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The vectors magnitude and direction is best understood when the dislocation-bearing crystal structure is first visualized without the dislocation, that is, the perfect crystal structure. In this perfect crystal structure, a rectangle whose lengths and widths are integer multiples of (the unit cell edge length) is drawn encompassing the site of the original dislocations origin. Once this encompassing rectangle is drawn, the dislocation can be introduced. This dislocation will have the effect of deforming, not only the perfect crystal structure, but the rectangle as well. The said rectangle could have one of its sides disjoined from the perpendicular side, severing the connection of the length and width line segments of the rectangle at one of the rectangles corners, and displacing each line segment from each other. What was once a rectangle before the dislocation was introduced is now an open geometric figure, whose opening defines the direction and magnitude of the Burgers vector. Specifically, the breadth of the opening defines the magnitude of the Burgers vector, and, when a set of fixed coordinates is introduced, an angle between the termini of the dislocated rectangles length line segment and width line segment may be specified.
When calculating the Burgers vector practically, one may draw a rectangular counterclockwise circuit (Burgers circuit) from a starting point to enclose the dislocation (see the picture above). The Burgers vector will be the vector to complete the circuit, i.e., from the end to the start of the circuit.
The direction of the vector depends on the plane of dislocation, which is usually on one of the closest-packed crystallographic planes.
The magnitude is usually represented by the equation (For BCC and FCC lattices only):
where is the unit cell edge length of the crystal, is the magnitude of the Burgers vector, and , , and are the components of the Burgers vector, the coefficient is owing to the fact that in BCC and FCC lattices, the shortest lattice vectors could be as expressed Comparatively, for simple cubic lattices, and hence the magnitude is represented by
Generally, the Burgers vector of a dislocation is defined by performing a line integral over the distortion field around the dislocation line
where the integration path is a Burgers circuit around the dislocation line, is the displacement field, and is the distortion field.
In most metallic materials, the magnitude of the Burgers vector for a dislocation is of a magnitude equal to the interatomic spacing of the material, since a single dislocation will offset the crystal lattice by one close-packed crystallographic spacing unit.
In edge dislocations, the Burgers vector and dislocation line are perpendicular to one another. In screw dislocations, they are parallel.
The Burgers vector is significant in determining the yield strength of a material by affecting solute hardening, precipitation hardening and work hardening.
The Burgers vector plays an important role in determining the direction of dislocation line. | 3 | Analytical Chemistry |
As of February 2016, the United States had no regulations governing mitochondrial donation, and Congress barred the FDA from evaluating any applications that involve implanting modified embryos into a woman.
The United Kingdom became the first country to legalize the procedure: the UK's chief medical officer recommended it be legalized in 2013; parliament passed The Human Fertilisation and Embryology (Mitochondrial Donation) Regulations in 2015, and the regulatory authority published regulations in 2016. | 1 | Biochemistry |
In most of its complexes, arenes bind in an η mode, with six nearly equidistant M-C bonds. The C-C-C angles are unperturbed vs the parent arene, but the C-C bonds are elongated by 0.2 Å. In the fullerene complex Ru(CO)(C), the fullerene binds to the triangular face of the cluster. | 0 | Organic Chemistry |
Joule heating is referred to as ohmic heating or resistive heating because of its relationship to Ohms Law. It forms the basis for the large number of practical applications involving electric heating. However, in applications where heating is an unwanted by-product of current use (e.g., load losses in electrical transformers) the diversion of energy is often referred to as resistive loss'. The use of high voltages in electric power transmission systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents. The ring circuits, or ring mains, used in UK homes are another example, where power is delivered to outlets at lower currents (per wire, by using two paths in parallel), thus reducing Joule heating in the wires. Joule heating does not occur in superconducting materials, as these materials have zero electrical resistance in the superconducting state.
Resistors create electrical noise, called Johnson–Nyquist noise. There is an intimate relationship between Johnson–Nyquist noise and Joule heating, explained by the fluctuation-dissipation theorem. | 7 | Physical Chemistry |
Carbon in the Earth's atmosphere exists in two main forms: carbon dioxide and methane. Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect. Methane produces a larger greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short-lived than carbon dioxide. Thus, carbon dioxide contributes more to the global greenhouse effect than methane.
Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (ocean, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity. It can then be absorbed by rocks through weathering. It also can acidify other surfaces it touches or be washed into the ocean.
Human activities over the past two centuries have increased the amount of carbon in the atmosphere by nearly 50% as of year 2020, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete.
In the far future (2 to 3 billion years), the rate at which carbon dioxide is absorbed into the soil via the carbonate–silicate cycle will likely increase due to expected changes in the sun as it ages. The expected increased luminosity of the Sun will likely speed up the rate of surface weathering. This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth's crust as carbonate. Once the concentration of carbon dioxide in the atmosphere falls below approximately 50 parts per million (tolerances vary among species), C photosynthesis will no longer be possible. This has been predicted to occur 600 million years from the present, though models vary.
Once the oceans on the Earth evaporate in about 1.1 billion years from now, plate tectonics will very likely stop due to the lack of water to lubricate them. The lack of volcanoes pumping out carbon dioxide will cause the carbon cycle to end between 1 billion and 2 billion years into the future. | 5 | Photochemistry |
Typical levels of beryllium that industries may release into the air are of the order of , averaged over a 30-day period, or of workroom air for an 8-hour work shift. Compliance with the current U.S. Occupational Safety and Health Administration (OSHA) permissible exposure limit for beryllium of has been determined to be inadequate to protect workers from developing beryllium sensitization and chronic beryllium disease. The American Conference of Governmental Industrial Hygienists (ACGIH), which is an independent organization of experts in the field of occupational health, has proposed a threshold limit value (TLV) of in a 2006 Notice of Intended Change (NIC). This TLV is 40 times lower than the current OSHA permissible exposure limit, reflecting the ACGIH analysis of best available peer-reviewed research data concerning how little airborne beryllium is required to cause sensitization and chronic beryllium disease.
Because it can be difficult to control industrial exposures to beryllium, it is advisable to use any methods possible to reduce airborne and surface contamination by beryllium, to minimize the use of beryllium and beryllium-containing alloys whenever possible, and to educate people about the potential hazards if they are likely to encounter beryllium dust or fumes. It is important to damp wipe metallographic preparation equipment to prevent accumulation of dry particles. Sectioning, grinding, and polishing must be performed under sufficiently vented hoods equipped with special filters.
On 29 January 2009, the Los Alamos National Laboratory announced it was notifying nearly 2,000 current and former employees and visitors that they may have been exposed to beryllium in the lab and may be at risk of disease. Concern over possible exposure to the material was first raised in November 2008, when a box containing beryllium was received at the laboratory's short-term storage facility. | 1 | Biochemistry |
The purpose of this method is to reduce carbon content from steel. This process is suitable for secondary steelmaking industry which recycling steel scrap that has variety of carbon content in their feedstock. This method aim to replace current conventional method that utilizing Basic Oxygen Furnace (BOF) to reduce carbon content of iron by blowing oxygen to make it react with carbon and forming CO.
In electrorefining, decarburization process happened in electrochemical cell that composed of inert electrode, slag and steel. During the process, current passing through the cell and made slag and steel melted. Oxygen ion from slag decompose and oxidize carbon on steel and to form CO. That decarburizing reaction is occurred in three steps as follow. (ads) means adsorbed intermediate
The total reaction from this cell is following this scheme
The SiO is come from the slag, based on the reaction above, beside producing CO gas, this method also producing pure silicon (depending on the slag). The benefit of this direct decarburization process is it does not produce CO but CO which is not considered as greenhouse gas. | 8 | Metallurgy |
The following example illustrates the potential of the photoacoustic technique: In the early 1970s, Patel and co-workers measured the temporal variation of the concentration of nitric oxide in the stratosphere at an altitude of 28 km with a balloon-borne photoacoustic detector. These measurements provided crucial data bearing on the problem of ozone depletion by man-made nitric oxide emission. Some of the early work relied on development of the RG theory by Rosencwaig and Gersho. | 7 | Physical Chemistry |
Haptophytes are similar and closely related to cryptophytes or heterokontophytes. Their chloroplasts lack a nucleomorph, their thylakoids are in stacks of three, and they synthesize chrysolaminarin sugar, which they store completely outside of the chloroplast, in the cytoplasm of the haptophyte. | 5 | Photochemistry |
* Noninvasive method for capturing mRNA from single cells in living, intact tissues for transcriptome analysis.
* Though other methods can be applied, such as laser capture microdissection and patch-pipette aspiration to isolate single cells, With TIVA Tags no damage to the cells and no tissue deformation from penetration of the pipette that may alter components of the transcriptional profile.
* Can be performed on various cell types, while existing methods depend on transgenic rodent models to identify cells of interest. | 1 | Biochemistry |
Expression of genes in mammals can be upregulated when signals are transmitted to the promoters associated with the genes. Cis-regulatory DNA sequences that are located in DNA regions distant from the promoters of genes can have very large effects on gene expression, with some genes undergoing up to 100-fold increased expression due to such a cis-regulatory sequence. These cis-regulatory sequences include enhancers, silencers, insulators and tethering elements. Among this constellation of sequences, enhancers and their associated transcription factor proteins have a leading role in the regulation of gene expression.
Enhancers are sequences of the genome that are major gene-regulatory elements. Enhancers control cell-type-specific gene expression programs, most often by looping through long distances to come in physical proximity with the promoters of their target genes. In a study of brain cortical neurons, 24,937 loops were found, bringing enhancers to promoters. Multiple enhancers, each often at tens or hundred of thousands of nucleotides distant from their target genes, loop to their target gene promoters and coordinate with each other to control expression of their common target gene.
The schematic illustration in this section shows an enhancer looping around to come into close physical proximity with the promoter of a target gene. The loop is stabilized by a dimer of a connector protein (e.g. dimer of CTCF or YY1), with one member of the dimer anchored to its binding motif on the enhancer and the other member anchored to its binding motif on the promoter (represented by the red zigzags in the illustration). Several cell function specific transcription factor proteins (in 2018 Lambert et al. indicated there were about 1,600 transcription factors in a human cell) generally bind to specific motifs on an enhancer and a small combination of these enhancer-bound transcription factors, when brought close to a promoter by a DNA loop, govern the level of transcription of the target gene. Mediator (coactivator) (a complex usually consisting of about 26 proteins in an interacting structure) communicates regulatory signals from enhancer DNA-bound transcription factors directly to the RNA polymerase II (RNAP II) enzyme bound to the promoter.
