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The Arrhenius equation gives the dependence of the rate constant of a chemical reaction on the absolute temperature as
where
* is the rate constant (frequency of collisions resulting in a reaction),
* is the absolute temperature,
* is the pre-exponential factor or Arrhenius factor or frequency factor. Arrhenius originally considered A to be a temperature-independent constant for each chemical reaction. However more recent treatments include some temperature dependence – see below.
* is the molar activation energy for the reaction,
* is the universal gas constant.
Alternatively, the equation may be expressed as
where
* is the activation energy for the reaction (in the same unit as kT),
* is the Boltzmann constant.
The only difference is the unit of : the former form uses energy per mole, which is common in chemistry, while the latter form uses energy per molecule directly, which is common in physics.
The different units are accounted for in using either the gas constant, , or the Boltzmann constant, , as the multiplier of temperature .
The unit of the pre-exponential factor are identical to those of the rate constant and will vary depending on the order of the reaction. If the reaction is first order it has the unit s, and for that reason it is often called the frequency factor or attempt frequency of the reaction. Most simply, is the number of collisions that result in a reaction per second, is the number of collisions (leading to a reaction or not) per second occurring with the proper orientation to react and is the probability that any given collision will result in a reaction. It can be seen that either increasing the temperature or decreasing the activation energy (for example through the use of catalysts) will result in an increase in rate of reaction.
Given the small temperature range of kinetic studies, it is reasonable to approximate the activation energy as being independent of the temperature. Similarly, under a wide range of practical conditions, the weak temperature dependence of the pre-exponential factor is negligible compared to the temperature dependence of the factor ; except in the case of "barrierless" diffusion-limited reactions, in which case the pre-exponential factor is dominant and is directly observable.
With this equation it can be roughly estimated that the rate of reaction increases by a factor of about 2 to 3 for every 10 °C rise in temperature, for common values of activation energy and temperature range.
The factor denotes the fraction of molecules with energy greater than or equal to . | 7 | Physical Chemistry |
Although black lights produce light in the UV range, their spectrum is mostly confined to the longwave UVA region, that is, UV radiation nearest in wavelength to visible light, with low frequency and therefore relatively low energy. While low, there is still some power of a conventional black light in the UVB range. UVA is the safest of the three spectra of UV light, although high exposure to UVA has been linked to the development of skin cancer in humans. The relatively low energy of UVA light does not cause sunburn. UVA is capable of causing damage to collagen fibers, however, so it does have the potential to accelerate skin aging and cause wrinkles. UVA can also destroy vitamin A in the skin.
UVA light has been shown to cause DNA damage, but not directly, like UVB and UVC. Due to its longer wavelength, it is absorbed less and reaches deeper into skin layers, where it produces reactive chemical intermediates such as hydroxyl and oxygen radicals, which in turn can damage DNA and result in a risk of melanoma. The weak output of black lights, however, is not considered sufficient to cause DNA damage or cellular mutations in the way that direct summer sunlight can, although there are reports that overexposure to the type of UV radiation used for creating artificial suntans on sunbeds can cause DNA damage, photoaging (damage to the skin from prolonged exposure to sunlight), toughening of the skin, suppression of the immune system, cataract formation and skin cancer.
UV-A can have negative effects on eyes in both the short-term and long-term. | 5 | Photochemistry |
Not all elements which are found in the human body in trace quantities play a role in life. Some of these elements are thought to be simple common contaminants without function (examples: caesium, titanium), while many others are thought to be active toxins, depending on amount (cadmium, mercury, lead, radioactives). In humans, arsenic is toxic, and its levels in foods and dietary supplements are closely monitored to reduce or eliminate its intake.
Some elements (silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Bromine is used by some (though not all) bacteria, fungi, diatoms, and seaweeds, and opportunistically in eosinophils in humans. One study has indicated bromine to be necessary to collagen IV synthesis in humans. Fluorine is used by a number of plants to manufacture toxins but only functions in humans as a local topical hardening agent in tooth enamel. | 1 | Biochemistry |
Salbutamol is typically used to treat bronchospasm (due to any cause—allergic asthma or exercise-induced), as well as chronic obstructive pulmonary disease. It is also one of the most common medicines used in rescue inhalers (short-term bronchodilators to alleviate asthma attacks).
As a β agonist, salbutamol also has use in obstetrics. Intravenous salbutamol can be used as a tocolytic to relax the uterine smooth muscle to delay premature labor. While preferred over agents such as atosiban and ritodrine, its role has largely been replaced by the calcium channel blocker nifedipine, which is more effective and better tolerated.
Salbutamol has been used to treat acute hyperkalemia, as it stimulates potassium flow into cells, thus lowering the potassium in the blood. | 4 | Stereochemistry |
Maritime New Zealand used the oil dispersant Corexit 9500 to help in the cleanup process. The dispersant was applied for only a week, after results proved inconclusive. | 2 | Environmental Chemistry |
When a crystalline material that contains atoms with uncompensated electron spins is cooled down, ordering of these spins generally occurs once the thermal energy is small enough not to overrule the interactions between neighboring spins. If the ordering does not exhibit the same symmetry as the original unit cell of the crystallographic lattice, a superstructure will result. In this case, the superspots are typically only visible in neutron diffraction patterns, because the neutron is scattered both by the nucleus and by the magnetic moments of the electron spins. | 3 | Analytical Chemistry |
Active transcription units are clustered in the nucleus, in discrete sites called transcription factories or euchromatin. Such sites can be visualized by allowing engaged polymerases to extend their transcripts in tagged precursors (Br-UTP or Br-U) and immuno-labeling the tagged nascent RNA. Transcription factories can also be localized using fluorescence in situ hybridization or marked by antibodies directed against polymerases. There are ~10,000 factories in the nucleoplasm of a HeLa cell, among which are ~8,000 polymerase II factories and ~2,000 polymerase III factories. Each polymerase II factory contains ~8 polymerases. As most active transcription units are associated with only one polymerase, each factory usually contains ~8 different transcription units. These units might be associated through promoters and/or enhancers, with loops forming a "cloud" around the factor. | 1 | Biochemistry |
Survivin is shown to be clearly regulated by the cell cycle, as its expression is found to be dominant only in the G2/M phase. This regulation exists at the transcriptional level, as there is evidence of the presence of cell-cycle-dependent element/cell-cycle gene homology region (CDE/CHR)boxes located in the survivin promoter region. Further evidence to support this mechanism of regulation includes the evidence that surivin is poly-ubiquinated and degraded by proteasomes during interphase of the cell cycle. Moreover, survivin has been shown to localize to components of the mitotic spindle during metaphase and anaphase of mitosis. Physical association between polymerized tubulin and survivin have been shown in vitro as well. It is also shown that post-transcriptional modification of survivin involving the phosphorylation of Thr34 leads to increased protein stability in the G2/M phase of the cell cycle.
It is known from Mirza et al. that repression of survivin by p53 is not a result of any cell cycle progressive regulation. The same experiment by Mirza et al. with regard to determining p53 suppression of survivin at the transcriptional level was repeated, but this time for cells arrested in different stages of the cell cycle. It was shown that, although p53 arrests the numbers of cells to different extents in different phases, the measured level of survivin mRNA and protein levels were the same across all the samples transfected with the wild-type p53. This shows that p53 acts in a cell-cycle independent manner to inhibit survivin expression. | 1 | Biochemistry |
In the electron transport chain, complex I (CI) catalyzes the reduction of ubiquinone (UQ) to ubiquinol (UQH) by the transfer of two electrons from reduced nicotinamide adenine dinucleotide (NADH) which translocates four protons from the mitochondrial matrix to the IMS:
Complex III (CIII) catalyzes the Q-cycle. The first step involving the transfer of two electrons from the UQH reduced by CI to two molecules of oxidized cytochrome c at the Q site. In the second step, two more electrons reduce UQ to UQH at the Q site. The total reaction is:
Complex IV (CIV) catalyzes the transfer of two electrons from the cytochrome c reduced by CIII to one half of a full oxygen. Utilizing one full oxygen in oxidative phosphorylation requires the transfer of four electrons. The oxygen will then consume four protons from the matrix to form water while another four protons are pumped into the IMS, to give a total reaction | 7 | Physical Chemistry |
The noradrenaline system consists of around 15,000 neurons, primarily in the locus coeruleus. This is diminutive compared to the more than 100 billion neurons in the brain. As with dopaminergic neurons in the substantia nigra, neurons in the locus coeruleus tend to be melanin-pigmented. Noradrenaline is released from the neurons, and acts on adrenergic receptors. Noradrenaline is often released steadily so that it can prepare the supporting glial cells for calibrated responses. Despite containing a relatively small number of neurons, when activated, the noradrenaline system plays major roles in the brain including involvement in suppression of the neuroinflammatory response, stimulation of neuronal plasticity through LTP, regulation of glutamate uptake by astrocytes and LTD, and consolidation of memory. | 1 | Biochemistry |
Mechanically stimulated gas emission (MSGE) is a complex phenomenon embracing various physical and chemical processes occurring on the surface and in the bulk of a solid under applied mechanical stress and resulting in emission of gases. MSGE is a part of a more general phenomenon of mechanically stimulated neutral emission. MSGE experiments are often performed in ultra-high vacuum. | 7 | Physical Chemistry |
Additionally, sea foam is a habitat for a number of marine microorganisms. Some research has shown the presence of various microphytoplanktonic, nanophytoplanktonic, and diatom groups in seafoam; the phytoplankton groups appeared in significantly higher abundance than in sea surface film and the top pelagic zone | 9 | Geochemistry |
Pelletizing is the process of compressing or molding a material into the shape of a pellet. A wide range of different materials are pelletized including chemicals, iron ore, animal compound feed, plastics, waste materials, and more. The process is considered an excellent option for the storage and transport of said materials. The technology is widely used in the powder metallurgy engineering and medicine industries. | 8 | Metallurgy |
Moshers acid, or α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) is a carboxylic acid which was first used by Harry Stone Mosher as a chiral derivatizing agent. It is a chiral molecule, consisting of R and S' enantiomers. | 4 | Stereochemistry |
At the end of the Medieval Era and into the Post-Medieval Era, new types of crucible designs and processes started. Smelting and melting crucibles types started to become more limited in designs which are produced by a few specialists. The main types used during the Post Medieval period are the Hessian crucibles which were made in the Hesse region in Germany. These are triangular vessels made on a wheel or within a mold using high alumina clay and tempered with pure quartz sand. Furthermore, another specialized crucible which was made at the same time was that of a graphite crucible from southern Germany. These had a very similar design to that of the triangular crucibles from Hesse but they also occur in conical forms. These crucibles were traded all across Europe and the New World.
