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TLC plates are usually commercially available, with standard particle size ranges to improve reproducibility. They are prepared by mixing the adsorbent, such as silica gel, with a small amount of inert binder like calcium sulfate (gypsum) and water. This mixture is spread as a thick slurry on an unreactive carrier sheet, usually glass, thick aluminum foil, or plastic. The resultant plate is dried and activated by heating in an oven for thirty minutes at 110 °C. The thickness of the absorbent layer is typically around 0.1–0.25 mm for analytical purposes and around 0.5–2.0 mm for preparative TLC. Other adsorbent coatings include aluminium oxide (alumina), or cellulose. | 3 | Analytical Chemistry |
This method cannot be applied if Mantoux test (tuberculin skin test) has been done within the last 40 days, because it can hamper the results of the ALS test. This test is used as a complementary test to other tests, e.g. chest X-ray, ESR, CRP, history of contact with active TB case, failure with conventional antibiotic treatment etc.; anti-TB therapy is not provided if only ALS test is positive. The reason is that this method is potentially an early biomarker of active infection. However, if a subject does not show any physical symptoms, the doctors cannot prescribe anti-TB treatment. | 1 | Biochemistry |
The LPB process includes a unique and patented way of analyzing, designing, and testing metallic components in order to develop the unique metal treatment necessary to improve performance and reduce metal fatigue, SCC, and corrosion fatigue failures. Lambda modifies the process and tooling for each component to provide the best results possible and to ensure that the apparatus reaches every inch on the component. With this practice of customization along with the closed-loop process control system, LPB has been shown to produce a maximum compression of 12mm, although the average is around 1-7+mm. LPB has even been shown to have the ability to produce through-thickness compression in blades and vanes, greatly increasing their damage tolerance over 10-fold, effectively mitigating most FOD and reducing inspection requirements. No material is removed during this process, even when correcting corrosion damage. LPB smooths surface asperities during machining, leaving an improved, almost mirror-like surface finish that is vastly better looking and better protected than even a newly manufactured component. | 8 | Metallurgy |
Between 3 and 10% of children taking amoxicillin (or ampicillin) show a late-developing (>72 hours after beginning medication and having never taken penicillin-like medication previously) rash, which is sometimes referred to as the "amoxicillin rash". The rash can also occur in adults and may rarely be a component of the DRESS syndrome.
The rash is described as maculopapular or morbilliform (measles-like; therefore, in medical literature, it is called "amoxicillin-induced morbilliform rash".). It starts on the trunk and can spread from there. This rash is unlikely to be a true allergic reaction and is not a contraindication for future amoxicillin usage, nor should the current regimen necessarily be stopped. However, this common amoxicillin rash and a dangerous allergic reaction cannot easily be distinguished by inexperienced persons, so a healthcare professional is often required to distinguish between the two.
A nonallergic amoxicillin rash may also be an indicator of infectious mononucleosis. Some studies indicate about 80–90% of patients with acute Epstein–Barr virus infection treated with amoxicillin or ampicillin develop such a rash. | 4 | Stereochemistry |
The diboron compound bis(pinacolato)diboron reacts with aromatic heterocycles or simple arenes to an arylboronate ester with iridium catalyst [IrCl(COD)] (a modification of Crabtree's catalyst) and base 4,4′-di-tert-butyl-2,2′-bipyridine in a C-H coupling reaction for example with benzene:
In one modification the arene reacts using only a stoichiometric equivalent rather than a large excess using the cheaper pinacolborane:
Unlike in ordinary electrophilic aromatic substitution (EAS) where electronic effects dominate, the regioselectivity in this reaction type is solely determined by the steric bulk of the iridium complex. This is exploited in a meta-bromination of m-xylene which by standard AES would give the ortho product: | 0 | Organic Chemistry |
Short-wave UV light is harmful to humans. In addition to causing sunburn and (over time) skin cancer, this light can produce extremely painful inflammation of the cornea of the eye, which may lead to temporary or permanent vision impairment. For this reason, the light produced by a germicidal lamp must be carefully shielded against direct viewing, with consideration of reflections and dispersed light. A February 2017 risk analysis of UVC lights concluded that ultraviolet light from these lamps can cause skin and eye problems. | 5 | Photochemistry |
Full article: Solubility pump
The oceans store the largest pool of reactive carbon on the planet as DIC, which is introduced as a result of the dissolution of atmospheric carbon dioxide into seawater – the solubility pump. Aqueous CO, carbonic acid, bicarbonate ion, and carbonate ion concentrations comprise dissolved inorganic carbon (DIC). DIC circulates throughout the whole ocean by Thermohaline circulation, which facilitates the tremendous DIC storage capacity of the ocean. The chemical equations below show the reactions that CO undergoes after it enters the ocean and transforms into its aqueous form.
Carbonic acid rapidly dissociates into free hydrogen ion (technically, hydronium) and bicarbonate.The free hydrogen ion meets carbonate, already present in the water from the dissolution of CaCO, and reacts to form more bicarbonate ion.The dissolved species in the equations above, mostly bicarbonate, make up the carbonate alkalinity system, the dominant contributor to seawater alkalinity. | 9 | Geochemistry |
More than 100 ECS student chapters are located in major universities in all of these regions as well as Asia, Europe, Latin America, the Middle East, North America, South Africa, and Southern Asia. | 7 | Physical Chemistry |
One theme of research in evolutionary developmental biology ("evo-devo") is investigating the role of enhancers and other cis-regulatory elements in producing morphological changes via developmental differences between species. | 1 | Biochemistry |
In materials science, grain growth is the increase in size of grains (crystallites) in a material at high temperature. This occurs when recovery and recrystallisation are complete and further reduction in the internal energy can only be achieved by reducing the total area of grain boundary. The term is commonly used in metallurgy but is also used in reference to ceramics and minerals. The behaviors of grain growth is analogous to the coarsening behaviors of grains, which implied that both of grain growth and coarsening may be dominated by the same physical mechanism. | 8 | Metallurgy |
The isopoly-molybdenum blues have been known for many years. They are the cause of the "blue waters" found near Idaho Springs, known to Native Americans. They were first documented by Scheele and Berzelius. The compounds responsible for the blue colour were not known until 1995. Before then it was well known that there were polymolybdates of Mo(VI). Molybdenum(VI)oxide, MoO, when dissolved in aqueous alkali forms the tetrahedral molybdate anion, . Dissolving molybdate salts in strong acid produces "molybdic acid", MoO·2HO. In between these extremes of pH, polymeric ions are produced which are mostly built from MoO octahedral units sharing corners and edges. Examples include , and , which contain the {(Mo)Mo}-type unit comprising a central MoO pentagonal bipyramid sharing edges with five MoO octahedra. The later unit occurs also in the giant mixed-valence molybdenum blue species [HMoO(HO)(SO)] (x ≈ 16) as well as in the cluster described in the next section. The molybdenum blue species are obtained by reduction of acidified molybdate(VI) solutions. | 7 | Physical Chemistry |
Learning and memory have levels of permanence, differing from other mental processes such as thought, language, and consciousness, which are temporary in nature. Learning and memory can be either accumulated slowly (multiplication tables) or rapidly (touching a hot stove), but once attained, can be recalled into conscious use for a long time. Rats subjected to one instance of contextual fear conditioning create an especially strong long-term memory. At 24 hours after training, 9.17% of the genes in the genomes of rat hippocampus neurons were found to be differentially methylated. This included more than 2,000 differentially methylated genes at 24 hours after training, with over 500 genes being demethylated. Similar results to that in the rat hippocampus were also obtained in mice with contextual fear conditioning.
The hippocampus region of the brain is where contextual fear memories are first stored (see figure of the brain, this section), but this storage is transient and does not remain in the hippocampus. In rats contextual fear conditioning is abolished when the hippocampus is subjected to hippocampectomy just one day after conditioning, but rats retain a considerable amount of contextual fear when hippocampectomy is delayed by four weeks. In mice, examined at 4 weeks after conditioning, the hippocampus methylations and demethylations were reversed (the hippocampus is needed to form memories but memories are not stored there) while substantial differential CpG methylation and demethylation occurred in cortical neurons during memory maintenance. There were 1,223 differentially methylated genes in the anterior cingulate cortex of mice four weeks after contextual fear conditioning. Thus, while there were many methylations in the hippocampus shortly after memory was formed, all these hippocampus methylations were demethylated as soon as four weeks later. | 1 | Biochemistry |
The binding constant, or affinity constant/association constant, is a special case of the equilibrium constant K, and is the inverse of the dissociation constant. It is associated with the binding and unbinding reaction of receptor (R) and ligand (L) molecules, which is formalized as:
:R + L RL
The reaction is characterized by the on-rate constant k and the off-rate constant k, which have units of M s and s, respectively. In equilibrium, the forward binding transition R + L → RL should be balanced by the backward unbinding transition RL → R + L. That is,
where [R], [L] and [RL] represent the concentration of unbound free receptors, the concentration of unbound free ligand and the concentration of receptor-ligand complexes. The binding constant K is defined by
An often considered quantity is the dissociation constant K ≡ , which has the unit of concentration, despite the fact that strictly speaking, all association constants are unitless values. The inclusion of units arises from the simplification that such constants are calculated solely from concentrations, which is not the case. Once chemical activity is factored into the correct form of the equation, a dimensionless value is obtained. For the binding of receptor and ligand molecules in solution, the molar Gibbs free energy ΔG, or the binding affinity is related to the dissociation constant K via
in which R is the ideal gas constant, T temperature and the standard reference concentration c</sup> = 1 mol/L. | 7 | Physical Chemistry |
As noted above, J.R. Partington points out that a state of thermodynamic equilibrium is stable against small transient perturbations. Without this condition, in general, experiments intended to study systems in thermodynamic equilibrium are in severe difficulties. | 7 | Physical Chemistry |
Measurements of primary productivity in the ocean can be made using this ratio. The concentration of oxygen dissolved in seawater varies according to biological processes (photosynthesis and respiration) as well as physical processes (air-sea gas exchange, temperature and pressure changes, lateral mixing and vertical diffusion). Argon concentrations, by contrast, vary only by physical processes.