Enhancers, when active, are generally transcribed from both strands of DNA with RNA polymerases acting in two different directions, producing two eRNAs as illustrated in the Figure. An inactive enhancer may be bound by an inactive transcription factor. Phosphorylation of the transcription factor may activate it and that activated transcription factor may then activate the enhancer to which it is bound (see small red star representing phosphorylation of a transcription factor bound to an enhancer in the illustration). An activated enhancer begins transcription of its RNA before activating a promoter to initiate transcription of messenger RNA from its target gene. | 1 | Biochemistry |
Organisms in all three domains of life, eukaryotes, bacteria and archaea, are able to carry out de novo biosynthesis of purines. This ability reflects the essentiality of purines for life. The biochemical pathway of synthesis is very similar in eukaryotes and bacterial species, but is more variable among archaeal species. A nearly complete, or complete, set of genes required for purine biosynthesis was determined to be present in 58 of the 65 archaeal species studied. However, also identified were seven archaeal species with entirely, or nearly entirely, absent purine encoding genes. Apparently the archaeal species unable to synthesize purines are able to acquire exogenous purines for growth., and are thus similar to purine mutants of eukaryotes, e.g. purine mutants of the Ascomycete fungus Neurospora crassa, that also require exogenous purines for growth. | 1 | Biochemistry |
The exact definition of spiciness is debated. Specifically, the orthogonality of the density with spiciness and the used scaling factor of potential temperature and salinity. McDougall claims that orthogonality should not be imposed because:
* There is no physical reason to impose orthogonality.
* Imposing orthogonality would necessarily depends on an arbitrary scaling factor of the salinity and temperature axes. In other words, spiciness would have different meanings for different (chosen) scaling factors.
* The meaning of spiciness changes with density. As a result, spiciness may only be useful over small vertical extensions in the surface layer.
McDougall is adopted by the Intergovernmental Oceanographic Commission (IOC), International Association for the Physical Sciences of the Oceans (IAPSO) and the Scientific Committee on Oceanic Research (SCOR) due to their implementation of spiciness in the TEOS-10.
Huang claims that the orthogonal system is superior to the non orthogonal system because the coordinates can be regarded as independent and distances between points can be calculated more easily.
McDougall recommended that the spiciness should not be used. Instead, they recommend that the variation of salinity should be used to differentiate between isopycnal water parcels and the stability ratio on vertical water columns for stability. | 7 | Physical Chemistry |
The BVS apparatus consists of a specimen surrounded by Helmholtz coils and isolated from external vibrations by a framework constructed from insulating foam and either lead or brass. The specimen is affixed with both a permanent magnet and a mirror. The orientation of the coils with respect to the magnet when a current is driven through them determines whether the specimen undergoes bending or torsion. Angular displacement of the specimen is measured by an interferometer that detects the spatial movement of a reflected laser. This spatial waveform is converted to an electrical one by a light detector and read out on an oscilloscope. This oscilloscope also displays the torque or force waveform from the capacitor driving the current in the Helmholtz coils. Phase delay is determined by comparing these waveforms.
Resonance is minimized through the use of short specimens—which have higher resonant frequencies—and by reducing the inertia (magnetic and mass moments) of the magnet. Cubic samarium-cobalt magnets are ideal for high frequency studies. Due to the sample geometry being a short rectangular bar or cylinder, the equation governing the resonance of the BVS specimen geometry has an exact analytic solution, which allows the technique to yield results even for high loss materials. This exact solution provides a relationship between dynamic moduli, angular displacement, and geometric parameters. The inherent lack of drift and friction in the apparatus is responsible for its large range of operating frequencies. | 7 | Physical Chemistry |
The electric dipole moment of the dioxygen molecule, is zero, but the molecule is paramagnetic with two unpaired electrons so that there are magnetic-dipole allowed transitions which can be observed by microwave spectroscopy. The unit electron spin has three spatial orientations with respect to the given molecular rotational angular momentum vector, K, so that each rotational level is split into three states, J = K + 1, K, and K - 1, each J state of this so-called p-type triplet arising from a different orientation of the spin with respect to the rotational motion of the molecule. The energy difference between successive J terms in any of these triplets is about 2 cm (60 GHz), with the single exception of J = 1←0 difference which is about 4 cm. Selection rules for magnetic dipole transitions allow transitions between successive members of the triplet (ΔJ = ±1) so that for each value of the rotational angular momentum quantum number K there are two allowed transitions. The O nucleus has zero nuclear spin angular momentum, so that symmetry considerations demand that K have only odd values. | 7 | Physical Chemistry |
* Antibiotics
** Actinomycin
** Bacitracin
** Calcium dependent antibiotic
** Daptomycin
** Vancomycin
** Teixobactin
** Tyrocidine
** Gramicidin
** Zwittermicin A
* Antibiotic precursors
** ACV-Tripeptide
* Cytostatics
** Epothilone
**Fabclavine
** Bleomycin
* Immunosuppressants
** Ciclosporin (Cyclosporine A)
* Siderophores
** Pyoverdine
** Enterobactin
** Myxochelin A
* Pigments
** Indigoidine
* Toxins
** Microcystins and
** Nodularins, cyanotoxins from cyanobacteria.
* Nitrogen storage polymers
** Cyanophycin – produced by some cyanobacteria
* Phytotoxins
** HC-toxin – a virulence factor made by the plant pathogenic fungus Cochliobolus (Helminthosporium) carbonum
** AM-toxin – made by the plant pathogenic fungus Alternaria alternata pv. Mali
** victorin – a chlorinated cyclic pentapeptide made by the pathogenic fungus Cochliobolus victoriae. Its nonribosomal synthesis has not been established. | 1 | Biochemistry |
Walsh diagrams were first introduced by A.D. Walsh, a British chemistry professor at the University of Dundee, in a series of ten papers in one issue of the Journal of the Chemical Society. Here, he aimed to rationalize the shapes adopted by polyatomic molecules in the ground state as well as in excited states, by applying theoretical contributions made by Mulliken. Specifically, Walsh calculated and explained the effect of changes in the shape of a molecule on the energy of molecular orbitals. Walsh diagrams are an illustration of such dependency, and his conclusions are what are referred to as the "rules of Walsh."
In his publications, Walsh showed through multiple examples that the geometry adopted by a molecule in its ground state primarily depends on the number of its valence electrons. He himself acknowledged that this general concept was not novel, but explained that the new data available to him allowed the previous generalizations to be expanded upon and honed. He also noted that Mulliken had previously attempted to construct a correlation diagram for the possible orbitals of a polyatomic molecule in two different nuclear configurations, and had even tried to use this diagram to explain shapes and spectra of molecules in their ground and excited states. However, Mulliken was unable to explain the reasons for the rises and falls of certain curves with increases in angle, thus Walsh claimed "his diagram was either empirical or based upon unpublished computations." | 7 | Physical Chemistry |
A Scuderi cycle is a thermodynamic cycle that is constructed out of the following series of thermodynamic processes:
* A-B and C-D (TOP and BOTTOM of the loop): a pair of quasi-parallel adiabatic processes
* D-A (LEFT side of the loop): a positively sloped, increasing pressure, increasing volume process
* B-C (RIGHT side of the loop): an isochoric process
The adiabatic processes are impermeable to heat: heat flows rapidly into the loop through the left expanding process, resulting in increasing pressure while volume is increasing; some of it flows back out through the right depressurizing process; the remaining heat does the work. | 7 | Physical Chemistry |
Wetland chemistry is largely affected by dredging, which can be done for a variety of purposes. Wetlands are areas within floodplains with both terrestrial and aquatic characteristics, including marshes, swamps, bogs, and others. It has been estimated that they occupy around 2.8x10 km, about 2.2% of the Earth’s surface, but other estimates are even higher. It has also been estimated to have a worth of $14.9 trillion and are responsible for 75% of commercial and 90% of recreational harvest of fish and shellfish in the United States. Wetlands also hold an important role in water purification, storm protection, industry, travel, research, education, and tourism. Being heavily used and traveled through, dredging is common and leads to continuation of long-term damage of the ecosystem and land loss, and ultimately a loss in industry, homes, and protection.
Wetlands undergo different chemical reactions depending on a variety of parameters, including salinity and pH. Redox reactions have a major effect on wetland ecosystems, as they depend heavily on salinity, pH, oxygen availability, and others. Common redox reactions in wetland include carbon, nitrogen, and sulfur transformations. Fluctuations in water flow and flooding can change the abundance of the oxidized or reduced species depending on the environment. Increased flooding and water flow can also change the availability of nutrients to local species. The further the wetlands change from their original states, the more difficult rebuilding land becomes. The types of mitigation efforts also change depending on the chemistry, so an understanding of the change is required for effective mitigation. | 9 | Geochemistry |
Concomitant or recent (previous fourteen days) monoamine oxidase inhibitor use can lead to hypertensive reactions, including hypertensive crises.
The antihypertensive effects of methyldopa, mecamylamine, reserpine, and veratrum alkaloids may be reduced by sympathomimetics. Beta-adrenergic antagonists may also interact with sympathomimetics. Increase of ectopic pacemaker activity can occur when pseudoephedrine is used concomitantly with digitalis. Antacids increase the rate of pseudoephedrine absorption, while kaolin decreases it. | 4 | Stereochemistry |
Regulation of glyceroneogenesis is a therapeutic target of type 2 diabetes treatment, specifically inhibiting it in the liver and increasing it in adipose tissues. Insulin down-regulates glyceroneogenesis in the liver, but it also suppresses it in adipose tissue. To restrict the release of free fatty acids from adipose tissues, glyceroneogenesis must be increased so they are re-esterified. Thiazolidinedione is a substance that only affects glyceroneogenesis in adipose tissue by increasing transcription of PEPC-K to up-regulate glyceroneogenesis. | 1 | Biochemistry |
Rotation of the reduction furnace may be a design choice intended to circulate the ore through the furnace. It can also play an active part in the chemical reaction by ensuring mixing between the reactants present. Rotary hearth processes, where the ore rests on a fixed bed and travels through a tunnel, fall into the first category. Rotary kiln processes, where the ore is mixed with coal at high temperature, constitute the second category. | 8 | Metallurgy |
RNA interference has been used to silence genes associated with several cancers. In in vitro studies of chronic myelogenous leukemia (CML), siRNA was used to cleave the fusion protein, BCR-ABL, which prevents the drug Gleevec (imatinib) from binding to the cancer cells. Cleaving the fusion protein reduced the amount of transformed hematopoietic cells that spread throughout the body by increasing the sensitivity of the cells to the drug. RNA interference can also be used to target specific mutants. For instance, siRNAs were able to bind specifically to tumor suppressor p53 molecules containing a single point mutation and destroy it, while leaving the wild-type suppressor intact.
