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Tropoelastin is a protein, of size 72kDa, that comes together via cross-links to form elastin in the extracellular matrix of the cell. The cross-link formation process is mediated by lysyl oxidase. One of the major reasons that elastin can withstand high levels of stress in the body without experiencing any physical deformation is that the underlying tropoelastin contains domains that are highly hydrophobic. These hydrophobic domains, consisting overwhelmingly of alanine, proline, glycine, and valine, tend towards instability and disorderliness, ensuring that the elastin does not lock into any specific confirmation. Thus, ELPs consisting of the Val-Pro-Gly-X-Gly monomeric units, which bear resemblance to the repetitive tropoelastin hydrophobic domains, are highly disordered below their T Even above their T in their aggregated state, ELPs are only partially ordered. This is due to the fact that the proline and glycine amino acids are present in high amounts in the ELP. Glycine, due to the lack of a bulky side chain, enables the biopolymer to be flexible and proline prevents the formation of stable hydrogen bonds in the ELP backbone. It is important to note, however, that certain segments of the ELP may be able to form instantaneous type II β turns, but these turns are not long-lasting and do not resemble true β sheets, when the NMR chemical shifts are compared. | 7 | Physical Chemistry |
There are other forms of metastable iron carbides that have been identified in tempered steel and in the industrial Fischer–Tropsch process. These include epsilon (ε) carbide, hexagonal close-packed FeC, precipitates in plain-carbon steels of carbon content > 0.2%, tempered at 100–200 °C. Non-stoichiometric ε-carbide dissolves above ~200 °C, where Hägg carbides and cementite begin to form. Hägg carbide, monoclinic FeC, precipitates in hardened tool steels tempered at 200–300 °C. It has also been found naturally as the mineral Edscottite in the Wedderburn meteorite | 8 | Metallurgy |
In an HCP crystal such as graphite, the two coordinates include the origin and the next plane up the c axis located at c/2, and hence , which gives us
From this it is convenient to define dummy variable , and from there consider the modulus squared so hence
This leads us to the following conditions for the structure factor: | 3 | Analytical Chemistry |
Iron, usually as Fe is a common constituent of river waters at very low levels. Higher iron concentrations in acidic springs or an anoxic hyporheic zone may cause visible orange/brown staining or semi-gelatinous precipitates of dense orange iron bacterial floc carpeting the river bed. Such conditions are very deleterious to most organisms and can cause serious damage in a river system.
Coal mining is also a very significant source of Iron both in mine-waters and from stocking yards of coal and from coal processing. Long abandoned mines can be a highly intractable source of high concentrations of Iron. Low levels of iron are common in spring waters emanating from deep-seated aquifers and maybe regarding as health giving springs. Such springs are commonly called Chalybeate springs and have given rise to a number of Spa towns in Europe and the United States. | 2 | Environmental Chemistry |
Unlike the majority of ACE inhibitors, captopril is not administered as a prodrug (the only other being lisinopril). About 70% of orally administered captopril is absorbed. Bioavailability is reduced by presence of food in stomach. It is partly metabolised and partly excreted unchanged in urine. Captopril also has a relatively poor pharmacokinetic profile. The short half-life necessitates dosing two or three times per day, which may reduce patient compliance. Captopril has a short half-life of 2–3 hours and a duration of action of 12–24 hours. | 4 | Stereochemistry |
The fixation of by RuBisCO is a multi-step process. First, a molecule (that is not the molecule that is eventually fixed) attaches to the uncharged ε-amino group of lysine 201 in the active site to form a carbamate. This carbamate then binds to the magnesium ion (Mg) in RuBisCOs active site. A molecule of RuBP then binds to the Mg ion. The bound RuBP then loses a proton to form a reactive, enodiolate species. The rate-limiting step of the Calvin-Benson cycle is the addition of CO to this 2,3-enediol form of RuBP. This is the stage where the intrinsic KIE of Rubisco occurs because a new C-C bond is formed. The newly formed 2-carboxy-3-keto-D-arabinitol 1,5-bisphosphate molecule is then hydrated and cleaved to form two molecules of 3-phosphoglycerate (3 PGA). 3 PGA is then converted into hexoses to be used in the photosynthetic organisms central metabolism.
The isotopic substitutions that can occur in this reaction are for carbon, oxygen, and/or hydrogen, though currently only a significant isotope effect is seen for carbon isotope substitution. Isotopes are atoms that have the same number of protons but varying numbers of neutrons. "Lighter" isotopes (like the stable carbon-12 isotope) have a smaller overall mass, and "heavier" isotopes (like the stable carbon-13 isotope or radioactive carbon-14 isotope) have a larger overall mass. Stable isotope geochemistry is concerned with how varying chemical and physical processes preferentially enrich or deplete stable isotopes. Enzymes like RuBisCO cause isotopic fractionation because molecules containing lighter isotopes have higher zero-point energies (ZPE), the lowest possible quantum energy state for a given molecular arrangement. For this reaction, CO has a lower ZPE than CO and sits lower in the potential energy well of the reactants. When enzymes catalyze chemical reactions, the lighter isotope is preferentially selected because it has a lower activation energy and is thus more energetically favorable to overcome the high potential-energy transition state and proceed through the reaction. Here, CO has a lower activation energy so more CO than CO goes through the reaction, resulting in the product (3 PGA) being lighter. | 7 | Physical Chemistry |
Except for fluorine that can only form unstable polyfluorides at low temperature, all other stable halogens (Cl, Br, I) can form several isopolyhalogen anions that are stable at room temperature, of which the most prominent example being triiodide. In all these anions, the halogen atoms of the same element bond to each other. | 0 | Organic Chemistry |
AdoMet is a methyl donor for transmethylation. It gives away its methyl group and is also the propylamino donor in polyamine biosynthesis. S-adenosylmethionine synthesis can be considered the rate-limiting step of the methionine cycle.
As a methyl donor SAM allows DNA methylation. Once DNA is methylated, it switches the genes off and therefore, S-adenosylmethionine can be considered to control gene expression.
SAM is also involved in gene transcription, cell proliferation, and production of secondary metabolites. Hence SAM synthetase is fast becoming a drug target, in particular for the following diseases: depression, dementia, vacuolar myelopathy, liver injury, migraine, osteoarthritis, and as a potential cancer chemopreventive agent.