This technique was first used by Craig and Hayward (1987) when they separated oxygen supersaturations into a biological and a physical component. This O/Ar supersaturation can be defined as
∆(O/Ar)=(c(O )/c(Ar)) / (c(O)/(c(Ar))) -1
where (∆O)/Ar is the difference between O production via photosynthesis and removal via respiration, c is the concentration of dissolved gas and c is the saturated concentration of the gas in water at a specific temperature, salinity and pressure.
Oxygen and argon concentrations can be compared using samples from water systems aboard ships using either a membrane inlet mass spectrometer (MIMS) or an equilibrator inlet mass spectrometer (EIMS). The measurements can then be used in conjunction with air-sea gas exchange values to calculate biologically induced air-sea O fluxes and net community production. | 3 | Analytical Chemistry |
Anthocyanins may have a protective role in plants against extreme temperatures. Tomato plants protect against cold stress with anthocyanins countering reactive oxygen species, leading to a lower rate of cell death in leaves. | 3 | Analytical Chemistry |
In biocatalysis, many reactions are sought that are absent in nature. To do this, enzymes with a small promiscuous activity towards the required reaction are identified and evolved via directed evolution or rational design.
An example of a commonly evolved enzyme is ω-transaminase which can replace a ketone with a chiral amine and consequently libraries of different homologues are commercially available for rapid biomining (eg. Codexis).
Another example is the possibility of using the promiscuous activities of cysteine synthase (cysM) towards nucleophiles to produce non-proteinogenic amino acids. | 1 | Biochemistry |
Prodynorphin, also known as proenkephalin B, is an opioid polypeptide hormone involved with chemical signal transduction and cell communication. The gene for prodynorphin is expressed in the endometrium and the striatum, and its gene map locus is 20pter-p12. Prodynorphin is a basic building-block of endorphins, the chemical messengers in the brain that appear most heavily involved in the anticipation and experience of pain and the formation of deep emotional bonds, and that are also critical in learning and memory.
The gene is thought to influence perception, as well as susceptibility to drug dependence, and is expressed more readily in human beings than in other primates. | 1 | Biochemistry |
FMRFamide patterning over the course of embryonic life has been recorded through immunofluorescence. Embryos at 0 to 144 hours (6 days) post cleavage were extracted from the egg mass and fixed in 4% PFA for 2 hrs at room temperature. Embryos older than 4 days were pre-treated with 0.5% ethylenediaminetetraacetic acid (EDTA) solution in PBS for 5 minutes to improve primary antibody penetration, and then fixed in 4% PFA for 2 hours at room temperature. Embryos were then washed overnight in 4% Triton X-100 in PBS and incubated in blocking solution, 1% normal goal serum in PBS, overnight at 4 °C.
After blocking overnight, embryos were treated with a primary antibody solution at 1:2000 (in NGS/PBS). Secondary antibody tagged with fluorescein isothiocyanate (FITC)-conjugated anti-rabbit IgG (polyclonal) was incubated at 1:20, overnight at 4 °C. Embryos were then mounted on a slide and viewed with fluorescent microscopy. | 1 | Biochemistry |
Two different variations of electron transport are used during photosynthesis:
*Noncyclic electron transport or non-cyclic photophosphorylation produces NADPH + H and ATP.
*Cyclic electron transport or cyclic photophosphorylation produces only ATP.
The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.
*Photosystem I uses light energy to reduce NADP to NADPH + H, and is active in both noncyclic and cyclic electron transport. In cyclic mode, the energized electron is passed down a chain that ultimately returns it (in its base state) to the chlorophyll that energized it.
*Photosystem II uses light energy to oxidize water molecules, producing electrons (e), protons (H), and molecular oxygen (O), and is only active in noncyclic transport. Electrons in this system are not conserved, but are rather continually entering from oxidized 2HO (O + 4 H + 4 e) and exiting with NADP when it is finally reduced to NADPH. | 5 | Photochemistry |
Post-transcriptional regulation is the control of gene expression at the RNA level. It occurs once the RNA polymerase has been attached to the gene's promoter and is synthesizing the nucleotide sequence. Therefore, as the name indicates, it occurs between the transcription phase and the translation phase of gene expression. These controls are critical for the regulation of many genes across human tissues. It also plays a big role in cell physiology, being implicated in pathologies such as cancer and neurodegenerative diseases. | 1 | Biochemistry |
The process of depolarization is entirely dependent upon the intrinsic electrical nature of most cells. When a cell is at rest, the cell maintains what is known as a resting potential. The resting potential generated by nearly all cells results in the interior of the cell having a negative charge compared to the exterior of the cell. To maintain this electrical imbalance, ions are transported across the cells plasma membrane. The transport of the ions across the plasma membrane is accomplished through several different types of transmembrane proteins embedded in the cells plasma membrane that function as pathways for ions both into and out of the cell, such as ion channels, sodium potassium pumps, and voltage-gated ion channels. | 7 | Physical Chemistry |
For a Bragg peak , the observed integrated intensity, , as determined from numerical integration is
where is the total number of data points in the range of the Bragg peak. The integrated intensity depends on multiple factors, and can be expressed as the following product:
where:
* : scale factor
* : multiplicity factor, which accounts for symmetrically equivalent points in the reciprocal lattice
* : Lorentz multiplier, defined by diffraction geometry
* : polarization factor
* : absorption multiplier
* : preferred orientation factor
* : extinction factor (often neglected as it is usually insignificant in powders)
* : structure factor as determined by the crystal structure of the material | 3 | Analytical Chemistry |
There are various important electrochemical processes in both nature and industry, like the coating of objects with metals or metal oxides through electrodeposition, the addition (electroplating) or removal (electropolishing) of thin layers of metal from an object's surface, and the detection of alcohol in drunk drivers through the redox reaction of ethanol. The generation of chemical energy through photosynthesis is inherently an electrochemical process, as is production of metals like aluminum and titanium from their ores. Certain diabetes blood sugar meters measure the amount of glucose in the blood through its redox potential. In addition to established electrochemical technologies (like deep cycle lead acid batteries) there is also a wide range of new emerging technologies such as fuel cells, large format lithium-ion batteries, electrochemical reactors and super-capacitors that are becoming increasingly commercial. Electrochemical or coulometric titrations were introduced for quantitative analysis of minute quantities in 1938 by the Hungarian chemists László Szebellédy and Zoltan Somogyi. Electrochemistry also has important applications in the food industry, like the assessment of food/package interactions, the analysis of milk composition, the characterization and the determination of the freezing end-point of ice-cream mixes, or the determination of free acidity in olive oil. | 7 | Physical Chemistry |
Rgg-like transcriptional regulators can be found in a variety of gram-positive bacteria Where ropB regulates speB protease production in S. pyogenes, a roughly equivalent secretory control mechanism can be seen in Rggs regulation of gtfG glucosyltransferase production in S. gordonii, in the manner in which gadR regulates acid resistance in Lactococcus lactis, how lasX regulates expression of lantibiotic lactocin S in Lactobacillus sakei, and MutRs regulation of mutacin in S. mutans Sequentially, these genes are all localized contiguously to their respective subject of regulation and share promoters localized contiguously to inverted repeat regions. | 1 | Biochemistry |
Liquefaction of gases is physical conversion of a gas into a liquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures, using, for example, turboexpanders. | 7 | Physical Chemistry |
Glow discharges can be used to analyze the elemental, and sometimes molecular, composition of solids, liquids, and gases, but elemental analysis of solids is the most common. In this arrangement, the sample is used as the cathode. As mentioned earlier, gas ions and atoms striking the sample surface knock atoms off of it, a process known as sputtering.
The sputtered atoms, now in the gas phase, can be detected by atomic absorption, but this is a comparatively rare strategy. Instead, atomic emission and mass spectrometry are usually used.
Collisions between the gas-phase sample atoms and the plasma gas pass energy to the sample atoms. This energy can excite the atoms, after which they can lose their energy through atomic emission. By observing the wavelength of the emitted light, the atom's identity can be determined. By observing the intensity of the emission, the concentration of atoms of that type can be determined.
Energy gained through collisions can also ionize the sample atoms. The ions can then be detected by mass spectrometry. In this case, it is the mass of the ions that identify the element and the number of ions that reflect the concentration. This method is referred to as glow discharge mass spectrometry (GDMS) and it has detection limits down to the sub-ppb range for most elements that are nearly matrix-independent. | 3 | Analytical Chemistry |
Vacuum arc remelting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. The VAR process has revolutionized the specialty traditional metallurgical techniques industry, and has made possible tightly-controlled materials used in biomedical, aviation and aerospace. | 8 | Metallurgy |
Cerium-doped lutetium aluminum perovskite (LuAP:Ce) single crystals were reported. The main property of those crystals is a large mass density of 8.4 g/cm, which gives short X- and gamma-ray absorption length. The scintillation light yield and the decay time with Cs radiation source are 11,400 photons/MeV and 17 ns, respectively. Those properties made LUAP:Ce scintillators attractive for commercials and they were used quite often in high energy physics experiments. Until eleven years later, one group in Japan proposed Ruddlesden-Popper solution-based hybrid organic-inorganic perovskite crystals as low-cost scintillators. However, the properties were not so impressive in comparison with LuAP:Ce. Until the next nine years, the solution-based hybrid organic-inorganic perovskite crystals became popular again through a report about their high light yields of more than 100,000 photons/MeV at cryogenic temperatures. Recent demonstration of perovskite nanocrystal scintillators for X-ray imaging screen was reported and it is triggering more research efforts for perovskite scintillators. Layered Ruddlesden-Popper perovskites have shown potential as fast novel scintillators with room temperature light yields up to 40,000 photons/MeV, fast decay times below 5 ns and negligible afterglow. In addition this class of materials have shown capability for wide-range particle detection, including alpha particles and thermal neutrons. | 3 | Analytical Chemistry |
One of the largest deliberate PCB spills in American history occurred in the summer of 1978 when 31,000 gallons (117 m^3) of PCB-contaminated oil were illegally sprayed by the Ward PCB Transformer Company in swaths along the roadsides of some of North Carolina highway shoulders in 14 counties and at the Fort Liberty Army Base. The crime, known as "the midnight dumpings", occurred over nearly two weeks, as drivers of a black-painted tanker truck drove down one side of rural Piedmont highways spraying PCB-laden waste and then up the other side the following night.