Receptors involved in mitogenic pathways that lead to the increased production of cancer cells there have also been targeted by siRNA molecules. The chemokine receptor chemokine receptor 4 (CXCR4), associated with the proliferation of breast cancer, was cleaved by siRNA molecules that reduced the number of divisions commonly observed by the cancer cells. Researchers have also used siRNAs to selectively regulate the expression of cancer-related genes. Antiapoptotic proteins, such as clusterin and survivin, are often expressed in cancer cells. Clusterin and survivin-targeting siRNAs were used to reduce the number of antiapoptotic proteins and, thus, increase the sensitivity of the cancer cells to chemotherapy treatments. In vivo studies are also being increasingly utilized to study the potential use of siRNA molecules in cancer therapeutics. For instance, mice implanted with colon adenocarcinoma cells were found to survive longer when the cells were pretreated with siRNAs that targeted B-catenin in the cancer cells. | 1 | Biochemistry |
The Hepatitis B viral protein X is believed to cause hepatocellular carcinoma through transformation, typically of liver cells. The viral DNA is incorporated into the host cell's genome causing rapid cell replication and tumor growth. | 1 | Biochemistry |
The production of BCF and similar chlorofluorocarbons has been banned in most countries since January 1, 1994 as part of the Montreal Protocol on ozone depleting substances. Halon 1211 is also a potent greenhouse gas with a 100-year global warming potential 2,070 times that of carbon dioxide and an atmospheric lifetime of 16.0 years.
Recycling of Halon 1211 allows it to remain in use, although parts availability is limited to a few manufacturers and can be an issue. Halon 1211 is still widely used in the United States, despite its high cost, with the US military being the biggest user. Europe and Australia have banned its use for all but "critical applications" such as aviation, military, and police use. The manufacture of UL Listed halon 1211 extinguishers was supposed to cease on October, 2009. The future listing is still in discussion. Halotron I, the replacement extinguishing agent, requires a larger volume to get the same ratings as 1211. | 2 | Environmental Chemistry |
Dimetcote should be applied by specific sprays. Here is a list of several suitable items of equipment typically used by manufacturers. | 8 | Metallurgy |
Ordering is the regularity in which atoms appear in a predictable lattice, as measured from one point. In a highly ordered, perfectly crystalline material, or single crystal, the location of every atom in the structure can be described exactly measuring out from a single origin. Conversely, in a disordered structure such as a liquid or amorphous solid, the location of the nearest and, perhaps, second-nearest neighbors can be described from an origin (with some degree of uncertainty) and the ability to predict locations decreases rapidly from there out. The distance at which atom locations can be predicted is referred to as the correlation length . A paracrystalline material exhibits a correlation somewhere between the fully amorphous and fully crystalline.
The primary, most accessible source of crystallinity information is X-ray diffraction and cryo-electron microscopy, although other techniques may be needed to observe the complex structure of paracrystalline materials, such as fluctuation electron microscopy in combination with density of states modeling of electronic and vibrational states. Scanning transmission electron microscopy can provide real-space and reciprocal space characterization of paracrystallinity in nanoscale material, such as quantum dot solids.
The scattering of X-rays, neutrons and electrons on paracrystals is quantitatively described by the theories of the ideal and real paracrystal.
Numerical differences in analyses of diffraction experiments on the basis of either of these two theories of paracrystallinity can often be neglected.
Just like ideal crystals, ideal paracrystals extend theoretically to infinity. Real paracrystals, on the other hand, follow the empirical α*-law, which restricts their size. That size is also indirectly proportional to the components of the tensor of the paracrystalline distortion. Larger solid state aggregates are then composed of micro-paracrystals. | 7 | Physical Chemistry |
Compounds with carbon phosphorus(III) multiple bonds are called phosphaalkenes (RC=PR) and phosphaalkynes (RC≡P). They are similar in structure, but not in reactivity, to imines (RC=NR) and nitriles (RC≡N), respectively. In the compound phosphorine, one carbon atom in benzene is replaced by phosphorus. Species of this type are relatively rare but for that reason are of interest to researchers. A general method for the synthesis of phosphaalkenes is by 1,2-elimination of suitable precursors, initiated thermally or by base such as DBU, DABCO, or triethylamine:
Thermolysis of MePH generates CH=PMe, an unstable species in the condensed phase. | 0 | Organic Chemistry |
CYC1 is a human gene that is conserved in chimpanzee, Rhesus monkey, dog, cow, mouse, rat, zebrafish, fruit fly, mosquito, C. elegans, S. cerevisiae, K. lactis, E. gossypii, S. pombe, N. crassa, A. thaliana, rice, and frog. There are orthologs of CYC1 in 137 known organisms.
In its structure and function, the cytochrome b-c1 complex bears extensive analogy to the cytochrome b6f complex of chloroplasts and cyanobacteria; cytochrome c1 plays an analogous role to cytochrome f, despite their different structures. | 1 | Biochemistry |
One of the important capabilities of using FTIR photoacoustic spectroscopy has been the ability to evaluate samples in their in situ state by infrared spectroscopy, which can be used to detect and quantify chemical functional groups and thus chemical substances. This is particularly useful for biological samples that can be evaluated without crushing to powder or subjecting to chemical treatments. Seashells, bone and such samples have been investigated. Using photoacoustic spectroscopy has helped evaluate molecular interactions in bone with osteogenesis imperfecta.
While most academic research has concentrated on high resolution instruments, some work has gone in the opposite direction. In the last twenty years, very low cost instruments for applications such as leakage detection and for the control of carbon dioxide concentration have been developed and commercialized. Typically, low cost thermal sources are used which are modulated electronically. Diffusion through semi-permeable disks instead of valves for gas exchange, low-cost microphones, and proprietary signal processing with digital signal processors have brought down the costs of these systems. The future of low-cost applications of photoacoustic spectroscopy may be the realization of fully integrated micromachined photoacoustic instruments.
The photoacoustic approach has been utilized to quantitatively measure macromolecules, such as proteins. The photoacoustic immunoassay labels and detects target proteins using nanoparticles that can generate strong acoustic signals. The photoacoustics-based protein analysis has also been applied for point-of-care testings.
Photoacoustic spectroscopy also has many military applications. One such application is the detection toxic chemical agents. The sensitivity of photoacoustic spectroscopy makes it an ideal analysis technique for detecting trace chemicals associated with chemical attacks.
LPAS sensors may be applied in industry, security (nerve agent and explosives detection), and medicine (breath analysis). | 7 | Physical Chemistry |
Heat can be used to trigger the electrocyclization topochemical polymerization. For example, Kana M. Sureshan et al. have developed a series of bio-compatible polymer crystals based on [3+2] Topochemical Azide-Alkyne Cycloaddition (TAAC) reaction and [3+2] topochemical ene-azide cycloaddition (TEAC) reaction. The monomers are polymerized by heating for a few days. Contrary to the light-initiated topochemical polymerization, the lower temperature and slower reaction rate would produce high quality polymer crystals. This is due to the fact that heat expansion is not obvious in lower temperature. | 7 | Physical Chemistry |
A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemoreceptor is a neuron, or in the form of a neurotransmitter that can activate a nerve fiber if the chemoreceptor is a specialized cell, such as taste receptors, or an internal peripheral chemoreceptor, such as the carotid bodies. In physiology, a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of carbon dioxide (hypercapnia) or a decrease in blood levels of oxygen (hypoxia), and transmits that information to the central nervous system which engages body responses to restore homeostasis.
In bacteria, chemoreceptors are essential in the mediation of chemotaxis. | 3 | Analytical Chemistry |
The basis of the reaction is similar to substrate-depletive clock reaction, except for the fact that rate k is very slow leading to the co-existing of substrates and clock species, so there is no need for substrate to be depleted to observe the change in color. The example for this clock is pentathionate/iodate reaction. | 7 | Physical Chemistry |
Most Grignard reactions are conducted in ethereal solvents, especially diethyl ether and THF. Grignard reagents react with 1,4-dioxane to give the diorganomagnesium compounds and insoluble coordination polymer and (R = organic group, X = halide):
This reaction exploits the Schlenk equilibrium, driving it toward the right. | 0 | Organic Chemistry |
Lawesson's reagent is commercially available. It can also be conveniently prepared in the laboratory by heating a mixture of anisole with phosphorus pentasulfide until the mixture is clear and no more hydrogen sulfide is formed, then recrystallized from toluene or xylene.
Samples give a strong odor of hydrogen sulfide owing to partial hydrolysis. One common and effective method of destroying the foul smelling residues is to use an excess of sodium hypochlorite (chlorine bleach). | 0 | Organic Chemistry |
The dead time is the time for the solutions to go from the mixing point to the observation point, it is the part of the kinetics which cannot be observed. So the lower the dead time, the more information the user can get. In older instruments this could be of the order of 1 ms, but improvements now allow a dead time of about 0.3 ms. | 7 | Physical Chemistry |
The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air being blown through the molten iron. The oxidation also raises the temperature of the iron mass and keeps it molten.
Related decarburizing with air processes had been used outside Europe for hundreds of years, but not on an industrial scale. One such process (similar to puddling) was known in the 11th century in East Asia, where the scholar Shen Kuo of that era described its use in the Chinese iron and steel industry. In the 17th century, accounts by European travelers detailed its possible use by the Japanese.