The refining of methods during the Medieval and Post-Medieval periods led to the invention of the cupel which resembles a small egg cup, made of ceramic or bone ash which was used to separate base metals from noble metals. This process is known as cupellation. Cupellation started long before the Post Medieval period, however, the first vessels made to carry out this process started in the 16th Century. Another vessel used for the same process is a scorifier which is similar to a cupel but slightly larger and removes the lead and leaves the noble metals behind. Cupels and scorifiers were mass-produced as after each reduction the vessels would have absorbed all of the lead and become fully saturated. These vessels were also used in the process of metallurgical assay where the noble metals are removed from a coin or a weight of metal to determine the amount of the noble metals within the object. | 3 | Analytical Chemistry |
Infrared radiation can pass through dry, clear air in the wavelength range of 8–13 µm. Materials that can absorb energy and radiate it in those wavelengths exhibit a strong cooling effect. Materials that can also reflect 95% or more of sunlight in the 200 nanometres to 2.5 µm range can exhibit cooling even in direct sunlight. | 7 | Physical Chemistry |
The first uncoupling protein discovered, UCP1, was discovered in the brown adipose tissues of hibernators and small rodents, which provide non-shivering heat to these animals. These brown adipose tissues are essential to maintaining the body temperature of small rodents, and studies with (UCP1)-knockout mice show that these tissues do not function correctly without functioning uncoupling proteins. In fact, these studies revealed that cold-acclimation is not possible for these knockout mice, indicating that UCP1 is an essential driver of heat production in these brown adipose tissues.
Elsewhere in the body, uncoupling protein activities are known to affect the temperature in micro-environments. This is believed to affect other proteins' activity in these regions, though work is still required to determine the true consequences of uncoupling-induced temperature gradients within cells.
The structure of human uncoupling protein 1 UCP1 has been solved by cryogenic-electron microscopy. The structure has the typical fold of a member of the SLC25 family. UCP1 is locked in a cytoplasmic-open state by guanosine triphosphate in a pH-dependent manner. | 1 | Biochemistry |
CBED was first introduced in 1939 by Kossel and Möllenstedt. The development of the Field Emission Gun (FEG) in the 1970s, the Scanning Transmission Electron Microscopy (STEM), energy filtering devices and so on, made possible smaller probe diameters and larger convergence angles, and all this made CBED more popular. In the seventies, CBED was being used for the determination of the point group and space group symmetries by Goodman and Lehmpfuh, and Buxton, and starting in 1985, CBED was used by Tanaka et al. for studying crystals structure. | 3 | Analytical Chemistry |
Diebold was born on 12 December 1961 in Kapfenberg, Austria. She spent much of her high school years reading, skiing, and agonizing over what to major in at the university. She ultimately settled on engineering physics, an area with good job prospects that was also general enough to accommodate a variety of future directions. After completing her diploma in engineering physics (TU Vienna, 1986), she became increasingly enthusiastic about experimental physics while working on her master's thesis, and ultimately completed a Doctor of Technology (Dr. techn.) in this area with Prof. Peter Varga (TU Vienna, 1990). | 7 | Physical Chemistry |
Methyl isocyanate (MIC) is highly flammable. MDI and TDI are much less flammable. Flammability of materials is a consideration in furniture design. The specific flammability hazard is noted on the safety data sheet (SDS) for specific isocyanates. | 0 | Organic Chemistry |
Calcium signaling in Arabidopsis is a calcium mediated signalling pathway that Arabidopsis plants use in order to respond to a stimuli. In this pathway, Ca works as a long range communication ion, allowing for rapid communication throughout the plant. Systemic changes in metabolites such as glucose and sucrose takes a few minutes after the stimulus, but gene transcription occurs within seconds. Because hormones, peptides and RNA travel through the vascular system at lower speeds than the plants response to wounds, indicates that Ca must be involved in the rapid signal propagation. Instead of local communication to nearby cells and tissues, Ca uses mass flow within the vascular system to help with rapid transport throughout the plant. Ca moving through the xylem and phloem acts through a “calcium signature” receptor system in cells where they integrate the signal and respond with the activation of defense genes. These calcium signatures encode information about the stimulus allowing the response of the plant to cater towards the type of stimulus. | 7 | Physical Chemistry |
In the history of thermodynamics, Thermodynamik chemischer Vorgänge (Chemical thermodynamic process) is a sequence of three papers (1882–1883) written by German physicist Hermann von Helmholtz. It is one of the founding papers in thermodynamics, along with Josiah Willard Gibbss 1876 paper "On the Equilibrium of Heterogeneous Substances'". Together they form the foundation of chemical thermodynamics as well as a large part of physical chemistry.
It was published in three parts in Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin ["Proceedings of the Royal Prussian Academy of Sciences"], and is available on HathiTrust and online archive of the Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin.
* First part: Die Thermodynamik chemischer Vorgänge, Submitted on 2 February 1882.
* Second part: Zur Thermodynamik chemischer Vorgänge, subtitled Folgerungen die galvanische Polarisation betreffend ["Conclusions concerning galvanic polarization"]. Submitted on 27 July 1882.
** (yes, the word "Die" was turned to "Zur", which might cause bibliographical confusion)
* Third part: Zur Thermodynamik chemischer Vorgänge, subtitled Versuche an Chlorzink-Kalomel-Elementen ["Experiments on zinc chloride calomel elements"]. Submitted on 31 May 1883. | 7 | Physical Chemistry |
One of the most important components of DSSC is the counter electrode. As stated before, the counter electrode is responsible for collecting electrons from the external circuit and introducing them back into the electrolyte to catalyze the reduction reaction of the redox shuttle, generally I to I. Thus, it is important for the counter electrode to not only have high electron conductivity and diffusive ability, but also electrochemical stability, high catalytic activity and appropriate band structure. The most common counter electrode material currently used is platinum in DSSCs, but is not sustainable owing to its high costs and scarce resources. Thus, much research has been focused towards discovering new hybrid and doped materials that can replace platinum with comparable or superior electrocatalytic performance. One such category being widely studied includes chalcogen compounds of cobalt, nickel, and iron (CCNI), particularly the effects of morphology, stoichiometry, and synergy on the resulting performance. It has been found that in addition to the elemental composition of the material, these three parameters greatly impact the resulting counter electrode efficiency. Of course, there are a variety of other materials currently being researched, such as highly mesoporous carbons, tin-based materials, gold nanostructures, as well as lead-based nanocrystals. However, the following section compiles a variety of ongoing research efforts specifically relating to CCNI towards optimizing the DSSC counter electrode performance. | 5 | Photochemistry |
The scleraxis protein is a member of the basic helix-loop-helix (bHLH) superfamily of transcription factors. Currently two genes ( and respectively) have been identified to code for identical scleraxis proteins. | 1 | Biochemistry |
mRNA processing acts as a form of post-transcriptional regulation, which mostly happens in eukaryotes. 3′ cleavage/polyadenylation and 5’ capping increase overall RNA stability, and the presence of 5’ cap allows ribosome binding for translation. RNA splicing allows the expression of various protein variants from the same gene.
Specific examples include:
* SR proteins
* Ribonucleoprotein
** hnRNP
** snRNP | 1 | Biochemistry |
An electric field pointing from left to right, for example, tends to pull nuclei to the right and electrons to the left. In another way of viewing it, if an electronic state has its electron disproportionately to the left, its energy is lowered, while if it has the electron disproportionately to the right, its energy is raised.
Other things being equal, the effect of the electric field is greater for outer electron shells, because the electron is more distant from the nucleus, so it travels farther left and farther right.
The Stark effect can lead to splitting of degenerate energy levels. For example, in the Bohr model, an electron has the same energy whether it is in the 2s state or any of the 2p states. However, in an electric field, there will be hybrid orbitals (also called quantum superpositions) of the 2s and 2p states where the electron tends to be to the left, which will acquire a lower energy, and other hybrid orbitals where the electron tends to be to the right, which will acquire a higher energy. Therefore, the formerly degenerate energy levels will split into slightly lower and slightly higher energy levels. | 7 | Physical Chemistry |
To the left is the NFPA diamond as determined by the Safety Data Sheet, or SDS, by Fisher Scientific. There is minimal risk in handling the chemical. | 3 | Analytical Chemistry |
The most common types of bioconjugation include coupling of a small molecule (such as biotin or a fluorescent dye) to a protein. Antibody-drug conjugates such as Brentuximab vedotin and Gemtuzumab ozogamicin are examples falling into this category.
Protein-protein conjugations, such as the coupling of an antibody to an enzyme, or the linkage of protein complexes, is also facilitated via bioconjugations.
Other less common molecules used in bioconjugation are oligosaccharides, nucleic acids, synthetic polymers such as polyethylene glycol, and carbon nanotubes. | 1 | Biochemistry |
The main advantage over other filtration systems is the reduction in energy consumption, up to 90% because no air flows through the discs due to the use of capillary force acting on the pores. Air breakthrough is prevented by the fine pores of the filter, thus allowing retention of higher vacuum levels. Therefore, the vacuum losses are less, which means the vacuum pump required is smaller than in conventional disc filters, thus minimizing operating costs. Power consumed by a vacuum ceramic filter with 45 m of filtration area is 15 kW while 170 kW is consumed by similar filters with cloth membranes.
Generally, conventional disc filters are not suitable for cake washing because the water quickly runs off the surface of the cake. As the cake solids are sprayed with a wash liquid to remove impurities, they are not suitable for conventional filtration systems where channelling or uneven distribution occurs, leading to cake cracking. However, cake washing has been proved to be more efficient with vacuum ceramic filters due to the steady flow profile and the even distribution of the cake.
A further advantage of vacuum ceramic filter is the high output capacity with a very low water content and drier filter cake. By comparison, the performance of a VDFK-3 ceramic filter was compared with the existing BOU-40 and BLN40-3 drum type vacuum filters to filter aluminium hydroxide. From the results, the average moisture content was 5% (abs? or rel?) lower when a vacuum ceramic filter was used.
Vacuum ceramic filters also have a longer service life while cloth filters have to be replaced, which eventually increases the moisture content of the cake, lowers the productivity and disturbs the production operations. In addition, the ceramic filter is both mechanically and chemically reliable enough to withstand regeneration.
Whilst the vacuum ceramic filter has proved to be a great innovation, there are still some limitations involved when operating the equipment. Ceramic filters exhibit large fluctuations in the recoiling washing pressure (0.05~0.35 MPa). This raises the short-term negative pressure and induces dilute acid due to the falling suck phenomenon. Therefore, the cleaning effect of the ceramic plates and the efficiency of the filter will be negatively affected. | 3 | Analytical Chemistry |
In chemistry, a metal carbonyl cluster is a compound that contains two or more metal atoms linked in part by metal–metal bonds and containing carbon monoxide (CO) as the exclusive or predominant ligand. The area is a subfield of metal carbonyl chemistry, and many metal carbonyl clusters are in fact prepared from simple metal carbonyls. Simple examples include Fe(CO), Fe(CO), and Mn(CO). High nuclearity clusters include [Rh(CO)H] and the stacked Pt triangules [Pt(CO)] (n = 2–6). | 7 | Physical Chemistry |
Petroleum, in one form or another, has been used since ancient times. More than 4300 years ago, bitumen was mentioned when the Sumerians used it to make boats. A tablet of the legend of the birth of Sargon of Akkad mentions a basket which was closed by straw and bitumen. More than 4000 years ago, according to Herodotus and Diodorus Siculus, asphalt was used in the construction of the walls and towers of Babylon; there were oil pits near Ardericca and Babylon, and a pitch spring on Zakynthos. Great quantities of it were found on the banks of the river Issus, one of the tributaries of the Euphrates. Ancient Persian tablets indicate the medicinal and lighting uses of petroleum in the upper levels of their society.