Under Governor James B. Hunt, Jr., state officials then erected large, yellow warning signs along the contaminated highways that read: "CAUTION: PCB Chemical Spills Along Highway Shoulders". The illegal dumping is believed to have been motivated by the passing of the Toxic Substances Control Act (TSCA), which became effective on August 2, 1978, and increased the expense of chemical waste disposal.
Within a couple of weeks of the crime, Robert Burns and his sons, Timothy and Randall, were arrested for dumping the PCBs along the roadsides. Burns was a business partner of Robert "Buck" Ward Jr., of the Ward PCB Transformer Company, in Raleigh. Burns and sons pleaded guilty to state and Federal criminal charges; Burns received a three to five-year prison sentence. Ward was acquitted of state charges in the dumping, but was sentenced to 18 months prison time for violation of TSCA.
Cleanup and disposal of the roadside PCBs generated controversy, as the Governor's plan to pick up the roadside PCBs and to bury them in a landfill in rural Warren County were strongly opposed in 1982 by local residents.
In October 2013, at the request of the South Carolina Department of Health and Environmental Control (SCDHEC), the City of Charlotte, North Carolina, decided to stop applying sewage sludge to land while authorities investigated the source of PCB contamination.
In February 2014, the City of Charlotte admitted PCBs have entered their sewage treatment centers as well.
After the 2013 SCDHEC had issued emergency regulations, the City of Charlotte discovered high levels of PCBs entering its sewage waste water treatment plants, where sewage is converted to sewage sludge. The city at first denied it had a problem, then admitted an "event" occurred in February 2014, and in April that the problem had occurred much earlier. The city stated that its very first test with a newly changed test method revealed very high PCB levels in its sewage sludge farm field fertilizer. Because of the widespread use of the contaminated sludge, SCDHEC subsequently issued PCB fish advisories for nearly all streams and rivers bordering farm fields that had been applied with city waste. | 2 | Environmental Chemistry |
Neuropeptides are synthesized from inactive precursor proteins called prepropeptides. Prepropeptides contain sequences for a family of distinct peptides and often contain duplicated copies of the same peptides, depending on the organism. In addition to the precursor peptide sequences, prepropeptides also contain a signal peptide, spacer peptides, and cleavage sites. The signal peptide sequence guides the protein to the secretory pathway, starting at the endoplasmic reticulum. The signal peptide sequence is removed in the endoplasmic reticulum, yielding a propeptide. The propeptide travels to the Golgi apparatus where it is proteolytically cleaved and processed into multiple peptides. Peptides are packaged into dense core vesicles, where further cleaving and processing, such as C-terminal amidation, can occur. Dense core vesicles are transported throughout the neuron and can release peptides at the synaptic cleft, cell body, and along the axon. | 1 | Biochemistry |
# Cells must identify and be near each other.
# Hemifusion occurs.
# Fusion pore in hemifusion structure opens, thus allowing for cell contents to merge.
# Cells completely join from pore expansion. | 1 | Biochemistry |
Julian Simon was a senior fellow at the Cato Institute and a professor of business and economics. In his book The Ultimate Resource 2 (first printed in 1981 and reprinted in 1998), he extensively criticizes the notion of "peak resources", and uses copper as one example. He argues that, even though "peak copper" has been a persistent scare since the early 20th century, "known reserves" grew at a rate that outpaced demand, and the price of copper was not rising but falling over the long run. For example, even though world production of copper in 1950 was only one-eighth of what it was in the early 2000s, known reserves were also much lower at the time – around 100 million metric tons – making it appear that the world would run out of copper in 40 to 50 years at most.
Simon's own explanation for this development is that the very notion of known reserves is deeply flawed, as it does not take into account changes in mining profitability. As richer mines are exhausted, developers turn their attention to poorer sources of the element and eventually develop cheap methods of extracting it, raising known reserves. Thus, for example, copper was so abundant 5000 years ago, occurring in pure form as well as in highly concentrated copper ores, that prehistoric peoples were able to collect and process it with very basic technology. As of the early 21st century, copper is commonly mined from ores that contain 0.3–0.6% copper by weight. Yet, despite the material being far less widespread, the cost of, for example, a copper pot was vastly lower in the late 20th century than 5000 years ago. | 8 | Metallurgy |
Bilirubin di-glucuronide is a conjugated form of bilirubin formed in bilirubin metabolism. The hydrophilic character of bilirubin diglucuronide enables it to be water-soluble. It is pumped across the hepatic canalicular membrane into the bile by the transporter MRP2. | 1 | Biochemistry |
The encapsulins are a family of bacterial proteins that serve as the main structural components of encapsulin nanocompartments. There are several different encapsulin proteins, including EncA, which forms the shell, and EncB, EncC, and EncD, which form the core.
Encapsulins are also used in synthetic biology. They are hard to discover due to their similarity to phage proteins. | 1 | Biochemistry |
Agarose gel electrophoresis is most commonly done horizontally in a subaquaeous mode whereby the slab gel is completely submerged in buffer during electrophoresis. It is also possible, but less common, to perform the electrophoresis vertically, as well as horizontally with the gel raised on agarose legs using an appropriate apparatus. The buffer used in the gel is the same as the running buffer in the electrophoresis tank, which is why electrophoresis in the subaquaeous mode is possible with agarose gel.
For optimal resolution of DNA greater than 2kb in size in standard gel electrophoresis, 5 to 8 V/cm is recommended (the distance in cm refers to the distance between electrodes, therefore this recommended voltage would be 5 to 8 multiplied by the distance between the electrodes in cm). Voltage may also be limited by the fact that it heats the gel and may cause the gel to melt if it is run at high voltage for a prolonged period, especially if the gel used is LMP agarose gel. Too high a voltage may also reduce resolution, as well as causing band streaking for large DNA molecules. Too low a voltage may lead to broadening of band for small DNA fragments due to dispersion and diffusion.
Since DNA is not visible in natural light, the progress of the electrophoresis is monitored using colored dyes. Xylene cyanol (light blue color) comigrates large DNA fragments, while Bromophenol blue (dark blue) comigrates with the smaller fragments. Less commonly used dyes include Cresol Red and Orange G which migrate ahead of bromophenol blue. A DNA marker is also run together for the estimation of the molecular weight of the DNA fragments. Note however that the size of a circular DNA like plasmids cannot be accurately gauged using standard markers unless it has been linearized by restriction digest, alternatively a supercoiled DNA marker may be used. | 1 | Biochemistry |
Gene expression in mammals is regulated by many cis-regulatory elements, including core promoters and promoter-proximal elements that are located near the transcription start sites of genes. Core promoters are sufficient to direct transcription
initiation, but generally have low basal activity. Other important cis-regulatory modules are localized in DNA regions that are distant from the transcription start sites. These include enhancers, silencers, insulators and tethering elements. Among this constellation of elements, enhancers and their associated transcription factors have a leading role in the regulation of gene expression. An enhancer localized in a DNA region distant from the promoter of a gene can have a very large effect on gene expression, with some genes undergoing up to 100-fold increased expression due to an activated enhancer.
Enhancers are regions of the genome that are major gene-regulatory elements. Enhancers control cell-type-specific gene expression programs, most often by looping through long distances to come in physical proximity with the promoters of their target genes. While there are hundreds of thousands of enhancer DNA regions, for a particular type of tissue only specific enhancers are brought into proximity with the promoters that they regulate. In a study of brain cortical neurons, 24,937 loops were found, bringing enhancers to their target promoters. Multiple enhancers, each often at tens or hundreds of thousands of nucleotides distant from their target genes, loop to their target gene promoters and can coordinate with each other to control the expression of their common target gene.
The schematic illustration in this section shows an enhancer looping around to come into close physical proximity with the promoter of a target gene. The loop is stabilized by a dimer of a connector protein (e.g. dimer of CTCF or YY1), with one member of the dimer anchored to its binding motif on the enhancer and the other member anchored to its binding motif on the promoter (represented by the red zigzags in the illustration). Several cell function specific transcription factors (there are about 1,600 transcription factors in a human cell) generally bind to specific motifs on an enhancer and a small combination of these enhancer-bound transcription factors, when brought close to a promoter by a DNA loop, govern level of transcription of the target gene. Mediator (a complex usually consisting of about 26 proteins in an interacting structure) communicates regulatory signals from enhancer DNA-bound transcription factors directly to the RNA polymerase II (pol II) enzyme bound to the promoter.
Enhancers, when active, are generally transcribed from both strands of DNA with RNA polymerases acting in two different directions, producing two Enhancer RNAs (eRNAs) as illustrated in the Figure. Like mRNAs, these eRNAs are usually protected by their 5′ cap. An inactive enhancer may be bound by an inactive transcription factor. Phosphorylation of the transcription factor may activate it and that activated transcription factor may then activate the enhancer to which it is bound (see small red star representing phosphorylation of transcription factor bound to enhancer in the illustration). An activated enhancer begins transcription of its RNA before activating transcription of messenger RNA from its target gene. | 1 | Biochemistry |
Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (- and -enantiomers), though only the -isomer exists naturally. The salts and esters of malic acid are known as malates. The malate anion is a metabolic intermediate in the citric acid cycle. | 1 | Biochemistry |
Silicon can form sigma bonds to other silicon atoms (and disilane is the parent of this class of compounds). However, it is difficult to prepare and isolate SiH (analogous to the saturated alkane hydrocarbons) with n greater than about 8, as their thermal stability decreases with increases in the number of silicon atoms. Silanes higher in molecular weight than disilane decompose to polymeric polysilicon hydride and hydrogen. But with a suitable pair of organic substituents in place of hydrogen on each silicon it is possible to prepare polysilanes (sometimes, erroneously called polysilenes) that are analogues of alkanes. These long chain compounds have surprising electronic properties - high electrical conductivity, for example - arising from sigma delocalization of the electrons in the chain.