The modern process is named after its inventor, the Englishman Henry Bessemer, who took out a patent on the process in 1856. The process was said to be independently discovered in 1851 by the American inventor William Kelly though the claim is controversial.
The process using a basic refractory lining is known as the "basic Bessemer process" or Gilchrist–Thomas process after the English discoverers Percy Gilchrist and Sidney Gilchrist Thomas. | 8 | Metallurgy |
One major feature found in protein structures is the addition of sugars (glycosylation) to specific amino acid residues by post translational modification. Complex sugar structures can be connected to these sites, and this can substantially modify the properties of these proteins, a main reason for their presence. Attached sugars can assist in folding some proteins to their correct shape; so, affecting a proteins’ structure is a possible outcome. SRCD is ideally well suited to determining any conformational differences that might arise from different ambient environments directly because of the extended wavelength range into the VUV region which provides greater information content. However, attached sugars can contribute to the SRCD signal because their transitions are located more towards the VUV end of the spectrum. This means that their presence can cause a problem in obtaining an accurate measure of the secondary structure content of the protein as a result. Matsuo. and Gekko produced the landmark study of VUVCD spectra of selected saccharides, thereby demonstrating that glycoproteins would have a contribution to their spectra from their sugar content. From this and further studies they demonstrated that the SRCD spectral characteristics that arose from sugars could be attributed to many factors within their conformations: the configuration of the hydroxyl group about the C1 atom of the saccharide (alpha or beta conformation, or almost axial or equatorial to the plane of the sugar ring respectively), the axial or equatorial positioning of the remaining hydroxyl groups, the trans or gauche nature of the C5 hydroxymethyl group, and the glycosidic linkage (either 1-4 or 1-6) between sugar monomers. Utilising this information, the Wallace group investigated the glycosylation of the voltage-gated sodium channel in experiments that relied on the fact that a CD(SRCD) spectrum of a mixture of components is the sum of all those components present. The aim was to establish if there were differences in the three-dimensional structure of the channel with and without sugars attached to the structure; did glycosylation play any significant role in the function of these channels when sugars were attached? Three experimental sets of SRCD spectra were collected; the non-glycosylated and glycosylated channel structures and a further one of the isolated sugar components that combined to form those attached to the channel. Taking away the spectrum of the non-glycosylated channel from that of the glycosylated they demonstrated that the resultant difference spectrum corresponded to that of the sugar components. This meant that there were no structural differences between the glycosylated and non-glycosylated channel structures, so sugar attachment played no key role in their function | 7 | Physical Chemistry |
A spiropyran is a type of organic chemical compound, known for photochromic properties that provide this molecule with the ability of being used in medical and technological areas. Spiropyrans were discovered in the early twentieth century. However, it was in the middle twenties when Fisher and Hirshbergin observed their photochromic characteristics and reversible reaction. In 1952, Fisher and co-workers announced for the first time photochromism in spiropyrans. Since then, there have been many studies on photochromic compounds that have continued up to the present. | 5 | Photochemistry |
Since liquids often have different boiling points, mixtures or solutions of liquids or gases can typically be separated by distillation, using heat, cold, vacuum, pressure, or other means. Distillation can be found in everything from the production of alcoholic beverages, to oil refineries, to the cryogenic distillation of gases such as argon, oxygen, nitrogen, neon, or xenon by liquefaction (cooling them below their individual boiling points). | 7 | Physical Chemistry |
The Prix Wilder-Penfield is an award by the government of Quebec that is part of the Prix du Québec, which "goes to scientists whose research aims fall within the field of biomedicine. These fields include the medical sciences, the natural sciences, and engineering". It is named in honour of Wilder Penfield. | 1 | Biochemistry |
See the review of mechanical defenses by Lucas et al., 2000, which remains relevant and well regarded in the subject . Many plants have external structural defenses that discourage herbivory. Structural defenses can be described as morphological or physical traits that give the plant a fitness advantage by deterring herbivores from feeding. Depending on the herbivores physical characteristics (i.e. size and defensive armor), plant structural defenses on stems and leaves can deter, injure, or kill the grazer. Some defensive compounds are produced internally but are released onto the plants surface; for example, resins, lignins, silica, and wax cover the epidermis of terrestrial plants and alter the texture of the plant tissue. The leaves of holly plants, for instance, are very smooth and slippery making feeding difficult. Some plants produce gummosis or sap that traps insects. | 1 | Biochemistry |
In the large intestine, the passage of the digesting food in the colon is a lot slower, taking from 30 to 40 hours until it is removed by defecation. The colon mainly serves as a site for the fermentation of digestible matter by the gut flora. The time taken varies considerably between individuals. The remaining semi-solid waste is termed feces and is removed by the coordinated contractions of the intestinal walls, termed peristalsis, which propels the excreta forward to reach the rectum and exit through the anus via defecation. The wall has an outer layer of longitudinal muscles, the taeniae coli, and an inner layer of circular muscles. The circular muscle keeps the material moving forward and also prevents any back flow of waste. Also of help in the action of peristalsis is the basal electrical rhythm that determines the frequency of contractions. The taeniae coli can be seen and are responsible for the bulges (haustra) present in the colon. Most parts of the GI tract are covered with serous membranes and have a mesentery. Other more muscular parts are lined with adventitia. | 1 | Biochemistry |
A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed energy in the form of light). Sometimes, the excited state is metastable, so the relaxation back down from the excited state to lower states is delayed (necessitating anywhere from a few nanoseconds to hours depending on the material). The process then corresponds to one of two phenomena: delayed fluorescence or phosphorescence. The correspondence depends on the type of transition and hence the wavelength of the emitted optical photon. | 5 | Photochemistry |
Many industrial peroxides are produced using hydrogen peroxide. Reactions with aldehydes and ketones yield a series of compounds depending on conditions. Specific reactions include addition of hydrogen peroxide across the C=O double bond:
In some cases, these hydroperoxides convert to give cyclic diperoxides:
Addition of this initial adduct to a second equivalent of the carbonyl:
Further replacement of alcohol groups:
Triphenylmethanol reacts with hydrogen peroxide gives the unusually stable hydroperoxide, . | 0 | Organic Chemistry |
The number concentration is defined as the number of entities of a constituent in a mixture divided by the volume of the mixture :
The SI unit is 1/m. | 3 | Analytical Chemistry |
TRFLP is one of several molecular methods aimed to generate a fingerprint of an unknown microbial community. Other similar methods include DGGE, TGGE, ARISA, ARDRA, PLFA, etc.
<br>These relatively high throughput methods were developed in order to reduce the cost and effort in analyzing microbial communities using a clone library. The method was first described by Avaniss-Aghajani et al in 1994 and later by Liu in 1997 which employed the amplification of the 16S rDNA target gene from the DNA of several isolated bacteria as well as environmental samples.
<br>Since then the method has been applied for the use of other marker genes such as the functional marker gene pmoA to analyze methanotrophic communities. | 1 | Biochemistry |
In aqueous solution the concentration of the hydroxide ion is related to the concentration of the hydrogen ion by
The first step in metal ion hydrolysis can be expressed in two different ways
It follows that . Hydrolysis constants are usually reported in the β form and therefore often have values much less than 1. For example, if and so that β = 10. In general when the hydrolysis product contains n hydroxide groups | 7 | Physical Chemistry |
Pentamethylcyclopentadiene gives rise to pentamethylcyclopentadienyl (Cp*) complexes. These ligands are more basic and more lipophilic. Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible. Well-studied ligands of this type include CRH (R = iso-Pr) and 1,2,4-CRH (R = tert-Bu). | 0 | Organic Chemistry |
Charge-exchange spectroscopy (abbreviated CES or CXS) is a technique commonly used in plasma diagnostics to analyze high-temperature controlled fusion plasmas. In fusion plasmas, the light elements tend to become fully ionized during operation, which makes it challenging to diagnose their properties using conventional optical diagnostics. To address this, a method was developed in the 1970s which involves the injection of a beam of neutral atoms, such as hydrogen or deuterium, into the plasma. This process results in the ionization of hydrogenic atoms the excitation of ions through charge exchange, as represented by the reaction:
where represents the various possible charged states of the ions in the plasma.
Optical fibers are then strategically positioned to create "chords", lines of sight along which the measurements are taken. These chords pass through regions both with and without the neutral beam. By subtracting the signals from these two chords, emissions not generated by the neutral beam can be inferred. This allows for the determination of ion properties, such as its temperature and density.
The technique can also be extended to include multiple chords to build spatial profiles of the plasma, such as its toroidal and poloidal rotation. This provides insights into how ions conduct heat and transport momentum within the plasma.
Charge-exchange spectroscopy is often referred to as charge-exchange recombination spectroscopy, which is acronymized as CXRS or CER. | 7 | Physical Chemistry |
mTECs as APCs reveal some drawbacks on population level. Their numbers in thymic medulla reach only 100,000 per 2-week-old thymus. Furthermore, average lifespan of mTECs does not exceed 2–3 days, probably due to only known PGE activator Autoimmune regulator (Aire), which requires for its proper function generation of DNA double strand breaks. And last but not least, each TRA is expressed only by 1-3% of mTEC population. These facts decrease the chance of efficient recessive or dominant tolerance. | 1 | Biochemistry |
As mentioned above, G-proteins may terminate their own activation due to their intrinsic GTP→GDP hydrolysis capability. However, this reaction proceeds at a slow rate (≈0.02 times/sec) and, thus, it would take around 50 seconds for any single G-protein to deactivate if other factors did not come into play. Indeed, there are around 30 isoforms of RGS proteins that, when bound to Gα through their GAP domain, accelerate the hydrolysis rate to ≈30 times/sec. This 1500-fold increase in rate allows for the cell to respond to external signals with high speed, as well as spatial resolution due to limited amount of second messenger that can be generated and limited distance a G-protein can diffuse in 0.03 seconds. For the most part, the RGS proteins are promiscuous in their ability to deactivate G-proteins, while which RGS is involved in a given signaling pathway seems more determined by the tissue and GPCR involved than anything else. In addition, RGS proteins have the additional function of increasing the rate of GTP-GDP exchange at GPCRs, (i.e., as a sort of co-GEF) further contributing to the time resolution of GPCR signaling.
