F. Crommie et al., and the subsequent Quantum mirage experiment, where the Kondo signature of an adatom was reflected from one focus to another in an elliptical quantum corral.
Atomic manipulation has also sparked interest as a computation platform. Andreas J. Heinrich et al. built logic gates out of molecular cascades of CO adsorbates, and Kalff et al. demonstrated a rewritable kilobyte memory made of individual atoms.
Recent experiments on artificial lattice structures have utilized atomic manipulation techniques to study the electronic properties of Lieb lattices, artificial graphene and Sierpiński triangles. | 7 | Physical Chemistry |
The decay of the positive muon into a positron and two neutrinos occurs via the weak interaction process after a mean lifetime of
τ = 2.197034(21) μs:
Parity violation in the weak interaction leads in this more complicated case (three body decay) to an anisotropic distribution of the positron emission with respect to the spin direction of the μ at the decay time. The positron emission probability is given by
where is the angle between the positron trajectory and the μ-spin, and is an intrinsic asymmetry parameter determined by the weak decay mechanism. This anisotropic emission constitutes in fact the basics for the μSR technique.
The average asymmetry is measured over a statistical ensemble of implanted muons and it depends on further experimental parameters, such as the beam spin polarization , close to one, as already mentioned. Theoretically =1/3 is obtained if all emitted positrons are detected with the same efficiency, irrespective of their energy. Practically, values of ≈ 0.25 are routinely obtained.
The muon spin motion may be measured over a time scale dictated by the muon decay, i.e. a few times τ, roughly 10 µs. The asymmetry in the muon decay correlates the positron emission and the muon spin directions. The simplest example is when the spin direction of all muons remains constant in time after implantation (no motion). In this case the asymmetry shows up as an imbalance between the positron counts in two equivalent detectors placed in front and behind the sample, along the beam axis. Each of them records an exponentially decaying rate as a function of the time t elapsed from implantation, according to
with for the detector looking towards and away from the spin arrow, respectively. Considering that the huge muon spin polarization is completely outside thermal equilibrium, a dynamical relaxation towards the equilibrium unpolarized state typically shows up in the count rate, as an additional decay factor in front of the experimental asymmetry parameter, A. A magnetic field parallel to the initial muon spin direction probes the dynamical relaxation rate as a function of the additional muon Zeeman energy, without introducing additional coherent spin dynamics. This experimental arrangement is called Longitudinal Field (LF) μSR.
A special case of LF μSR is Zero Field (ZF) μSR, when the external magnetic field is zero. This experimental condition is particularly important since it allows to probe any internal quasi-static (i.e. static on the muon time-scale) magnetic field of field distribution at the muon site. Internal quasi-static fields may appear spontaneously, not induced by the magnetic response of the sample to an external field They are produced by disordered nuclear magnetic moments or, more importantly, by ordered electron magnetic moments and orbital currents.
Another simple type of μSR experiment is when implanted all muon spins precess coherently around the external magnetic field of modulus , perpendicular to the beam axis, causing the count unbalance to oscillate at the corresponding Larmor frequency between the same two detectors, according to
Since the Larmor frequency is , with a gyromagnetic ratio Mrad(sT), the frequency spectrum obtained by means of this experimental arrangement provides a direct measure of the internal magnetic field intensity distribution. The distribution produces an additional decay factor of the experimental asymmetry A. This method is usually referred to as Transverse Field (TF) μSR.
A more general case is when the initial muon spin direction (coinciding with the detector axis) forms an angle with the field direction. In this case the muon spin precession describes a cone which results in both a longitudinal component, , and a transverse precessing component, , of the total asymmetry. ZF μSR experiments in the presence of a spontaneous internal field fall into this category as well. | 7 | Physical Chemistry |
Density functional theory seeks to solve for an approximate form of the electronic density of a system. In general, atoms are split into ionic cores and valence electrons. The ionic cores (nuclei plus non-bonding electrons) are assumed to be stable and are treated as a single object. Each valence electron is treated separately. Thus, for example, a Lithium atom is treated as two bodies – Li+ and e- – while oxygen is treated as three bodies, namely O and 2e.
The “true” ground state of a crystal system is generally unsolvable. However, the variational theorem assures us that any guess as to the electronic state function of a system will overestimate the ground state energy. Thus, by beginning with a suitably parametrized guess and minimizing the energy with respect to each of those parameters, an extremely accurate prediction may be made. The question as to what one's initial guess should be is a topic of active research.
In the large majority of crystal systems, electronic relaxation times are orders of magnitude shorter than ionic relaxation times. Thus, an iterative scheme is adopted. First, the ions are considered fixed and the electronic state is relaxed by considering the ionic and electron-electron pair potentials. Next, the electronic states are considered fixed and the ions are allowed to move under the influence of the electronic and ion-ion pair potentials. When the decrease in energy between two iterative steps is sufficiently small, the structure of the crystal is considered solved. | 3 | Analytical Chemistry |
The He Jiankui affair is a scientific and bioethical controversy concerning the use of genome editing following its first use on humans by Chinese scientist He Jiankui, who edited the genomes of human embryos in 2018. He became widely known on 26 November 2018 after he announced that he had created the first human genetically edited babies. He was listed in the Time 100 most influential people of 2019. The affair led to ethical and legal controversies, resulting in the indictment of He and two of his collaborators, Zhang Renli and Qin Jinzhou. He eventually received widespread international condemnation.
He Jiankui, working at the Southern University of Science and Technology (SUSTech) in Shenzhen, China, started a project to help people with HIV-related fertility problems, specifically involving HIV-positive fathers and HIV-negative mothers. The subjects were offered standard in vitro fertilisation services and in addition, use of CRISPR gene editing (CRISPR/Cas9), a technology for modifying DNA. The embryos genomes were edited to remove the CCR5 gene in an attempt to confer genetic resistance to HIV. The clinical project was conducted secretly until 25 November 2018, when MIT Technology Review' broke the story of the human experiment based on information from the Chinese clinical trials registry. Compelled by the situation, he immediately announced the birth of genome-edited babies in a series of five YouTube videos the same day. The first babies, known by their pseudonyms Lulu () and Nana (), are twin girls born in October 2018, and the second birth or the third baby born was in 2019, named Amy. He reported that the babies were born healthy.
His actions received widespread criticism, and included concern for the girls' well-being. After his presentation on the research at the Second International Summit on Human Genome Editing at the University of Hong Kong on 28 November 2018, Chinese authorities suspended his research activities the following day. On 30 December 2019, Chinese authorities announced that he was found guilty of forging documents and unethical conduct; he was sentenced to three years in prison with a three-million-yuan fine (US$430,000). Zhang Renli and Qin Jinzhou received an 18-month prison sentence, a 500,000-yuan fine and were banned from working in assisted reproductive technology for life.
He Jiankui has been variously referred to as a "rogue scientist", "China's Dr Frankenstein", and a "mad genius". The impact of human gene editing on resistance to HIV infection and other body functions in experimental infants remains controversial. The World Health Organization had issued three reports on the guidelines of human genome editing since 2019, and the Chinese government has prepared regulations since May 2019. | 1 | Biochemistry |
P (systematically araBp) is a promoter found in bacteria and especially as part of plasmids used in laboratory studies. The promoter is a part of the arabinose operon whose name derives from the genes it regulates transcription of: araB, araA, and araD. In E. coli, the P promoter is adjacent to the P promoter (systematically araCp), which transcribes the araC gene in the opposite direction. araC encodes the AraC protein, which regulates activity of both the P and P promoters. The cyclic AMP receptor protein CAP binds between the P and P promoters, stimulating transcription of both when bound by cAMP. | 1 | Biochemistry |
The term kagome lattice was coined by Japanese physicist Kôdi Husimi, and first appeared in a 1951 paper by his assistant Ichirō Shōji.
The kagome lattice in this sense consists of the vertices and edges of the trihexagonal tiling.
Despite the name, these crossing points do not form a mathematical lattice.
A related three dimensional structure formed by the vertices and edges of the quarter cubic honeycomb, filling space by regular tetrahedra and truncated tetrahedra, has been called a hyper-kagome lattice. It is represented by the vertices and edges of the quarter cubic honeycomb, filling space by regular tetrahedra and truncated tetrahedra. It contains four sets of parallel planes of points and lines, each plane being a two dimensional kagome lattice. A second expression in three dimensions has parallel layers of two dimensional lattices and is called an orthorhombic-kagome lattice. The trihexagonal prismatic honeycomb represents its edges and vertices.
Some minerals, namely jarosites and herbertsmithite, contain two-dimensional layers or three-dimensional kagome lattice arrangement of atoms in their crystal structure. These minerals display novel physical properties connected with geometrically frustrated magnetism. For instance, the spin arrangement of the magnetic ions in CoVO rests in a kagome lattice which exhibits fascinating magnetic behavior at low temperatures. Quantum magnets realized on Kagome metals have been discovered to exhibit many unexpected electronic and magnetic phenomena. It is also proposed that SYK behavior can be observed in two dimensional kagome lattice with impurities.
The term is much in use nowadays in the scientific literature, especially by theorists studying the magnetic properties of a theoretical kagome lattice.