Even silicon–silicon pi bonds are possible. However, these bonds are less stable than the carbon analogues. Disilane and longer silanes are quite reactive compared to alkanes. Disilene and disilynes are quite rare, unlike alkenes and alkynes. Examples of disilynes, long thought to be too unstable to be isolated were reported in 2004. | 0 | Organic Chemistry |
Conditionally essential nutrients are certain organic molecules that can normally be synthesized by an organism, but under certain conditions in insufficient quantities. In humans, such conditions include premature birth, limited nutrient intake, rapid growth, and certain disease states. Inositol, taurine, arginine, glutamine and nucleotides are classified as conditionally essential and are particularly important in neonatal diet and metabolism. | 9 | Geochemistry |
A major focus in the field of protein engineering is on creating DNA libraries that sample regions of sequence space, often with the goal of finding mutants of proteins with enhanced functions compared to the wild type. These libraries are created either by using a wild type sequence as a template and applying one or more mutagenesis techniques to make different variants of it, or by creating proteins from scratch using artificial gene synthesis. These libraries are then screened or selected, and ones with improved phenotypes are used for the next round of mutagenesis. | 1 | Biochemistry |
Copper serves catalytic and structural roles in many essential enzymes in biology. In the context of catalytic activity, copper proteins function as electron or oxygen carriers, oxidases, mono- and dioxygenases and nitrite reductases. In particular, copper-containing enzymes include hemocyanins, one flavor of superoxide dismutase (SOD), metallothionein, cytochrome c oxidase, multicopper oxidase and particulate methane monooxygenase (pMMO). | 9 | Geochemistry |
Azobilirubin is a coloured compound formed by the condensation of diazotized sulfanilic acid with bilirubin in the van den Bergh reaction. The quantity of bilirubin in patients with jaundice can be determined by the formation of azobilirubin in the presence of methanol.
The Van den Bergh chemical reaction which is used to measure bilirubin levels, couples bilirubin with diazotized sulfanilic acid. This reaction produced azo pigments, or azobilirubin. The presence of azobilirubin is best indicated by the emergence of a pink-purple color. The intensity of the color will also indicate how much bilirubin is in the blood. Color markers and indicators can be changed. Adding alkaline tartrate can make the purple azobilirubin into a blue azobilirubin. If we have high levels of bilirubin in the blood, the Van den Bergh chemical reaction is used to determine if the bilirubin is conjugated or unconjugated.
Azobilirubin can also be used to determine how much conjugated bilirubin is the blood compared to that of unconjugated bilirubin. This process is done by using both forms of bilirubin alongside a diazo reagent and a caffeine-benzoate reagent. Conjugated bilirubin with the azo reagent will react, whilst the unconjugated bilirubin will not react with the azo reagent. | 1 | Biochemistry |
The phase diagram shows the importance of the electrode gap distance to the performance of electrochemical reactions. For traditional macrosystems, where the electrode gap distance is much larger than the Debye-length, two half-reactions are decoupled and cannot influence each other. Normally the electrochemical current is limited by a slow diffusion step. When the gap distance is reduced to around the Debye-length, a large electric field can form between the two electrodes (due to double layers and the two regions overlapping with each other); this enhances the mass transport rate. In this region the electrolysis current is very sensitive to the gap distance and the reactions are migration-rate limited. When the gap distance is further reduced to the deep-sub-Debye-length region, the mass transport can be enhanced further to a level even faster than the electron-transfer step. In this region, even when we shrink the gap distance further, the current cannot be enlarged any more, meaning that the current has reached saturation. Here the two half-reactions are coupled together and the reactions are limited by the electron-transfer steps.
Therefore, by just adjusting the gap distance, the fundamental performance of the electrochemical reactions can be significantly changed. | 7 | Physical Chemistry |
Historic instruments are the first backscattering spectrometer that was a temporary setup at FRM I
and the backscattering spectrometer BSS (also called PI) at the DIDO reactor of the Forschungszentrum Jülich (decommissioned). | 7 | Physical Chemistry |
Eukaryotes belonging to some divergent lineages have very special organizations of the ATP synthase. A euglenozoa ATP synthase forms a dimer with a boomerang-shaped F head like other mitochondrial ATP synthases, but the F subcomplex has many unique subunits. It uses cardiolipin. The inhibitory IF also binds differently, in a way shared with trypanosomatida. | 5 | Photochemistry |
The journal is abstracted and indexed in:
According to the Journal Citation Reports, the journal has a 2021 impact factor of 4.290. | 7 | Physical Chemistry |
The important sulfur cycle is a biogeochemical cycle in which the sulfur moves between rocks, waterways and living systems. It is important in geology as it affects many minerals and in life because sulfur is an essential element (CHNOPS), being a constituent of many proteins and cofactors, and sulfur compounds can be used as oxidants or reductants in microbial respiration. The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes.
Steps of the sulfur cycle are:
* Mineralization of organic sulfur into inorganic forms, such as hydrogen sulfide (HS), elemental sulfur, as well as sulfide minerals.
* Oxidation of hydrogen sulfide, sulfide, and elemental sulfur (S) to sulfate ().
* Reduction of sulfate to sulfide.
* Incorporation of sulfide into organic compounds (including metal-containing derivatives).
* Disproportionation of sulfur compounds (elemental sulfur, sulfite, thiosulfate) into sulfate and hydrogen sulfide.
These are often termed as follows:
:Assimilative sulfate reduction (see also sulfur assimilation) in which sulfate () is reduced by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and –2 in R–SH.
:Desulfurization in which organic molecules containing sulfur can be desulfurized, producing hydrogen sulfide gas (HS, oxidation state = –2). An analogous process for organic nitrogen compounds is deamination.
:Oxidation of hydrogen sulfide produces elemental sulfur (S), oxidation state = 0. This reaction occurs in the photosynthetic green and purple sulfur bacteria and some chemolithotrophs. Often the elemental sulfur is stored as polysulfides.
:Oxidation in elemental sulfur by sulfur oxidizers produces sulfate.
:Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide.
:Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate. | 9 | Geochemistry |
Oxygen saturation (symbol S) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water. The standard unit of oxygen saturation is percent (%).
Oxygen saturation can be measured regionally and noninvasively. Arterial oxygen saturation (Sa) is commonly measured using pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS. This technique can be applied on both muscle and brain. | 3 | Analytical Chemistry |
In electrochemistry, a thermogalvanic cell is a kind of galvanic cell in which heat is employed to provide electrical power directly. These cells are electrochemical cells in which the two electrodes are deliberately maintained at different temperatures. This temperature difference generates a potential difference between the electrodes. The electrodes can be of identical composition and the electrolyte solution homogeneous. This is usually the case in these cells. This is in contrast to galvanic cells in which electrodes and/or solutions of different composition provide the electromotive potential. As long as there is a difference in temperature between the electrodes a current will flow through the circuit. A thermogalvanic cell can be seen as analogous to a concentration cell but instead of running on differences in the concentration/pressure of the reactants they make use of differences in the "concentrations" of thermal energy. The principal application of thermogalvanic cells is the production of electricity from low-temperature heat sources (waste heat and solar heat). Their energetic efficiency is low, in the range of 0.1% to 1% for conversion of heat into electricity. | 7 | Physical Chemistry |
In 1941, the year in which they graduated from the University of Oxford, Cornforth married Rita Harriet Harradence (b. 1915), with whom he had one son, John, and two daughters, Brenda and Philippa. Cornforth had met Harradence after she had broken a Claisen flask in their second year at the University of Sydney; Cornforth, with his expertise of glassblowing and the use of a blowpipe, mended the break. Rita Cornforth died on 6 November 2012, at home with her family around her, following a long illness.
On an important author or paper that was integral to his success, Cornforth stated that he was particularly impressed by the works of German chemist Hermann Emil Fischer.
Cornforth died in Sussex on 8 December 2013. at the age of 96. Cornforth is survived by his three children and four grandchildren. He was a sceptic and an atheist. | 0 | Organic Chemistry |
The most important aspect of disulfide bonds is their scission, as the bond is usually the weakest bond in a molecule. Many specialized organic reactions have been developed to cleave the bond.
A variety of reductants reduce disulfides to thiols. Hydride agents are typical reagents, and a common laboratory demonstration "uncooks" eggs with sodium borohydride. Alkali metals effect the same reaction more aggressively: followed by protonation of the resulting metal thiolate:
In biochemistry labwork, thiols such as β-mercaptoethanol (β-ME) or dithiothreitol (DTT) serve as reductants through thiol-disulfide exchange. The thiol reagents are used in excess to drive the equilibrium to the right:
The reductant tris(2-carboxyethyl)phosphine (TCEP) is useful, beside being odorless compared to β-ME and DTT, because it is selective, working at both alkaline and acidic conditions (unlike DTT), is more hydrophilic and more resistant to oxidation in air. Furthermore, it is often not needed to remove TCEP before modification of protein thiols.
In Zincke cleavage, halogens oxidize disulfides to a sulfenyl halide:
More unusually, oxidation of disulfides gives first thiosulfinates and then thiosulfonates:
:RSSR + [O] → RS(=O)SR
:RS(=O)SR + [O] → RS(=O)SR | 0 | Organic Chemistry |
Pathological changes associated with CYN poisoning were reported to be in four distinct stages: inhibition of protein synthesis, proliferation of membranes, lipid accumulation within cells, and finally cell death. Examination of mice livers removed at autopsy showed that on intraperitoneal injection of CYN, after 16 hours ribosomes from the rough endoplasmic reticulum (rER) had detached, and at 24 hours, marked proliferation of the membrane systems of the smooth ER and Golgi apparatus had occurred. At 48 hours, small lipid droplets had accumulated in the cell bodies, and at 100 hours, hepatocytes in the hepatic lobules were destroyed beyond function.