This article discusses the protein domains that make up the SAM synthetase enzyme and how these domains contribute to its function. More specifically, this article explores the shared pseudo-3-fold symmetry that makes the domains well-adapted to their functions.
This enzyme catalyses the following chemical reaction
: ATP + L-methionine + HO phosphate + diphosphate + S-adenosyl-L-methionine | 1 | Biochemistry |
The iron pillar of Dhar was originally longer than the iron pillar of Delhi. After the Muslim conquest of Dhar, it broke into at least two pieces. The smaller piece was planted at the Dilawar Khan's Mosque in Mandu. The larger piece was erected in front of the Lat Masjid constructed by Dilawar Khan in 1405.
In 1531 CE, Dhar came under the control of Bahadur Shah, the Sultan of Gujarat. He made an attempt to carry the large piece to Gujarat. In this process, this part of the pillar toppled and fragmented into two pieces.
Now, three fragments of the pillar are placed horizontally on a platform near Lat Masjid. These fragments were moved by ASI to their present position in 1980. The combined height of the three fragments is , and their total weight is estimated at . Thus, the original pillar must have been almost twice as high and at least heavier than the iron pillar of Delhi. At the time of its erection, it was probably the tallest and the largest forge-welded iron pillar in the world. | 8 | Metallurgy |
Unlike typical diffusion, anomalous diffusion is described by a power law, where is the so-called generalized diffusion coefficient and is the elapsed time. The classes of anomalous diffusions are classified as follows:
* α < 1: subdiffusion. This can happen due to crowding or walls. For example, a random walker in a crowded room, or in a maze, is able to move as usual for small random steps, but cannot take large random steps, creating subdiffusion. This appears for example in protein diffusion within cells, or diffusion through porous media. Subdiffusion has been proposed as a measure of macromolecular crowding in the cytoplasm.
* α = 1: Brownian motion.
* : superdiffusion. Superdiffusion can be the result of active cellular transport processes or due to jumps with a heavy-tail distribution.
* α = 2: ballistic motion. The prototypical example is a particle moving at constant velocity: .
* : hyperballistic. It has been observed in optical systems.
In 1926, using weather balloons, Lewis Fry Richardson demonstrated that the atmosphere exhibits super-diffusion. In a bounded system, the mixing length (which determines the scale of dominant mixing motions) is given by the Von Kármán constant according to the equation , where is the mixing length, is the Von Kármán constant, and is the distance to the nearest boundary. Because the scale of motions in the atmosphere is not limited, as in rivers or the subsurface, a plume continues to experience larger mixing motions as it increases in size, which also increases its diffusivity, resulting in super-diffusion. | 7 | Physical Chemistry |
Eukaryotic DNA with a typical length of dozens of centimeters should be orderly packed to be readily accessible inside the micrometer-size nucleus. In most eukaryotes, DNA is arranged in the cell nucleus with the help of histones. In this case, the basic level of DNA compaction is the nucleosome, where the double helix is wrapped around the histone octamer containing two copies of each histone H2A, H2B, H3 and H4. Linker histone H1 binds the DNA between nucleosomes and facilitates packaging of the 10 nm "beads on the string" nucleosomal chain into a more condensed 30 nm fiber. Most of the time, between cell divisions, chromatin is optimized to allow easy access of transcription factors to active genes, which are characterized by a less compact structure called euchromatin, and to alleviate protein access in more tightly packed regions called heterochromatin. During the cell division, chromatin compaction increases even more to form chromosomes, which can cope with large mechanical forces dragging them into each of the two daughter cells. Many aspects of transcription are controlled by chemical modification on the histone proteins, known as the histone code.
Chromosome scaffold has important role to hold the chromatin into compact chromosome. Chromosome scaffold is made of proteins including condensin, topoisomerase IIα and kinesin family member 4 (KIF4)
Dinoflagellates are very divergent eukaryotes in terms of how they package their DNA. Their chromosomes are packed in a liquid-crystalline state. They have lost many of the conserved histone genes, using mostly dinoflagellate viral nucleoproteins (DVNPs) or bacteria-derived dinoflagellate histone-like proteins (HLPs) for packaging instead. It is unknown how they control access to genes; those do retain histone have a special histone code. | 1 | Biochemistry |
Specific volume is the inverse of mass concentration only in the case of pure substances, for which mass concentration is the same as the density of the pure-substance: | 3 | Analytical Chemistry |
One of the most important features of chemical synapses is that they are the site of action for the majority of psychoactive drugs. Synapses are affected by drugs, such as curare, strychnine, cocaine, morphine, alcohol, LSD, and countless others. These drugs have different effects on synaptic function, and often are restricted to synapses that use a specific neurotransmitter. For example, curare is a poison that stops acetylcholine from depolarizing the postsynaptic membrane, causing paralysis. Strychnine blocks the inhibitory effects of the neurotransmitter glycine, which causes the body to pick up and react to weaker and previously ignored stimuli, resulting in uncontrollable muscle spasms. Morphine acts on synapses that use endorphin neurotransmitters, and alcohol increases the inhibitory effects of the neurotransmitter GABA. LSD interferes with synapses that use the neurotransmitter serotonin. Cocaine blocks reuptake of dopamine and therefore increases its effects. | 1 | Biochemistry |
The process itself is regulated through both positive and negative termination factors, usually through modification of the hairpin structure. This is accomplished through interactions with single stranded RNA that corresponds to the upstream area of the loop, resulting in disruption of the termination process. Furthermore, there is some implication that the nut site may also contribute to regulation, as it is involved in recruitment of some critical components in the formation of the hairpin. | 1 | Biochemistry |
Samples are dissolved or suspended in a "cocktail" containing a solvent (historically aromatic organics such as xylene or toluene, but more recently less hazardous solvents are used), typically some form of a surfactant, and "fluors" or scintillators which produce the light measured by the detector. Scintillators can be divided into primary and secondary phosphors, differing in their luminescence properties.