See also: Kagome crests. | 3 | Analytical Chemistry |
Four versions of Ficks law for binary gas mixtures are given below. These assume: thermal diffusion is negligible; the body force per unit mass is the same on both species; and either pressure is constant or both species have the same molar mass. Under these conditions, Ref. shows in detail how the diffusion equation from the kinetic theory of gases reduces to this version of Ficks law:
where is the diffusion velocity of species . In terms of species flux this is
If, additionally, , this reduces to the most common form of Fick's law,
If (instead of or in addition to ) both species have the same molar mass, Fick's law becomes
where is the mole fraction of species . | 7 | Physical Chemistry |
This technique brings together protein and precipitation solutions without premixing them, but instead, injecting them through either sides of a channel, allowing equilibrium through diffusion. The two solutions come into contact in a reagent chamber, both at their maximum concentrations, initiating spontaneous nucleation. As the system comes into equilibrium, the level of supersaturation decreases, favouring crystal growth. | 3 | Analytical Chemistry |
A reductone is a special class of organic compounds. They are enediols with a carbonyl group adjacent to the enediol group, i.e. RC(OH)=C(OH)-C(O)R. The enediol structure is stabilized by the resonance resulting from the tautomerism with the adjacent carbonyl. Therefore, the chemical equilibrium produces mainly the enediol form rather than the keto form.
Reductones are reducing agents, thus efficacious antioxidants. Some are fairly strong acids.
Examples of reductones are tartronaldehyde, reductic acid and ascorbic acid. | 0 | Organic Chemistry |
The secretion pattern of thyrotropin (TSH) is shaped by infradian, circadian and ultradian rhythms. Infradian rhythmis are mainly represented by circannual variation mirroring the seasonality of thyroid function. Circadian rhythms lead to a peak secretion (acrophase) around midnight and nadir concentrations around noon and in the early afternoon. A similar pattern is observed for triiodothyronine, however with a phase shift. Pulsatile release contributes to the ultradian rhythm of TSH concentration with about 10 pulses per 24 hours. The amplitude of the circadian and ultradian rhythms is reduced in severe non-thyroidal illness syndrome (TACITUS).
Contemporary theories assume that autocrine and paracrine (ultrashort) feedback mechanisms controlling TSH secretion within the anterior pituitary gland are a major factor contributing to the evolution of its pulsatility. | 1 | Biochemistry |
The genes provided in this section are only a small number of the vast amount of oscillating genes found in the world. These genes were selected because they were determined to be the some of most important genes in regulating the circadian rhythm of their respective classification. | 1 | Biochemistry |
The extent of reaction is a useful quantity in computations with equilibrium reactions. Consider the reaction
:2 A ⇌ B + 3 C
where the initial amounts are , and the equilibrium amount of A is 0.5 mol. We can calculate the extent of reaction in equilibrium from its definition
In the above, we note that the stoichiometric number of a reactant is negative. Now when we know the extent, we can rearrange the equation and calculate the equilibrium amounts of B and C. | 7 | Physical Chemistry |
Sir William Herschel was one of the first to advocate the use of fingerprinting in the identification of criminal suspects. While working for the Indian Civil Service, he began to use thumbprints on documents as a security measure to prevent the then-rampant repudiation of signatures in 1858.
In 1877 at Hooghly (near Kolkata), Herschel instituted the use of fingerprints on contracts and deeds, and he registered government pensioners fingerprints to prevent the collection of money by relatives after a pensioners death.
In 1880, Henry Faulds, a Scottish surgeon in a Tokyo hospital, published his first paper on the subject in the scientific journal Nature, discussing the usefulness of fingerprints for identification and proposing a method to record them with printing ink. He established their first classification and was also the first to identify fingerprints left on a vial. Returning to the UK in 1886, he offered the concept to the Metropolitan Police in London, but it was dismissed at that time.
Faulds wrote to Charles Darwin with a description of his method, but, too old and ill to work on it, Darwin gave the information to his cousin, Francis Galton, who was interested in anthropology. Having been thus inspired to study fingerprints for ten years, Galton published a detailed statistical model of fingerprint analysis and identification and encouraged its use in forensic science in his book Finger Prints. He had calculated that the chance of a "false positive" (two different individuals having the same fingerprints) was about 1 in 64 billion.
Juan Vucetich, an Argentine chief police officer, created the first method of recording the fingerprints of individuals on file. In 1892, after studying Galtons pattern types, Vucetich set up the worlds first fingerprint bureau. In that same year, Francisca Rojas of Necochea was found in a house with neck injuries whilst her two sons were found dead with their throats cut. Rojas accused a neighbour, but despite brutal interrogation, this neighbour would not confess to the crimes. Inspector Alvarez, a colleague of Vucetich, went to the scene and found a bloody thumb mark on a door. When it was compared with Rojas' prints, it was found to be identical with her right thumb. She then confessed to the murder of her sons.
A Fingerprint Bureau was established in Calcutta (Kolkata), India, in 1897, after the Council of the Governor General approved a committee report that fingerprints should be used for the classification of criminal records. Working in the Calcutta Anthropometric Bureau, before it became the Fingerprint Bureau, were Azizul Haque and Hem Chandra Bose. Haque and Bose were Indian fingerprint experts who have been credited with the primary development of a fingerprint classification system eventually named after their supervisor, Sir Edward Richard Henry. The Henry Classification System, co-devised by Haque and Bose, was accepted in England and Wales when the first United Kingdom Fingerprint Bureau was founded in Scotland Yard, the Metropolitan Police headquarters, London, in 1901. Sir Edward Richard Henry subsequently achieved improvements in dactyloscopy.
In the United States, Henry P. DeForrest used fingerprinting in the New York Civil Service in 1902, and by December 1905, New York City Police Department Deputy Commissioner Joseph A. Faurot, an expert in the Bertillon system and a fingerprint advocate at Police Headquarters, introduced the fingerprinting of criminals to the United States. | 3 | Analytical Chemistry |
In catenane nomenclature, a number in square brackets precedes the word "catenane" in order to indicate how many rings are involved. Discrete catenanes up to a [7]catenane have been synthesised. | 6 | Supramolecular Chemistry |
Depending on the general substrate on which the assay principle is applied:
#Bioassay: when the response is biological activity of live objects. Examples include
##in vivo, whole organism (e.g. mouse or other subject injected with a drug)
##ex vivo body part (e.g. leg of a frog)
##ex vivo organ (e.g. heart of a dog)
##ex vivo part of an organ (e.g. a segment of an intestine).
##tissue (e.g. limulus lysate)
##cell (e.g. platelets)
#Ligand binding assay when a ligand (usually a small molecule) binds a receptor (usually a large protein).
#Immunoassay when the response is an antigen antibody binding type reaction. | 1 | Biochemistry |
In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures are used. In these, one end of the bimetallic strip is mechanically fixed and attached to an electrical power source, while the other (moving) end carries an electrical contact. In adjustable thermostats another contact is positioned with a regulating knob or lever. The position so set controls the regulated temperature, called the set point.
Some thermostats use a mercury switch connected to both electrical leads. The angle of the entire mechanism is adjustable to control the set point of the thermostat.
Depending upon the application, a higher temperature may open a contact (as in a heater control) or it may close a contact (as in a refrigerator or air conditioner).
The electrical contacts may control the power directly (as in a household iron) or indirectly, switching electrical power through a relay or the supply of natural gas or fuel oil through an electrically operated valve. In some natural gas heaters the power may be provided with a thermocouple that is heated by a pilot light (a small, continuously burning, flame). In devices without pilot lights for ignition (as in most modern gas clothes dryers and some natural gas heaters and decorative fireplaces) the power for the contacts is provided by reduced household electrical power that operates a relay controlling an electronic ignitor, either a resistance heater or an electrically powered spark generating device. | 8 | Metallurgy |
In organic chemistry, the Baker–Nathan effect is observed with reaction rates for certain chemical reactions with certain substrates where the order in reactivity cannot be explained solely by an inductive effect of substituents.
This effect was described in 1935 by John W. Baker and W. S. Nathan. They examined the chemical kinetics for the reaction of pyridine with benzyl bromide to form a pyridinium salt, and a series of benzyl bromides having different alkyl groups as substituents at the para position.
The reaction is facilitated by electron-releasing substituents (the inductive effect) and in general the observed order (with decreasing reactivity) is tert-butyl > isopropyl > ethyl > methyl. The observed order in this particular reaction however was methyl > ethyl> isopropyl > tert-butyl. In 1935 Baker and Nathan explained the observed difference in terms of a conjugation effect and in later years after the advent of hyperconjugation (1939) as its predecessor.
A fundamental problem with the effect is that differences in the observed order are relatively small and therefore difficult to measure accurately. Other researchers have found similar results or very different results. An alternative explanation for the effect is differential solvation as orders invert on going from the solution phase to the gas phase.
Today, the conjugation of neighbouring pi orbitals and polarised sigma bonds is known as hyperconjugation. Numerous anomalous physical measurements, including bond lengths and dipole moments, have been examined through this concept. The original formulation of the Baker-Nathan effect is no longer employed due to more logical reasons for rate accelerations in solutions and its historical context is discussed by Saltzman. | 7 | Physical Chemistry |
Aluminium-based nanogalvanic alloys are characterized by their galvanic microstructure, which comprises an anodic matrix consisting of aluminum, an aluminum alloy, and a cathodic dispersed phase of another metal composition. These other metals may be tin, magnesium, silicon, bismuth, lead, gallium, indium, zinc, carbon, or a mixture of these metals. These alloys produce hydrogen gas when the cathodic disperse phase forms galvanic couples with the anodic matrix and the resulting galvanic metal microstructure comes in contact with water or any liquid containing water. The nanostructured galvanic couple, with aluminium as the anode and the other metal element as the cathode, rapidly disturbs the formation of the native oxide layer and continually exposes fresh aluminium surfaces to hydrolysis.