In addition, the GPCR may be desensitized itself. This can occur as:
# a direct result of ligand occupation, wherein the change in conformation allows recruitment of GPCR-Regulating Kinases (GRKs), which go on to phosphorylate various serine/threonine residues of IL-3 and the C-terminal tail. Upon GRK phosphorylation, the GPCR's affinity for β-arrestin (β-arrestin-1/2 in most tissues) is increased, at which point β-arrestin may bind and act to both sterically hinder G-protein coupling as well as initiate the process of receptor internalization through clathrin-mediated endocytosis. Because only the liganded receptor is desensitized by this mechanism, it is called homologous desensitization
# the affinity for β-arrestin may be increased in a ligand occupation and GRK-independent manner through phosphorylation of different ser/thr sites (but also of IL-3 and the C-terminal tail) by PKC and PKA. These phosphorylations are often sufficient to impair G-protein coupling on their own as well.
# PKC/PKA may, instead, phosphorylate GRKs, which can also lead to GPCR phosphorylation and β-arrestin binding in an occupation-independent manner. These latter two mechanisms allow for desensitization of one GPCR due to the activities of others, or heterologous desensitization. GRKs may also have GAP domains and so may contribute to inactivation through non-kinase mechanisms as well. A combination of these mechanisms may also occur.
Once β-arrestin is bound to a GPCR, it undergoes a conformational change allowing it to serve as a scaffolding protein for an adaptor complex termed AP-2, which in turn recruits another protein called clathrin. If enough receptors in the local area recruit clathrin in this manner, they aggregate and the membrane buds inwardly as a result of interactions between the molecules of clathrin, in a process called opsonization. Once the pit has been pinched off the plasma membrane due to the actions of two other proteins called amphiphysin and dynamin, it is now an endocytic vesicle. At this point, the adapter molecules and clathrin have dissociated, and the receptor is either trafficked back to the plasma membrane or targeted to lysosomes for degradation.
At any point in this process, the β-arrestins may also recruit other proteins—such as the non-receptor tyrosine kinase (nRTK), c-SRC—which may activate ERK1/2, or other mitogen-activated protein kinase (MAPK) signaling through, for example, phosphorylation of the small GTPase, Ras, or recruit the proteins of the ERK cascade directly (i.e., Raf-1, MEK, ERK-1/2) at which point signaling is initiated due to their close proximity to one another. Another target of c-SRC are the dynamin molecules involved in endocytosis. Dynamins polymerize around the neck of an incoming vesicle, and their phosphorylation by c-SRC provides the energy necessary for the conformational change allowing the final "pinching off" from the membrane. | 1 | Biochemistry |
Beryllides of cobalt and nickel have metallurgical importance as the precipitated phase in beryllium copper alloys. These materials are nonsparking, which allows them to be used in certain hazardous environments.
In nuclear technology, beryllides are investigated as neutron multipliers. Unlike metallic Be, materials such as BeTi are more resistant to oxidation by water but retain the neutron-multiplying properties of the predominant isotope Be. | 8 | Metallurgy |
One of the largest implementations of Fischer–Tropsch technology is in Bintulu, Malaysia. This Shell facility converts natural gas into low-sulfur Diesel fuels and food-grade wax. The scale is . | 0 | Organic Chemistry |
The earliest evidence for iron-making is a small number of iron fragments with the appropriate amounts of carbon admixture found in the Proto-Hittite layers at Kaman-Kalehöyük and dated to 2200–2000 BCE. Souckova-Siegolová (2001) shows that iron implements were made in Central Anatolia in very limited quantities around 1800 BCE and were in general use by elites, though not by commoners, during the New Hittite Empire (~1400–1200 BCE).
Archaeologists have found indications of iron working in Ancient Egypt, somewhere between the Third Intermediate Period and 23rd Dynasty (ca. 1100–750 BCE). Significantly though, they have found no evidence of iron ore smelting in any (pre-modern) period. In addition, very early instances of carbon steel were in production around 2000 years ago (around the first-century CE.) in northwest Tanzania, based on complex preheating principles. These discoveries are significant for the history of metallurgy.
Most early processes in Europe and Africa involved smelting iron ore in a bloomery, where the temperature is kept low enough so that the iron does not melt. This produces a spongy mass of iron called a bloom, which then must be consolidated with a hammer to produce wrought iron. The earliest evidence to date for the bloomery smelting of iron is found at Tell Hammeh, Jordan ([http://www.ironsmelting.net/www/smelting/]), and dates to 930 BCE (C14 dating). | 8 | Metallurgy |
Sulfuric acid can be used to produce hydrogen from water:
The compounds of sulfur and iodine are recovered and reused, hence the process is called the sulfur–iodine cycle. This process is endothermic and must occur at high temperatures, so energy in the form of heat has to be supplied. The sulfur–iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy. It is an alternative to electrolysis, and does not require hydrocarbons like current methods of steam reforming. But note that all of the available energy in the hydrogen so produced is supplied by the heat used to make it. | 7 | Physical Chemistry |
The consolidation stage of mucoadhesion involves the establishment of adhesive interactions to reinforce strong or prolonged adhesion. When moisture is present, mucoadhesive materials become activated and the system becomes plasticized. This stimulus allows the mucoadhesive molecules to separate and break free while proceeding to link up by weak van der Waals and hydrogen bonds. Consolidation factors are essential for the surface when exposed to significant dislodging stresses. Multiple mucoadhesion theories exist that explain the consolidation stage, the main two which focus on macromolecular interpenetration and dehydration. | 1 | Biochemistry |
While chemical kinetics is concerned with the rate of a chemical reaction, thermodynamics determines the extent to which reactions occur. In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal (the principle of dynamic equilibrium) and the concentrations of the reactants and products no longer change. This is demonstrated by, for example, the Haber–Bosch process for combining nitrogen and hydrogen to produce ammonia. Chemical clock reactions such as the Belousov–Zhabotinsky reaction demonstrate that component concentrations can oscillate for a long time before finally attaining the equilibrium. | 7 | Physical Chemistry |
*Vorobiev E., Lebovka N., (2008). Electrotechnologies for Extraction from Food Plants and Biomaterials, . | 3 | Analytical Chemistry |
Doppler cooling, which is usually accompanied by a magnetic trapping force to give a magneto-optical trap, is by far the most common method of laser cooling. It is used to cool low density gases down to the Doppler cooling limit, which for rubidium-85 is around 150 microkelvins.
In Doppler cooling, initially, the frequency of light is tuned slightly below an electronic transition in the atom. Because the light is detuned to the "red" (i.e., at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion. In each scattering event the atom loses a momentum equal to the momentum of the photon. If the atom, which is now in the excited state, then emits a photon spontaneously, it will be kicked by the same amount of momentum, but in a random direction. Since the initial momentum change is a pure loss (opposing the direction of motion), while the subsequent change is random, the probable result of the absorption and emission process is to reduce the momentum of the atom, and therefore its speed—provided its initial speed was larger than the recoil speed from scattering a single photon. If the absorption and emission are repeated many times, the average speed, and therefore the kinetic energy of the atom, will be reduced. Since the temperature of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms. | 7 | Physical Chemistry |
Low plasticity burnishing (LPB) is a method of metal improvement that provides deep, stable surface compressive residual stresses with little cold work for improved damage tolerance and metal fatigue life extension. Improved fretting fatigue and stress corrosion performance has been documented, even at elevated temperatures where the compression from other metal improvement processes relax. The resulting deep layer of compressive residual stress has also been shown to improve high cycle fatigue (HCF) and low cycle fatigue (LCF) performance. | 8 | Metallurgy |
Because mechanisms that control the accumulation of damage through the lifetime of a cell are essential to longevity, it is logical that caretaker and gatekeeper genes play a significant role in cellular aging. Increased activity of caretaker genes postpones aging, increasing lifespan. This is because of the regulatory function associated with caretaker genes in maintaining the stability of the genome. The actions of caretaker genes contribute to increasing lifespan of the cell.
A specific purpose of caretaker genes has been outlined in chromosomal duplication. Caretakers have been identified as crucial to encoding products that maintain the telomeres. It is believed that degradation of telomeres, the ends of chromosomes, through repeated cell cycle divisions, is a main component of cellular aging and death.
It has been suggested that gatekeeper genes confer beneficial anti-cancer affects but may provide deleterious effects that increase aging. This is because young organisms experiencing times of rapid growth necessitate significant anti-cancer mechanisms. As the organism ages, however, these formerly beneficial pathways become deleterious by inducing apoptosis in cells of renewable tissues, causing degeneration of the structure. Studies have shown an increased expression of pro-apoptotic genes in age-related pathologies. This is because the products of gatekeeper genes are directly involved in coding for cellular growth and proliferation.
However, dysfunctional caretaker genes do not always lead to a cancerous phenotype. For example, defects in nucleotide excision repair pathways are associated with premature aging phenotypes in diseases such as Xeroderma pigmentosum and Trichothiodystrophy. These patients exhibit brittle hair, nails, scaly skin, and hearing loss – characteristics associated with simple human aging. This is important because the nucleotide excision repair pathway is a mechanism thought to be encoded by a caretaker gene. Geneticists studying these premature-aging syndromes propose that caretaker genes that determine cell fate also play a significant role in aging. Accumulation of DNA damage with age may be especially prevalent in the central nervous system because of low DNA repair capability in postmitotic brain tissue.
Similarly, gatekeeper genes have been identified as having a role in aging disorders that exhibit mutations in such genes without an increased susceptibility to cancer. Experiments with mice that have increased gatekeeper function in the p53 gene show reduced cancer incidence (due to the protective activities of products encoded by p53) but a faster rate of aging.
Cellular senescence, also encoded by a gatekeeper gene, is arrest of the cell cycle in the G1 phase. Qualitative differences have been found between senescent cells and normal cells, including differential expression of cytokines and other factors associated with inflammation. It is believed that this may contribute, in part, to cellular aging.