Beta particles emitted from the isotopic sample transfer energy to the solvent molecules: the π cloud of the aromatic ring absorbs the energy of the emitted particle. The energized solvent molecules typically transfer the captured energy back and forth with other solvent molecules until the energy is finally transferred to a primary scintillator. The primary phosphor will emit photons following absorption of the transferred energy. Because that light emission may be at a wavelength that does not allow efficient detection, many cocktails contain secondary phosphors that absorb the fluorescence energy of the primary phosphor and re-emit at a longer wavelength. Two widely used primary and secondary
fluors are 2,5-diphenyloxazole (PPO) with an emission maximum of 380 nm and 1,4-bis-2-(5-phenyloxazolyl)benzene (POPOP) with an emission maximum of 420 nm.
The radioactive samples and cocktail are placed in small transparent or translucent (often glass or plastic) vials that are loaded into an instrument known as a liquid scintillation counter. Newer machines may use 96-well plates with individual filters in each well. Many counters have two photo multiplier tubes connected in a coincidence circuit. The coincidence circuit assures that genuine light pulses, which reach both photomultiplier tubes, are counted, while spurious pulses (due to line noise, for example), which would only affect one of the tubes, are ignored.
Counting efficiencies under ideal conditions range from about 30% for tritium (a low-energy beta emitter) to nearly 100% for phosphorus-32, a high-energy beta emitter. Some chemical compounds (notably chlorine compounds) and highly colored samples can interfere with the counting process. This interference, known as "quenching", can be overcome through data correction or through careful sample preparation. | 5 | Photochemistry |
Slow or deep carbon cycling is an important process, though it is not as well-understood as the relatively fast carbon movement through the atmosphere, terrestrial biosphere, ocean, and geosphere. The deep carbon cycle is intimately connected to the movement of carbon in the Earth's surface and atmosphere. If the process did not exist, carbon would remain in the atmosphere, where it would accumulate to extremely high levels over long periods of time. Therefore, by allowing carbon to return to the Earth, the deep carbon cycle plays a critical role in maintaining the terrestrial conditions necessary for life to exist.
Furthermore, the process is also significant simply due to the massive quantities of carbon it transports through the planet. In fact, studying the composition of basaltic magma and measuring carbon dioxide flux out of volcanoes reveals that the amount of carbon in the mantle is actually greater than that on the Earths surface by a factor of one thousand. Drilling down and physically observing deep-Earth carbon processes is evidently extremely difficult, as the lower mantle and core extend from 660 to 2,891 km and 2,891 to 6,371 km deep into the Earth respectively. Accordingly, not much is conclusively known regarding the role of carbon in the deep Earth. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the elements movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of said layer. Furthermore, techniques like seismology have led to a greater understanding of the potential presence of carbon in the Earth's core. | 5 | Photochemistry |
People can reduce the number of mosquito bites they receive (to a greater or lesser degree) by:
* Using a mosquito net
* Wearing long clothing that covers the skin and is tucked in to seal up holes
* Avoiding the outdoors during dawn and dusk, when mosquitos are most active
* Keeping air moving to prevent mosquitos from landing, such as by using a fan
* Wearing light-colored clothing (light objects are harder for mosquitos to detect)
* Reducing exercise, which reduces output of carbon dioxide used by mosquitos for detection | 1 | Biochemistry |
The Orbiting Carbon Observatory (OCO) is a NASA satellite mission intended to provide global space-based observations of atmospheric carbon dioxide (). The original spacecraft was lost in a launch failure on 24 February 2009, when the payload fairing of the Taurus rocket which was carrying it failed to separate during ascent. The added mass of the fairing prevented the satellite from reaching orbit. It subsequently re-entered the atmosphere and crashed into the Indian Ocean near Antarctica. The replacement satellite, Orbiting Carbon Observatory-2, was launched 2 July 2014 aboard a Delta II rocket. The Orbiting Carbon Observatory-3, a stand-alone payload built from the spare OCO-2 flight instrument, was installed on the International Space Station Kibō Exposed Facility in May 2019. | 2 | Environmental Chemistry |
Germicidal lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. If the quartz envelope transmits shorter wavelengths, such as the 185 nm mercury emission line, they can also be used wherever ozone is desired, for example, in the sanitizing systems of hot tubs and aquariums. They are also used by geologists to provoke fluorescence in mineral samples, aiding in their identification. In this application, the light produced by the lamp is usually filtered to remove as much visible light as possible, leaving just the UV light. Germicidal lamps are also used in waste water treatment in order to kill microorganisms.
The light produced by germicidal lamps is also used to erase EPROMs; the ultraviolet photons are sufficiently energetic to allow the electrons trapped on the transistors' floating gates to tunnel through the gate insulation, eventually removing the stored charge that represents binary ones and zeroes. | 5 | Photochemistry |
Artificial upwelling or downwelling is an approach that would change the mixing layers of the ocean. Encouraging various ocean layers to mix can move nutrients and dissolved gases around, offering avenues for geoengineering. Mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient rich water to the surface, triggering blooms of algae, which store carbon when they grow and export carbon when they die. This produces results somewhat similar to iron fertilization. One side-effect is a short-term rise in , which limits its attractiveness.