The size of the particles that make up the cathodic disperse phase can range from less than 50 nanometers in length to less than 1000 nanometers in length. No additional health hazards have been observed with the handling of the nanogalvanic powders. The by-products of the powder reaction with water was also found to be non-toxic. In terms of performance, the aluminium-based nanogalvanic alloys were demonstrated to produce 1000 ml. of hydrogen gas per gram of aluminium in less than 1 minute and 1340 ml—100% of the theoretical yield at 295 K and 1 atm.—in 3 minutes without the need for hazardous or costly materials, or additional processes. Aluminium-based nanogalvanic alloys can be manufactured by means of low energy ball milling at room temperature or at lower temperatures and remain stable at standard temperature, pressure, and humidity levels.
In 2017, ARL researchers discovered that the hydrogen generation rate increases by almost two-fold when the aluminium-based nanogalvanic alloy powder comes in contact with urine, when compared with pure water. | 2 | Environmental Chemistry |
Sterilization is an important factor to consider during the fermentation of foods. Failing to completely remove any microbes from equipment and storing vessels may result in the multiplication of harmful organisms within the ferment, potentially increasing the risks of food borne illnesses like botulism. However, botulism in vegetable ferments is only possible when not properly canned. The production of off smells and discoloration may be indications that harmful bacteria may have been introduced to the food.
Alaska has witnessed a steady increase of cases of botulism since 1985. It has more cases of botulism than any other state in the United States of America. This is caused by the traditional Alaska Native practice of allowing animal products such as whole fish, fish heads, walrus, sea lion, and whale flippers, beaver tails, seal oil, and birds, to ferment for an extended period of time before being consumed. The risk is exacerbated when a plastic container is used for this purpose instead of the old-fashioned, traditional method, a grass-lined hole, as the Clostridium botulinum bacteria thrive in the anaerobic conditions created by the air-tight enclosure in plastic.
The World Health Organization has classified pickled foods as possibly carcinogenic, based on epidemiological studies. Other research found that fermented food contains a carcinogenic by-product, ethyl carbamate (urethane). "A 2009 review of the existing studies conducted across Asia concluded that regularly eating pickled vegetables roughly doubles a person's risk for esophageal squamous cell carcinoma." | 1 | Biochemistry |
One way of understanding the carcinogenic effects of benzene is by examining the products of biological oxidation. Pure benzene, for example, oxidizes in the body to produce an epoxide, benzene oxide, which is not excreted readily and can interact with DNA to produce harmful mutations. | 2 | Environmental Chemistry |
2-amino-4-deoxychorismate dehydrogenase - 2-dehydropantolactone reductase (B-specific) - 2-methylacyl-CoA dehydrogenase - 2-nitropropane dioxygenase - 2-oxobutyrate synthase - (2,3-dihydroxybenzoyl)adenylate synthase - 2,4-Dihydroxy-1,4-benzoxazin-3-one-glucoside dioxygenase - 2010107G12Rik - 27-hydroxycholesterol 7alpha-monooxygenase - 3 end - 3 flanking region - 3-hydroxy-2-methylpyridinecarboxylate dioxygenase - 3-Ketosteroid 9alpha-monooxygenase - 3-oxoacyl-(acyl-carrier-protein) reductase (NADH) - (3,5-dihydroxyphenyl)acetyl-CoA 1,2-dioxygenase - 3(or 17)a-hydroxysteroid dehydrogenase - 3110001I22Rik - 3alpha-hydroxyglycyrrhetinate dehydrogenase - 4932414N04Rik - 3alpha-hydroxysteroid dehydrogenase (A-specific) - 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoyl-CoA 24-hydroxylase - 3alpha,7alpha,12alpha-trihydroxycholestan-26-al 26-oxidoreductase - 4-Cresol dehydrogenase (hydroxylating) - 4-Hydroxycyclohexanecarboxylate dehydrogenase - 4-hydroxyphenylacetaldehyde oxime monooxygenase - 4-hydroxyphenylpyruvate oxidase - 4-Nitrophenol 4-monooxygenase - 4933425L06Rik - 5 end - 5 flanking region - 5-pyridoxate dioxygenase - 6-endo-hydroxycineole dehydrogenase - 7-deoxyloganin 7-hydroxylase - 7beta-hydroxysteroid dehydrogenase (NADP+) - 8-oxocoformycin reductase - 12beta-hydroxysteroid dehydrogenase - 25-hydroxycholesterol 7α-hydroxylase - | 1 | Biochemistry |
Kai Simons honours and awards include:
* 1975 Federation of European Biochemical Society Anniversary Prize
* 1984 Honorary Professor, University of Heidelberg, Germany
* 1989 Lamport Visiting Professor (Columbia, New York)
* 1990 Histochemical Society Lecture
* 1990 Keith R. Porter Lecture, American Society of Cell Biology
* 1991 Anders Jahre Prize for Medical Research
* 1991 NICHD Lectureship in Cell Biology
* 1993 Carl Zeiss Lecture, German Society for Cell Biology
* 1994 Harvey Society Lecture
* 1996 Dunham Lecturer, Harvard University
* 1996 Foreign Member of the American Academy of Arts and Sciences
* 1997 Academia Europaea
* 1997 Adam Neville Lecture, University of Dundee, UK
* 1997 Foreign Member of the National Academy of Sciences, USA
* 1997 Runeberg Prize, Finland
* 1998 Academy of Sciences Leopoldina
* 1998 Choh Hao Li Memorial Lecture in Biochemical Endocrinology, University of California, Berkeley
* 1999 Doctor Honoris Causa, University of Oulu, Finland
* 2000 President of the European Life Scientist Organisation
* 2001 Honorary member of the German Society for Cell Biology
* 2001 ISI Highly cited researcher
* 2001 Schleiden Medal of Academy Leopoldina
* 2002 Hoppe-Seyler Lecture, Society of Biology and Molecular Biology Germany
* 2003 Albert Wander Prize, University of Bern, Switzerland
* 2003 Matti Äyräpää Prize, Finland
* 2003 Doctor Honoris Causa, University of Leuven, Belgium
* 2003 Honorary Professor at TU Dresden
* 2003 Prix Mondial Nessim Habif, University of Geneva, Switzerland
* 2004 Virchow Lecture and Medal, University of Würzburg, Germany
* 2005 Foundation Day Lecture, Centre for Cellular and Molecular Biology, Hyderabad, India
* 2005 van Deenen Medal, Utrecht, The Netherlands
* 2006 A.I. Virtanen Medal, Finland
* 2006 Honorary member of the Societas Biochemica, Biophysica et Microbiologica Fenniae
* 2007 Order of Merit of the Free State of Saxony
* 2010 Doctor honoris causa, University of Kuopio, Finland
* 2011 Doctor honoris causa, University of Geneva, Switzerland
* 2016 Robert Koch Medal
Kai Simons was and is also a member of numerous societies, committees and organisations, as well as an editor for several scientific journals. | 1 | Biochemistry |
Phosphonates are esters of phosphonic acid and have the general formula RP(=O)(OR'). Phosphonates have many technical applications, a well-known member being glyphosate, better known as Roundup. With the formula (HO)P(O)CHNHCHCOH, this derivative of glycine is one of the most widely used herbicides. Bisphosphonates are a class of drugs to treat osteoporosis. The nerve gas agent sarin, containing both C–P and F–P bonds, is a phosphonate.
Phosphinates feature two P–C bonds, with the general formula RP(=O)(OR'). A commercially significant member is the herbicide glufosinate. Similar to glyphosate mentioned above, it has the structure CHP(O)(OH)CHCHCH(NH)COH.
The Michaelis–Arbuzov reaction is the main method for the synthesis of these compounds. For example, dimethylmethylphosphonate (see figure above) arises from the rearrangement of trimethylphosphite, which is catalyzed by methyl iodide. In the Horner–Wadsworth–Emmons reaction and the Seyferth–Gilbert homologation, phosphonates are used in reactions with carbonyl compounds. The Kabachnik–Fields reaction is a method for the preparation of aminophosphonates. These compounds contain a very inert bond between phosphorus and carbon. Consequently, they hydrolyze to give phosphonic and phosphinic acid derivatives, but not phosphate. | 0 | Organic Chemistry |
Kaede is a photoactivatable fluorescent protein naturally originated from a stony coral, Trachyphyllia geoffroyi. Its name means "maple" in Japanese. With the irradiation of ultraviolet light (350–400 nm), Kaede undergoes irreversible photoconversion from green fluorescence to red fluorescence.