In sum, although mechanisms encoded by gatekeeper and caretaker genes to protect individuals from cancer early in life, namely induction of apoptosis or senescence, later in life these functions may promote the aging phenotype. | 1 | Biochemistry |
Electronic entropy is the entropy of a system attributable to electrons' probabilistic occupation of states. This entropy can take a number of forms. The first form can be termed a density of states based entropy. The Fermi–Dirac distribution implies that each eigenstate of a system, , is occupied with a certain probability, . As the entropy is given by a sum over the probabilities of occupation of those states, there is an entropy associated with the occupation of the various electronic states. In most molecular systems, the energy spacing between the highest occupied molecular orbital and the lowest unoccupied molecular orbital is usually large, and thus the probabilities associated with the occupation of the excited states are small. Therefore, the electronic entropy in molecular systems can safely be neglected. Electronic entropy is thus most relevant for the thermodynamics of condensed phases, where the density of states at the Fermi level can be quite large, and the electronic entropy can thus contribute substantially to thermodynamic behavior. A second form of electronic entropy can be attributed to the configurational entropy associated with localized electrons and holes. This entropy is similar in form to the configurational entropy associated with the mixing of atoms on a lattice.
Electronic entropy can substantially modify phase behavior, as in lithium ion battery electrodes, high temperature superconductors, and some perovskites. It is also the driving force for the coupling of heat and charge transport in thermoelectric materials, via the Onsager reciprocal relations. | 7 | Physical Chemistry |
In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO from the atmosphere and, in a process called the Calvin cycle, uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is
Carbon fixation produces the three-carbon sugar intermediate, which is then converted into the final carbohydrate products. The simple carbon sugars photosynthesis produces are then used to form other organic compounds, such as the building material cellulose, the precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration. The latter occurs not only in plants but also in animals when the carbon and energy from plants is passed through a food chain.
The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate, to yield two molecules of a three-carbon compound, glycerate 3-phosphate, also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of ATP and NADPH produced during the light-dependent stages, is reduced to glyceraldehyde 3-phosphate. This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose phosphate. Most (five out of six molecules) of the glyceraldehyde 3-phosphate produced are used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "recycled" often condense to form hexose phosphates, which ultimately yield sucrose, starch, and cellulose, as well as glucose and fructose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids. | 5 | Photochemistry |
Disulfides can undergo dynamic exchange reactions with free thiols. The reaction is well documented within the realm of DCvC, and is one of the first reactions demonstrated to have dynamic properties. The application of disulfide chemistry has the added advantage of being a biological motif. Cysteine residues can form disulfide bonds in natural systems. | 6 | Supramolecular Chemistry |
Methylene is also a common ligand in coordination compounds, such as copper methylene .
Methylene can bond as a terminal ligand, which is called methylidene, or as a bridging ligand, which is called methanediyl. | 0 | Organic Chemistry |
The DNA of one organism is labelled, then mixed with the unlabelled DNA to be compared against. The mixture is incubated to allow DNA strands to dissociate and then cooled to form renewed hybrid double-stranded DNA. Hybridized sequences with a high degree of similarity will bind more firmly, and require more energy to separate them: i.e. they separate when heated at a higher temperature than dissimilar sequences, a process known as "DNA melting".
To assess the melting profile of the hybridized DNA, the double-stranded DNA is bound to a column or filter and the mixture is heated in small steps. At each step, the column or filter is washed; sequences that melt become single-stranded and wash off. The temperatures at which labelled DNA comes off reflects the amount of similarity between sequences (and the self-hybridization sample serves as a control). These results are combined to determine the degree of genetic similarity between organisms.
One method was introduced for hybridizing large numbers of DNA samples against large numbers of DNA probes on a single membrane. These samples would have to be separated in their own lanes inside the membranes and then the membrane would have to be rotated to a different angle where it would result in simultaneous hybridization with many different DNA probes. | 1 | Biochemistry |
There is a strong scientific consensus that greenhouse effect due to carbon dioxide is a main driver of climate change. Following is an illustrative model meant for a pedagogical purpose, showing the main physical determinants of the effect.
Under this understanding, global warming is determined by a simple energy budget: In the long run, Earth emits radiation in the same amount as it receives from the sun. However, the amount emitted depends both on Earths temperature and on its albedo: The more reflective the Earth in a certain wavelength, the less radiation it would both receive and emit in this wavelength; the warmer the Earth, the more radiation it emits. Thus changes in the albedo may have an effect on Earths temperature, and the effect can be calculated by assuming a new steady state would be arrived at.
In most of the electromagnetic spectrum, atmospheric carbon dioxide either blocks the radiation emitted from the ground almost completely, or is almost transparent, so that increasing the amount of carbon dioxide in the atmosphere, e.g. doubling the amount, will have negligible effects. However, in some narrow parts of the spectrum this is not so; doubling the amount of atmospheric carbon dioxide will make Earth's atmosphere relatively opaque to in these wavelengths, which would result in Earth emitting light in these wavelengths from the upper layers of the atmosphere, rather from lower layers or from the ground. Since the upper layers are colder, the amount emitted would be lower, leading to warming of Earth until the reduction in emission is compensated by the rise in temperature.
Furthermore, such warming may cause a feedback mechanism due to other changes in Earth's albedo, e.g. due to ice melting. | 2 | Environmental Chemistry |
The roasting process is generally undertaken in combination with reverberatory furnaces. In the roaster, the copper concentrate is partially oxidised to produce "calcine". Sulfur dioxide is liberated. The stoichiometry of the reaction is:
Roasting generally leaves more sulfur in the calcined product (15% in the case of the roaster at Mount Isa Mines) than a sinter plant leaves in the sintered product (about 7% in the case of the Electrolytic Refining and Smelting smelter).
As of 2005, roasting is no longer common in copper concentrate treatment because its combination with reverberatory furnaces is not energy efficient and the SO concentration in the roaster offgas is too dilute for cost-effective capture. Direct smelting is now favored, and uses the following smelting technologies: flash smelting, Isasmelt, Noranda, Mitsubishi or El Teniente furnaces. | 8 | Metallurgy |
Bacteriorhodopsin (Bop) is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy. | 5 | Photochemistry |
Selenols are organic compounds that contain the functional group with the connectivity . Selenols are sometimes also called selenomercaptans and selenothiols. Selenols are one of the principal classes of organoselenium compounds. A well-known selenol is the amino acid selenocysteine. | 0 | Organic Chemistry |
In the field of cell biology, the method of partial cloning (PCL) converts a fully differentiated old somatic cell into a partially reprogrammed young cell that retains all the specialised functions of the differentiated old cell but is simply younger. The method of PCL reverses characteristics associated with old cells. For example, old, senescent, cells rejuvenated by PCL are free of highly condensed senescence-associated heterochromatin foci (SAHF) and re-acquire the proliferation potential of young cells. The method of PCL thus rejuvenates old cells without de-differentiation and passage through an embryonic, pluripotent, stage. | 1 | Biochemistry |
Iodobenzene dichloride (PhICl) is a complex of iodobenzene with chlorine. As a reagent for organic chemistry, it is used as an oxidant and chlorinating agent. | 0 | Organic Chemistry |
Significant nonspecific protein adsorption during implantation can cause adverse effects. However, some proteins can be beneficial in stabilizing the implant by reducing micro-motion and implant migration, as well as improving the signal quality through increased neuron connection; improving the long-term performance. Instead of relying on the native cells to secrete these proteins, they can be added to the surface of the material prior to implantation. The surface modification of biomaterials with proteins has been done with great success in various regions of the body. However, since the anatomy of the brain is different from the rest of the body, the types of proteins that must be used in these applications vary from those used elsewhere. Proteins like laminin that promotes neuronal outgrowth and L1 that promotes axonal outgrowth have shown great promise in surface modification applications; L1 more so than laminin because of the decreased attachment associated with astrocytes – the cells responsible for glial scar formation.
Proteins are typically added to the material surface via self-assembled monolayer (SAM) formation. | 7 | Physical Chemistry |
Between 1857 and 1862, Macadam served as honorary secretary of the Philosophical Institute of Victoria, which then became the Royal Society of Victoria in 1860, and was appointed vice-president of it in 1863. He was editor of first five volumes of the societys Transactions. He was active in erecting the Societys Meeting Hall (their present building) and was involved in the institute's initiative to obtain a royal charter. He saw both happen while he held office, when in January 1860, the Philosophical Institute became the Royal Society of Victoria and met in their new building. | 3 | Analytical Chemistry |
Diversity-generating retroelements (DGRs) are a family of retroelements that were first found in Bordetella phage (BPP-1), and since been found in bacteria (e.g.Treponema denticola and Legionella pneumophila), Archaea, Archaean viruses (e.g. ANMV-1), temperate phages (e.g. Hankyphage and CrAss-like phage), and lytic phages. DGRs benefit their host by mutating particular regions of specific target proteins, for instance, phage tail fiber in BPP-1, lipoprotein in legionella pneumophila ( the pathogen behind Legionnaires disease), and TvpA in Treponema denticola (oral-associated periopathogen). An error-prone reverse transcriptase is responsible for generating these hypervariable regions in target proteins (Mutagenic retrohoming). In mutagenic retrohoming, a mutagenized cDNA (containing substantial A to N mutations) is reverse transcribed from a template region (TR), and is replaced with a segment similar to the template region called variable region (VR). Accessory variability determinant (Avd) protein is another component of DGRs, and its complex formation with the error-prone RT is of importance to mutagenic rehoming.
DGRs are beneficial to the evolution and survival of their host. A large fraction of Faecalibacterium prausnitzii phages contain DGRs that are believed to have a role in phage adaptability to the digestive system, as patients with inflammatory bowel disease (IBD), have more phages, but less F.prausnitzii in their stool samples compared to healthy individuals, suggesting that these phages activate during the illness, and that they may trigger F.prausnitzii depletion. Several tools have been implemented to identify DGRs, such as DiGReF, DGRscan, MetaCSST, and myDGR | 1 | Biochemistry |
The RBS in prokaryotes is a region upstream of the start codon. This region of the mRNA has the consensus 5-AGGAGG-3, also called the Shine-Dalgarno (SD) sequence. The complementary sequence (CCUCCU), called the anti-Shine-Dalgarno (ASD) is contained in the 3’ end of the 16S region of the smaller (30S) ribosomal subunit. Upon encountering the Shine-Dalgarno sequence, the ASD of the ribosome base pairs with it, after which translation is initiated.