Mixing layers involve transporting the denser and colder deep ocean water to the surface mixed layer. As the ocean temperature decreases with depth, more carbon dioxide and other compounds are able to dissolve in the deeper layers. This can be induced by reversing the oceanic carbon cycle through the use of large vertical pipes serving as ocean pumps, or a mixer array. When the nutrient rich deep ocean water is moved to the surface, algae bloom occurs, resulting in a decrease in carbon dioxide due to carbon intake from phytoplankton and other photosynthetic eukaryotic organisms. The transfer of heat between the layers will also cause seawater from the mixed layer to sink and absorb more carbon dioxide. This method has not gained much traction as algae bloom harms marine ecosystems by blocking sunlight and releasing harmful toxins into the ocean. The sudden increase in carbon dioxide on the surface level will also temporarily decrease the pH of the seawater, impairing the growth of coral reefs. The production of carbonic acid through the dissolution of carbon dioxide in seawater hinders marine biogenic calcification and causes major disruptions to the oceanic food chain. | 5 | Photochemistry |
These clusters consist of at least two different (semi)metallic elements, and possess more direct metal-metal than metal-ligand contacts. The suffix "oid" designate that such clusters possess at a molecular scale, atom arrangements that appear in bulk intermetallic compounds with high coordination numbers of the atoms, such as for example in Laves phase and Hume-Rothery phases. Ligand-free intermetalloid clusters include also endohedrally filled Zintl clusters. A synonym for ligand-stabilized intermetalloid clusters is "molecular alloy". The clusters appear as discrete units in intermetallic compounds separated from each other by electropositive atoms such as [Sn@Cu@Sn], as soluble ions [As@Ni@As] or as ligand-stabilized molecules such as [Mo(ZnCH)(ZnCp*)]. | 7 | Physical Chemistry |
Bottromycin was first isolated from Streptomyces bottropensis in 1957. It has since been identified in at least two other members of the genus Streptomyces; members of Streptomyces are known to be prolific producers of secondary metabolites. Bottromycin has a unique structure, consisting of the macrocyclic amidine linkage and four β-methylated amino acids. Bottromycin blocks aminoacyl tRNA binding to the ribosome by binding to the A site of the 50s subunit. Although bottromycin was discovered over 50 years ago, there was a lack of research following initial studies on bottromycin until recent years. The lack of research is potentially a result of bottromycin's low stability in blood plasma. However, the unique structure and mode of action have recently made bottromycin a more target for drug development, especially given the rise of antibiotic resistance. | 0 | Organic Chemistry |
Oxygen is probably the most important chemical constituent of surface water chemistry, as all aerobic organisms require it for survival. It enters the water mostly via diffusion at the water-air interface. Oxygens solubility in water decreases as water temperature increases. Fast, turbulent streams expose more of the waters surface area to the air and tend to have low temperatures and thus more oxygen than slow, backwaters. Oxygen is a by-product of photosynthesis, so systems with a high abundance of aquatic algae and plants may also have high concentrations of oxygen during the day. These levels can decrease significantly during the night when primary producers switch to respiration. Oxygen can be limiting if circulation between the surface and deeper layers is poor, if the activity of animals is very high, or if there is a large amount of organic decay occurring such as following Autumn leaf-fall.
Most other atmospheric inputs come from man-made or anthropogenic sources the most significant of which are the oxides of sulphur produced by burning sulphur rich fuels such as coal and oil which give rise to acid rain. The chemistry of sulphur oxides is complex both in the atmosphere and in river systems. However the effect on the overall chemistry is simple in that it reduces the pH of the water making it more acidic. The pH change is most marked in rivers with very low concentrations of dissolved salts as these cannot buffer the effects of the acid input. Rivers downstream of major industrial conurbations are also at greatest risk. In parts of Scandinavia and West Wales and Scotland many rivers became so acidic from oxides of sulphur that most fish life was destroyed and pHs as low as pH4 were recorded during critical weather conditions. | 2 | Environmental Chemistry |
Blocking antibodies can be used in a variety of medical and scientific manners, thus far been to treat cancer, Graves' disease, and prevent the growth of malaria in mosquitoes. | 1 | Biochemistry |
Immunosequencing in its modern context started being discussed in scientific literature in the early 2010s with the advent of more powerful gene sequencing techniques. | 1 | Biochemistry |
Agostic interactions are best demonstrated by crystallography. Neutron diffraction data have shown that C−H and M┄H bond distances are 5-20% longer than expected for isolated metal hydride and hydrocarbons. The distance between the metal and the hydrogen is typically 1.8–2.3 Å, and the M┄H−C angle is in the range of 90°–140°. The presence of a H NMR signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to hydride ligands. The coupling constant J is typically lowered to 70–100 Hz versus the 125 Hz expected for a normal sp carbon–hydrogen bond. | 0 | Organic Chemistry |
Deflagrations are often used in engineering applications when the force of the expanding gas is used to move an object such as a projectile down a barrel, or a piston in an internal combustion engine. Deflagration systems and products can also be used in mining, demolition and stone quarrying via gas pressure blasting as a beneficial alternative to high explosives. | 7 | Physical Chemistry |
Classically, the kinetic energy of rotation is
:where
:: is the angular momentum
:: is the moment of inertia of the molecule
For microscopic, atomic-level systems like a molecule, angular momentum can only have specific discrete values given by
:where is a non-negative integer and is the reduced Planck constant.
Also, for a diatomic molecule the moment of inertia is
:where
:: is the reduced mass of the molecule and
:: is the average distance between the centers of the two atoms in the molecule.
So, substituting the angular momentum and moment of inertia into E, the rotational energy levels of a diatomic molecule are given by: | 4 | Stereochemistry |
Directional solidification uses a thermal gradient to promote nucleation of metal grains on a low temperature surface, as well as to promote their growth along the temperature gradient. This leads to grains elongated along the temperature gradient, and significantly greater creep resistance parallel to the long grain direction. In polycrystalline turbine blades, directional solidification is used to orient the grains parallel to the centripetal force. It is also known as dendritic solidification. | 8 | Metallurgy |
Dichlorodifluoromethane (R-12) is a colorless gas usually sold under the brand name Freon-12, and a chlorofluorocarbon halomethane (CFC) used as a refrigerant and aerosol spray propellant. In compliance with the Montreal Protocol, its manufacture was banned in developed countries (non-article 5 countries) in 1996, and in developing countries (Article 5 countries) in 2010 out of concerns about its damaging effect on the ozone layer. Its only allowed usage is as a fire retardant in submarines and aircraft. It is soluble in many organic solvents. R-12 cylinders are colored white. | 2 | Environmental Chemistry |
The earliest example of a [4+4] photocycloaddition was the photodimerization of anthracene, discovered in 1936. This reaction has remained one of the most reliable photocycloadditions and is the basis of continuing investigation.
Other polycyclic aromatics undergo anthracene-like photocycloaddition, such as acridizinium salts. Biologically active natural products include pleuromutilin, kalmanol, crenulide, and longipenol.