Kaede is a homotetrameric protein with the size of 116 kDa. The tetrameric structure was deduced as its primary structure is only 28 kDa. This tetramerization possibly makes Kaede have a low tendency to form aggregates when fused to other proteins. | 1 | Biochemistry |
First, the coverage and quality of an interactome has to be evaluated. Interactomes are never complete, given the limitations of experimental methods. For instance, it has been estimated that typical Y2H screens detect only 25% or so of all interactions in an interactome. The coverage of an interactome can be assessed by comparing it to benchmarks of well-known interactions that have been found and validated by independent assays. Other methods filter out false positives calculating the similarity of known annotations of the proteins involved or define a likelihood of interaction using the subcellular localization of these proteins. | 1 | Biochemistry |
The nomenclature is based on [n × m] G, n corresponds to the number of ligands above the metal ion level, m the number below ones. In case of using only one ligand type, the homoleptic grid is formed in a square [nxn] structure. When using different ligands arise heteroleptic complexes, however, compete with the homoleptic. The number of metal ions is always n + m. | 6 | Supramolecular Chemistry |
The primary concerns to take into consideration when using a CDA in NMR spectroscopy are kinetic resolution, racemization during the derivatization reaction and that the reagent should have 100% optical purity. Kinetic resolution is especially significant when determining optical purity, but it is somewhat negligible when the CDA is being used to assign the absolute configuration of an optically pure substrate. Kinetic resolution can be overcome using excess of the CDA. Racemization can occur to either the CDA or the substrate and in both cases it has the potential to significantly affect the results. | 4 | Stereochemistry |
Pricing for progesterone can vary depending location, insurance coverage, discount coupons, quantity, shortages, manufacturers, brand or generic versions, different pharmacies, and so on. As of currently, 30 capsules of 100 mg of the generic version, Prometrium, from CVS Pharmacy is around $40 without any discounts or insurance applied. The brand version, Progesterone, is around $450 for 30 capsules without any discounts or insurance applied. In comparison, Walgreens offers 30 capsules of 100 mg in the generic version for $51 without insurance or coupons applied. The brand name costs around $431 for 30 capsules of 100 mg. | 0 | Organic Chemistry |
Many secondary amines react with carbon disulfide and sodium hydroxide to form dithiocarbamate salts:
Ammonia reacts with CS similarly:
:2 NH + CS → HNCSNH
Dithiocarbamate salts are pale colored solids that are soluble in water and polar organic solvents. | 0 | Organic Chemistry |
Access to oil was and still is a major factor in several military conflicts of the 20th century, including World War II, during which oil facilities were a major strategic asset and were extensively bombed. The German invasion of the Soviet Union included the goal to capture the Baku oilfields, as it would provide much-needed oil supplies for the German military which was suffering from blockades.
Oil exploration in North America during the early 20th century later led to the U.S. becoming the leading producer by mid-century. As petroleum production in the U.S. peaked during the 1960s, the United States was surpassed by Saudi Arabia and the Soviet Union in total output.
In 1973, Saudi Arabia and other Arab nations imposed an oil embargo against the United States, United Kingdom, Japan and other Western nations which supported Israel in the Yom Kippur War of October 1973. The embargo caused an oil crisis. This was followed by the 1979 oil crisis, which was caused by a drop in oil production in the wake of the Iranian Revolution and caused oil prices to more than double. | 7 | Physical Chemistry |
The adsorption of ensemble molecules on a surface or interface can be divided into two processes: adsorption and desorption. If the adsorption rate wins the desorption rate, the molecules will accumulate over time giving the adsorption curve over time. If the desorption rate is larger, the number of molecules on the surface will decrease over time. The adsorption rate is dependent on the temperature, the diffusion rate of the solute (related to mean free path for pure gas), and the energy barrier between the molecule and the surface. The diffusion and key elements of the adsorption rate can be calculated using Fick's laws of diffusion and Einstein relation (kinetic theory).
Under ideal conditions, when there is no energy barrier and all molecules that diffuse and collide with the surface get adsorbed, the number of molecules adsorbed at a surface of area on an infinite area surface can be directly integrated from Fick's second law differential equation to be:
where is the surface area (unit m), is the number concentration of the molecule in the bulk solution (unit #/m), is the diffusion constant (unit m/s), and is time (unit s). Further simulations and analysis of this equation show that the square root dependence on the time is originated from the decrease of the concentrations near the surface under ideal adsorption conditions. Also, this equation only works for the beginning of the adsorption when a well-behaved concentration gradient forms near the surface. Correction on the reduction of the adsorption area and slowing down of the concentration gradient evolution have to be considered over a longer time.
Under real experimental conditions, the flow and the small adsorption area always make the adsorption rate faster than what this equation predicted, and the energy barrier will either accelerate this rate by surface attraction or slow it down by surface repulsion. Thus, the prediction from this equation is often a few to several orders of magnitude away from the experimental results. Under special cases, such as a very small adsorption area on a large surface, and under chemical equilibrium when there is no concentration gradience near the surface, this equation becomes useful to predict the adsorption rate with debatable special care to determine a specific value of in a particular measurement.
The desorption of a molecule from the surface depends on the binding energy of the molecule to the surface and the temperature. The typical overall adsorption rate is thus often a combined result of the adsorption and desorption. | 7 | Physical Chemistry |
Semicarbazide products (semicarbazones and thiosemicarbazones) are known to have an activity of antiviral, antiinfective and antineoplastic through binding to copper or iron in cells. | 0 | Organic Chemistry |
In chemistry, a regular solution is a solution whose entropy of mixing is equal to that of an ideal solution with the same composition, but is non-ideal due to a nonzero enthalpy of mixing. Such a solution is formed by random mixing of components of similar molar volume and without strong specific interactions, and its behavior diverges from that of an ideal solution by showing phase separation at intermediate compositions and temperatures (a miscibility gap). Its entropy of mixing is equal to that of an ideal solution with the same composition, due to random mixing without strong specific interactions. For two components
where is the gas constant, the total number of moles, and the mole fraction of each component. Only the enthalpy of mixing is non-zero, unlike for an ideal solution, while the volume of the solution equals the sum of volumes of components. | 7 | Physical Chemistry |
Gestonorone caproate is a potent, long-acting, and pure progestogen, possessing no androgenic, anabolic, antiandrogenic, estrogenic, antiestrogenic, glucocorticoid, mineralocorticoid, or teratogenic effects. It is approximately 20 to 25 times more potent than progesterone or hydroxyprogesterone caproate in animal bioassays when all are given by subcutaneous injection. In humans, 100 or 200 mg intramuscular gestonorone caproate has been said to be equivalent to 1,000 mg intramuscular hydroxyprogesterone caproate. Hence, gestonorone caproate is approximately 5- to 10-fold more potent than hydroxyprogesterone caproate in humans. The biological effects of gestonorone caproate in women have been studied.
Like other potent progestins, gestonorone caproate possesses potent antigonadotropic activity and is capable of markedly suppressing the gonadal production and circulating levels of sex hormones such as testosterone and estradiol. A clinical study found that 400 mg/week intramuscular gestonorone caproate suppressed testosterone levels by 75% in men, while orchiectomy as a comparator reduced testosterone levels by 91%. Levels of luteinizing hormone, conversely, remained unchanged. In general, progestogens can maximally suppress testosterone levels by about 70 to 80%. In accordance with its lack of glucocorticoid activity, gestonorone caproate has no anticorticotropic effects, and does not influence the secretion of adrenocorticotropic hormone.
17α-Hydroxyprogesterone has weak progestogenic activity, but C17α esterification results in higher progestogenic activity. Of a variety of different esters, the caproate (hexanoate) ester was found to have the strongest progestogenic activity, and this formed the basis for the development of gestonorone caproate, as well as other caproate progestogen esters such as hydroxyprogesterone caproate.
Gestonorone caproate has been found to decrease the weights of the prostate gland and seminal vesicles by 40 to 70% in adult male rats. It has been shown in canines to mediate these effects both via its antigonadotropic effects and by direct actions in these tissues. Gestonorone caproate decreases the uptake of testosterone into the prostate gland. It has also been found to have direct antiproliferative effects on human ovarian cancer cells in vitro.
Gestonorone caproate has been reported to act to some extent as a 5α-reductase inhibitor, similarly to progesterone. | 4 | Stereochemistry |
C3 carbon fixation is prone to photorespiration (PR) during dehydration, accumulating toxic glycolate products. In the 2000s scientists used computer simulation combined with an optimization algorithm to figure out what parts of the metabolic pathway may be tuned to improve photosynthesis. According to simulation, improving glycolate metabolism would help significantly to reduce photorespiration.
Instead of optimizing specific enzymes on the PR pathway for glycolate degradation, South et al. decided to bypass PR altogether. In 2019, they transferred Chlamydomonas reinhardtii glycolate dehydrogenase and Cucurbita maxima malate synthase into the chloroplast of tobacco (a model organism). These enzymes, plus the chloroplasts own, create a catabolic cycle: acetyl-CoA combines with glyoxylate to form malate, which is then split into pyruvate and CO; the former in turn splits into acetyl-CoA and CO. By forgoing all transport among organelles, all the CO released will go into increasing the CO concentration in the chloroplast, helping with refixation. The end result is 24% more biomass. An alternative using E. coli' glycerate pathway produced a smaller improvement of 13%. They are now working on moving this optimization into other crops like wheat. | 5 | Photochemistry |
Commercially available salts thiamine chloride and thiamine nitrate are produced at scales of thousands of tons annually by chemistry-based manufacturing processes in Europe and Asia. These salts are supplied for formulations for supplementation of human diet and as feed additives for cattle, swine, poultry and fish.
Research for potential biotechnology-based production of thiamine has resulted in patent applications claiming fermentation using recombinant microorganisms modified to deregulate feedback inhibition and allow release of thiamine forms to the media as demonstrated at small scale.