The process of protein synthesis inhibition has been shown to be irreversible, however is not conclusively the method of cytotoxicity of the compound. Froscio et al.. proposed that CYN has at least two separate modes of action: the previously reported protein synthesis inhibition, and an as-yet unclear method of causing cell death. It has been shown that cells can survive for long periods (up to 20 hours) with 90% inhibition of protein synthesis, and still maintain viability. Since CYN is cytotoxic within 16–18 hours it has been suggested that other mechanisms are the cause of cell death.
Cytochrome P450 has been implicated in the toxicity of CYN, as blocking the action of P450 reduces the toxicity of CYN. It has been proposed that an activated P450-derived metabolite (or metabolites) of CYN is the main cause of toxicity. Shaw et al.. demonstrated that the toxin could be metabolised in vivo, resulting in bound metabolites in the liver tissue, and that damage was more prevalent in rat hepatocytes than other cell types.
Due to the structure of CYN, which includes sulfate, guanidine and uracil groups, it has been suggested that CYN acts on DNA or RNA. Shaw et al.. reported covalent binding of CYN or its metabolites to DNA in mice, and DNA strand breakage has also been observed. Humpage et al. also supported this, and in addition postulated that CYN (or a metabolite) acts on either the spindle or centromeres during cell division, inducing loss of whole chromosomes.
The uracil group of CYN has been identified as a pharmacophore of the toxin. In two experiments, the vinylic hydrogen atom on the uracil ring was replaced with a chlorine atom to form 5-chlorocylindrospermopsin, and the uracil group was truncated to a carboxylic acid, to form cylindrospermic acid (Figure 6). Both products were assessed as being non-toxic, even at 50 times the LD of CYN. In the previous determination of the structure of deoxycylindrospermopsin, a toxicity assessment of the compound was carried out. Mice injected intraperitoneally with four times the 5-day median lethal dose of CYN showed no toxic effects. As this compound was shown to be relatively abundant, it was concluded that this analogue was comparatively non-toxic. Given that both CYN and epiCYN are toxic, the hydroxyl group on the uracil bridge can be considered necessary for toxicity. As yet, the relative toxicities of CYN and epiCYN have not been compared. | 0 | Organic Chemistry |
Simultaneously theoretical and computational approaches provided data necessary for identification of new spectra and interpretation of observed line intensities. New laboratory and theoretical data become very useful for spectral observation in space. It was a real upheaval of works on MIA in USA, England, France, Italy, Israel, Sweden, Russia and other countries
A new page in the spectroscopy of MIA may be dated as 1986 with development of EBIT (Levine and Marrs, LLNL) due to a favorable composition of modern high technologies such as cryogenics, ultra-high vacuum, superconducting magnets, powerful electron beams and semiconductor detectors. Very quickly EBIT sources were created in many countries (see NIST summary for many details as well as reviews.)
A wide field of spectroscopic research with EBIT is enabled including achievement of highest grades of ionization (U), wavelength measurement, hyperfine structure of energy levels, quantum electrodynamic studies, ionization cross-sections (CS) measurements, electron-impact excitation CS, X-ray polarization, relative line intensities, dielectronic recombination CS, magnetic octupole decay, lifetimes of forbidden transitions, charge-exchange recombination, etc. | 7 | Physical Chemistry |
In organic chemistry, the Cornforth rearrangement is a rearrangement reaction of a 4-acyloxazole in which the group attached to an acyl on position 4 and the substituent on position 5 of an oxazole ring exchange places. It was first reported in 1949, and is named for John Cornforth. The reaction is used in the synthesis of amino acids, where the corresponding oxazoles occur as intermediates. | 0 | Organic Chemistry |
In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context.
This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material. Many non-brittle metals with a reasonably high melting point as well as several polymers can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at low temperatures, such as indium, however others can be strengthened only via work hardening, such as pure copper and aluminum. | 8 | Metallurgy |
*1983, 1984 NASA Lewis Research Center Summer Faculty Fellow
*1985, 1986, 1987: Senior Visiting Scientist, Burroughs Wellcome Co.
*1987: East Carolina University Sigma XI Helms Award for outstanding Research
*1999: East Carolina University 5-Year Achievement Research/Creative Activity Award
*2001: East Carolina University College of Arts and Sciences Distinguished Professor of Chemistry, Lifetime Achievement Award
*2003: Eastern Analytical Symposium Award for Achievement in Chemometrics.
*2010: Applied Spectroscopy William F. Meggers Award for outstanding paper appearing in Applied Spectroscopy
*2014: Distinguished Alumni Award, Cleveland State University | 3 | Analytical Chemistry |
This puzzle was mentioned by W. W. Rouse Ball in the third, 1896, edition of his book Mathematical Recreations And Problems Of Past And Present Times, and is said to have been a favorite problem of Lewis Carroll. | 7 | Physical Chemistry |
The effects of electrostatic and steric interactions of the substituents as well as orbital interactions such as hyperconjugation are responsible for the relative stability of conformers and their transition states. The contributions of these factors vary depending on the nature of the substituents and may either contribute positively or negatively to the energy barrier. Computational studies of small molecules such as ethane suggest that electrostatic effects make the greatest contribution to the energy barrier; however, the barrier is traditionally attributed primarily to steric interactions.
In the case of cyclic systems, the steric effect and contribution to the free energy can be approximated by A values, which measure the energy difference when a substituent on cyclohexane in the axial as compared to the equatorial position. In large (>14 atom) rings, there are many accessible low-energy conformations which correspond to the strain-free diamond lattice. | 4 | Stereochemistry |
Faster GC methods have shorter times but Kovats indexes of the compounds may be conserved if proper method translation is applied.
Temperatures of the temperature program stay the same, but ramps and times change when using a smaller column or faster carrier gas.
If column dimensions Length×diameter×film are divided by 2 and gas velocity is doubled by using H2 in place of Helium, the hold times must be divided by 4 and the ramps must be multiplied by 4 to keep the same index and the same retention temperature for the same compound analyzed. Method translation rules are incorporated in some chromatography data systems. | 3 | Analytical Chemistry |
Soil that is well aggregated has a range of pore sizes. Each pore size plays a role in soil's physical functioning. Large pores drain rapidly and are needed for good air exchange during wet periods, preventing oxygen deficiency that can drown plants and increase pest problems. Oxygen-deficient wet soils increase denitrification – conversion of nitrogen to gaseous forms. In degraded soil large pores are compressed into small ones.
Small pores are critical for water retention and help a crop endure dry periods with minimal yield loss. | 9 | Geochemistry |
2-(Morpholinothio)benzothiazole is used as an accelerator/stabilizer for vulcanization, or the manufacture of rubber products. It is the precursor to NMOR in the vulcanization process, as it is nitrosated by ambient sources of the nitro group present in the manufacturing process. As such, workers and others exposed to the rubber industry or its byproducts are exposed to higher levels of NMOR than the general population, raising their risk of cancer. | 0 | Organic Chemistry |
β-carotene can be extracted from fungus Blakeslea trispora, marine algae Dunaliella salina or genetically modified yeast Saccharomyces cerevisiae, starting with xylose as a substrate. Chemical synthesis uses either a method developed by BASF or a Grignard reaction utilized by Hoffman-La Roche.