[4+4] photocycloaddition approaches to the creation of cyclooctanoid systems offer several appealing features, including convergent assembly of the eight-membered ring from two relatively simpler four-carbon fragments, formation of two new carbon-carbon bonds, and the potential for introduction of up to four new stereocenters in one step. | 5 | Photochemistry |
Lipid rafts are loosely defined as clusters of cholesterol and saturated lipids forming regions of lipid heterogeneity in cellular membranes (e.g., the ganglioside GM1). The association of proteins to lipid rafts is cholesterol dependent and regulates the proteins' function (e.g., substrate presentation). | 1 | Biochemistry |
Another group suggests that primitive cells might have formed inside inorganic clay microcompartments, which can provide an ideal container for the synthesis and compartmentalization of complex organic molecules. Clay-armored bubbles form naturally when particles of montmorillonite clay collect on the outer surface of air bubbles under water. This creates a semi permeable vesicle from materials that are readily available in the environment. The authors remark that montmorillonite is known to serve as a chemical catalyst, encouraging lipids to form membranes and single nucleotides to join into strands of RNA. Primitive reproduction can be envisioned when the clay bubbles burst, releasing the lipid membrane-bound product into the surrounding medium. | 9 | Geochemistry |
In biochemistry, ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) is a chemical compound used to observe the reaction kinetics of specific enzymes. A common use for it is in the enzyme-linked immunosorbent assay (ELISA) to detect the binding of molecules to each other.
It is commonly used as a substrate with hydrogen peroxide for a peroxidase enzyme (such as horseradish peroxidase) or alone with blue multicopper oxidase enzymes (such as laccase or bilirubin oxidase). Its use allows the reaction kinetics of peroxidases themselves to be followed. In this way it also can be used to indirectly follow the reaction kinetics of any hydrogen peroxide-producing enzyme, or to simply quantify the amount of hydrogen peroxide in a sample.
The formal reduction potentials for ABTS are high enough for it to act as an electron donor for the reduction of oxo species such as molecular oxygen and hydrogen peroxide, particularly at the less-extreme pH values encountered in biological catalysis. Under these conditions, the sulfonate groups are fully deprotonated and the mediator exists as a dianion.
:ABTS + e → ABTS E° = 0.67 V vs SHE
:ABTS + e → ABTS E° = 1.08 V vs SHE
This compound is chosen because the enzyme facilitates the reaction with hydrogen peroxide, turning it into a green and soluble end-product. Its new absorbance maximum of 420 nm light (ε = 3.6 × 10 M cm) can easily be followed with a spectrophotometer, a common laboratory instrument.
It is sometimes used as part of a glucose estimating reagent when finding glucose concentrations of solutions such as blood serum.
ABTS is also frequently used by the food industry and agricultural researchers to measure the antioxidant capacities of foods. In this assay, ABTS is converted to its radical cation by addition of sodium persulfate. This radical cation is blue in color and absorbs light at 415, 645, 734 and 815 nm. The ABTS radical cation is reactive towards most antioxidants including phenolics, thiols and Vitamin C. During this reaction, the blue ABTS radical cation is converted back to its colorless neutral form. The reaction may be monitored spectrophotometrically. This assay is often referred to as the Trolox equivalent antioxidant capacity (TEAC) assay. The reactivity of the various antioxidants tested are compared to that of Trolox, which is a water-soluble analog of vitamin E. | 1 | Biochemistry |
Glycosides are also classified according to the chemical nature of the aglycone. For purposes of biochemistry and pharmacology, this is the most useful classification. | 0 | Organic Chemistry |
Traditional methods of measuring CDOM include UV-visible spectroscopy (absorbance) and fluorometry (fluorescence). Optical proxies have been developed to characterize sources and properties of CDOM, including specific ultraviolet absorbance at 254 nm (SUVA) and spectral slopes for absorbance, and the fluorescence index (FI), biological index (BIX), and humification index (HIX) for fluorescence. Excitation emission matrices (EEMs) can be resolved into components in a technique called parallel factor analysis (PARAFAC), where each component is often labelled as "humic-like", "protein-like", etc. As mentioned above, remote sensing is the newest technique to detect CDOM from space. | 0 | Organic Chemistry |
Microarrays can be manufactured in different ways, depending on the number of probes under examination, costs, customization requirements, and the type of scientific question being asked. Arrays from commercial vendors may have as few as 10 probes or as many as 5 million or more micrometre-scale probes. | 1 | Biochemistry |
Two monosaccharides with equivalent molecular graphs (same chain length and same carbonyl position) may still be distinct stereoisomers, whose molecules differ in spatial orientation. This happens only if the molecule contains a stereogenic center, specifically a carbon atom that is chiral (connected to four distinct molecular sub-structures). Those four bonds can have any of two configurations in space distinguished by their handedness. In a simple open-chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and (in ketoses) the carbon with the keto group.
For example, the triketose H(CHOH)(C=O)(CHOH)H (glycerone, dihydroxyacetone) has no stereogenic center, and therefore exists as a single stereoisomer. The other triose, the aldose H(C=O)(CHOH)H (glyceraldehyde), has one chiral carbon—the central one, number 2—which is bonded to groups −H, −OH, −C(OH)H, and −(C=O)H. Therefore, it exists as two stereoisomers whose molecules are mirror images of each other (like a left and a right glove). Monosaccharides with four or more carbons may contain multiple chiral carbons, so they typically have more than two stereoisomers. The number of distinct stereoisomers with the same diagram is bounded by 2, where c is the total number of chiral carbons.
The Fischer projection is a systematic way of drawing the skeletal formula of an acyclic monosaccharide so that the handedness of each chiral carbon is well specified. Each stereoisomer of a simple open-chain monosaccharide can be identified by the positions (right or left) in the Fischer diagram of the chiral hydroxyls (the hydroxyls attached to the chiral carbons).
Most stereoisomers are themselves chiral (distinct from their mirror images). In the Fischer projection, two mirror-image isomers differ by having the positions of all chiral hydroxyls reversed right-to-left. Mirror-image isomers are chemically identical in non-chiral environments, but usually have very different biochemical properties and occurrences in nature.
While most stereoisomers can be arranged in pairs of mirror-image forms, there are some non-chiral stereoisomers that are identical to their mirror images, in spite of having chiral centers. This happens whenever the molecular graph is symmetrical, as in the 3-ketopentoses H(CHOH)(CO)(CHOH)H, and the two halves are mirror images of each other. In that case, mirroring is equivalent to a half-turn rotation. For this reason, there are only three distinct 3-ketopentose stereoisomers, even though the molecule has two chiral carbons.