The use of petroleum in ancient China dates back to more than 2000 years ago. The I Ching, one of the earliest Chinese writings, cites that oil in its raw state, without refining, was first discovered, extracted, and used in China in the first century BCE. In addition, the Chinese were the first to record the use of petroleum as fuel as early as the fourth century BCE. By 347 CE, oil was produced from bamboo-drilled wells in China.
In the 7th century, petroleum was among the essential ingredients for Greek fire, an incendiary projectile weapon that was used by Byzantine Greeks against Arab ships, which were then attacking Constantinople. Crude oil was also distilled by Persian chemists, with clear descriptions given in Arabic handbooks such as those of Abu Bakr al-Razi (Rhazes). The streets of Baghdad were paved with tar, derived from petroleum that became accessible from natural fields in the region.
In the 9th century, oil fields were exploited in the area around modern Baku, Azerbaijan. These fields were described by the Arab geographer Abu Bakr al-Razi in the 10th century, and by Marco Polo in the 13th century, who described the output of those wells as hundreds of shiploads. Arab and Persian chemists also distilled crude oil to produce flammable products for military purposes. Through Islamic Spain, distillation became available in Western Europe by the 12th century. It has also been present in Romania since the 13th century, being recorded as păcură.
Sophisticated oil pits, deep, were dug by the Seneca people and other Iroquois in Western Pennsylvania as early as 1415–1450. The French General Louis-Joseph de Montcalm encountered Seneca using petroleum for ceremonial fires and as a healing lotion during a visit to Fort Duquesne in 1750.
Early British explorers to Myanmar documented a flourishing oil extraction industry based in Yenangyaung that, in 1795, had hundreds of hand-dug wells under production.
Merkwiller-Pechelbronn is said to be the first European site where petroleum has been explored and used. The still active Erdpechquelle, a spring where petroleum appears mixed with water has been used since 1498, notably for medical purposes. | 7 | Physical Chemistry |
Parker graduated from Senn High School in Chicago. She went on to receive a B.A. from Northwestern University (1966). While in college she won an award from the student chapter of the American Institute of Chemists for her essay "Chemistry as a Profession" making her the first woman to receive this award. She earned her Ph.D. from Stanford University in 1970. Following her Ph.D. she was a postdoctoral research at Columbia University. From 1973 until 2001 Parker was in the chemistry department at Brown University. In 2001 she moved to Stony Brook University, and in 2017 she was named a distinguished professor at Stony Brook University. | 0 | Organic Chemistry |
The MAP2 family is involved in the development of neurons, mostly present during early stages of axon formation then disappear later. However they exist in mature dendrites as well. Different forms of MAP2s are formed by different post-translational modifications of the mRNA. | 1 | Biochemistry |
Ractopamine use as a feed additive is authorized in the United States, Canada, and Mexico. In the US, ractopamine is allowed to be used at a feed concentration of 5–20 mg/kg feed for finishing pigs and in dosages of 5–10 mg/kg feed for finishing pigs heavier than 109 kg. The maximum residue limit for ractopamine for meat in the USA is 50 parts per billion (ppb), or five times the standard set by the Codex Alimentarius. Ractopamine was approved by the FDA in 1999 for use in swine, in 2003 for use in cattle, and in 2008 for use in turkeys.
In 2015, the USDA approved of a new label, "No ractopamine — a beta-agonist growth promotant" to be used.
The Canadian Food Inspection Agency (CFIA) maintains the Canadian Ractopamine-Free Pork Certification Program (CRFPCP) so that Canadian exports to Asian countries are not disallowed by their authorities. Hundreds of commercial feed facilities, including some from overseas, are enrolled in the CRFPCP, a programme that is essentially based on self-certification and backed up by a randomized testing policy.
Currently, the label for USDA organic means no synthetic compounds can be used other than those on the list of allowed synthetics; therefore, ractopamine would not be allowed in certified organic production.
For Canadian domestic consumption of non-CRFPCP pork, ractopamine hydrochloride is permitted to be sold over the counter with applications in meal or pellet feed for finishing barrows and gilts (up to 10 mg/kg for last six weeks), confined finishing cattle (up to 30 mg/kg for last 42 days), and finishing heavy turkeys (up to 9 mg/kg for 14 days). The withdrawal period was set to 0 days. | 4 | Stereochemistry |
In Amphipholis squamata, bioluminescence has been observed to come from the spines emanating from the arms from photocytes within the spinal ganglia. Acetylcholine has been found to be able to stimulate the photocytes to produce light. | 1 | Biochemistry |
A stepwise model for the emergence of chemiosmosis, a key element in the origin of life on earth, proposes that primordial organisms used thermal cycling as an energy source (thermosynthesis), functioning essentially as a heat engine:
:self-organized convection in natural waters causing thermal cycling →
:: added β-subunit of F ATP Synthase
:: (generated ATP by thermal cycling of subunit during suspension in convection cell: thermosynthesis) →
::: added membrane and F ATP Synthase moiety
::: (generated ATP by change in electrical polarization of membrane during thermal cycling: thermosynthesis) →
:::: added metastable, light-induced electric dipoles in membrane
:::: (primitive photosynthesis) →
::::: added quinones and membrane-spanning light-induced electric dipoles
::::: (today's bacterial photosynthesis, which makes use of chemiosmosis). | 1 | Biochemistry |
Energy spectral density describes how the energy of a signal or a time series is distributed with frequency. Here, the term energy is used in the generalized sense of signal processing; that is, the energy of a signal is:
The energy spectral density is most suitable for transients—that is, pulse-like signals—having a finite total energy. Finite or not, Parsevals theorem (or Plancherels theorem) gives us an alternate expression for the energy of the signal:
where:
is the value of the Fourier transform of at frequency (in Hz). The theorem also holds true in the discrete-time cases. Since the integral on the left-hand side is the energy of the signal, the value of can be interpreted as a density function multiplied by an infinitesimally small frequency interval, describing the energy contained in the signal at frequency in the frequency interval .
Therefore, the energy spectral density of is defined as:
The function and the autocorrelation of form a Fourier transform pair, a result also known as the Wiener–Khinchin theorem (see also Periodogram).
As a physical example of how one might measure the energy spectral density of a signal, suppose represents the potential (in volts) of an electrical pulse propagating along a transmission line of impedance , and suppose the line is terminated with a matched resistor (so that all of the pulse energy is delivered to the resistor and none is reflected back). By Ohm's law, the power delivered to the resistor at time is equal to , so the total energy is found by integrating with respect to time over the duration of the pulse. To find the value of the energy spectral density at frequency , one could insert between the transmission line and the resistor a bandpass filter which passes only a narrow range of frequencies (, say) near the frequency of interest and then measure the total energy dissipated across the resistor. The value of the energy spectral density at is then estimated to be . In this example, since the power has units of V Ω, the energy has units of V s Ω = J, and hence the estimate of the energy spectral density has units of J Hz, as required. In many situations, it is common to forget the step of dividing by so that the energy spectral density instead has units of V Hz.
This definition generalizes in a straightforward manner to a discrete signal with a countably infinite number of values such as a signal sampled at discrete times :
where is the discrete-time Fourier transform of The sampling interval is needed to keep the correct physical units and to ensure that we recover the continuous case in the limit But in the mathematical sciences the interval is often set to 1, which simplifies the results at the expense of generality. (also see normalized frequency) | 7 | Physical Chemistry |
Diprotonation of [CBH] gives the neutral carborane CBH. Pyrolysis of this nido cluster gives closo-CBH. Chromate-oxidation of [CBH] results in deboronation, giving the CBH. This carborane features two CH vertices. | 7 | Physical Chemistry |
Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In addition to playing a major role in the activation of host immune responses, chemokines are important for biological processes, including morphogenesis and wound healing, as well as in the pathogenesis of diseases like cancers.
Cytokine proteins are classified as chemokines according to behavior and structural characteristics. In addition to being known for mediating chemotaxis, chemokines are all approximately 8–10 kilodaltons in mass and have four cysteine residues in conserved locations that are key to forming their 3-dimensional shape.
These proteins have historically been known under several other names including the SIS family of cytokines, SIG family of cytokines, SCY family of cytokines, Platelet factor-4 superfamily or intercrines. Some chemokines are considered pro-inflammatory and can be induced during an immune response to recruit cells of the immune system to a site of infection, while others are considered homeostatic and are involved in controlling the migration of cells during normal processes of tissue maintenance or development. Chemokines are found in all vertebrates, some viruses and some bacteria, but none have been found in other invertebrates.
Chemokines have been classified into four main subfamilies: CXC, CC, CX3C and C. All of these proteins exert their biological effects by interacting with G protein-linked transmembrane receptors called chemokine receptors, that are selectively found on the surfaces of their target cells. | 1 | Biochemistry |
Many general purpose vectors such as pUC19 usually include a system for detecting the presence of a cloned DNA fragment, based on the loss of an easily scored phenotype. The most widely used is the gene coding for E. coli β-galactosidase, whose activity can easily be detected by the ability of the enzyme it encodes to hydrolyze the soluble, colourless substrate X-gal (5-bromo-4-chloro-3-indolyl-beta-d-galactoside) into an insoluble, blue product (5,5-dibromo-4,4-dichloro indigo). Cloning a fragment of DNA within the vector-based lacZα sequence of the β-galactosidase prevents the production of an active enzyme. If X-gal is included in the selective agar plates, transformant colonies are generally blue in the case of a vector with no inserted DNA and white in the case of a vector containing a fragment of cloned DNA. | 1 | Biochemistry |
The Hirao coupling (also called the Hirao reaction or the Hirao cross-coupling) is the chemical reaction involving the palladium-catalyzed cross-coupling of a dialkyl phosphite and an aryl halide to form a phosphonate.
This reaction is named after Toshikazu Hirao and is related to the Michaelis-Arbuzov reaction. In contrast to the classic Michaelis-Arbuzov reaction, which is limited to alkyl phosphonates, the Hirao coupling can also deliver aryl phosphonates. | 0 | Organic Chemistry |
[https://iifiir.org/en/fridoc FRIDOC] is the most comprehensive database in the world dedicated to refrigeration. It contains over 110,000 references to documents in all domains of refrigeration.
A large number of the documents referenced in FRIDOC are scientific and technical. FRIDOC also contains many review articles, documents on economic data and statistics, articles dealing with regulations and standardization, etc. | 7 | Physical Chemistry |
Aerogels may be used as a potential low-cost PDRC material scalable for mass production. Some aerogels can also be considered a more environmentally friendly alternative to other materials, with degradable potential and the absence of toxic chemicals. Aerogels can also be useful as a thermal insulation material to reduce solar absorption and parasitic heat gain to improve the cooling performance of PDRCs.
* Yue et al. (2022), superhydrophobic waste paper-based (cellulose) aerogel, solar reflectance 93%, thermal emittance 91%, reduced daytime temperatures up to 8.5 °C below ambient in outdoor test, in a building energy simulation the aerogel "showed that 43.4% of cooling energy on average could be saved compared to the building baseline consumption" in China if widely implemented.
* Liu et al. (2022), degradable and superhydrophobic stereo-complex poly (lactic acid) aerogel with low thermal conductivity, solar reflectance 89%, heat emittance 93%, reduced daytime temperatures 3.5ᵒC below ambient, "opens an environmentally sustainable pathway to radiative cooling applications."