The world market for synthetic retinol is primarily for animal feed, leaving approximately 13% for a combination of food, prescription medication and dietary supplement use. Industrial methods for the production of retinol rely on chemical synthesis. The first industrialized synthesis of retinol was achieved by the company Hoffmann-La Roche in 1947. In the following decades, eight other companies developed their own processes. β-ionone, synthesized from acetone, is the essential starting point for all industrial syntheses. Each process involves elongating the unsaturated carbon chain. Pure retinol is extremely sensitive to oxidization and is prepared and transported at low temperatures and oxygen-free atmospheres. When prepared as a dietary supplement or food additive, retinol is stabilized as the ester derivatives retinyl acetate or retinyl palmitate. Prior to 1999, three companies, Roche, BASF and Rhone-Poulenc controlled 96% of global vitamin A sales. In 2001, the European Commission imposed total fines of 855.22 million euros on these and five other companies for their participation in eight distinct market-sharing and price-fixing cartels that dated back to 1989. Roche sold its vitamin division to DSM in 2003. DSM and BASF have the major share of industrial production. A biosynthesis alternative utilizes genetically engineered yeast species Saccharomyces cerevisiae to synthesize retinal and retinol, using xylose as a starting substrate. This was accompished by having the yeast first synthesize β-carotene and then the cleaving enzyme β-carotene 15,15'-dioxygenase to yield retinal. | 1 | Biochemistry |
When p75NTR initiates apoptosis, NGF binding to Tropomyosin receptor kinase A (TrkA) can negate p75NTR apoptotic effects. p75NTR c-Jun kinase pathway activation (which causes apoptosis) is suppressed when NGF binds to TrkA. p75NTR activation of NF-kB, which promotes survival, is unaffected by NGF binding to TrkA. | 1 | Biochemistry |
Flammable substances include, but are not limited to:
* Gasoline - Petrol / a complicated mixture of hydrocarbons that includes isomers of octane, CH
* Ethanol / CHCHOH
* Rubber
* Isopropyl alcohol / CHCH(OH)CH
* Methanol / CHOH
* Wood
* Acetone / CHCOCH
* Paper
* Nitromethane / CHNO | 7 | Physical Chemistry |
*[http://rruff.geo.arizona.edu/AMS/amcsd.php American Mineralogist Crystal Structure Database (AMCSD)] (contents: crystal structures of minerals, access: free, size: large)
*[http://www.ccdc.cam.ac.uk/products/csd/ Cambridge Structural Database (CSD)] (contents: crystal structures of organics and metal-organics, access: restricted, size: very large)
*Crystallography Open Database (COD) (contents: crystal structures of organics, metalorganics, minerals, inorganics, metals, alloys, and intermetallics, access: free, size: very large)
*[http://crystallography.io/ COD+] (Web Interface for COD) (contents: crystal structures of organics, metalorganics, minerals, inorganics, metals, alloys, and intermetallics, access: free, size: very large)
*[http://www.iza-structure.org/databases/ Database of Zeolite Structures] (contents: crystal structures of zeolites, access: free, size: small)
*[http://www.cryst.ehu.es/icsdb/index.html Incommensurate Structures Database] (contents: incommensurate structures, access: free, size: small)
*Inorganic Crystal Structure Database (ICSD) (contents: crystal structures of minerals and inorganics, access: restricted, size: large)
*[http://materialsproject.org MaterialsProject Database] (contents: crystal structures of inorganic compounds, access: free, size: large)
*[https://mpds.io Materials Platform for Data Science (MPDS) or PAULING FILE] (contents: critically evaluated crystal structures, as well as physical properties and phase diagrams, from the world scientific literature, access: partially free, size: very large)
*[http://materialsweb.org MaterialsWeb Database] (contents: crystal structures of inorganic 2D materials and bulk compounds, access: free, size: large)
*[https://web.archive.org/web/20080622165920/http://tothcanada.com/databases.htm Metals Structure Database (CRYSTMET)] (contents: crystal structures of metals, alloys, and intermetallics, access: restricted, size: large)
*[http://webmineral.com/ Mineralogy Database] (contents: crystal structures of minerals, access: free, size: medium)
*[http://database.iem.ac.ru/mincryst/index.php MinCryst] (contents: crystal structures of minerals, access: free, size: medium)
*[https://web.archive.org/web/20080516022153/http://www.nist.gov/srd/nist83.htm NIST Structural Database NIST Structural Database] (contents: crystal structures of metals, alloys, and intermetallics, access: restricted, size: large)
*[https://web.archive.org/web/20080610153131/http://www.nist.gov/srd/nist42.htm NIST Surface Structure Database] (contents: surface and interface structures, access: restricted, size: small-medium)
*[http://ndbserver.rutgers.edu/ Nucleic Acid Database] (contents: crystal and molecular structures of nucleic acids, access: free, size: medium)
*[http://www.crystalimpact.com/pcd/Default.htm Pearson's Crystal Data] (contents: crystal structures of inorganics, minerals, salts, oxides, hydrides, metals, alloys, and intermetallics, access: restricted, size: very large)
*[http://www.wwpdb.org/ Worldwide Protein Data Bank (PDB)] (contents: crystal and molecular structures of biological macromolecules, access: free, size: very large)
*[http://nanocrystallography.research.pdx.edu/search/wcd/ Wiki Crystallography Database (WCD)] (contents: crystal structures of organics, metalorganics, minerals, inorganics, metals, alloys, and intermetallics, access: free, size: medium) | 7 | Physical Chemistry |
Metabolic flux analysis (MFA) using stable isotope labeling is an important tool for explaining the flux of certain elements through the metabolic pathways and reactions within a cell. An isotopic label is fed to the cell, then the cell is allowed to grow utilizing the labeled feed. For stationary metabolic flux analysis the cell must reach a steady state (the isotopes entering and leaving the cell remain constant with time) or a quasi-steady state (steady state is reached for a given period of time). The isotope pattern of the output metabolite is determined. The output isotope pattern provides valuable information, which can be used to find the magnitude of flux, rate of conversion from reactants to products, through each reaction.
The figure demonstrates the ability to use different labels to determine the flux through a certain reaction. Assume the original metabolite, a three carbon compound, has the ability to either split into a two carbon metabolite and one carbon metabolite in one reaction then recombine or remain a three carbon metabolite. If the reaction is provided with two isotopes of the metabolite in equal proportion, one completely labeled (blue circles), commonly known as uniformly labeled, and one completely unlabeled (white circles). The pathway down the left side of the diagram does not display any change in the metabolites, while the right side shows the split and recombination. As shown, if the metabolite only takes the pathway down the left side, it remains in a 50–50 ratio of uniformly labeled to unlabeled metabolite. If the metabolite only takes the right side new labeling patterns can occur, all in equal proportion. Other proportions can occur depending on how much of the original metabolite follows the left side of the pathway versus the right side of the pathway. Here the proportions are shown for a situation in which half of the metabolites take the left side and half the right, but other proportions can occur. These patterns of labeled atoms and unlabeled atoms in one compound represent isotopomers. By measuring the isotopomer distribution of the differently labeled metabolites, the flux through each reaction can be determined.
MFA combines the data harvested from isotope labeling with the stoichiometry of each reaction, constraints, and an optimization procedure resolve a flux map. The irreversible reactions provide the thermodynamic constraints needed to find the fluxes. A matrix is constructed that contains the stoichiometry of the reactions. The intracellular fluxes are estimated by using an iterative method in which simulated fluxes are plugged into the stoichiometric model. The simulated fluxes are displayed in a flux map, which shows the rate of reactants being converted to products for each reaction. In most flux maps, the thicker the arrow, the larger the flux value of the reaction. | 7 | Physical Chemistry |
The Semipalatinsk Test Site, also known as "The Polygon", was the primary testing venue for the Soviet Union's nuclear weapons. It is located on the steppe in northeast Kazakhstan (then the Kazakh SSR), south of the valley of the Irtysh River. The scientific buildings for the test site were located around 150 km west of the town of Semipalatinsk (later renamed Semey), near the border of East Kazakhstan Province and Pavlodar Province with most of the nuclear tests taking place at various sites further to the west and south, some as far as into Karagandy Province.
The Soviet Union conducted 456 nuclear tests at Semipalatinsk from 1949 until 1989 with little regard for their effect on the local people or environment. The full impact of radiation exposure was hidden for many years by Soviet authorities and has only come to light since the test site closed in 1991.
From 1996 to 2012, a secret joint operation of Kazakh, Russian, and American nuclear scientists and engineers secured the waste plutonium in the tunnels of the mountains. | 2 | Environmental Chemistry |
The story of ledeburite begins in the late 19th century when Adolf Ledebur, a pioneering German metallurgist, embarked on a journey to unravel the complexities of steel microstructures. In 1882, Ledebur identified a distinct microconstituent in high-carbon steels, characterized by its unique lamellar structure. This discovery marked the birth of ledeburite, named in honor of the scientist whose keen observations laid the foundation for understanding the intricate world within steel. | 8 | Metallurgy |
While the colloidal gold assay is the most sensitive in-solution colorimetric protein assay, it may be equally sensitive or surpassed in sensitivity by fluorescent protein assays such as the CBQCA, FQ, NanoOrange, Quant-iT, and EZQ assays. | 1 | Biochemistry |
CPC offers direct scale-up from analytical apparatuses (few milliliters) to industrial apparatuses (several liters) for fast batch-production. CPC seems particularly suited to accommodate aqueous two-phase solvent systems. Generally, CPC instruments can retain solvent systems that are not well-retained in a hydrodynamic instrument due to small differences in density between the phases. It has been very helpful for the development of CPC instrumentation to visualize the flow patterns which give rise to the mixing and settling in the CPC chamber with an asynchronous camera and a stroboscope triggered by the CPC rotor. | 3 | Analytical Chemistry |
The first experimental account and analysis of gene expression noise in prokaryotes is from Becskei & Serrano and from Alexander van Oudenaardens lab. The first experimental account and analysis of gene expression noise in eukaryotes is from James J. Collinss lab. | 1 | Biochemistry |
The chemical character of azo violet may be attributed to its azo group (-N=N-), six-membered rings, and hydroxyl side groups. Due to steric repulsions, azo violet is most stable in the trans-configuration, but isomerization of azo dyes by irradiation is not uncommon. The para-position tautomerization of azo violet provides mechanical insight into the behavior of the compound in an acidic environment, and thus its use as a basic pH indicator.
The predicted H-NMR of pure azo violet shows the hydroxyl protons as the most deshielded and acidic protons. The participation of these hydroxyl groups electron-donation to the conjugated π system likewise influences azo violets λ and pK value. | 3 | Analytical Chemistry |
Aluminothermy started from the experiments of Russian scientist Nikolay Beketov at the University of Kharkiv in Ukraine, who proved that aluminium restored metals from their oxides under high temperatures. The reaction was first used for the carbon-free reduction of metal oxides. The reaction is highly exothermic, but it has a high activation energy since strong interatomic bonds in the solids must be broken first. The oxide was heated with aluminium in a crucible in a furnace. The runaway reaction made it possible to produce only small quantities of material. Hans Goldschmidt improved the aluminothermic process between 1893 and 1898, by igniting the mixture of fine metal oxide and aluminium powder by a starter reaction without heating the mixture externally. The process was patented in 1898 and used extensively in the later years for rail track welding. | 8 | Metallurgy |
While LCP was the first and sole method used to produce poly(phthalaldehyde), its usage nowadays has dramatically decreased in favor of other polymerization techniques which allow a better control over the polymer properties including molar mass and thermal stability. | 7 | Physical Chemistry |
In atmospheric chemistry, mixing ratio usually refers to the mole ratio r, which is defined as the amount of a constituent n divided by the total amount of all other constituents in a mixture:
The mole ratio is also called amount ratio.
If n is much smaller than n (which is the case for atmospheric trace constituents), the mole ratio is almost identical to the mole fraction. | 3 | Analytical Chemistry |
It is important to be able to predict the onset of gelation, since it is an irreversible process that dramatically changes the properties of the system. | 7 | Physical Chemistry |
Sphingosine-1-phosphate (S1P), like Sph, is composed of a single hydrophobic chain and has sufficient solubility to move between membranes. S1P is formed by phosphorylation of sphingosine by sphingosine kinase (SK). The phosphate group of the product can be detached (dephosphorylated) to regenerate sphingosine via S1P phosphatase enzymes or S1P can be broken down by S1P lyase enzymes to ethanolamine phosphate and hexadecenal. Similar to Sph, its second messenger function is not yet clear. However, there is substantial evidence that implicates S1P to cell survival, cell migration, and inflammation. Certain growth-inducing proteins such as platelet-derived growth factor (PDGF), insulin-like growth factor (IGF) and vascular endothelial growth factor (VEGF) promote the formation of SK enzymes, leading to increased levels of S1P. Other factors that induce SK include cellular communication molecules called cytokines, such as tumor necrosis factor α (TNFα) and interleukin-1 (IL-1), hypoxia or lack of oxygen supply in cells, oxidized low-density lipoproteins (oxLDL) and several immune complexes.