Thiamine forms and their bio-precursors are produced at very large scale in biological matrices such as yeast, grains, plants and meats widely consumed as food and feed. Research into genetic modification of plants. has led to higher levels of thiamine in foodstuffs, such as rice. Use of thiamine forms and their bio-precursors by various means such as seed coating or soil and foliar fertilization to improve plant growth and properties are being investigated. | 1 | Biochemistry |
Trifluoroperacetic acid can be easily prepared by an Organic Syntheses process of treating trifluoroacetic anhydride with a concentrated (90%) aqueous solution of hydrogen peroxide:
:trifluoroacetic anhydride| + hydrogen peroxide| → + trifluoroacetic acid|
As the anhydride will form trifluoroacetic acid in contact with water, an excess of the anhydride also serves to remove the solvent from the peroxide reactant:
: + water| → 2
A more dilute hydrogen peroxide solution (30%) can be used to form trifluoroperacetic acid for some reactions from trifluoroacetic acid.
In order to avoid the danger of handling pure or highly concentrated solutions of hydrogen peroxide, hydrogen peroxide – urea can be used to give the peracid. This method involves no water, so it gives a completely anhydrous peracid, which is an advantage when the presence of water leads to side reactions during certain oxidation reactions.
: + hydrogen peroxide - urea| → + + urea|
In cases where a pH buffering agent is needed for a synthesis and where the presence of water is tolerated, another approach has been developed. Reacting trifluoroacetic anhydride with sodium percarbonate, , yields trifluoroperacetic acid and sodium carbonate, obviating the need for an additional buffer.
:3 + 4 sodium percarbonate| → 6 + 4 sodium carbonate| + 3
Trifluoroperacetic acid can also be generated in situ, allowing it to react promptly with the target substrate rather than pre-synthesizing a batch of the reagent for later use. | 0 | Organic Chemistry |
In matrix-assisted laser desorption and ionization (MALDI), the sample is incorporated in a chemical matrix that is capable of absorbing energy from a laser. Similar to SIMS, ionization happens in vacuum. Laser irradiation ablates the matrix material from the surface and results in charged gas phase matrix particles, the analyte molecules are ionized from this charged chemical matrix. Liu et al. used MALDI-MS to detect eight phospholipids from single A549 cells. MALDI MS imaging can be used for spatial metabolomics and single cell analysis. | 1 | Biochemistry |
There are a number of disorders which disrupt normal functioning of ion channels and have disastrous consequences for the organism. Genetic and autoimmune disorders of ion channels and their modifiers are known as channelopathies. See :Category:Channelopathies for a full list.
* Shaker gene mutations cause a defect in the voltage gated ion channels, slowing down the repolarization of the cell.
* Equine hyperkalaemic periodic paralysis as well as human hyperkalaemic periodic paralysis (HyperPP) are caused by a defect in voltage-dependent sodium channels.
* Paramyotonia congenita (PC) and potassium-aggravated myotonias (PAM)
* Generalized epilepsy with febrile seizures plus (GEFS+)
* Episodic ataxia (EA), characterized by sporadic bouts of severe discoordination with or without myokymia, and can be provoked by stress, startle, or heavy exertion such as exercise.
* Familial hemiplegic migraine (FHM)
* Spinocerebellar ataxia type 13
* Long QT syndrome is a ventricular arrhythmia syndrome caused by mutations in one or more of presently ten different genes, most of which are potassium channels and all of which affect cardiac repolarization.
* Brugada syndrome is another ventricular arrhythmia caused by voltage-gated sodium channel gene mutations.
* Polymicrogyria is a developmental brain malformation caused by voltage-gated sodium channel and NMDA receptor gene mutations.
* Cystic fibrosis is caused by mutations in the CFTR gene, which is a chloride channel.
* Mucolipidosis type IV is caused by mutations in the gene encoding the TRPML1 channel
* Mutations in and overexpression of ion channels are important events in cancer cells. In Glioblastoma multiforme, upregulation of gBK potassium channels and ClC-3 chloride channels enables glioblastoma cells to migrate within the brain, which may lead to the diffuse growth patterns of these tumors. | 1 | Biochemistry |
The increase in intensity of the Raman signal for adsorbates on particular surfaces occurs because of an enhancement in the electric field provided by the surface. When the incident light in the experiment strikes the surface, localized surface plasmons are excited. The field enhancement is greatest when the plasmon frequency, ω, is in resonance with the radiation ( for spherical particles). In order for scattering to occur, the plasmon oscillations must be perpendicular to the surface; if they are in-plane with the surface, no scattering will occur. It is because of this requirement that roughened surfaces or arrangements of nanoparticles are typically employed in SERS experiments as these surfaces provide an area on which these localized collective oscillations can occur. SERS enhancement can occur even when an excited molecule is relatively far apart from the surface which hosts metallic nanoparticles enabling surface plasmon phenomena.
The light incident on the surface can excite a variety of phenomena in the surface, yet the complexity of this situation can be minimized by surfaces with features much smaller than the wavelength of the light, as only the dipolar contribution will be recognized by the system. The dipolar term contributes to the plasmon oscillations, which leads to the enhancement. The SERS effect is so pronounced because the field enhancement occurs twice. First, the field enhancement magnifies the intensity of incident light, which will excite the Raman modes of the molecule being studied, therefore increasing the signal of the Raman scattering. The Raman signal is then further magnified by the surface due to the same mechanism that excited the incident light, resulting in a greater increase in the total output. At each stage the electric field is enhanced as E, for a total enhancement of E.
The enhancement is not equal for all frequencies. For those frequencies for which the Raman signal is only slightly shifted from the incident light, both the incident laser light and the Raman signal can be near resonance with the plasmon frequency, leading to the E enhancement. When the frequency shift is large, the incident light and the Raman signal cannot both be on resonance with ω, thus the enhancement at both stages cannot be maximal.
The choice of surface metal is also dictated by the plasmon resonance frequency. Visible and near-infrared radiation (NIR) are used to excite Raman modes. Silver and gold are typical metals for SERS experiments because their plasmon resonance frequencies fall within these wavelength ranges, providing maximal enhancement for visible and NIR light. Copper's absorption spectrum also falls within the range acceptable for SERS experiments. Platinum and palladium nanostructures also display plasmon resonance within visible and NIR frequencies. | 7 | Physical Chemistry |
Cell–cell interaction refers to the direct interactions between cell surfaces that play a crucial role in the development and function of multicellular organisms.
These interactions allow cells to communicate with each other in response to changes in their microenvironment. This ability to send and receive signals is essential for the survival of the cell. Interactions between cells can be stable such as those made through cell junctions. These junctions are involved in the communication and organization of cells within a particular tissue. Others are transient or temporary such as those between cells of the immune system or the interactions involved in tissue inflammation. These types of intercellular interactions are distinguished from other types such as those between cells and the extracellular matrix. The loss of communication between cells can result in uncontrollable cell growth and cancer. | 1 | Biochemistry |
Ioliomics (from a portmanteau of ions and liquids) is the study of ions in liquids (or liquid phases) and stipulated with fundamental differences of ionic interactions. Ioliomics covers a broad research area concerning structure, properties and applications of ions involved in various biological and chemical systems. The concept of this research discipline is related to other comprehensive research fields, such as genomics, proteomics, glycomics, petroleomics, etc., where the suffix -omics is used for describing the comprehensiveness of data. | 1 | Biochemistry |
Hemiacetals and hemiketals may be thought of as intermediates in the reaction between alcohols and aldehydes or ketones, with the final product being an acetal or a ketal:
:RC=O + ROH ⇌ RC(OH)(OR)
:RC(OH)(OR) + ROH ⇌ RC(OR') + HO
Usually, the second reaction is unfavorable. In the presence of a dehydrating agent, it proceeds. | 0 | Organic Chemistry |
Vitamin D is carried via the blood to the liver, where it is converted into the prohormone calcifediol. Circulating calcifediol may then be converted into calcitriol the biologically active form of vitamin D in the kidneys.
Whether synthesized in the skin or ingested, vitamin D is hydroxylated in the liver at position 25 (upper right of the molecule) to form 25-hydroxycholecalciferol (calcifediol or 25(OH)D). This reaction is catalyzed by the microsomal enzyme vitamin D 25-hydroxylase, the product of the CYP2R1 human gene, and expressed by hepatocytes. Once made, the product is released into the plasma, where it is bound to an α-globulin carrier protein named the vitamin D-binding protein.
Calcifediol is transported to the proximal tubules of the kidneys, where it is hydroxylated at the 1-α position (lower right of the molecule) to form calcitriol (1,25-dihydroxycholecalciferol, 1,25(OH)D). The conversion of calcifediol to calcitriol is catalyzed by the enzyme 25-hydroxyvitamin D 1-alpha-hydroxylase, which is the product of the CYP27B1 human gene. The activity of CYP27B1 is increased by parathyroid hormone, and also by low calcium or phosphate. Following the final converting step in the kidney, calcitriol is released into the circulation. By binding to vitamin D-binding protein, calcitriol is transported throughout the body, including to the intestine, kidneys, and bones. Calcitriol is the most potent natural ligand of the vitamin D receptor, which mediates most of the physiological actions of vitamin D. In addition to the kidneys, calcitriol is also synthesized by certain other cells, including monocyte-macrophages in the immune system. When synthesized by monocyte-macrophages, calcitriol acts locally as a cytokine, modulating body defenses against microbial invaders by stimulating the innate immune system. | 1 | Biochemistry |
Fluoride (F) and acetate (CHCOO) anions are among the most important ones in the context of human health and well-being. The former, though used extensively in health care, is known for its toxicity and corrosiveness. The latter can cause alkalosis and affect metabolic pathways beyond a certain concentration. Hence, it is crucial to develop methods to detect these anions in aqueous media. Bhat et al. constructed an INHIBIT gate with receptors that bind selectively to F and CHCOO anions. The system used changes in absorbance as a colorimetric-based output to detect the concentration of anions.