* Li et al. (2022), low-cost silica-alumina nanofibrous aerogels (SAFAs) synthesized by electrospinning, solar reflectance 95%, heat emittance 93%, reduced daytime temperatures 5ᵒC below ambient, "the SAFAs exhibit high compression fatigue resistance, robust fire resistance and excellent thermal insulation" with "low cost and high performance," shows potential for further studies. | 7 | Physical Chemistry |
These different definitions have true physical meaning because different techniques in physical polymer chemistry often measure just one of them. For instance, osmometry measures number average molar mass and small-angle laser light scattering measures mass average molar mass. is obtained from viscosimetry and by sedimentation in an analytical ultra-centrifuge. The quantity a in the expression for the viscosity average molar mass varies from 0.5 to 0.8 and depends on the interaction between solvent and polymer in a dilute solution. In a typical distribution curve, the average values are related to each other as follows:
The dispersity (also known as the polydispersity index) of a sample is defined as divided by and gives an indication just how narrow a distribution is.
The most common technique for measuring molecular mass used in modern times is a variant of high-pressure liquid chromatography (HPLC) known by the interchangeable terms of size exclusion chromatography (SEC) and gel permeation chromatography (GPC). These techniques involve forcing a polymer solution through a matrix of cross-linked polymer particles at a pressure of up to several hundred bar. The limited accessibility of stationary phase pore volume for the polymer molecules results in shorter elution times for high-molecular-mass species. The use of low dispersity standards allows the user to correlate retention time with molecular mass, although the actual correlation is with the Hydrodynamic volume. If the relationship between molar mass and the hydrodynamic volume changes (i.e., the polymer is not exactly the same shape as the standard) then the calibration for mass is in error.
The most common detectors used for size exclusion chromatography include online methods similar to the bench methods used above. By far the most common is the differential refractive index detector that measures the change in refractive index of the solvent. This detector is concentration-sensitive and very molecular-mass-insensitive, so it is ideal for a single-detector GPC system, as it allows the generation of mass v's molecular mass curves. Less common but more accurate and reliable is a molecular-mass-sensitive detector using multi-angle laser-light scattering - see static light scattering. These detectors directly measure the molecular mass of the polymer and are most often used in conjunction with differential refractive index detectors. A further alternative is either low-angle light scattering, which uses a single low angle to determine the molar mass, or Right-angle-light laser scattering in combination with a viscometer, although this latter technique does not give an absolute measure of molar mass but one relative to the structural model used.
The molar mass distribution of a polymer sample depends on factors such as chemical kinetics and work-up procedure. Ideal step-growth polymerization gives a polymer with dispersity of 2. Ideal living polymerization results in a dispersity of 1. By dissolving a polymer an insoluble high molar mass fraction may be filtered off resulting in a large reduction in and a small reduction in , thus reducing dispersity. | 7 | Physical Chemistry |
This geometry has been applied to black hole thermodynamics, with some physically relevant results. The most physically significant case is for the Kerr black hole in higher dimensions, where the curvature singularity signals thermodynamic instability, as found earlier by conventional methods.
The entropy of a black hole is given by the well-known Bekenstein–Hawking formula
where is Boltzmanns constant, the speed of light, Newtons constant and is the area of the event horizon of the black hole. Calculating the Ruppeiner geometry of the black hole's entropy is, in principle, straightforward, but it is important that the entropy should be written in terms of extensive parameters,
where is ADM mass of the black hole and are the conserved charges and runs from 1 to n. The signature of the metric reflects the sign of the hole's specific heat. For a Reissner-Nordström black hole, the Ruppeiner metric has a Lorentzian signature which corresponds to the negative heat capacity it possess, while for the BTZ black hole, we have a Euclidean signature. This calculation cannot be done for the Schwarzschild black hole, because its entropy is
which renders the metric degenerate. | 7 | Physical Chemistry |
In 1804, Dalton explained his atomic theory to his friend and fellow chemist Thomas Thomson, who published an explanation of Daltons theory in his book A System of Chemistry in 1807. According to Thomson, Daltons idea first occurred to him when experimenting with "olefiant gas" (ethylene) and "carburetted hydrogen gas" (methane). Dalton found that "carburetted hydrogen gas" contains twice as much hydrogen per measure of carbon as "olefiant gas", and concluded that a molecule of "olefiant gas" is one carbon atom and one hydrogen atom, and a molecule of "carburetted hydrogen gas" is one carbon atom and two hydrogen atoms. In reality, an ethylene molecule has two carbon atoms and four hydrogen atoms (CH), and a methane molecule has one carbon atom and four hydrogen atoms (CH). In this particular case, Dalton was mistaken about the formulas of these compounds, and it wasnt his only mistake. But in other cases, he got their formulas right. The following examples come from Daltons own books A New System of Chemical Philosophy (in two volumes, 1808 and 1817):
Example 1 — tin oxides: Dalton identified two types of tin oxide. One is a grey powder that Dalton referred to as "the protoxide of tin", which is 88.1% tin and 11.9% oxygen. The other is a white powder which Dalton referred to as "the deutoxide of tin", which is 78.7% tin and 21.3% oxygen. Adjusting these figures, in the grey powder there is about 13.5 g of oxygen for every 100 g of tin, and in the white powder there is about 27 g of oxygen for every 100 g of tin. 13.5 and 27 form a ratio of 1:2. These compounds are known today tin(II) oxide (SnO) and tin(IV) oxide (SnO). In Daltons terminology, a "protoxide" is a molecule containing a single oxygen atom, and a "deutoxide" molecule has two. Tin oxides are actually crystals, they dont exist in molecular form.
Example 2 — iron oxides: Dalton identified two oxides of iron. There is one type of iron oxide that is a black powder which Dalton referred to as "the protoxide of iron", which is 78.1% iron and 21.9% oxygen. The other iron oxide is a red powder, which Dalton referred to as "the intermediate or red oxide of iron" which is 70.4% iron and 29.6% oxygen. Adjusting these figures, in the black powder there is about 28 g of oxygen for every 100 g of iron, and in the red powder there is about 42 g of oxygen for every 100 g of iron. 28 and 42 form a ratio of 2:3. These compounds are iron(II) oxide (FeO) and iron(III) oxide (FeO). Dalton described the "intermediate oxide" as being "2 atoms protoxide and 1 of oxygen", which adds up to two atoms of iron and three of oxygen. That averages to one and a half atoms of oxygen for every iron atom, putting it midway between a "protoxide" and a "deutoxide". As with tin oxides, iron oxides are crystals.
Example 3 — nitrogen oxides: Dalton was aware of three oxides of nitrogen: "nitrous oxide", "nitrous gas", and "nitric acid". These compounds are known today as nitrous oxide, nitric oxide, and nitrogen dioxide respectively. "Nitrous oxide" is 63.3% nitrogen and 36.7% oxygen, which means it has 80 g of oxygen for every 140 g of nitrogen. "Nitrous gas" is 44.05% nitrogen and 55.95% oxygen, which means there are 160 g of oxygen for every 140 g of nitrogen. "Nitric acid" is 29.5% nitrogen and 70.5% oxygen, which means it has 320 g of oxygen for every 140 g of nitrogen. 80 g, 160 g, and 320 g form a ratio of 1:2:4. The formulas for these compounds are NO, NO, and NO.
The earliest definition of Dalton's observation appears in an 1807 chemistry encyclopedia:
Dalton's atomic theory garnered widespread interest but not universal acceptance shortly after he published it because the law of multiple proportions by itself was not complete proof of the existence of atoms. Over the course of the 19th century, other discoveries in the fields of chemistry and physics would give atomic theory more credence, such that by the end of the 19th century it had found universal acceptance. | 7 | Physical Chemistry |
Somewhat like the parent amines, imines are mildly basic and reversibly protonate to give iminium salts:
:RC=NR + H [RC=NHR]
Alternatively, primary imines are sufficiently acidic to allow N-alkylation, as illustrated with benzophenone imine:
:(CH)C=NH + CHLi → (CH)C=NLi + CH
:(CH)C=NLi + CHI → (CH)C=NCH + LiI | 0 | Organic Chemistry |
The following is an example of the algorithm for determining the axis/angle representation of misorientation between two texture components given as Euler angles:
:Copper [90,35,45]
:S3 [59,37,63]
The first step is converting the Euler angle representation, to an orientation matrix by:
where and represent and of the respective Euler component. This yields the following orientation matrices:
The misorientation is then:
The axis/angle description (with the axis as a unit vector) is related to the misorientation matrix by:
(There are errors in the similar formulae for the components of r given in the book by Randle and Engler (see refs.), which will be corrected in the next edition of their book. The above are the correct versions, note a different form for these equations has to be used if Θ = 180 degrees.)
For the copper—S misorientation given by , the axis/angle description is 19.5° about [0.689,0.623,0.369], which is only 2.3° from <221>. This result is only one of the 1152 symmetrically related possibilities but does specify the misorientation. This can be verified by considering all possible combinations of orientation symmetry (including switching symmetry). | 3 | Analytical Chemistry |
While the above techniques use a spatially extended, wide incident beam, section topography is based on a narrow beam on the order of some 10 micrometers (in one or, in the case of pinhole topography with a pencil beam, in both lateral dimensions). Section topographs therefore investigate only a restricted volume of the sample.
On its path through the crystal, the beam is diffracted at different depths, each one contributing to image formation on a different location on the detector (film). Section topography can therefore be used for depth-resolved defect analysis.
In section topography, even perfect crystals display fringes. The technique is very sensitive to crystalline defects and strain, as these distort the fringe pattern in the topograph. Quantitative analysis can be performed with the help of image simulation by computer algorithms, usually based on the Takagi-Taupin equations.
An enlarged synchrotron X-ray transmission section topograph on the right shows a diffraction image of the section of a sample having a gallium nitride (GaN) layer grown by metal-organic vapour phase epitaxy on sapphire wafer. Both the epitaxial GaN layer and the sapphire substrate show numerous defects. The GaN layer actually consists of about 20 micrometers wide small-angle grains connected to each other. Strain in the epitaxial layer and substrate is visible as elongated streaks parallel to the diffraction vector direction. The defects on the underside of the sapphire wafer section image are surface defects on the unpolished backside of the sapphire wafer. Between the sapphire and GaN the defects are interfacial defects. | 3 | Analytical Chemistry |
Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of a distinct subset of RNA. All are structurally and mechanistically related to each other and to bacterial RNAP:
Eukaryotic chloroplasts contain an RNAP very highly similar to bacterial RNAP ("plastid-encoded polymerase, PEP"). They use sigma factors encoded in the nuclear genome.