S1P is probably formed at the inner leaflet of the plasma membrane in response to TNFα and other receptor activity-altering compounds called agonists. S1P, being present in low nanomolar concentrations in the cell, has to interact with high-affinity receptors that are capable of sensing their low levels. So far, the only identified receptors for S1P are the high-affinity G protein-coupled receptors (GPCRs), also known as S1P receptors (S1PRs). S1P is required to reach the extracellular side (outer leaflet) of the plasma membrane to interact with S1PRs and launch typical GPCR signaling pathways. However, the zwitterionic headgroup of S1P makes it unlikely to flip-flop spontaneously. To overcome this difficulty, the ATP-binding cassette (ABC) transporter C1 (ABCC1) serves as the "exit door" for S1P. On the other hand, the cystic fibrosis transmembrane regulator (CFTR) serves as the means of entry for S1P into the cell. In contrast to its low intracellular concentration, S1P is found in high nanomolar concentrations in serum where it is bound to albumin and lipoproteins. Inside the cell, S1P can induce calcium release independent of the S1PRs—the mechanism of which remains unknown. To date, the intracellular molecular targets for S1P are still unidentified.
The SK1-S1P pathway has been extensively studied in relation to cytokine action, with multiple functions connected to effects of TNFα and IL-1 favoring inflammation. Studies show that knockdown of key enzymes such as S1P lyase and S1P phosphatase increased prostaglandin production, parallel to increase of S1P levels. This strongly suggests that S1P is the mediator of SK1 action and not subsequent compounds. Research done on endothelial and smooth muscle cells is consistent to the hypothesis that S1P has a crucial role in regulating endothelial cell growth, and movement. Recent work on a sphingosine analogue, FTY270, demonstrates its ability to act as a potent compound that alters the activity of S1P receptors (agonist). FTY270 was further verified in clinical tests to have roles in immune modulation, such as that on multiple sclerosis. This highlights the importance of S1P in the regulation of lymphocyte function and immunity. Most of the studies on S1P are used to further understand diseases such as cancer, arthritis and inflammation, diabetes, immune function and neurodegenerative disorders. | 1 | Biochemistry |
Dithionite is used in conjunction with complexing agents (for example, citric acid) to reduce iron(III) oxy-hydroxide into soluble iron(II) compounds and to remove amorphous iron(III)-bearing mineral phases in soil analyses (selective extraction).
The decomposition of dithionite produces reduced species of sulfur that can be very aggressive for the corrosion of steel and stainless steel. Thiosulfate () is known to induce pitting corrosion, whereas sulfide (S, HS) is responsible for stress corrosion cracking (SCC). | 8 | Metallurgy |
*Alfred P. Sloan Research Fellowship, 1971–1973
*Alexander von Humboldt Senior Scientist Award, 1984
*Member of the Latvian Academy of Sciences, 1992
*Paul Walden Medal, 1997
*Herbert C. Brown Award for Creative Research in Synthetic Methods, 2004
*Grand Medal of the Latvian Academy of Sciences, 2005
*Order of the Three Stars, Republic of Latvia, 2006
*Elected fellow of the American Chemical Society, 2011 | 0 | Organic Chemistry |
It is difficult to do meaningful validation of an individual, purely computational, macromolecular model in the absence of experimental data for that molecule, because the model with the best geometry and conformational score may not be the one closest to the right answer. Therefore, much of the emphasis in validation of computational modeling is in assessment of the methods. To avoid bias and wishful thinking, double-blind prediction competitions have been organized, the original example of which (held every 2 years since 1994) is CASP (Critical Assessment of Structure Prediction) to evaluate predictions of 3D protein structure for newly solved crystallographic or NMR structures held in confidence until the end of the relevant competition. The major criterion for CASP evaluation is a weighted score called GDT-TS for the match of Calpha positions between the predicted and the experimental models. | 1 | Biochemistry |
The last stage of transcription is termination, which leads to the dissociation of the complete transcript and the release of RNA polymerase from the template DNA.The process differs for each of the three RNA polymerases.
The mechanism of termination is the least understood of the three transcription stages. | 1 | Biochemistry |
Real crystals feature defects or irregularities in the ideal arrangements described above and it is these defects that critically determine many of the electrical and mechanical properties of real materials. | 3 | Analytical Chemistry |
One group proposed to use the material to create digital circuits made of quantum dots by removing hydrogen atoms from the silicon surface. | 6 | Supramolecular Chemistry |
The onset of acidosis during periods of intense exercise is commonly attributed to accumulation of hydrogens that are dissociated from lactate. Previously, lactic acid was thought to cause fatigue. From this reasoning, the idea of lactate production being a primary cause of muscle fatigue during exercise was widely adopted. A closer, mechanistic analysis of lactate production under “anaerobic” conditions shows that there is no biochemical evidence for the production of lactate through LDH contributing to acidosis. While LDH activity is correlated to muscle fatigue, the production of lactate by means of the LDH complex works as a system to delay the onset of muscle fatigue. George Brooks and Colleagues at UC Berkeley where the lactate shuttle was discovered showed that lactate was actually a metabolic fuel not a waste product or the cause of fatigue.
LDH works to prevent muscular failure and fatigue in multiple ways. The lactate-forming reaction generates cytosolic NAD+, which feeds into the glyceraldehyde 3-phosphate dehydrogenase reaction to help maintain cytosolic redox potential and promote substrate flux through the second phase of glycolysis to promote ATP generation. This, in effect, provides more energy to contracting muscles under heavy workloads. The production and removal of lactate from the cell also ejects a proton consumed in the LDH reaction- the removal of excess protons produced in the wake of this fermentation reaction serves to act as a buffer system for muscle acidosis. Once proton accumulation exceeds the rate of uptake in lactate production and removal through the LDH symport, muscular acidosis occurs. | 1 | Biochemistry |
Downs' process is an electrochemical method for the commercial preparation of metallic sodium, in which molten NaCl is electrolyzed in a special apparatus called the Downs cell. The Downs cell was invented in 1923 (patented: 1924) by the American chemist James Cloyd Downs (1885–1957). | 8 | Metallurgy |
Thermal mass is ideally placed within a building where it is shielded from direct solar gain but exposed to the building occupants. It is therefore most commonly associated with solid concrete floor slabs in naturally ventilated or low-energy mechanically ventilated buildings where the concrete soffit is left exposed to the occupied space.
During the day heat is gained from the sun, the occupants of the building, and any electrical lighting and equipment, causing the air temperatures within the space to increase, but this heat is absorbed by the exposed concrete slab above, thus limiting the temperature rise within the space to be within acceptable levels for human thermal comfort. In addition the lower surface temperature of the concrete slab also absorbs radiant heat directly from the occupants, also benefiting their thermal comfort.
By the end of the day the slab has in turn warmed up, and now, as external temperatures decrease, the heat can be released and the slab cooled down, ready for the start of the next day. However this "regeneration" process is only effective if the building ventilation system is operated at night to carry away the heat from the slab. In naturally ventilated buildings it is normal to provide automated window openings to facilitate this process automatically. | 7 | Physical Chemistry |
Dische's diphenylamine reagent consists of diphenylamine, glacial acetic acid, sulfuric acid, and ethanol.
When heated with DNA, it turns blue in the presence of DNA. A more intense blue color indicates a greater concentration of DNA. | 3 | Analytical Chemistry |
The Stern–Volmer relationship, named after Otto Stern and Max Volmer, allows the kinetics of a photophysical intermolecular deactivation process to be explored.
Processes such as fluorescence and phosphorescence are examples of intramolecular deactivation processes. An intermolecular deactivation is where the presence of another chemical species can accelerate the decay rate of a chemical in its excited state. In general, this process can be represented by a simple equation:
or
where A is one chemical species, Q is another (known as a quencher) and * designates an excited state.
The kinetics of this process follows the Stern–Volmer relationship:
Where is the intensity, or rate of fluorescence, without a quencher, is the intensity, or rate of fluorescence, with a quencher, is the quencher rate coefficient, is the lifetime of the emissive excited state of A without a quencher present, and is the concentration of the quencher.
For diffusion-limited quenching (i.e., quenching in which the time for quencher particles to diffuse toward and collide with excited particles is the limiting factor, and almost all such collisions are effective), the quenching rate coefficient is given by , where is the ideal gas constant, is temperature in kelvins is the viscosity of the solution. This formula is derived from the Stokes–Einstein relation and is only useful in this form in the case of two spherical particles of identical radius that react every time they approach a distance R, which is equal to the sum of their two radii. The more general expression for the diffusion limited rate constant is
Where and are the radii of the two molecules and is an approach distance at which unity reaction efficiency is expected (this is an approximation).
In reality, only a fraction of the collisions with the quencher are effective at quenching, so the true quenching rate coefficient must be determined experimentally. | 7 | Physical Chemistry |
SOCl is highly reactive and can violently release hydrochloric acid upon contact with water and alcohols. It is also a controlled substance under the Chemical Weapons Convention, where it is listed as a Schedule 3 substance, since it is used in the manufacture of G-series nerve agents and the Meyer and Meyer–Clarke methods of producing sulfur-based mustard gases. | 0 | Organic Chemistry |
Research International produces and sells the TacBio particle detection system. The system samples the air continuously and monitors for aerosol particle or bio-particle levels
It does not identify the type of bio-particle detected but will notify the mailroom attendant if either excess particles or bio-particles are present. This is an early warning system that gives the mailroom supervisor enough time to notice a potential threat and to test for biological agents or airborne threats before mail is delivered to a recipient. | 3 | Analytical Chemistry |
The name lactone derives from the ring compound called lactide, which is formed from the dehydration of 2-hydroxypropanoic acid (lactic acid) CH-CH(OH)-COOH. Lactic acid, in turn, derives its name from its original isolation from soured milk (Latin: lac, lactis). The name was coined in 1844 by the French chemist Théophile-Jules Pelouze, who first obtained it as a derivative of lactic acid. An internal dehydration reaction within the same molecule of lactic acid would have produced alpha-propiolactone, a lactone with a 3-membered ring.