Distinct stereoisomers that are not mirror-images of each other usually have different chemical properties, even in non-chiral environments. Therefore, each mirror pair and each non-chiral stereoisomer may be given a specific monosaccharide name. For example, there are 16 distinct aldohexose stereoisomers, but the name "glucose" means a specific pair of mirror-image aldohexoses. In the Fischer projection, one of the two glucose isomers has the hydroxyl at left on C3, and at right on C4 and C5; while the other isomer has the reversed pattern. These specific monosaccharide names have conventional three-letter abbreviations, like "Glu" for glucose and "Thr" for threose.
Generally, a monosaccharide with n asymmetrical carbons has 2 stereoisomers. The number of open chain stereoisomers for an aldose monosaccharide is larger by one than that of a ketose monosaccharide of the same length. Every ketose will have 2 stereoisomers where n > 2 is the number of carbons. Every aldose will have 2 stereoisomers where n > 2 is the number of carbons.
These are also referred to as epimers which have the different arrangement of −OH and −H groups at the asymmetric or chiral carbon atoms (this does not apply to those carbons having the carbonyl functional group). | 0 | Organic Chemistry |
Although fluorescent dyes may not have the same sensitivity as radioactive probes, they are able to show real-time activity of molecules in action. Moreover, radiation and appropriate handling is no longer a concern.
With the development of fluorescent tagging, fluorescence microscopy has allowed the visualization of specific proteins in both fixed and live cell images. Localization of specific proteins has led to important concepts in cellular biology such as the functions of distinct groups of proteins in cellular membranes and organelles. In live cell imaging, fluorescent tags enable movements of proteins and their interactions to be monitored.
Latest advances in methods involving fluorescent tags have led to the visualization of mRNA and its localization within various organisms. Live cell imaging of RNA can be achieved by introducing synthesized RNA that is chemically coupled with a fluorescent tag into living cells by microinjection. This technique was used to show how the oskar mRNA in the Drosophila embryo localizes to the posterior region of the oocyte. | 1 | Biochemistry |
The original commercial AFM-IR instruments required most samples to be thicker than 50 nm to achieve sufficient sensitivity. Sensitivity improvements were achieved using specialized cantilever probes with an internal resonator and by wavelet based signal processing techniques. Sensitivity was further improved by Lu et al. by using quantum cascade laser (QCL) sources. The high repetition rate of the QCL allows absorbed infrared light to continuously excite the AFM tip at a "contact resonance" of the AFM cantilever. This resonance-enhanced AFM-IR, in combination with electric field enhancement from metallic tips and substrates led to the demonstration of AFM-IR spectroscopy and compositional imaging of films as thin as single self-assembled monolayers. AFM-IR has also been integrated with other sources including a picosecond OPO offering a tuning range 1.55 μm to 16 μm (from 6450 cm to 625 cm).
In its initial development, with samples deposited on transparent prisms and using OPO laser sources, the sensitivity of AFM-IR was limited to a minimal thickness of the sample of circa 50-100 nm as mentioned above. The advent of quantum cascade lasers (QCL) and the use of the electromagnetic field enhancement between metallic probes and substrates have improved the sensitivity and spatial resolution of AFM-IR down to the measurement of large (>0.3 μm) and flat (~2–10 nm) self-assembled monolayers, where still hundreds of molecules are present. Ruggeri et al. have recently developed off-resonance, low power and short pulse AFM-IR (ORS-nanoIR) to prove the acquisition of infrared absorption spectra and chemical maps at the single molecule level, in the case of macromolecular assemblies and large protein molecules with a spatial resolution of ca. 10 nm. | 3 | Analytical Chemistry |
Squalene is a biochemical precursor to both steroids and hopanoids. For sterols, the squalene conversion begins with oxidation (via squalene monooxygenase) of one of its terminal double bonds, resulting in 2,3-oxidosqualene. It then undergoes an enzyme-catalysed cyclisation to produce lanosterol, which can be elaborated into other steroids such as cholesterol and ergosterol in a multistep process by the removal of three methyl groups, the reduction of one double bond by NADPH and the migration of the other double bond. In many plants, this is then converted into stigmasterol, while in many fungi, it is the precursor to ergosterol.
The biosynthetic pathway is found in many bacteria, and most eukaryotes, though has not been found in Archaea. | 1 | Biochemistry |
Prebiotically plausible lactones, such as ε-caprolactone and δ-valerolactone, have been shown to oligomerize without the usage of catalysts forming oligomers that may have been relevant during the origin of life. | 0 | Organic Chemistry |
Dexlansoprazole is used to heal and maintain healing of erosive esophagitis and to treat heartburn associated with gastroesophageal reflux disease (GERD). It lasts longer than lansoprazole, to which it is chemically related, and needs to be taken less often. There is no good evidence that it works better than other PPIs. | 4 | Stereochemistry |
The coding region can be modified in order to regulate gene expression.
Alkylation is one form of regulation of the coding region. The gene that would have been transcribed can be silenced by targeting a specific sequence. The bases in this sequence would be blocked using alkyl groups, which create the silencing effect.
While the regulation of gene expression manages the abundance of RNA or protein made in a cell, the regulation of these mechanisms can be controlled by a regulatory sequence found before the open reading frame begins in a strand of DNA. The regulatory sequence will then determine the location and time that expression will occur for a protein coding region.
RNA splicing ultimately determines what part of the sequence becomes translated and expressed, and this process involves cutting out introns and putting together exons. Where the RNA spliceosome cuts, however, is guided by the recognition of splice sites, in particular the 5' splicing site, which is one of the substrates for the first step in splicing. The coding regions are within the exons, which become covalently joined together to form the mature messenger RNA. | 1 | Biochemistry |
In dehydrohalogenation reactions, the halogen and an adjacent proton are removed from halocarbons, thus forming an alkene. For example, with bromoethane and sodium hydroxide (NaOH) in ethanol, the hydroxide ion HO abstracts a hydrogen atom. A Bromide ion is then lost, resulting in ethene, HO and NaBr. Thus, haloalkanes can be converted to alkenes. Similarly, dihaloalkanes can be converted to alkynes.