Variations of the 5-AGGAGG-3 sequence have been found in Archaea as highly conserved 5′-GGTG-3′ regions, 5 basepairs upstream of the start site. Additionally, some bacterial initiation regions, such as rpsA in E.coli completely lack identifiable SD sequences. | 1 | Biochemistry |
Epoxides are uncommon in nature. They arise usually via oxygenation of alkenes by the action of cytochrome P450. (but see also the short-lived epoxyeicosatrienoic acids which act as signalling molecules. and similar epoxydocosapentaenoic acids, and epoxyeicosatetraenoic acids.) | 0 | Organic Chemistry |
The vibronic spectra of diatomic molecules in the gas phase also show rotational fine structure. Each line in a vibrational progression will show P- and R-branches. For some electronic transitions there will also be a Q-branch. The transition energies, expressed in wavenumbers, of the lines for a particular vibronic transition are given, in the rigid rotor approximation, that is, ignoring centrifugal distortion, by
Here are rotational constants and are rotational quantum numbers. (For B also, a double prime indicates the ground state and a single prime an electronically excited state.) The values of the rotational constants may differ appreciably because the bond length in the electronic excited state may be quite different from the bond length in the ground state, because of the operation of the Franck-Condon principle. The rotational constant is inversely proportional to the square of the bond length. Usually as is true when an electron is promoted from a bonding orbital to an antibonding orbital, causing bond lengthening. But this is not always the case; if an electron is promoted from a non-bonding or antibonding orbital to a bonding orbital, there will be bond-shortening and .
The treatment of rotational fine structure of vibronic transitions is similar to the treatment of rotation-vibration transitions and differs principally in the fact that the ground and excited states correspond to two different electronic states as well as to two different vibrational levels. For the P-branch , so that
Similarly for the R-branch , and
Thus, the wavenumbers of transitions in both P- and R-branches are given, to a first approximation, by the single formula
Here positive values refer to the R-branch (with ) and negative values refer to the P-branch (with ). The wavenumbers of the lines in the P-branch, on the low wavenumber side of the band origin at increase with . In the R-branch, for the usual case that , as increases the wavenumbers at first lie increasingly on the high wavenumber side of the band origin but then start to decrease, eventually lying on the low wavenumber side. The Fortrat diagram illustrates this effect. In the rigid rotor approximation the line wavenumbers lie on a parabola which has a maximum at
The line of highest wavenumber in the R-branch is known as the band head. It occurs at the value of which is equal to the integer part of , or of .
When a Q-branch is allowed for a particular electronic transition, the lines of the Q-branch correspond to the case , and wavenumbers are given by
The Q-branch then consists of a series of lines with increasing separation between adjacent lines as increases. When the Q-branch lies to lower wavenumbers relative to the vibrational line. | 7 | Physical Chemistry |
The development of laser heating began only 8 years after Charles Weir, of the National Bureau of Standards (NBS), made the first diamond anvil cell and Alvin Van Valkenburg, NBS, realized the potential of being able to see the sample while under pressure. William Bassett and his colleague Taro Takahashi focused a laser beam on the sample while under pressure. The first laser heating system used a single 7 joule pulsed ruby laser that heated the sample to 3000 °C while at 260 kilobars. This was sufficient to convert graphite to diamond. The major flaws within the first system related to control and temperature measurement.
Temperature measurement was initially done by Basset using an optical pyrometer to measure the intensity of the incandescent light from the sample. Colleagues at UC Berkeley were better able to utilize the black-body radiation and more accurately measure the temperature. The hot spot produced by the laser also created large thermal gradients in between the portions of sample that were hit by the focused laser and those that were not. The solution to this problem is ongoing but advances have been made with the introduction of a double-sided approach. | 7 | Physical Chemistry |
In elemental analysis by ICP-MS, ICP-AES, GFAA, and Flame AA, dilute nitric acid (0.5–5.0%) is used as a matrix compound for determining metal traces in solutions. Ultrapure trace metal grade acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.
It is also typically used in the digestion process of turbid water samples, sludge samples, solid samples as well as other types of unique samples which require elemental analysis via ICP-MS, ICP-OES, ICP-AES, GFAA and flame atomic absorption spectroscopy. Typically these digestions use a 50% solution of the purchased mixed with Type 1 DI Water.
In electrochemistry, nitric acid is used as a chemical doping agent for organic semiconductors, and in purification processes for raw carbon nanotubes. | 3 | Analytical Chemistry |
There are four ways to crystallize a racemate; three of which H. W. B. Roozeboom had distinguished by 1899:
;Conglomerate (sometimes racemic conglomerate):If the molecules of the substance have a much greater affinity for the same enantiomer than for the opposite one, a mechanical mixture of enantiomerically pure crystals will result. The mixture of enantiomerically pure R and S crystals forms a eutectic mixture. Consequently, the melting point of the conglomerate is always lower than that of the pure enantiomer. Addition of a small amount of one enantiomer to the conglomerate increases the melting point. Roughly 10% of racemic chiral compounds crystallize as conglomerates.
;Racemic compound (sometimes true racemate):If molecules have a greater affinity for the opposite enantiomer than for the same enantiomer, the substance forms a single crystalline phase in which the two enantiomers are present in an ordered 1:1 ratio in the elementary cell. Adding a small amount of one enantiomer to the racemic compound decreases the melting point. But the pure enantiomer can have a higher or lower melting point than the compound. A special case of racemic compounds are kryptoracemic compounds (or kryptoracemates), in which the crystal itself has handedness (is enantiomorphic), despite containing both enantiomorphs in a 1:1 ratio.
;Pseudoracemate (sometimes racemic solid solution): When there is no big difference in affinity between the same and opposite enantiomers, then in contrast to the racemic compound and the conglomerate, the two enantiomers will coexist in an unordered manner in the crystal lattice. Addition of a small amount of one enantiomer changes the melting point slightly or not at all.
;Quasiracemate: A quasiracemate is a co-crystal of two similar but distinct compounds, one of which is left-handed and the other right-handed. Although chemically different, they are sterically similar (isosteric) and are still able to form a racemic crystalline phase. One of the first such racemates studied, by Pasteur in 1853, forms from a 1:2 mixture of the bis ammonium salt of (+)-tartaric acid and the bis ammonium salt of (−)-malic acid in water. Re-investigated in 2008, the crystals formed are dumbbell-shape with the central part consisting of ammonium (+)-bitartrate, whereas the outer parts are a quasiracemic mixture of ammonium (+)-bitartrate and ammonium (−)-bimalate. | 4 | Stereochemistry |
Dye-sensitized solar cells (DSSCs) are made of low-cost materials and do not need elaborate manufacturing equipment, so they can be made in a DIY fashion. In bulk it should be significantly less expensive than older solid-state cell designs. DSSC's can be engineered into flexible sheets and although its conversion efficiency is less than the best thin film cells, its price/performance ratio may be high enough to allow them to compete with fossil fuel electrical generation.
Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material, which is adsorbed onto a thin film of titanium dioxide. The dye-sensitized solar cell depends on this mesoporous layer of nanoparticulate titanium dioxide (TiO) to greatly amplify the surface area (200–300 m/g , as compared to approximately 10 m/g of flat single crystal) which allows for a greater number of dyes per solar cell area (which in term in increases the current). The photogenerated electrons from the light absorbing dye are passed on to the n-type and the holes are absorbed by an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows more flexible use of materials and is typically manufactured by screen printing or ultrasonic nozzles, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light and the cell casing is difficult to seal due to the solvents used in assembly. Due to this reason, researchers have developed solid-state dye-sensitized solar cells that use a solid electrolyte to avoid leakage. The first commercial shipment of DSSC solar modules occurred in July 2009 from G24i Innovations. | 7 | Physical Chemistry |
Several forms (vitamers) of vitamin D exist. The two major forms are vitamin D or ergocalciferol, and vitamin D or cholecalciferol. Vitamin D without a subscript refers to either D or D, or both, and is known collectively as calciferol.
Vitamin D was chemically characterized in 1931. In 1935, the chemical structure of vitamin D was defined and shown to result from the ultraviolet irradiation of 7-dehydrocholesterol. A chemical nomenclature for vitamin D forms was recommended in 1981, but alternative names remain in common use.
Chemically, the various forms of vitamin D are secosteroids, that is, steroids in which one of the bonds in the steroid rings is broken. The structural difference between vitamin D and vitamin D is in the side chain, which contains a double bond, between carbons 22 and 23, and a methyl group on carbon 24 in vitamin D. Many vitamin D analogues have been synthesized. | 1 | Biochemistry |
The properties of RNA make the idea of the RNA world hypothesis conceptually plausible, though its general acceptance as an explanation for the origin of life requires further evidence. RNA is known to form efficient catalysts and its similarity to DNA makes clear its ability to store information. Opinions differ, however, as to whether RNA constituted the first autonomous self-replicating system or was a derivative of a still-earlier system. One version of the hypothesis is that a different type of nucleic acid, termed pre-RNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later. On the other hand, the discovery in 2009 that activated pyrimidine ribonucleotides can be synthesized under plausible prebiotic conditions suggests that it is premature to dismiss the RNA-first scenarios. Suggestions for simple pre-RNA nucleic acids have included peptide nucleic acid (PNA), threose nucleic acid (TNA) or glycol nucleic acid (GNA). Despite their structural simplicity and possession of properties comparable with RNA, the chemically plausible generation of "simpler" nucleic acids under prebiotic conditions has yet to be demonstrated. | 9 | Geochemistry |
In the mechanical stream of thinking about closed systems, heat transferred is defined as a calculated residual amount of energy transferred after the energy transferred as work has been determined, assuming for the calculation the law of conservation of energy, without reference to the concept of temperature. There are five main elements of the underlying theory.
*The existence of states of thermodynamic equilibrium, determinable by precisely one (called the non-deformation variable) more variable of state than the number of independent work (deformation) variables.
*That a state of internal thermodynamic equilibrium of a body have a well defined internal energy, that is postulated by the first law of thermodynamics.
*The universality of the law of conservation of energy.
*The recognition of work as a form of energy transfer.
*The universal irreversibility of natural processes.