Wen and coworkers also designed an INHIBIT molecular logic gate with Fe and EDTA as the inputs and a fluorescence output for the detection of ferric ions in solutions. The fluorescence of the system is quenched if and only if Fe input is present and EDTA is absent.
Heavy metal ions provide a persistent threat to human health because of their inherent toxicity and low degradability. Several molecular logic gate-based systems have been constructed to detect ions such as Cd, Hg/Pb, and Ag. In their work, Chen and coworkers demonstrated that logic gate-based systems could be used to detect Cd ions in rice samples, thereby widening the scope of the field to safety detection in food materials, too. | 6 | Supramolecular Chemistry |
George H. Miley is a professor of nuclear engineering and a cold fusion researcher who claims to have replicated the Patterson power cell. During the 2011 World Green Energy Symposium, Miley stated that his device continuously produces several hundred watts of power. Earlier results by Miley have not convinced mainstream researchers, who believe that they can be explained by contamination or by misinterpretation of data.
On Good Morning America, Quintin Bowles, professor of mechanical engineering at the University of Missouri–Kansas City, claimed in 1996 to have successfully replicated the Patterson power cell. In the book Voodoo Science, Bowles is quoted as having stated: "It works, we just don't know how it works."
A replication has been attempted at Earthtech, using a CETI supplied kit. They were not able to replicate the excess heat. They looked for cold fusion products, but only found traces of contamination in the electrolyte. | 7 | Physical Chemistry |
The OEPCR method uses a small amount of the gene encoding the monomeric ELP unit and leads to the amplification of this segment to a great extent. This amplification is due to the fact that the initial segment added to the reaction functions as a template, from which identical gene segments can be synthesized. The process will result in the production of double-stranded DNA encoding the ELP of interest. One major bottleneck associated with this method is the potentially low fidelity associated with the Taq polymerase used. This might lead to replication from the template in which the wrong nucleotides are incorporated into the growing DNA strand. | 7 | Physical Chemistry |
As of 2017, 7% of genetic therapy trials targeted infectious diseases. 69.2% of trials targeted HIV, 11% hepatitis B or C, and 7.1% malaria. | 1 | Biochemistry |
Inevitably, [4+4] photocycloadditions carry the side reaction of [2+2] photocycloadditions. However, since these reactions are reversible, the most stable product may be formed through thermodynamic control. | 5 | Photochemistry |
It is believed that thyroid hormones evolved in the Urbilaterian well before the development of the thyroid itself and molluscs, echinoderms, cephalochordates and ascidians all use such hormones. Cnidarians also respond to Thyroid hormone despite being parahoxozoans rather than bilaterians.
Insects use hormones similar to thyroid hormone using iodine.
Phosphorylated tyrosines created with tyrosine kinases are fundamental signalling molecules in all animals and in choanoflagellates. | 1 | Biochemistry |
Weatherization generally does not cause indoor air quality problems by adding new pollutants to the air. (There are a few exceptions, such as caulking, that can sometimes emit pollutants.) However, measures such as installing storm windows, weather stripping, caulking, and blown-in wall insulation can reduce the amount of outdoor air infiltrating into a home. Consequently, after weatherization, concentrations of indoor air pollutants from sources inside the home can increase.
Weatherization may have a negative impact on indoor air quality, if done improperly, exacerbating respiratory conditions especially among occupants with pre-existing respiratory illnesses. This may occur because of a drastic decrease in air exchange rate in the home, introduction of new chemicals, and poor management of indoor moisture due to a poorly performed weatherization work. Low air exchange rates may lead to higher concentrations of pollutants in the air when ventilation is not sufficiently addressed during weatherization work. However, the situation may be different in case of a house situated in an area with high outdoor air pollution levels such as in close proximity (<200 m) from a busy major road. In such a scenario, a more airtight building envelope can actually offer protection against infiltration of outdoor air pollution. The same is true for the protection offered by tighter building envelopes during wildfire events that cause elevated levels of outdoor air pollution. | 7 | Physical Chemistry |
Solar energy conversion describes technologies devoted to the transformation of solar energy to other (useful) forms of energy, including electricity, fuel, and heat. It covers light-harvesting technologies including traditional semiconductor photovoltaic devices (PVs), emerging photovoltaics, solar fuel generation via electrolysis, artificial photosynthesis, and related forms of photocatalysis directed at the generation of energy rich molecules.
Fundamental electro-optical aspects in several emerging solar energy conversion technologies for generation of both electricity (photovoltaics) and solar fuels constitute an active area of current research. | 7 | Physical Chemistry |
Although the concept of U-value (or U-factor) is universal, U-values can be expressed in different units. In most countries, U-value is expressed in SI units, as watts per square metre-kelvin:
:W/(m⋅K)
In the United States, U-value is expressed as British thermal units (Btu) per hour-square feet-degrees Fahrenheit:
:Btu/(h⋅ft⋅°F)
Within this article, U-values are expressed in SI unless otherwise noted. To convert from SI to US customary values, divide by 5.678.
Well-insulated parts of a have a low thermal transmittance whereas poorly insulated parts of a building have a high thermal transmittance. Losses due to thermal radiation, thermal convection and thermal conduction are taken into account in the U-value. Although it has the same units as heat transfer coefficient, thermal transmittance is different in that the heat transfer coefficient is used to solely describe heat transfer in fluids while thermal transmittance is used to simplify an equation that has several different forms of thermal resistances.
It is described by the equation:
:Φ = A × U × (T - T)
where Φ is the heat transfer in watts, U is the thermal transmittance, T is the temperature on one side of the structure, T is the temperature on the other side of the structure and A is the area in square metres.
Thermal transmittances most walls and roofs can be calculated using ISO 6946, unless there is metal bridging the insulation in which case it can be calculated using ISO 10211. For most ground floors it can be calculated using ISO 13370. For most windows the thermal transmittance can be calculated using ISO 10077 or ISO 15099. ISO 9869 describes how to measure the thermal transmittance of a structure experimentally.
Choice of materials and quality of installation has a critical impact on the window insulation results. The frame and double sealing of the window system are the actual weak points in the window insulation.
Typical thermal transmittance values for common building structures are as follows:
*Single glazing: 5.7 W/(m⋅K)
*Single glazed windows, allowing for frames: 4.5 W/(m⋅K)
*Double glazed windows, allowing for frames: 3.3 W/(m⋅K)
*Double glazed windows with advanced coatings: 2.2 W/(m⋅K)
*Double glazed windows with advanced coatings and frames: 1.2 W/(m⋅K)
*Triple glazed windows, allowing for frames: 1.8 W/(m⋅K)
*Triple glazed windows, with advanced coatings and frames: 0.8 W/(m⋅K)
*Well-insulated roofs: 0.10 W/(m⋅K)
*Poorly insulated roofs: 1.0 W/(m⋅K)
*Well-insulated walls: 0.15 W/(m⋅K)
*Poorly insulated walls: 2 W/(m⋅K)
*Well-insulated floors: 0.2 W/(m⋅K)
*Poorly insulated floors: 1.0 W/(m⋅K)
In practice the thermal transmittance is strongly affected by the quality of workmanship and if insulation is fitted poorly, the thermal transmittance can be considerably higher than if insulation is fitted well | 7 | Physical Chemistry |
Immunomagnetic separation (IMS) is a laboratory tool that can efficiently isolate cells out of body fluid or cultured cells. It can also be used as a method of quantifying the pathogenicity of food, blood or feces. DNA analysis have supported the combined use of both this technique and Polymerase Chain Reaction (PCR). Another laboratory separation tool is the affinity magnetic separation (AMS), which is more suitable for the isolation of prokaryotic cells.
IMS deals with the isolation of cells, proteins, and nucleic acids through the specific capture of biomolecules through the attachment of small-magnetized particles, beads, containing antibodies and lectins. These beads are coated to bind to targeted biomolecules, gently separated and goes through multiple cycles of washing to obtain targeted molecules bound to these super paramagnetic beads, which can differentiate based on strength of magnetic field and targeted molecules, are then eluted to collect supernatant and then are able to determine the concentration of specifically targeted biomolecules. IMS obtains certain concentrations of specific molecules within targeted bacteria.
A mixture of cell population will be put into a magnetic field where cells then are attached to super paramagnetic beads, specific example are Dynabeads (4.5-μm), will remain once excess substrate is removed binding to targeted antigen. Dynabeads consists of iron-containing cores, which is covered by a thin layer of a polymer shell allowing the absorption of biomolecules. The beads are coated with primary antibodies, specific-specific antibodies, lectins, enzymes, or streptavidin; the linkage between magnetized beads coated materials are cleavable DNA linker cell separation from the beads when the culturing of cells is more desirable.