In related reactions, 1,2-dibromocompounds are debrominated by zinc dust to give alkenes and geminal dihalides can react with strong bases to give carbenes. | 0 | Organic Chemistry |
The EFI is developing an integrated sequence-structure based strategy for functional assignment by predicting the substrate specificities of unknown members of mechanistically diverse enzyme superfamilies. The approach leverages conserved features within a given superfamily such as known chemistry, identity of active site functional groups, and composition of specificity-determining residues, motifs, or structures to predict function but relies on multidisciplinary expertise to streamline, refine, and test the predictions. The integrated sequence-strategy under development will be generally applicable to deciphering the ligand specificities of any functionally unknown protein. | 1 | Biochemistry |
β-Hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the β-carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty (or vacant) site cis to the alkyl group for this reaction to occur. Moreover, for facile cleavage of the C–H bond, a d electron pair is needed for donation into the σ* orbital of the C–H bond. Thus, d metals alkyls are generally more stable to β-hydride elimination than d and higher metal alkyls and may form isolable agostic complexes, even if an empty coordination site is available.
The β-hydride elimination can either be a vital step in a reaction or an unproductive side reaction. The Shell higher olefin process relies on β-hydride elimination to produce α-olefins which are used to produce detergents. Illustrative of a sometimes undesirable β-hydride elimination, β-hydride elimination in Ziegler–Natta polymerization results in polymers of decreased molecular weight. In the case of nickel- and palladium-catalyzed couplings of aryl halides with alkyl Grignard reagents, the β-hydride elimination can lower the yield. The production of branched polymers from ethylene relies on chain walking, a key step of which is β-hydride elimination.
In some cases, β-hydride elimination is the first in a series of steps. For instance in the synthesis of RuHCl(CO)(PPh) from ruthenium trichloride, triphenylphosphine and 2-methoxyethanol, an intermediate alkoxide complex undergoes a β-hydride elimination to form the hydride ligand and the pi-bonded aldehyde which then is later converted into the carbonyl (carbon monoxide) ligand. | 0 | Organic Chemistry |
Smolková-Keulemansová became one of the leading experts in the field of chromatography. She was the first professor of chemistry in the Czech Republic and one of the first in Europe. Not only did she continue her studies in chemistry, but she also focused on polarography, a PhD focused on gas chromatography and a DrSc concentrated on inclusion compounds in chromatography. In the early 1970s, inclusion complex formations in selective analytical separations became a major focus of Smolková-Keulemansovás, her first choice being cyclodextrins, but moving on with urea and thiourea for the separation of isomers. Her research on cyclodextrins started soon after her methods focused on gas chromatography, high-performance liquid chromatography and electromigration. Her research became more widespread and she was asked to add many monographs on cyclodextrins, one of them being for a compendium on supramolecular chemistry edited by Jean-Marie Lehn. She has written and co-written 140 original papers and numerous reviews and has contributed to many books, including her work in Journal of High-Resolution Chromatography, "A Few Milestones on the Journey of Chromatography", and an article in the journal Chromatographia', "Study of retention of isomeric aromatic hydrocarbons on GTCB and cyclodextrins".
Smolková-Keulemansová died on 27 February 2024, at the age of 96. | 3 | Analytical Chemistry |
Conservative replacements in proteins often have a better effect on function than non-conservative replacements. The reduced effect of conservative replacements on function can also be seen in the occurrence of different replacements in nature. Non-conservative replacements between proteins are far more likely to be removed by natural selection due to their deleterious effects. | 1 | Biochemistry |
* 2014 Monomers, polymers and articles containing the same from sugar derived compounds
* 2018 Isosorbide-based polymethacrylates | 1 | Biochemistry |
In metallurgy, recovery is a process by which a metal or alloys deformed grains can reduce their stored energy by the removal or rearrangement of defects in their crystal structure. These defects, primarily dislocations, are introduced by plastic deformation of the material and act to increase the yield strength of a material. Since recovery reduces the dislocation density, the process is normally accompanied by a reduction in a materials strength and a simultaneous increase in the ductility. As a result, recovery may be considered beneficial or detrimental depending on the circumstances.
Recovery is related to the similar processes of recrystallization and grain growth, each of them being stages of annealing. Recovery competes with recrystallization, as both are driven by the stored energy, but is also thought to be a necessary prerequisite for the nucleation of recrystallized grains. It is so called because there is a recovery of the electrical conductivity due to a reduction in dislocations. This creates defect-free channels, giving electrons an increased mean free path. | 8 | Metallurgy |
The speed of sound in seawater is about 1,500 m/s (whereas the speed of sound is usually around 330 m/s in air at roughly 101.3 kPa pressure, 1 atmosphere), and varies with water temperature, salinity, and pressure. The thermal conductivity of seawater is 0.6 W/mK at 25 °C and a salinity of 35 g/kg.
The thermal conductivity decreases with increasing salinity and increases with increasing temperature. | 9 | Geochemistry |
In Kinematical theory an approximation is made that the electrons are only scattered once. For transmission electron diffraction it is common to assume a constant thickness , and also what is called the Column Approximation (e.g. references and further reading). For a perfect crystal the intensity for each diffraction spot is then:<math display="block">I_{g} = \left|\phi(\mathbf k)\right|^2 \propto \left|F_{g}\frac{\sin(\pi t s_z)}{\pi s_z}\right|^2
is the magnitude of the excitation error along z, the distance along the beam direction (z-axis by convention) from the diffraction spot to the Ewald sphere, and is the structure factor:the sum being over all the atoms in the unit cell with the form factors, the reciprocal lattice vector, is a simplified form of the Debye–Waller factor, and is the wavevector for the diffraction beam which is:for an incident wavevector of , as in Figure 6 and above. The excitation error comes in as the outgoing wavevector has to have the same modulus (i.e. energy) as the incoming wavevector . The intensity in transmission electron diffraction oscillates as a function of thickness, which can be confusing; there can similarly be intensity changes due to variations in orientation and also structural defects such as dislocations. If a diffraction spot is strong it could be because it has a larger structure factor, or it could be because the combination of thickness and excitation error is "right". Similarly the observed intensity can be small, even though the structure factor is large. This can complicate interpretation of the intensities. By comparison, these effects are much smaller in x-ray diffraction or neutron diffraction because they interact with matter far less and often Bragg's law is adequate.