Chloroplast also contain a second, structurally and mechanistically unrelated, single-subunit RNAP ("nucleus-encoded polymerase, NEP"). Eukaryotic mitochondria use POLRMT (human), a nucleus-encoded single-subunit RNAP. Such phage-like polymerases are referred to as RpoT in plants. | 1 | Biochemistry |
There have been a variety of long-overlap-based assembly methods developed in recent years. One of the most commonly used methods, the Gibson assembly method, was developed in 2009, and provides a one-pot DNA assembly method that does not require the use of restriction enzymes or integrases. Other similar overlap-based assembly methods include Circular Polymerase Extension Cloning (CPEC), Sequence and Ligase Independent Cloning (SLIC) and Seamless Ligation Cloning Extract (SLiCE). Despite the presence of many overlap assembly methods, the Gibson assembly method is still the most popular. Besides the methods listed above, other researchers have built on the concepts used in Gibson assembly and other assembly methods to develop new assembly strategies like the Modular Overlap-Directed Assembly with Linkers (MODAL) strategy, or the Biopart Assembly Standard for Idempotent Cloning (BASIC) method. | 1 | Biochemistry |
Similar to Pourbaix diagrams for the speciation of redox species as a function of the redox potential and the pH, ionic partition diagrams indicate in which phase an acid or a base is predominantly present in a biphasic system as a function of the Galvani potential difference
between the two phases and the pH of the aqueous solution | 7 | Physical Chemistry |
Synchrotron radiation circular dichroism spectroscopy, commonly referred to as SRCD and also known as VUV-circular dichroism or VUVCD spectroscopy, is a powerful extension to the technique of circular dichroism (CD) spectroscopy, often used to study structural properties of biological molecules such as proteins and nucleic acids. The physical principles of SRCD are essentially identical to those of CD, in that the technique measures the difference in absorption (ΔA) of left (A) and right (A) circularly polarized light (ΔA=A-A) by a sample in solution. To obtain a CD(SRCD) spectrum the sample must be innately optically active (chiral), or, in some way be induced to have chiral properties, as only then will there be an observable difference in absorption of the left and right circularly polarized light. The major advantages of SRCD over CD arise from the ability to measure data over an extended wavelength range into the vacuum ultra violet (VUV) end of the spectrum. As these measurements are utilizing a light source with a higher photon flux (quantity of light stricking a given surface area) than a bench-top CD machine it means data are more accurate at these extended wavelengths because there is a larger signal over the background noise (the signal-to-noise ratio) and, generally, less sample is needed when recording the spectra and there is more information content available in the data. Many beamlines now exist around the world to enable the measurement of SRCD data. | 7 | Physical Chemistry |
In 2008, a California physician transferred 12 embryos to a woman who gave birth to octuplets (Suleman octuplets). This led to accusations that a doctor is willing to endanger the health and even life of people in order to gain money. Robert Winston, professor of fertility studies at Imperial College London, had called the industry "corrupt" and "greedy" stating that "one of the major problems facing us in healthcare is that IVF has become a massive commercial industry," and that "what has happened, of course, is that money is corrupting this whole technology", and accused authorities of failing to protect couples from exploitation: "The regulatory authority has done a consistently bad job. Its not prevented the exploitation of people, its not put out very good information to couples, it's not limited the number of unscientific treatments people have access to". The IVF industry has been described as a market-driven construction of health, medicine and the human body.
The industry has been accused of making unscientific claims, and distorting facts relating to infertility, in particular through widely exaggerated claims about how common infertility is in society, in an attempt to get as many couples as possible and as soon as possible to try treatments (rather than trying to conceive naturally for a longer time). This risks removing infertility from its social context and reducing the experience to a simple biological malfunction, which not only can be treated through bio-medical procedures, but should be treated by them. | 1 | Biochemistry |
DNA nanotechnology is an area of current research that uses the bottom-up, self-assembly approach for nanotechnological goals. DNA nanotechnology uses the unique molecular recognition properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. DNA is thus used as a structural material rather than as a carrier of biological information, to make structures such as complex 2D and 3D lattices (both tile-based as well as using the "DNA origami" method) and three-dimensional structures in the shapes of polyhedra. These DNA structures have also been used as templates in the assembly of other molecules such as gold nanoparticles and streptavidin proteins. | 6 | Supramolecular Chemistry |
Ionic bipolar transistors can be made from two conical channels with the smallest opening in nano-scaled dimension. By introducing opposite surface charges at each side, it is able to rectify ionic current as an ionic diode. An ionic bipolar transistor is built by combining two ionic diodes and forming a PNP junction along the inner surface of the channel. While the ionic current is from emitter end to collector end, the strength of the current can be modulated by the base electrode. The surface charge at the channel wall can be modified using chemical methods, by changing the electrolyte concentration or pH value. | 7 | Physical Chemistry |
Neuropeptides are often co-released with other neurotransmitters and neuropeptides to modulate synaptic activity. Synaptic vesicles and dense core vesicles can have differential activation properties for release, resulting in context-dependent corelease combinations. For example, insect motor neurons are glutamatergic and some contain dense core vesicles with proctolin. At low frequency activation, only glutamate is released, yielding fast and rapid excitation of the muscle. At high frequency activation however, dense core vesicles release proctolin, inducing prolonged contractions. Thus, neuropeptide release can be fine-tuned to modulate synaptic activity in certain contexts.
Some regions of the nervous system are specialized to release distinctive sets of peptides. For example, the hypothalamus and the pituitary gland release peptides (e.g. TRH, GnRH, CRH, SST) that act as hormones In one subpoplation of the arcuate nucleus of the hypothalamus, three anorectic peptides are co-expressed: α-melanocyte-stimulating hormone (α-MSH), galanin-like peptide, and cocaine-and-amphetamine-regulated transcript (CART), and in another subpopulation two orexigenic peptides are co-expressed, neuropeptide Y and agouti-related peptide (AGRP). These peptides are all released in different combinations to signal hunger and satiation cues.
The following is a list of neuroactive peptides coreleased with other neurotransmitters. Transmitter names are shown in bold.
Norepinephrine (noradrenaline).
In neurons of the A2 cell group in the nucleus of the solitary tract), norepinephrine co-exists with:
* Galanin
* Enkephalin
* Neuropeptide Y
GABA
* Somatostatin (in the hippocampus)
* Cholecystokinin
* Neuropeptide Y (in the arcuate nucleus)
Acetylcholine
* VIP
* Substance P
Dopamine
* Cholecystokinin
* Neurotensin
* Glucagon-like peptide-1 (in the nucleus accumbens)
Epinephrine (adrenaline)
* Neuropeptide Y
* Neurotensin
Serotonin (5-HT)
* Substance P
* TRH
* Enkephalin
Some neurons make several different peptides. For instance,
vasopressin co-exists with dynorphin and galanin in magnocellular neurons of the supraoptic nucleus and paraventricular nucleus, and with CRF (in parvocellular neurons of the paraventricular nucleus)
Oxytocin in the supraoptic nucleus co-exists with enkephalin, dynorphin, cocaine-and amphetamine regulated transcript (CART) and cholecystokinin. | 1 | Biochemistry |
N-Ethylmaleimide (NEM) is an organic compound that is derived from maleic acid. It contains the amide functional group, but more importantly it is an alkene that is reactive toward thiols and is commonly used to modify cysteine residues in proteins and peptides. | 1 | Biochemistry |
Enantioselective benzylic functionalization methods use the complexed chromium tri(carbonyl) moiety essentially as a chiral auxiliary. Approach of the functionalizing reagent anti to the ----chromium tri(carbonyl) fragment leads to a single diastereomer of the product complex. After removal of the chromium group with light or an oxidizing agent such as iodine, a nearly enantiopure product remains. See the Scope and Limitations section below for several methods for diastereoselective benzylic functionalization. | 0 | Organic Chemistry |
A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen peroxide (HO) is then formed. Peroxisomes owe their name to hydrogen peroxide generating and scavenging activities. They perform key roles in lipid metabolism and the reduction of reactive oxygen species.
Peroxisomes are involved in the catabolism of very long chain fatty acids, branched chain fatty acids, bile acid intermediates (in the liver), D-amino acids, and polyamines. Peroxisomes also play a role in the biosynthesis of plasmalogens: ether phospholipids critical for the normal function of mammalian brains and lungs. Peroxisomes contain approximately 10% of the total activity of two enzymes (Glucose-6-phosphate dehydrogenase and 6-Phosphogluconate dehydrogenase) in the pentose phosphate pathway, which is important for energy metabolism. It is vigorously debated whether peroxisomes are involved in isoprenoid and cholesterol synthesis in animals. Other peroxisomal functions include the glyoxylate cycle in germinating seeds ("glyoxysomes"), photorespiration in leaves, glycolysis in trypanosomes ("glycosomes"), and methanol and amine oxidation and assimilation in some yeasts. | 1 | Biochemistry |
12-Crown-4 can be synthesized using a modified Williamson ether synthesis, using LiClO as a templating cation:
: (CHOCHCHCl) + (CHOH) + 2 NaOH → (CHCHO) + 2 NaCl + 2 HO
It also forms from the cyclic oligomerization of ethylene oxide in the presence of gaseous boron trifluoride. | 6 | Supramolecular Chemistry |
There is an intimate connection of quantum thermodynamics with the theory of open quantum systems.
Quantum mechanics inserts dynamics into thermodynamics, giving a sound foundation to finite-time-thermodynamics.
The main assumption is that the entire world is a large closed system, and therefore, time evolution
is governed by a unitary transformation generated by a global Hamiltonian. For the combined system
bath scenario, the global Hamiltonian can be decomposed into:
where is the system Hamiltonian, is the bath Hamiltonian and is the system-bath interaction.
The state of the system is obtained from a partial trace over the combined system and bath:
Reduced dynamics is an equivalent description of the system dynamics utilizing only system operators.
Assuming Markov property for the dynamics the basic equation of motion for an open quantum system is the Lindblad equation (GKLS):
is a (Hermitian) Hamiltonian part and :
is the dissipative part describing implicitly through system operators the influence of the bath on the system.
The Markov property imposes
that the system and bath are uncorrelated at all times .
The L-GKS equation is unidirectional and leads any initial state to a steady state solution which is an invariant of the equation of motion .
The Heisenberg picture supplies a direct link to quantum thermodynamic observables. The dynamics of a system observable represented by the operator, , has the form:
where the possibility that the operator, is explicitly time-dependent, is included. | 7 | Physical Chemistry |
To first order, reduced iron favors isotopically light iron and oxidized iron favors isotopically heavy iron. This effect has been studied in regards to the abiotic oxidation of Fe to Fe, which results in fractionation. The mineral ferrihydrite, which forms in acidic aquatic conditions, is precipitated via the oxidation of aqueous Fe to Fe. Precipitated ferrihydrite has been found to be enriched in the heavy isotopes by 0.45‰ per atomic mass unit with respect to the starting material. This indicates that heavier iron isotopes are preferentially precipitated as a result of oxidizing processes.
Theoretical calculations in combination with experimental data have also aimed to quantify the fractionation between Fe(III) and Fe(II) in HCl. Based on modeling, the fractionation factor between the two species is temperature dependent: | 9 | Geochemistry |
Chitin is used in many industrial processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods and food emulsions. Processes to size and strengthen paper employ chitin and chitosan. | 1 | Biochemistry |
When a photon is absorbed, 11-cis-retinal is transformed to all-trans-retinal, and it moves to the exit site of rhodopsin. It will not leave the opsin protein until another fresh chromophore comes to replace it, except for in the ABCR pathway. Whilst still bound to the opsin, all-trans-retinal is transformed into all-trans-retinol by all-trans-Retinol Dehydrogenase. It then proceeds to the cell membrane of the rod, where it is chaperoned to the Retinal Pigment Epithelium (RPE) by Interphotoreceptor retinoid-binding protein (IRBP). It then enters the RPE cells, and is transferred to the Cellular Retinol Binding Protein (CRBP) chaperone.