Many of these beads have the same principles of separation; however, the presence and different strength s of magnetic fields requires certain sizes of beads, based on the ramifications of the separation of the cell population. The larger sized beads (>2μm) are the most commonly used range that was produced by Dynal (Dynal [UK] Ltd., Wirral, Mersyside, UK; Dynal, Inc., Lake Success, NY). Where as smaller beads (<100 nm) are mostly used for MACS system that was produced by Miltenyi Biotech (Miltenyi Biotech Ltd., Bisley, Surrey, UK; Miltenyi Biotech Inc., Auburn, CA).
Immunomagnetic separation is used in a variety of scientific fields including molecular biology, microbiology, and immunology. (3) This technique of separation does not only consist of separation of cells within the blood, but can also be used for techniques of separation from primary tumors and in metastases research, through separation into component parts, creating a singular-cell delay, then allowing the suitable antibody to label the cell. In metastasis research this separation technique may become necessary to separate when given a cell population and wanting to isolate tumors cells in tumors, peripheral blood, and bone marrow. | 1 | Biochemistry |
Using non-linear least squares minimization, the following system is solved:
where is the calculated intensity and is the observed intensity of a point in the powder pattern, , is a scale factor, and is the number of measured data points. The minimized function is given by:
where is the weight, and from the previous equation is unity (since is usually absorbed in the phase scale factor). The summation extends to all data points. Considering the peak shape functions and accounting for the overlapping of Bragg peaks because of the one-dimensionality of XRD data, the expanded form of the above equation for the case of a single phase measured with a single wavelength becomes:
where:
* is the background at the data point.
* is the phase scale factor.
* is the number of Bragg reflections contributing to the intensity of the reflection.
* is the integrated intensity of the Bragg peak.
* is the peak shape function.
For a material that contains several phases (), the contribution from each is accounted for by modifying the above equation as follows:
It can easily be seen from the above equations that experimentally minimizing the background, which holds no useful structural information, is paramount for a successful profile fitting. For a low background, the functions are defined by contributions from the integrated intensities and peak shape parameters. But with a high background, the function being minimized depends on the adequacy of the background and not integrated intensities or peak shapes. Thus, a structure refinement cannot adequately yield structural information in the presence of a large background.
It is also worth noting the increased complexity brought forth by the presence of multiple phases. Each additional phase adds to the fitting, more Bragg peaks, and another scale factor tied to corresponding structural parameters, and peak shape. Mathematically they are easily accounted for, but practically, due to the finite accuracy and limited resolution of experimental data, each new phase can lower the quality and stability of the refinement. It is advantageous to use single phase materials when interested in finding precise structural parameters of a material. However, since the scale factors of each phase are determined independently, Rietveld refinement of multi phase materials can quantitatively examine the mixing ratio of each phase in the material. | 3 | Analytical Chemistry |
Bahadurs publications were ambivalently received, and the overall attention of the scientific community seemed limited since Krishna Bahadur and his co-workers reported that the Jeewanus are alive (a striking statement), the team changed the protocols frequently and documented them somewhat idiosyncratically. Bahadur defined "living units" as "[...] those which grow, multiply, and are metabolically active in a systematic, harmonious, and synchronized manner". Then, NASAs Exobiology Division tasked two biologists in 1967 to review and evaluate the literature so far published by Krishna Bahadur (not to replicate the experiments) on the synthesis and characteristics of the Jeewanu. The two NASA biologists did not debate whether these three criteria are an adequate definition of life, but whether the Jeewanu satisfy these criteria. The NASA report concluded that "the evidence presented on these three points is on the whole unconvincing". The report also stated that the postulated existence of these living units has not been proved and "the nature and properties of the Jeewanu remains to be clarified."
In the 1980s, the Hungarian chemist Tibor Gánti discussed the Jeewanu at length in his chemoton theory—an abstract model of autocatalytic chemical reactions—published first in Hungarian and translated into English in 2003. In the context of self-organizing structures, Gánti considered the Jeewanu a promising model system to understand the origin and fundamentals of life, and one that had never received due attention. In 2011, a German scientist stated that the Jeewanu story pertains to concepts of life, its beginnings, as well as possible artificially created cells.
Experimental duplication work published in 2013 by Gupta and Rai reported that their size varies from 0.5 μ to 3.5 μ in diameter, growth from within, metabolic activities, and "the presence of RNA-like material". The authors stated that the RNA-like material detected in the Jeewanu protocells support the RNA world hypothesis. | 9 | Geochemistry |
MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though. First, MRI has a sensitivity of around 10 mol/L mol/L which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the difference between atoms in the high energy state and the low energy state is very small. For example, at 1.5 tesla, a typical field strength for clinical MRI, the difference between high and low energy states is approximately 9 molecules per 2 million. Improvements to increase MR sensitivity include increasing magnetic field strength, and hyperpolarization via optical pumping or dynamic nuclear polarization. There are also a variety of signal amplification schemes based on chemical exchange that increase sensitivity.
To achieve molecular imaging of disease biomarkers using MRI, targeted MRI contrast agents with high specificity and high relaxivity (sensitivity) are required. To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. To enhance the sensitivity of the contrast agents, these targeting moieties are usually linked to high payload MRI contrast agents or MRI contrast agents with high relaxivities. | 1 | Biochemistry |
Phytomining would in principle cause minimal environmental effects compared to mining. Phytomining could also remove low-grade heavy metals from mine waste. | 2 | Environmental Chemistry |
A drugs distribution coefficient strongly affects how easily the drug can reach its intended target in the body, how strong an effect it will have once it reaches its target, and how long it will remain in the body in an active form. Hence, the log P of a molecule is one criterion used in decision-making by medicinal chemists in pre-clinical drug discovery, for example, in the assessment of druglikeness of drug candidates. Likewise, it is used to calculate lipophilic efficiency in evaluating the quality of research compounds, where the efficiency for a compound is defined as its potency, via measured values of pIC or pEC, minus its value of log P'. | 7 | Physical Chemistry |
The preferential alignment is a criterion of an orientation of a molecule or atom. The preferential alignment can be related to the formation of the crystal structure of an amorphous structure.
For a polymer material with liquid crystals, the liquid crystals are molecules shaped like rigid rods. Just as logs being floated down a river tend to travel parallel to the direction of the river, liquid crystals have a preferential alignment with each other. At high temperatures, this alignment is disrupted and the material is said to be in the isotropic state. At lower temperatures, the alignment will take place and the liquid crystals are said to be in the pneumatic state [Hoong.C.C]. | 3 | Analytical Chemistry |
Meta-selective C–H functionalization refers to the regioselective reaction of a substituted aromatic ring on the C–H bond meta to the substituent.
Substituted aromatic ring is an important type of substructure in pharmaceuticals and industrial compounds. Thus, synthetic methods towards substituted aromatic rings are always of great interest to chemists.
Traditionally, regioselectivity on the aromatic ring is achieved by the electronic effect of substituents. Taking the well-known Friedel–Craft electrophilic aromatic substitution as example, electron donating groups direct the electrophile to ortho-/para-position while electron withdrawing groups direct the electrophile to meta-position. However, with complicated systems, electronic difference between different C–H bonds can be subtle and electronic directing effect alone could become less synthetically useful.
The fast development of C–H activation in the past few decades provides synthetic chemists with the powerful tools to synthesize functionalized aromatic compounds with high selectivity. The widely used approach to achieve ortho-selectivity involves metal-chelating directing groups, which forms a relatively stable 6- or 7-membered cyclic pre-transition state to bring the metal catalyst to the proximity of the ortho-hydrogen. However, applying the same strategy to meta- or para- C-H functionalization does not work because the corresponding cyclophane-like cyclic pre-transition state is highly strained. Thus, while ortho-selectivity has been achieved by numerous catalytic systems, meta- and para-selectivity remains a challenge.