*The existence of adiabatic enclosures.
*The existence of walls permeable only to heat.
Axiomatic presentations of this stream of thinking vary slightly, but they intend to avoid the notions of heat and of temperature in their axioms. It is essential to this stream of thinking that heat is not presupposed as being measurable by calorimetry. It is essential to this stream of thinking that, for the specification of the thermodynamic state of a body or closed system, in addition to the variables of state called deformation variables, there be precisely one extra real-number-valued variable of state, called the non-deformation variable, though it should not be axiomatically recognized as an empirical temperature, even though it satisfies the criteria for one. | 7 | Physical Chemistry |
The catalytic site of NPP consists of a two-metal-ion (bimetallo) Zn catalytic core. These Zn catalytic components are thought to stabilize the transition state of the NPP phosphoryl transfer reaction. | 1 | Biochemistry |
In classic polarography, the standard addition method involves creating two samples – one sample without any spikes, and another one with spikes. By comparing the current measured from two samples, the amount of analyte in the unknown is determined. This approach was the first reported use of standard addition, and was introduced by a German mining chemist, Hans Hohn, in 1937. In his polarography practical book, titled Chemische Analysen mit dem Polargraphen, Hohn referred this method as Eizhusatzes, which translates to "calibration addition" in English. Later in the German literature, this method was called as Standardzugabe, meaning "standard addition" in English.
Modern polarography typically involves using three solutions: the standard solution, the unknown solution, and a mixture of the standard and unknown solution. By measuring any two of these solutions, the unknown concentration is calculated.
As polarographic standard addition involves using only one solution with the standard added – the two-level design, polarographers always refer to the method as singular, standard addition. | 3 | Analytical Chemistry |
Residual topology is a descriptive stereochemical term to classify a number of intertwined and interlocked molecules, which cannot be disentangled in an experiment without breaking of covalent bonds, while the strict rules of mathematical topology allow such a disentanglement. Examples of such molecules are rotaxanes, catenanes with covalently linked rings (so-called pretzelanes), and open knots (pseudoknots) which are abundant in proteins.
The term "residual topology" was suggested on account of a striking similarity of these compounds to the well-established topologically nontrivial species, such as catenanes and knotanes (molecular knots). The idea of residual topological isomerism introduces a handy scheme of modifying the molecular graphs and generalizes former efforts of systemization of mechanically bound and bridged molecules. | 6 | Supramolecular Chemistry |
Addition of tricyclohexylphosphine to the carbene complex (PPh)(Cl)Ru=C(CHCOMe) results in olefin extrusion and yields an air stable anionic carbido complex. This species displaces a dimethyl sulfide ligand from PdCl(SMe) to give the μ-carbido bimetallic complex (PCy)ClRu≡C-PdCl(SMe). Spark towards a novel type of bonding was proposed following empirical observations wherein the carbido-palladium interaction could be readily disturbed. Reversible coordination ensues upon exposure of the bimetallic complex to carbon monoxide. Additionally, no coordination occurs if the anionic carbido complex contains bulky ligands such as HIMes. This indicates that the thermodynamic sink towards making the C-M bond is not very favorable, suggesting a weak interaction. Although not intuitive, characterization of this type of bonding can be inferred if C NMR shifts are observed to be far downfield, and C-M bond lengths are similar to those of complexes proven to contain carbon-based σ-donor ligands such as [(EtHIm)PdCl(μ-Cl)] | 0 | Organic Chemistry |
A uniform structure is identified by each sphere having the same number of contacting neighbours. This gives each sphere an identical neighbourhood. In the example image on the side each sphere has six neighbouring contacts.
The number of contacts is best visualised in the rolled-out contact network. It is created by rolling out the contact network into a plane of height and azimuthal angle of each sphere. For a uniform structure such as the one in the example image, this leads to a regular hexagonal lattice. Each dot in this pattern represents a sphere of the packing and each line a contact between adjacent spheres.
For all uniform structures above a diameter ratio of , the regular hexagonal lattice is its characterising feature since this lattice type has the maximum number of contacts. For different uniform structures the rolled-out contact pattern only varies by a rotation in the plane. Each uniform structure is thus distinguished by its periodicity vector , which is defined by the phyllotactic triplet . | 3 | Analytical Chemistry |
The conversion of nitrogen gas () into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. Between 5 and 10 billion kg per year are fixed by lightning strikes, but most fixation is done by free-living or symbiotic bacteria known as diazotrophs. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds. Most biological nitrogen fixation occurs by the activity of molybdenum (Mo)-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two-component enzyme that has multiple metal-containing prosthetic groups. An example of free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form such symbioses. Today, about 30% of the total fixed nitrogen is produced industrially using the Haber-Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia. | 1 | Biochemistry |
An outron is a nucleotide sequence at the 5' end of the primary transcript of a gene that is removed by a special form of RNA splicing during maturation of the final RNA product. Whereas intron sequences are located inside the gene, outron sequences lie outside the gene. | 1 | Biochemistry |
The existence of intermolecular forces was first postulated by Johannes Diderik van der Waals in 1873. However, Nobel laureate Hermann Emil Fischer developed supramolecular chemistry's philosophical roots. In 1894, Fischer suggested that enzyme–substrate interactions take the form of a "lock and key", the fundamental principles of molecular recognition and host–guest chemistry. In the early twentieth century non-covalent bonds were understood in gradually more detail, with the hydrogen bond being described by Latimer and Rodebush in 1920.
The use of these principles led to an increasing understanding of protein structure and other biological processes. For instance, the important breakthrough that allowed the elucidation of the double helical structure of DNA occurred when it was realized that there are two separate strands of nucleotides connected through hydrogen bonds. The use of non-covalent bonds is essential to replication because they allow the strands to be separated and used to template new double stranded DNA. Concomitantly, chemists began to recognize and study synthetic structures based on non-covalent interactions, such as micelles and microemulsions.
Eventually, chemists were able to take these concepts and apply them to synthetic systems. The breakthrough came in the 1960s with the synthesis of the crown ethers by Charles J. Pedersen. Following this work, other researchers such as Donald J. Cram, Jean-Marie Lehn and Fritz Vögtle became active in synthesizing shape- and ion-selective receptors, and throughout the 1980s research in the area gathered a rapid pace with concepts such as mechanically interlocked molecular architectures emerging.
The importance of supramolecular chemistry was established by the 1987 Nobel Prize for Chemistry which was awarded to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen in recognition of their work in this area. The development of selective "host–guest" complexes in particular, in which a host molecule recognizes and selectively binds a certain guest, was cited as an important contribution.
In the 1990s, supramolecular chemistry became even more sophisticated, with researchers such as James Fraser Stoddart developing molecular machinery and highly complex self-assembled structures, and Itamar Willner developing sensors and methods of electronic and biological interfacing. During this period, electrochemical and photochemical motifs became integrated into supramolecular systems in order to increase functionality, research into synthetic self-replicating system began, and work on molecular information processing devices began. The emerging science of nanotechnology also had a strong influence on the subject, with building blocks such as fullerenes, nanoparticles, and dendrimers becoming involved in synthetic systems. | 6 | Supramolecular Chemistry |
Organisms inherit their genes from their parents. Asexual organisms simply inherit a complete copy of their parent's genome. Sexual organisms have two copies of each chromosome because they inherit one complete set from each parent. | 1 | Biochemistry |
Although not usually isolated as such, the salt ammonium carbamate is produced on a large scale as an intermediate in the production of the commodity chemical urea from ammonia and carbon dioxide. | 0 | Organic Chemistry |
Perhaps the most direct measurement of s character in a bonding orbital between hydrogen and carbon is via the H−C coupling constants determined from NMR spectra. Theory predicts that J values correlates with s character. In particular, the one bond C-H coupling constant J is related to the fractional s character of the carbon hybrid orbital used to form the bond through the empirical relationship , where is the s character. (For instance the pure sp hybrid atomic orbital found in the C-H bond of methane would have 25% s character resulting in an expected coupling constant of 500 Hz × 0.25 = 125 Hz, in excellent agreement with the experimentally determined value.)
As the electronegativity of the substituent increases, the amount of p character directed towards the substituent increases as well. This leaves more s character in the bonds to the methyl protons, which leads to increased J coupling constants. | 4 | Stereochemistry |
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential adsorption of compounds to the adsorbent; compounds move through the column at different rates, allowing them to be separated into fractions. The technique is widely applicable, as many different adsorbents (normal phase, reversed phase, or otherwise) can be used with a wide range of solvents. The technique can be used on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling. Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column.
A thin-layer chromatograph can show how a mixture of compounds will behave when purified by column chromatography. The separation is first optimised using thin-layer chromatography before performing column chromatography. | 3 | Analytical Chemistry |
Bumblebees (Bombus spp.), like the honeybee (Apis spp.) collect nectar and pollen from flowers and store them for food. Many individuals must be recruited to forage for food to provide for the hive. Some bee species have highly developed ways of communicating with each other about the location and quality of food resources ranging from physical to chemical displays.
While honey bees are known for their specialized dances, such as the waggle dance which recruit other bees to the precise location of the food source, bumblebees are not capable of transmitting this type of detailed information. Instead, the nest serves as a hub where bees receive information about the foraging bouts of her conspecifics. Differences between the communication methods of honeybees and bumblebees are mainly due to differences in colony size and nest structure. Bumblebees are distinct from honeybees because they lack receiver bees (bees in the nest which receive pollen and nectar from incoming foragers during unloading) and are not capable of trophallaxis (the transfer of nectar from one bee to another). They deposit collected nectar directly into the honey pots and don't share information of the quality of the resource with other bees through nectar transfer. Another bee may sample the nectar brought into the nest, and if the colony is in need of food or the nectar is high quality she will likely go out foraging herself. Other means of alerting passive bees to a potentially rewarding resource include releasing pheromone signals and increasing physical activity.
For information on communication methods in honey bees, see bee learning and communication. | 1 | Biochemistry |
Thermal radiation is characteristically different from conduction and convection in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Thermal radiation is a type of electromagnetic radiation which is often modeled by the propagation of waves. These waves have the standard wave properties of frequency, and wavelength, which are related by the equationwhere is the speed of light in the medium. | 7 | Physical Chemistry |
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