In 1880 the German chemist Wilhelm Rudolph Fittig extended the name "lactone" to all intramolecular carboxylic esters. | 0 | Organic Chemistry |
A comparison between a pulsed and non-pulsed dc current electrolysers was explored in 1993 by Shaaban, that demonstrated a non-pulsed current used the least electrical power. This opposes the previous and future works conducted.
The experimental electrolyser separated the anolyte and catholyte compartments and used a 324-Naflon membrane to allow the ion exchange. The distance between the anode, made with platinum coated titanium, and the cathode, stainless steel, was 3mm and was immersed in a 10 weight percent sulfuric acid electrolyte. He conducted tests under several different frequencies that included 0.01 Hz, 0.5 kHz, 5 kHz, i kHz, 10 kHz, 25 kHz, and 40 kHz and with four duty cycles, 10, 25, 50, and 80%.
Initial observations revealed that the off-period resulted in a reversal in polarity, causing the reaction to reverse. This effected the cathode, which displayed a 2g loss after experimentation. A diode was input into the circuit to rectify the polarity. However, the cell was prevented from dropping to 0 V during the off-period, maintaining a higher value of 2.3V. This further impacted the experiment, distorting the square wave produced by the function generator Shaaban used, as the electrical potential provided needed to overcome the cell voltage of 2.3V before current could flow. Bokris et al. records that current would continue to flow, discharging ions from the EDL, but this was contradicted in this experiment. This only occurred when the diode was in place but it prevented a current spike in the duty cycle as well.
With a 10% duty cycle at a 1 kHz pulse, temperature increases of nearly 7 °C greater than in the non-pulsed experimental electrolysis, were found. Temperature increases can prevent the circuit
Calculating the power consumption, it was determined a non-pulsed current had power demand losses of 3.5%, and a pulsed current resulted in 13 - 16% losses. It also opposes the idea from Bockris et al. that the effectiveness of non-pulsed dc current electrolysis increases by a factor of 2 when a pulsed current is applied. | 7 | Physical Chemistry |
Alternatively, the general statement can be formulated in terms of the total number of electrons using simple rules of divisibility by a straightforward analysis of two cases.
First, consider the case where the total number of electrons is 4n + 2:
:4n + 2 = a(4q + 2) + b(4p + 2) + c(4t) + d(4r),
where a, b, c, and d are coefficients indicating the number of each type of component. This equation implies that one of, but not both, a or b is odd, for if a and b are both even or both odd, then the sum of the four terms is 0 (mod 4).
The generalized statement of the Woodward–Hoffmann rules states that a + d is odd if the reaction is allowed. Now, if a is even, then this implies that d is odd. Since b is odd in this case, the number of antarafacial components, b + d, is even. Likewise, if a is odd, then d is even. Since b even in this case, the number of antarafacial components, b + d, is again even. Thus, regardless of the initial assumption of parity for a and b, the number of antarafacial components is even when the electron count is 4n + 2. Contrariwise,, b + d is odd.
In the case where the total number of electrons is 4n, similar arguments (omitted here) lead to the conclusion that the number of antarafacial components b + d must be odd in the allowed case and even in the forbidden case.
Finally, to complete the argument, and show that this new criterion is truly equivalent to the original criterion, one needs to argue the converse statements as well, namely, that the number of antarafacial components b + d and the electron count (4n + 2 or 4n) implies the parity of a + d that is given by the Woodward–Hoffmann rules (odd for allowed, even for forbidden). Another round of (somewhat tedious) case analyses will easily show this to be the case. The pericyclic selection rule states:
In this formulation, the electron count refers to the entire reacting system, rather than to individual components, as enumerated in Woodward and Hoffmann's original statement. In practice, an even or odd number of antarafacial components usually means zero or one antarafacial components, respectively, as transition states involving two or more antarafacial components are typically disfavored by strain. As exceptions, certain intramolecular reactions may be geometrically constrained in such a way that enforces an antarafacial trajectory for multiple components. In addition, in some cases, e.g., the Cope rearrangement, the same (not necessarily strained) transition state geometry can be considered to contain two supra or two antara π components, depending on how one draws the connections between orbital lobes. (This ambiguity is a consequence of the convention that overlap of either both interior or both exterior lobes of a σ component can be considered to be suprafacial.)
This alternative formulation makes the equivalence of the Woodward–Hoffmann rules to the Dewar–Zimmerman analysis (see below) clear. An even total number of phase inversions is equivalent to an even number of antarafacial components and corresponds to Hückel topology, requiring 4n + 2 electrons for aromaticity, while an odd total number of phase inversions is equivalent to an odd number of antarafacial components and corresponds to Möbius topology, requiring 4n electrons for aromaticity. To summarize aromatic transition state theory: Thermal pericyclic reactions proceed via (4n + 2)-electron Hückel or (4n)-electron Möbius transition states.
As a mnemonic, the above formulation can be further restated as the following: | 7 | Physical Chemistry |
In supramolecular chemistry, host–guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host–guest chemistry encompasses the idea of molecular recognition and interactions through non-covalent bonding. Non-covalent bonding is critical in maintaining the 3D structure of large molecules, such as proteins and is involved in many biological processes in which large molecules bind specifically but transiently to one another.
Although non-covalent interactions could be roughly divided into those with more electrostatic or dispersive contributions, there are few commonly mentioned types of non-covalent interactions: ionic bonding, hydrogen bonding, van der Waals forces and hydrophobic interactions.
Host-guest interaction has raised dramatical attention since it was discovered. It is an important field, because many biological processes require the host-guest interaction, and it can be useful in some material designs. There are several typical host molecules, such as, cyclodextrin, crown ether, et al. | 6 | Supramolecular Chemistry |
Some acetylides are notoriously explosive. Formation of acetylides poses a risk in handling of gaseous acetylene in presence of metals such as mercury, silver or copper, or alloys with their high content (brass, bronze, silver solder). | 0 | Organic Chemistry |
According to the adsorption behavior, the adsorption of gases on solids can be classified into three temperature ranges relative to :
1.Subcritical region (T)
2.Near-critical region (+10)
3. The region T>+10
Isotherms in the first region will show the feature of subcritical adsorption. Isotherms in the second region will show the feature of mechanism transition. Isotherms in the third region will show the feature of supercritical adsorption. The transition will take a continuous way if the isotherms in both sides of the critical temperature belong to the same type, such as adsorption on microporous activated carbon. However, discontinuous transition could be observed on isotherms in the second region if there is a transformation of isotherm types, such as adsorption on mesoporous silica gel. The decisive factor in such a classification of adsorption is merely temperature, irrespective of pressure. This is because a fluid cannot undergo a transition to a liquid phase at above-critical temperature, regardless of the pressure applied. This fundamental law determines the different adsorption mechanism for the subcritical and supercritical regions. For the subcritical region, the highest equilibrium pressure of adsorption is the saturation pressure of adsorbate. Beyond condensation happens. Adsorbate in the adsorbed phase is largely in liquid state, based on which different adsorption and thermodynamic theories as well as their applications were developed. For supercritical region, condensation cannot happen, no matter how great the pressure is. | 7 | Physical Chemistry |
For many workers in the field, a reactor is an item which is too expensive; instead, it is common to use a neutron source which uses a combination of an alpha emitter and beryllium. These sources tend to be much weaker than reactors. | 3 | Analytical Chemistry |
Alexander Zaytsev first published his observations regarding the products of elimination reactions in Justus Liebigs Annalen der Chemie in 1875. Although the paper contained some original research done by Zaytsevs students, it was largely a literature review and drew heavily upon previously published work. In it, Zaytsev proposed a purely empirical rule for predicting the favored regiochemistry in the dehydrohalogenation of alkyl iodides, though it turns out that the rule is applicable to a variety of other elimination reactions as well. While Zaytsevs paper was well referenced throughout the 20th century, it was not until the 1960s that textbooks began using the term "Zaytsev's rule".
Zaytsev was not the first chemist to publish the rule that now bears his name. Aleksandr Nikolaevich Popov published an empirical rule similar to Zaytsevs in 1872, and presented his findings at the University of Kazan in 1873. Zaytsev had cited Popovs 1872 paper in previous work and worked at the University of Kazan, and was thus probably aware of Popovs proposed rule. In spite of this, Zaytsevs 1875 Liebigs Annalen paper makes no mention of Popov's work.
Any discussion of Zaytsevs rule would be incomplete without mentioning Vladimir Vasilyevich Markovnikov. Zaytsev and Markovnikov both studied under Alexander Butlerov, taught at the University of Kazan during the same period, and were bitter rivals. Markovnikov, who published in 1870 what is now known as Markovnikovs rule, and Zaytsev held conflicting views regarding elimination reactions: the former believed that the least substituted alkene would be favored, whereas the latter felt the most substituted alkene would be the major product. Perhaps one of the main reasons Zaytsev began investigating elimination reactions was to disprove his rival. Zaytsev published his rule for elimination reactions just after Markovnikov published the first article in a three-part series in Comptes Rendus detailing his rule for addition reactions. | 7 | Physical Chemistry |
The reverse Krebs cycle (also known as the reverse tricarboxylic acid cycle, the reverse TCA cycle, or the reverse citric acid cycle, or the reductive tricarboxylic acid cycle, or the reductive TCA cycle)
is a sequence of chemical reactions that are used by some bacteria to produce carbon compounds from carbon dioxide and water by the use of energy-rich reducing agents as electron donors.
The reaction is the citric acid cycle run in reverse. Where the Krebs cycle takes carbohydrates and oxidizes them to CO and water, the reverse cycle takes CO and HO to make carbon compounds.
This process is used by some bacteria (such as Aquificota) to synthesize carbon compounds, sometimes using hydrogen, sulfide, or thiosulfate as electron donors. This process can be seen as an alternative to the fixation of inorganic carbon in the reductive pentose phosphate cycle which occurs in a wide variety of microbes and higher organisms. | 1 | Biochemistry |
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