In recent years, new strategies that override the electronic and steric bias have been developed to address meta-C–H functionalization. However, before these discoveries, synthesis of meta-substituted aromatic compounds could be either limited or cumbersome. For example, before the development of the C–H activation involving one-pot synthetic route to meta-substituted phenol derivatives by Maleczka and co-workers, the traditional synthesis requires 10 steps from TNT. Some early attempts utilize steric and electronic effects to achieve meta-selectivity. However, they are either limited to certain structure of substrates or are not highly selective. In recent years, several highly selective meta-C-H functionalization strategies have been reported which can override the intrinsic electronic and steric properties of the substrates and can apply to a wide range of substrate derivatives. The development of the modern meta-C-H functionalization strategies “open doors for numerous possibilities” for synthesis and catalyst development. | 0 | Organic Chemistry |
The effluent from the tailings from the mining of sulfidic minerals has been described as "the largest environmental liability of the mining industry". These tailings contain large amounts of pyrite (FeS) and Iron(II) sulfide (FeS), which are rejected from the sought-after ores of copper and nickel, as well as coal. Although harmless underground, these minerals are reactive toward air in the presence of microorganisms, which if not properly managed lead to acid mine drainage. | 8 | Metallurgy |
Chloromethane is an abundant organohalogen, anthropogenic or natural, in the atmosphere. Natural sources produce an estimated 4,100,000,000 kg/yr. | 2 | Environmental Chemistry |
Coral is a calcifying organism, putting it at high risk for decay and slow growth rates as ocean acidification increases. Aragonite assists the coral as they build their skeletons because it is another form of calcium carbonate (CaCO) that is more soluble. When the pH of the water decreases, aragonite decreases as well, leading to the loss of calcium carbonate uptake in corals. Levels of aragonite have decreased by 16% since industrialization and could be lower in some portions of the Great Barrier Reef due to the current, which allows northern corals to take up more aragonite than southern corals. Aragonite is predicted to reduce by 0.1 by 2100 which could greatly hinder coral growth. Since 1990, calcification rates of Porites, a common large reef-building coral in the Great Barrier Reef, have decreased by 14.2% annually. Aragonite levels across the Great Barrier Reef itself are not equal; due to currents and circulation, some portions of the Great Barrier Reef can have half as much aragonite as others. Levels of aragonite are also affected by calcification and production, which can vary from reef to reef.If atmospheric carbon dioxide reaches 560 ppm, most ocean surface waters will be adversely undersaturated with respect to aragonite, and the pH will have reduced by about 0.24 units, from almost 8.2 today to just over 7.9. At this point (sometime in the third quarter of this century, at current rates of carbon dioxide increase), only a few parts of the Pacific will have levels of aragonite saturation adequate for coral growth. Additionally, if atmospheric carbon dioxide reaches 800 ppm, the ocean surface water pH decrease will be 0.4 units, and the total dissolved carbonate ion concentration will have decreased by at least 60%. Recent estimates state that with business-as-usual emission levels, the atmospheric carbon dioxide could reach 800 ppm by the year 2100. At this point, it is almost certain that all the reefs in the world will be in erosional states. Increasing the pH and replicating pre-industrialization ocean chemistry conditions in the Great Barrier Reef, however, led to an increase in coral growth rates of 7%. | 9 | Geochemistry |
The National Oceanic Atmospheric Admiration in the United States has created a forecasting tool for regions such as the Great Lakes. | 2 | Environmental Chemistry |
Stakeholder Program on Agent Detection Assays (SPADA) brings together expert stakeholders from the biothreat community to foster a comprehensive and uniform approach to scientific analysis and detection of biothreat agents. | 3 | Analytical Chemistry |
The global steel industry has been going through major changes since 1970. China has emerged as a major producer and consumer, as has India to a lesser extent. Consolidation has been rapid in Europe. According to the 2019 International Energy Agency (IEA) report, the iron and steel industry directly contributed 2.6 Gt to global CO emissions and accounted for 7% of global energy demand. Singapore is the worlds main trading hub for iron, with about 90% of the worlds iron ore derivatives traded on their stock exchange. | 8 | Metallurgy |
Usually, transition metal NHC complexes are prepared less directly. A popular method entails transmetallation of silver-NHC complexes. Such reagents are generated by the reaction of silver(I) oxide with the imidazolium salt. | 0 | Organic Chemistry |
Expanded nucleotides and their oligomeric helices share many properties with their natural B-DNA counterparts, including their pairing preference: A with T, C with G. The various differences in chemical properties between xDNA and B-DNA support the hypothesis that the benzene ring which expands x-nucleobases is not, in fact, chemically inert. xDNA is more hydrophobic than B-DNA, and also has a smaller HOMO-LUMO gap (distance between the highest occupied molecular orbital and lowest unoccupied molecular orbital) as a result of modified saturation. xDNA has higher melting temperatures than B-DNA (a mixed decamer of xA and T has a melting temperature of 55.6 °C, 34.3 °C higher than the same decamer of A and T), and exhibits an "all-or-nothing" melting behaviour. | 1 | Biochemistry |
Sublimation is historically used as a generic term to describe a two-step phase transition ― a solid-to-gas transition (sublimation in a more precise definition) followed by a gas-to-solid transition (deposition). (See below) | 3 | Analytical Chemistry |
Oligosaprobes are organisms that inhabit clean water or water that is only slightly polluted by organic matter. Oxidation processes predominate in such waters owing to an excess of dissolved oxygen. Nitrates are among the nitrogen compounds present; there is little carbonic acid and no hydrogen sulfide. Oligosaprobic environments are aquatic environments rich in dissolved oxygen and (relatively) free from decayed organic matter. | 9 | Geochemistry |
The regulation of gene expression in eukaryotes is achieved through the interaction of several levels of control that acts both locally to turn on or off individual genes in response to a specific cellular need and globally to maintain a chromatin-wide gene expression pattern that shapes cell identity. Because eukaryotic genome is wrapped around histones to form nucleosomes and higher-order chromatin structures, the substrates for transcriptional machinery are in general partially concealed. Without regulatory proteins, many genes are expressed at low level or not expressed at all. Transcription requires displacement of the positioned nucleosomes to enable the transcriptional machinery to gain access of the DNA.
All steps in the transcription are subject to some degree of regulation. Transcription initiation in particular is the primary level at which gene expression is regulated. Targeting the rate-limiting initial step is the most efficient in terms of energy costs for the cell. Transcription initiation is regulated by cis-acting elements (enhancers, silencers, isolators) within the regulatory regions of the DNA, and sequence-specific trans-acting factors that act as activators or repressors. Gene transcription can also be regulated post-initiation by targeting the movement of the elongating polymerase. | 1 | Biochemistry |
For each component in a binary mixture, one could make a vapor–liquid equilibrium diagram. Such a diagram would graph liquid mole fraction on a horizontal axis and vapor mole fraction on a vertical axis. In such VLE diagrams, liquid mole fractions for components 1 and 2 can be represented as and respectively, and vapor mole fractions of the corresponding components are commonly represented as and . Similarly for binary mixtures in these VLE diagrams:
: and
Such VLE diagrams are square with a diagonal line running from the () corner to the () corner for reference.
These types of VLE diagrams are used in the McCabe–Thiele method to determine the number of equilibrium stages (or theoretical plates) needed to distill a given composition binary feed mixture into one distillate fraction and one bottoms fraction. Corrections can also be made to take into account the incomplete efficiency of each tray in a distillation column when compared to a theoretical plate. | 7 | Physical Chemistry |
Following the first publication, Pockels' studies of surface films accelerated. She continued to correspond with Lord Rayleigh. These communications emphasized her findings concerning the importance of purity and cleanliness of the equipment, including a recognition of difficulties in her own experimentation regarding previously unrecognized contamination.
Pockels pointed out that even airborne dust can affect results with her experimental apparatus. She recognized that impurities can affect reproducibility of experimental findings. Pockels developed a refined method of assessing monolayer films consisting of deposition of the compound on interest as a solution in benzene on the water surface in her sliding trough. Through this line of experimentation, she measured the thickness of certain monolayer films as being 13Å. With the aid of Lord Rayleigh, Pockelss second publication appeared in the journal Nature' in 1892.
Pockels described the calming effect that oils can have on bodies of water, an effect first investigated in the published literature by Benjamin Franklin. Her research extended to investigations of other surface phenomena including capillarity and contact angles.
Pockels published 14 scientific papers, mostly in German journals, the last one being published in 1926. She was eventually recognized as a pioneer in the emerging field of surface science. Following the death of her brother, Friedrich Carl Alwin Pockels in 1913 and her own ill health, Agnes Pockels lost contact with many professional scientists and ceased original research.
Pockels never received a formal appointment for her scientific endeavors. Nevertheless, she published a number of scholarly papers and eventually received recognition as a pioneer in the new field of surface science. Commentators wrote: "When Langmuir received the Nobel Prize for Chemistry in 1932, for his work in investigating monolayers on solids and on liquids, part of his achievement was [...] founded on original experiments first made with a button and a thin tray, by a young lady of 18 who had had no formal scientific training." | 7 | Physical Chemistry |
Allylic strain (also known as A strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin (a synonym for an alkene) with an allylic substituent on the other end. If the substituents (R and R') are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other. Allylic strain was first recognized in the literature in 1965 by Johnson and Malhotra. The authors were investigating cyclohexane conformations including endocyclic and exocylic double bonds when they noticed certain conformations were disfavored due to the geometry constraints caused by the double bond. Organic chemists capitalize on the rigidity resulting from allylic strain for use in asymmetric reactions. | 4 | Stereochemistry |
It is generally believed that the mechanical and other properties of the crystal are also pertinent to the subject matter, and that crystal morphology provides the missing link between growth kinetics and physical properties. The necessary thermodynamic apparatus was provided by Josiah Willard Gibbs study of heterogeneous equilibrium. He provided a clear definition of surface energy, by which the concept of surface tension is made applicable to solids as well as liquids. He also appreciated that an anisotropic surface free energy implied a non-spherical equilibrium shape, which should be thermodynamically defined as the shape which minimizes the total surface free energy'.
It may be instructional to note that whisker growth provides the link between the mechanical phenomenon of high strength in whiskers and the various growth mechanisms which are responsible for their fibrous morphologies. (Prior to the discovery of carbon nanotubes, single-crystal whiskers had the highest tensile strength of any materials known). Some mechanisms produce defect-free whiskers, while others may have single screw dislocations along the main axis of growth—producing high strength whiskers.
The mechanism behind whisker growth is not well understood, but seems to be encouraged by compressive mechanical stresses including mechanically induced stresses, stresses induced by diffusion of different elements, and thermally induced stresses. Metal whiskers differ from metallic dendrites in several respects. Dendrites are fern-shaped like the branches of a tree, and grow across the surface of the metal. In contrast, whiskers are fibrous and project at a right angle to the surface of growth, or substrate. | 3 | Analytical Chemistry |
Subsets and Splits