When inside the RPE cell, bound to CRBP, the all-trans-retinol is esterified by Lecithin Retinol Acyltransferase (LRAT) to form a retinyl ester. The retinyl esters of the RPE are chaperoned by a protein known as RPE65. It is in this form that the RPE stores most of its retinoids, as the RPE stores 2-3 times more retinoids than the neural retina itself. When further chromophore is required, the retinyl esters are acted on by isomerohydrolase to produce 11-cis-retinol, which is transferred to the Cellular retinaldehyde binding protein (CRALBP). 11-cis-Retinol is transformed into 11-cis retinal by 11-cis-retinol dehydrogenase, then it is shipped back to the photoreceptor cells via IRBP. There, it replaces the spent chromophore in opsin molecules, rendering the opsin photosensitive. | 1 | Biochemistry |
First, adhesive from the HybriWell is peeled off and the HybriWell is attached over the area of the slide printed with the gelatin-DNA solution. Second, 200ul of transfection mix is pipetted into one of the HybriWell ports; the mixture will distribute evenly over the array. The array is then incubated, with temperature and time dependent on the cell types used. Third, the transfection mix is pipetted away and the HybriWell removed with a thin-tipped forceps. Fourth, the printed slide treated with transfection reagent is placed into a square dish with the printed side facing up. Fifth, the harvested cells are gently poured onto the slides (not on the printed areas). Finally, the dish is placed in a 37°C, 5% CO humidified incubator and incubated overnight. | 1 | Biochemistry |
* Cells that are to be studied need to be collected.
* Breaking the cell membranes open exposes the DNA along with the cytoplasm within (cell lysis).
** Lipids from the cell membrane and the nucleus are broken down with detergents and surfactants.
** Breaking down proteins by adding a protease (optional).
** Breaking down RNA by adding an RNase (optional).
* The solution is treated with a concentrated salt solution (saline) to make debris such as broken proteins, lipids, and RNA clump together.
* Centrifugation of the solution, which separates the clumped cellular debris from the DNA.
* DNA purification from detergents, proteins, salts, and reagents is used during the cell lysis step. The most commonly used procedures are:
**Ethanol precipitation usually by ice-cold ethanol or isopropanol. Since DNA is insoluble in these alcohols, it will aggregate together, giving a pellet upon centrifugation. Precipitation of DNA is improved by increasing ionic strength, usually by adding sodium acetate.
** Phenol–chloroform extraction in which phenol denatures proteins in the sample. After centrifugation of the sample, denatured proteins stay in the organic phase while the aqueous phase containing nucleic acid is mixed with chloroform to remove phenol residues from the solution.
** Minicolumn purification relies on the fact that the nucleic acids may bind (adsorption) to the solid phase (silica or other) depending on the pH and the salt concentration of the buffer.
Cellular and histone proteins bound to the DNA can be removed either by adding a protease or having precipitated the proteins with sodium or ammonium acetate or extracted them with a phenol-chloroform mixture before the DNA precipitation.
After isolation, the DNA is dissolved in a slightly alkaline buffer, usually in a TE buffer, or in ultra-pure water. | 1 | Biochemistry |
This is a table of surface tension values for some interfaces at the indicated temperatures. Note that the SI units millinewtons per meter (mN·m) are equivalent to the cgs units dynes per centimetre (dyn·cm). | 7 | Physical Chemistry |
Regulatory networks allow bacteria to adapt to almost every environmental niche on earth. A network of interactions among diverse types of molecules including DNA, RNA, proteins and metabolites, is utilised by the bacteria to achieve regulation of gene expression. In bacteria, the principal function of regulatory networks is to control the response to environmental changes, for example nutritional status and environmental stress. A complex organization of networks permits the microorganism to coordinate and integrate multiple environmental signals.
One example stress is when the environment suddenly becomes poor of nutrients. This triggers a complex adaptation process in bacteria, such as E. coli. After this environmental change, thousands of genes change expression level. However, these changes are predictable from the topology and logic of the gene network that is reported in RegulonDB. Specifically, on average, the response strength of a gene was predictable from the difference between the numbers of activating and repressing input transcription factors of that gene. | 1 | Biochemistry |
A common and practical expression of the Beer–Lambert law relates the optical attenuation of a physical material containing a single attenuating species of uniform concentration to the optical path length through the sample and absorptivity of the species. This expression is:where
* is the absorbance
* is the molar attenuation coefficient or absorptivity of the attenuating species
* is the optical path length
* is the concentration of the attenuating species
A more general form of the Beer–Lambert law states that, for attenuating species in the material sample,or equivalently thatwhere
* is the attenuation cross section of the attenuating species in the material sample;
* is the number density of the attenuating species in the material sample;
* is the molar attenuation coefficient or absorptivity of the attenuating species in the material sample;
* is the amount concentration of the attenuating species in the material sample;
* is the path length of the beam of light through the material sample.
In the above equations, the transmittance of material sample is related to its optical depth and to its absorbance by the following definitionwhere
* is the radiant flux transmitted by that material sample;
* is the radiant flux received by that material sample.
Attenuation cross section and molar attenuation coefficient are related byand number density and amount concentration byor equivalentlyCases of non-uniform attenuation occur in atmospheric science applications and radiation shielding theory for instance.
The law tends to break down at very high concentrations, especially if the material is highly scattering. Absorbance within range of 0.2 to 0.5 is ideal to maintain linearity in the Beer–Lambert law. If the radiation is especially intense, nonlinear optical processes can also cause variances. The main reason, however, is that the concentration dependence is in general non-linear and Beer's law is valid only under certain conditions as shown by derivation below. For strong oscillators and at high concentrations the deviations are stronger. If the molecules are closer to each other interactions can set in. These interactions can be roughly divided into physical and chemical interactions. Physical interaction do not alter the polarizability of the molecules as long as the interaction is not so strong that light and molecular quantum state intermix (strong coupling), but cause the attenuation cross sections to be non-additive via electromagnetic coupling. Chemical interactions in contrast change the polarizability and thus absorption. | 7 | Physical Chemistry |
Peroxisomes are small (0.1–1 µm diameter) subcellular compartments (organelles) with a fine, granular matrix and surrounded by a single biomembrane which are located in the cytoplasm of a cell. Compartmentalization creates an optimized environment to promote various metabolic reactions within peroxisomes required to sustain cellular functions and viability of the organism.
The number, size and protein composition of peroxisomes are variable and depend on cell type and environmental conditions. For example, in bakers yeast (S. cerevisiae'), it has been observed that, with good glucose supply, only a few, small peroxisomes are present. In contrast, when the yeasts were supplied with long-chain fatty acids as sole carbon source up to 20 to 25 large peroxisomes can be formed. | 1 | Biochemistry |
Dry regions such as western Asia, north Africa, Australia and the southwestern United States are ideal for PDRC application due to the relative lack of humidity and cloud cover in both winter and summer. The cooling potential for desert regions has been estimated at "in the higher range of 80–110 W/m2," as per Aili et al. and 120 W/m2 as per Yin et al. The Sahara Desert and western Asia is the largest area on Earth with a high cooling potential in both winter and summer.
The cooling potential of desert regions risks being relatively unfulfilled due to very low population densities, which may lower interest in applying PDRCs for local cooling. However, in the event of global implementation, lowly populated or unpopulated desert climates may be an important "land surface contribution to the planetary albedo" which could "reduce air temperature near the surface, if not the whole atmosphere." | 7 | Physical Chemistry |
Reuterin is an intermediate in the metabolism of glycerol to 1,3-propanediol catalysed by the coenzyme B12-dependent glycerol dehydratase.
Reuterin is a potent antimicrobial compound produced by Lactobacillus reuteri. It is an intermediate in the metabolism of glycerol to 1,3-propanediol catalysed by the coenzyme B12-dependent diol dehydrase. It inhibits the growth of some harmful Gram-negative and Gram-positive bacteria, along with yeasts, molds, and protozoa. L. reuteri can secrete sufficient amounts of reuterin to inhibit the growth of harmful gut organisms, without killing beneficial gut bacteria, allowing L. reuteri to remove gut invaders while keeping normal gut flora intact.
Reuterin is water-soluble, effective in a wide range of pH, resistant to proteolytic and lipolytic enzymes, and has been studied as a food preservative or auxiliary therapeutic agent.
Reuterin as an extracted compound has been shown capable of killing Escherichia coli O157:H7 and Listeria monocytogenes, with the addition of lactic acid increasing its efficacy. It has also been demonstrated to kill Escherichia coli O157:H7 when produced by L. reuteri. | 1 | Biochemistry |
The discovery of dephosphorylation came from a series of experiments examining the enzyme phosphorylase isolated from rabbit skeletal muscle. In 1955, Edwin Krebs and Edmond Fischer used radiolabeled ATP to determine that phosphate is added to the serine residue of phosphorylase to convert it from its b to a form via phosphorylation. Subsequently, Krebs and Fischer showed that this phosphorylation is part of a kinase cascade. Finally, after purifying the phosphorylated form of the enzyme, phosphorylase a, from rabbit liver, ion exchange chromatography was used to identify phosphoprotein phosphatase I and II.
Since the discovery of these dephosphorylating proteins, the reversible nature of phosphorylation and dephosphorylation has been associated with a broad range of functional proteins, primarily enzymatic, but also including nonenzymatic proteins. Edwin Krebs and Edmond Fischer won the 1992 Nobel Prize in Physiology or Medicine for the discovery of reversible protein phosphorylation. | 1 | Biochemistry |
In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.
Upon crystallization from water, or water-containing solvents, many compounds incorporate water molecules in their crystalline frameworks. Water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.
Compared to inorganic salts, proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is not uncommon for proteins. | 3 | Analytical Chemistry |
Open-chain monosaccharides with same molecular graph may exist as two or more stereoisomers. The Fischer projection is a systematic way of drawing the skeletal formula of an open-chain monosaccharide so that each stereoisomer is uniquely identified.
Two isomers whose molecules are mirror-images of each other are identified by prefixes - or -, according to the handedness of the chiral carbon atom that is farthest from the carbonyl. In the Fischer projection, that is the second carbon from the bottom; the prefix is - or - according to whether the hydroxyl on that carbon lies to the right or left of the backbone, respectively.
If the molecular graph is symmetrical (H(CHOH)(CO)(CHOH)H) and the two halves are mirror images of each other, then the molecule is identical to its mirror image, and there is no - form.
A distinct common name, such as "glucose" or "ribose", is traditionally assigned to each pair of mirror-image stereoisomers, and to each achiral stereoisomer. These names have standard three-letter abbreviations, such as Glc for glucose and Rib for ribose.
Another nomenclature uses the systematic name of the molecular graph, a - or - prefix to indicate the position of the last chiral hydroxyl on the Fischer diagram (as above), and another italic prefix to indicate the positions of the remaining hydroxyls relative to the first one, read from bottom to top in the diagram, skipping the keto group if any. These prefixes are attached to the systematic name of the molecular graph. So for example, -glucose is -gluco-hexose, -ribose is -ribo-pentose, and -psicose is -ribo-hexulose. Note that, in this nomenclature, mirror-image isomers differ only in the / prefix, even though all their hydroxyls are reversed.
The following tables shows the Fischer projections of selected monosaccharides (in open-chain form), with their conventional names. The table shows all aldoses with 3 to 6 carbon atoms, and a few ketoses. For chiral molecules, only the - form (with the next-to-last hydroxyl on the right side) is shown; the corresponding forms have mirror-image structures. Some of these monosaccharides are only synthetically prepared in the laboratory and not found in nature. | 0 | Organic Chemistry |
Founded as the Association of Clinical Biochemists, the association has evolved as biochemistry has changed with advances in laboratory medicine. Recognizing an increasing number of medical members, the name was changed in 2005 to Association for Clinical Biochemistry. In 2007 the "Association of Clinical Scientists in Immunology" merged with the ACB. The membership expanded in 2010 with the merger with the "Association of Clinical Microbiologists". The broader nature of the membership contributed to the renaming of the ACB to its current name at the annual meeting in 2013. | 1 | Biochemistry |
Photoelectron photoion coincidence spectroscopy (PEPICO) is a combination of photoionization mass spectrometry and photoelectron spectroscopy. It is largely based on the photoelectric effect. Free molecules from a gas-phase sample are ionized by incident vacuum ultraviolet (VUV) radiation. In the ensuing photoionization, a cation and a photoelectron are formed for each sample molecule. The mass of the photoion is determined by time-of-flight mass spectrometry, whereas, in current setups, photoelectrons are typically detected by velocity map imaging. Electron times-of-flight are three orders of magnitude smaller than those of ions, which allows electron detection to be used as a time stamp for the ionization event, starting the clock for the ion time-of-flight analysis. In contrast with pulsed experiments, such as REMPI, in which the light pulse must act as the time stamp, this allows to use continuous light sources, e.g. a discharge lamp or a synchrotron light source. No more than several ion–electron pairs are present simultaneously in the instrument, and the electron–ion pairs belonging to a single photoionization event can be identified and detected in delayed coincidence. | 7 | Physical Chemistry |
Consider a simple branched pathway with all three steps irreversible. Such a pathway will admit two elementary modes which are indicated in thicked (or red) reaction lines.
Because both and are irreversible, and elementary mode lying on both these reactions is not possible since it would mean one reactions going against its thermodynamic direction. Each mode in this system satisfies the three conditions described above. The first condition is steady state, that is for each mode , it has to be true that .
Algebraically the two modes are given by:
By substituting each of these vectors into , it is easy to show that condition one is satisfied. For condition two we must ensure that all reactions that are irreversible have positive entries in the corresponding elements of the elementary modes. Since all three reactions in the branch are irreversible and all entries in the elementary modes are positive, condition two is satisfied.
Finally, to satisfy condition three, we must ask whether we can decompose the two elementary modes into other paths that can sustain a steady state while using the same non-zero entries in the elementary mode. In this example, it is impossible to decompose the elementary modes any further without disrupting the ability to sustain a steady state. Therefore, with all three conditions satisfied, we can conclude that the two vectors shown above are elementary modes.
All possible flows through a network can be constructed from linear combinations of the elementary modes, that is:
such that the entire space of flows through a network can be described. must be greater than or equal to zero to ensure that irreversible steps aren't inadvertently made to go in the reverse direction. For example, the following is a possible steady-state flow in the branched pathway.
If one of the outflow steps in the simple branched pathway is made reversible, an additional elementary mode becomes available, representing the flow between the two outflow branches. An additional mode emerges because, with only the first two modes, it is impossible to represent a flow between the two branches because the scaling factor, , cannot be negative (which would be required to reverse the flow). | 1 | Biochemistry |
The uranium is then stripped from the DEHPA/kerosene solution with hydrochloric acid, hydrofluoric acid, or carbonate solutions. Sodium carbonate solutions effectively strip uranium from the organic layer, but the sodium salt of DEHPA is somewhat soluble in water, which can lead to loss of the extractant. | 3 | Analytical Chemistry |
Modafinil is researched as a remedy to improve abstinence in people with cocaine addiction. Morning-dosed modafinil has been shown to normalize sleep parameters in individuals experiencing cocaine withdrawal. | 4 | Stereochemistry |
In chemistry, molecules with a non-collinear arrangement of two adjacent bonds have bent molecular geometry, also known as angular or V-shaped. Certain atoms, such as oxygen, will almost always set their two (or more) covalent bonds in non-collinear directions due to their electron configuration. Water (HO) is an example of a bent molecule, as well as its analogues. The bond angle between the two hydrogen atoms is approximately 104.45°. Nonlinear geometry is commonly observed for other triatomic molecules and ions containing only main group elements, prominent examples being nitrogen dioxide (NO), sulfur dichloride (SCl), and methylene (CH).
This geometry is almost always consistent with VSEPR theory, which usually explains non-collinearity of atoms with a presence of lone pairs. There are several variants of bending, where the most common is AXE where two covalent bonds and two lone pairs of the central atom (A) form a complete 8-electron shell. They have central angles from 104° to 109.5°, where the latter is consistent with a simplistic theory which predicts the tetrahedral symmetry of four sp hybridised orbitals. The most common actual angles are 105°, 107°, and 109°: they vary because of the different properties of the peripheral atoms (X).
Other cases also experience orbital hybridisation, but in different degrees. AXE molecules, such as SnCl, have only one lone pair and the central angle about 120° (the centre and two vertices of an equilateral triangle). They have three sp orbitals. There exist also sd-hybridised AX compounds of transition metals without lone pairs: they have the central angle about 90° and are also classified as bent. (See further discussion at VSEPR theory#Complexes with strong d-contribution). | 4 | Stereochemistry |
Several other quantities can be used to describe the composition of a mixture. These should not be called concentrations. | 3 | Analytical Chemistry |
A sulfoxide, R−S(O)−R, is the S-oxide of a sulfide ("sulfide oxide"), a sulfone, R−S(O)−R, is the S,S-dioxide of a sulfide, a thiosulfinate, R−S(O)−S−R, is the S-oxide of a disulfide, and a thiosulfonate, R−S(O)−S−R, is the S,S-dioxide of a disulfide. All of these compounds are well known with extensive chemistry, e.g., dimethyl sulfoxide, dimethyl sulfone, and allicin (see drawing). | 9 | Geochemistry |
Early fluctuations in oxygen concentration had little direct effect on life, with mass extinctions not observed until around the start of the Cambrian period, . The presence of provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen created new possibilities for life to explore. Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume. 430-million-year-old fossilized charcoal produced by wildfires show that the atmospheric oxygen levels in the Silurian must have been equivalent to, or possibly above, present day levels. The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of various arthropods, including insects, millipedes and scorpions. Whilst human activities, such as the burning of fossil fuels, affect relative carbon dioxide concentrations, their effect on the much larger concentration of oxygen is less significant.
The Great Oxygenation Event had the first major effect on the course of evolution. Due to the rapid buildup of oxygen in the atmosphere, many organisms not reliant on oxygen to live died.
The concentration of oxygen in the atmosphere is often cited as a possible contributor to large-scale evolutionary phenomena, such as the Avalon explosion, the Cambrian explosion, trends in animal body size, and other diversification and extinction events.
Data show an increase in biovolume soon after the Great Oxygenation Event by more than 100-fold and a moderate correlation between atmospheric oxygen and maximum body size later in the geological record. The large size of many arthropods in the Carboniferous period, when the oxygen concentration in the atmosphere reached 35%, has been attributed to the limiting role of diffusion in these organisms metabolism. But Haldanes essay points out that it would only apply to insects. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies – for instance, the appearance of flying competitors such as pterosaurs, birds and bats.
Rising oxygen concentrations have been cited as one of several drivers for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen concentrations and the rate of evolution is not clearly evident. The most celebrated link between oxygen and evolution occurs at the end of the last of the Snowball Earth glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen concentrations and before the evolution of nitrogen fixation, biologically-available nitrogen compounds were in limited supply and periodic "nitrogen crises" could render the ocean inhospitable to life. Significant concentrations of oxygen were just one of the prerequisites for the evolution of complex life. Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a concentration was only reached immediately before metazoa first appeared in the fossil record. Further, anoxic or otherwise chemically "inhospitable" oceanic conditions that resemble those supposed to inhibit macroscopic life re-occur at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent effect on lifeforms at these times. This might suggest that the geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian – perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of planktivory.
An oxygen-rich atmosphere can release phosphorus and iron from rock, by weathering, and these elements then become available for sustenance of new species whose metabolisms require these elements as oxides. | 9 | Geochemistry |
Klaus Schmiegel (born June 28, 1939), is a German chemist best known for his work in organic chemistry, which led to the invention of Prozac, a widely used antidepressant. | 0 | Organic Chemistry |
In some cases of natural phenols, they are present in vegetative foliage to discourage herbivory, such as in the case of Western poison oak. | 0 | Organic Chemistry |
The chiral switch concept is illustrated in the diagram. This chiral switch is from (±)-ibuprofen to (S)-(+)-ibuprofen (dexibuprofen). The nonsteroidal anti-inflammatory drug (NSAID) ibuprofen was the first chiral drug of the NSAID class to be switched to the single-enantiomer version in 1994. The switch was done based on the fact that the (S)-ibuprofen, the eutomer, was over 100-fold more potent as an inhibitor of cycloxygenase-1 (COX-1) enzyme than (R)-ibuprofen. Moreover, ibuprofen, when administered as the racemate, the active (R)-enantiomer undergoes partial unidirectional chiral inversion (approximately 60%) to the (S)-enantiomer. Therefore, the use of the single (S)-ibuprofen was expected to give faster onset of action at a lower dosage. Further, while choosing the chiral drug candidate for a chiral switch one should take a look at the chiral inversion tendency of the molecule. For instance, thalidomide, the sedative drug, undergoes bidirectional chiral inversion or racemization in biological systems. In such cases chiral switching efforts will be pointless. | 4 | Stereochemistry |
The Bradford assay, a colorimetric protein assay, is based on an absorbance shift of the dye Coomassie brilliant blue G-250. The Coomassie brilliant blue G-250 dye exists in three forms: anionic (blue), neutral (green), and cationic (red). Under acidic conditions, the red form of the dye is converted into its blue form, binding to the protein being assayed. If theres no protein to bind, then the solution will remain brown. The dye forms a strong, noncovalent complex with the proteins carboxyl group by van der Waals force and amino group through electrostatic interactions. During the formation of this complex, the red form of Coomassie dye first donates its free electron to the ionizable groups on the protein, which causes a disruption of the proteins native state, consequently exposing its hydrophobic pockets. These pockets in the proteins tertiary structure bind non-covalently to the non-polar region of the dye via the first bond interaction (van der Waals forces) which position the positive amine groups in proximity with the negative charge of the dye. The bond is further strengthened by the second bond interaction between the two, the ionic interaction. When the dye binds to the protein, it causes a shift from 465 nm to 595 nm, which is why the absorbance readings are taken at 595 nm.
The cationic (unbound) form is green / red and has an absorption spectrum maximum historically held to be at 465 nm. The anionic bound form of the dye which is held together by hydrophobic and ionic interactions, has an absorption spectrum maximum historically held to be at 595 nm. The increase of absorbance at 595 nm is proportional to the amount of bound dye, and thus to the amount (concentration) of protein present in the sample.
Unlike other protein assays, the Bradford protein assay is less susceptible to interference by various chemical compounds such as sodium, potassium or even carbohydrates like sucrose, that may be present in protein samples. An exception of note is elevated concentrations of detergent. Sodium dodecyl sulfate (SDS), a common detergent, may be found in protein extracts because it is used to lyse cells by disrupting the membrane lipid bilayer and to denature proteins for SDS-PAGE. While other detergents interfere with the assay at high concentration, the interference caused by SDS is of two different modes, and each occurs at a different concentration. When SDS concentrations are below critical micelle concentration (known as CMC, 0.00333%W/V to 0.0667%) in a Coomassie dye solution, the detergent tends to bind strongly with the protein, inhibiting the protein binding sites for the dye reagent. This can cause underestimations of protein concentration in solution. When SDS concentrations are above CMC, the detergent associates strongly with the green form of the Coomassie dye, causing the equilibrium to shift, thereby producing more of the blue form. This causes an increase in the absorbance at 595 nm independent of protein presence.
Other interference may come from the buffer used when preparing the protein sample. A high concentration of buffer will cause an overestimated protein concentration due to depletion of free protons from the solution by conjugate base from the buffer. This will not be a problem if a low concentration of protein (subsequently the buffer) is used.
In order to measure the absorbance of a colorless compound a Bradford assay must be performed. Some colorless compounds such as proteins can be quantified at an Optical Density of 280 nm due to the presence of aromatic rings such as Tryptophan, Tyrosine and Phenylalanine but if none of these amino acids are present then the absorption cannot be measured at 280 nm. | 3 | Analytical Chemistry |
Antibody (or immunoglobulin) structure is made up of two heavy-chains and two light-chains. These chains are held together by disulfide bonds. The arrangement or processes that put together different parts of this antibody molecule play important role in antibody diversity and production of different subclasses or classes of antibodies. The organization and processes take place during the development and differentiation of B cells. That is, the controlled gene expression during transcription and translation coupled with the rearrangements of immunoglobulin gene segments result in the generation of antibody repertoire during development and maturation of B cells. | 1 | Biochemistry |
The durability assessment has been implemented in European design codes at the beginning of the 90s. It is required for designers to include the effects of long-term corrosion of steel rebar during the design stage, in order to avoid unacceptable damages during the service life of the structure. Different approaches are then available for the durability design. | 8 | Metallurgy |
Production of prebiotic organic compounds at hydrothermal vents is estimated to be 1x10 kg yr. While a large amount of key prebiotic compounds, such as methane, are found at vents, they are in far lower concentrations than estimates of a Miller-Urey Experiment environment. In the case of methane, the production rate at vents is around 2-4 orders of magnitude lower than predicted amounts in a Miller-Urey Experiment surface atmosphere.
Other arguments against an oceanic vent setting for the origin of life include the inability to concentrate prebiotic materials due to strong dilution from seawater. This open-system cycles compounds through minerals that make up vents, leaving little residence time to accumulate. All modern cells rely on phosphates and potassium for nucleotide backbone and protein formation respectively, making it likely that the first life forms also shared these functions. These elements were not available in high quantities in the Archaean oceans as both primarily come from the weathering of continental rocks on land, far from vent settings. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation to form macromolecules. Further, these polymers were encapsulated in vesicles after condensation, which would not happen in saltwater because of the high concentrations of ions. | 9 | Geochemistry |
Polar substituent constants describe the way a substituent will influence a reaction through polar (inductive, field, and resonance) effects. To determine σ Taft studied the hydrolysis of methyl esters (RCOOMe). The use of ester hydrolysis rates to study polar effects was first suggested by Ingold in 1930. The hydrolysis of esters can occur through either acid and base catalyzed mechanisms, both of which proceed through a tetrahedral intermediate. In the base catalyzed mechanism the reactant goes from a neutral species to negatively charged intermediate in the rate determining (slow) step, while in the acid catalyzed mechanism a positively charged reactant goes to a positively charged intermediate.
Due to the similar tetrahedral intermediates, Taft proposed that under identical conditions any steric factors should be nearly the same for the two mechanisms and therefore would not influence the ratio of the rates. However, because of the difference in charge buildup in the rate determining steps it was proposed that polar effects would only influence the reaction rate of the base catalyzed reaction since a new charge was formed. He defined the polar substituent constant σ* as:
where log(k/k is the ratio of the rate of the base catalyzed reaction compared to the reference reaction, log(k/k is ratio of a rate of the acid catalyzed reaction compared to the reference reaction, and ρ* is a reaction constant that describes the sensitivity of the reaction series. For the definition reaction series, ρ* was set to 1 and R = methyl was defined as the reference reaction (σ* = zero). The factor of 1/2.48 is included to make σ* similar in magnitude to the Hammett σ values. | 7 | Physical Chemistry |
One of the most interesting questions is if there is a threshold in reaction energy and/or volume size which needs to be exceeded in order to form a domain in which quarks can move freely. It is natural to expect that if such a threshold exists the particle yields/ratios we have shown above should indicate that. One of the most accessible signatures would be the relative Kaon yield ratio. A possible structure has been predicted, and indeed, an unexpected structure is seen in the ratio of particles comprising the positive kaon K (comprising anti s-quarks and up-quark) and positive pion particles, seen in the figure (solid symbols). The rise and fall (square symbols) of the ratio has been reported by the CERN NA49. The reason the negative kaon particles do not show this "horn" feature is that the s-quarks prefer to hadronize bound in the Lambda particle, where the counterpart structure is observed. Data point from BNL–RHIC–STAR (red stars) in figure agree with the CERN data.
In view of these results the objective of ongoing NA61/SHINE experiment at CERN SPS and the proposed low energy run at BNL RHIC where in particular the STAR detector can search for the onset of production of quark–gluon plasma as a function of energy in the domain where the horn maximum is seen, in order to improve the understanding of these results, and to record the behavior of other related quark–gluon plasma observables. | 7 | Physical Chemistry |
In June 2009, ATSDR and NCEH launched a joint project, the [https://www.atsdr.cdc.gov/nationalconversation/ National Conversation on Public Health and Chemical Exposures]. The goal of the National Conversation is to develop recommendations for ways ATSDR and other government agencies can improve their efforts to protect the public from harmful chemical exposures. To foster a productive dialogue, ATSDR encouraged broad public participation in the National Conversation and welcomed involvement from all interested stakeholders, including government agencies, public health professionals, environmental organizations, community leaders, business and industry representatives, tribal groups, and other interested citizens. The National Conversation is led by a 40-person Leadership Council that includes experts in various areas related to environmental public health. In addition, there are six work groups, which also have a diverse membership, to research and propose recommendations on certain key areas. To encourage involvement from community groups, interested citizens, and the general public, ATSDR developed a [http://www.atsdr.cdc.gov/nationalconversation/docs/Toolkit_complete.pdf community toolkit] to assist community leaders in holding discussions to solicit feedback and ideas for the National Conversation. ATSDR plans to release its final action agenda in early 2011. | 1 | Biochemistry |
In the sandwich hybridization ELISA assay format, the antigen ligand and antibodies in ELISA are replaced with a nucleic acid analyte, complementary oligonucleotide capture and detection probes.
Generally, in the case of nucleic acid hybridization, monovalent salt concentration and temperature are controlled for hybridization and wash stringency, contrary to a traditional ELISA, where the salt concentration will usually be fixed for the binding and wash steps (i.e. PBS or TBS). Thus, optimal salt concentration in hybridization assays varies dependent upon the length and base composition of the analyte, capture and detection probes. | 1 | Biochemistry |
Class II AMGs encode for peripheral functions absent from the KEGG metabolic pathways. This includes genes typically involved in transport and assembly. Major representatives of this class are involved in balancing TCA cycle intermediates. Additionally, the acquisition of biogenic elements outside of carbon like phosphate, governed by pstS, are prevalent for this class. Confidence of AMG identification for Class II AMGs is reduced without a database for reference. | 1 | Biochemistry |
The inner mitochondrial membrane is impermeable to fatty acids and a specialized carnitine carrier system operates to transport activated fatty acids from cytosol to mitochondria.
Once activated, the acyl CoA is transported into the mitochondrial matrix. This occurs via a series of similar steps:
# Acyl CoA is conjugated to carnitine by carnitine acyltransferase I (palmitoyltransferase) I located on the outer mitochondrial membrane
# Acyl carnitine is shuttled inside by a translocase
# Acyl carnitine (such as Palmitoylcarnitine) is converted to acyl CoA by carnitine acyltransferase (palmitoyltransferase) II located on the inner mitochondrial membrane. The liberated carnitine returns to the cytosol.
It is important to note that carnitine acyltransferase I undergoes allosteric inhibition as a result of malonyl-CoA, an intermediate in fatty acid biosynthesis, in order to prevent futile cycling between beta-oxidation and fatty acid synthesis.
The mitochondrial oxidation of fatty acids takes place in three major steps:
# β-oxidation occurs to convert fatty acids into 2-carbon acetyl-CoA units.
# Acetyl-CoA enters into TCA cycle to yield generate reduced NADH and reduced FADH.
# Reduced cofactors NADH and FADH participate in the electron transport chain in the mitochondria to yield ATP. There is no direct participation of the fatty acid. | 1 | Biochemistry |
There have been suggestions that leaf shedding may be a response that provides protection against diseases and certain kinds of pests such as leaf miners and gall forming insects. Other responses such as the change of leaf colors prior to fall have also been suggested as adaptations that may help undermine the camouflage of herbivores. Autumn leaf color has also been suggested to act as an honest warning signal of defensive commitment towards insect pests that migrate to the trees in autumn. | 1 | Biochemistry |
The Coblentz Society is a non-profit scientific organization named after William Weber Coblentz which is involved in fostering the understanding and application of vibrational spectroscopy. The Society provides education, awards and recognitions enhancing the understanding of molecular (vibrational) spectroscopy. The organization was founded in 1954 and is incorporated in the state of Connecticut. Originally considered to be the Infrared Society at its founding, the Coblentz Society has expanded its technical reach into Raman spectroscopy as the technique has become more accessible to both researchers and more casual users. The Society is the oldest organization in the United States specifically dedicated to the profession and activities associated with vibrational spectroscopy. The Coblentz Society is also the infrared and Raman technical affiliate of the Society for Applied Spectroscopy.
The Awards sponsored by The Coblentz Society include:
*The Coblentz Award is to recognize the contributions by a young professional spectroscopist to the fundamental understanding of vibrational spectroscopy
*The Craver Award is to recognize the efforts of young professional spectroscopists in the field of applied analytical spectroscopy
*The Williams–Wright Award is unique in that it recognizes the lifetime accomplishments of an industrial spectroscopist
*The Bomem-Michelson Award is currently inactive but was designed to recognize advancements in the field of vibrational spectroscopy
*The Lippincott Award for the advancement of spectroscopy from an optical perspective (co-sponsored by Optica and the Society for Applied Spectroscopy). | 7 | Physical Chemistry |
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