This form is a reasonable first approximation which is qualitatively correct in many cases, but more accurate forms including multiple scattering (dynamical diffraction) of the electrons are needed to properly understand the intensities. | 7 | Physical Chemistry |
A Paul trap that uses an oscillating quadrupole field to trap ions radially and a static potential to confine ions axially. The quadrupole field is realized by four parallel electrodes laying in the -axis positioned at the corners of a square in the -plane. Electrodes diagonally opposite each other are connected and an a.c. voltage is applied. Using Maxwells equations, the electric field produced by this potential is electric field . Applying Newtons second law to an ion of charge and mass in this a.c. electric field, we can find the force on the ion using . We wind up with
Assuming that the ion has zero initial velocity, two successive integrations give the velocity and displacement as
where is a constant of integration. Thus, the ion oscillates with angular frequency and amplitude proportional to the electric field strength and is confined radially.
Working specifically with a linear Paul trap, we can write more specific equations of motion. Along the -axis, an analysis of the radial symmetry yields a potential
The constants and are determined by boundary conditions on the electrodes and satisfies Laplace's equation . Assuming the length of the electrodes is much greater than their separation , it can be shown that
Since the electric field is given by the gradient of the potential, we get that
Defining , the equations of motion in the -plane are a simplified form of the Mathieu equation, | 7 | Physical Chemistry |
Visual phototransduction in invertebrates like the fruit fly differs from that of vertebrates, described up to now. The primary basis of invertebrate phototransduction is the PI(4,5)P cycle. Here, light induces the conformational change into rhodopsin and converts it into meta-rhodopsin. This helps in dissociation of G-protein complex. Alpha sub-unit of this complex activates the PLC enzyme (PLC-beta) which hydrolyze the PIP2 into DAG. This hydrolysis leads to opening of TRP channels and influx of calcium.
Invertebrate photoreceptor cells differ morphologically and physiologically from their vertebrate counterparts. Visual stimulation in vertebrates causes a hyperpolarization (weakening) of the photoreceptor membrane potential, whereas invertebrates experience a depolarization with light intensity. Single-photon events produced under identical conditions in invertebrates differ from vertebrates in time course and size. Likewise, multi-photon events are longer than single-photon responses in invertebrates. However, in vertebrates, the multi-photon response is similar to the single-photon response. Both phyla have light adaptation and single-photon events are smaller and faster. Calcium plays an important role in this adaptation. Light adaptation in vertebrates is primarily attributable to calcium feedback, but in invertebrates cyclic AMP is another control on dark adaptation. | 1 | Biochemistry |
Version 1.0 of Rfam was launched in 2003 and contained 25 ncRNA families and annotated about 50 000 ncRNA genes. In 2005, version 6.1 was released and contained 379 families annotating over 280 000 genes. In August 2012, version 11.0 contained 2208 RNA families, while the current version (14.9, released in November 2022) annotates 4108 families. | 1 | Biochemistry |
Nitrates and nitrites are wastes produced by nitrifying bacteria, just as sulfur and sulfates are produced by the sulfur-reducing bacteria and sulfate-reducing bacteria. Insoluble iron waste can be made by iron bacteria by using soluble forms. In plants, resins, fats, waxes, and complex organic chemicals are exuded from plants, e.g., the latex from rubber trees and milkweeds. Solid waste products may be manufactured as organic pigments derived from breakdown of pigments like hemoglobin, and inorganic salts like carbonates, bicarbonates, and phosphate, whether in ionic or in molecular form, are excreted as solids.
Animals dispose of solid waste as feces. | 1 | Biochemistry |
Immunologic adjuvants are substances, administered in conjunction with a vaccine, that stimulate the immune system and increase the response to the vaccine. Squalene is not itself an adjuvant, but it has been used in conjunction with surfactants in certain adjuvant formulations.
An adjuvant using squalene is Seqirus proprietary MF59, which is added to influenza vaccines to help stimulate the human bodys immune response through production of CD4 memory cells. It is the first oil-in-water influenza vaccine adjuvant to be commercialised in combination with a seasonal influenza virus vaccine. It was developed in the 1990s by researchers at Ciba-Geigy and Chiron; both companies were subsequently acquired by Novartis. Novartis was later acquired by CSL Bering and created the company Seqirus. It is present in the form of an emulsion and is added to make the vaccine more immunogenic. However, the mechanism of action remains unknown. MF59 is capable of switching on a number of genes that partially overlap with those activated by other adjuvants. How these changes are triggered is unclear; to date, no receptors responding to MF59 have been identified. One possibility is that MF59 affects the cell behaviour by changing the lipid metabolism, namely by inducing accumulation of neutral lipids within the target cells. An influenza vaccine called [https://www.fluad.ca/patient/en/index.html?gclid=Cj0KCQiAzMGNBhCyARIsANpUkzPPnohR9VXZ2WLu-FjLHTTuYljtqZti7Qybbq-nr59zQ3gcvSBUwlwaAng1EALw_wcB FLUAD] which used MF59 as an adjuvant was approved for use in the US in people 65 years of age and older, beginning with the 2016–2017 flu season.
A 2009 meta-analysis assessed data from 64 clinical trials of influenza vaccines with the squalene-containing adjuvant MF59 and compared them to the effects of vaccines with no adjuvant. The analysis reported that the adjuvated vaccines were associated with slightly lower risks of chronic diseases, but that neither type of vaccines altered the rate of autoimmune diseases; the authors concluded that their data "supports the good safety profile associated with MF59-adjuvated influenza vaccines and suggests there may be a clinical benefit over non-MF59-containing vaccines". | 1 | Biochemistry |
The Bradford assay is a colorimetric assay that measures protein concentration. The reagent Coomassie brilliant blue turns blue when it binds to arginine and aromatic amino acids present in proteins, thus increasing the absorbance of the sample. The absorbance is measured using a spectrophotometer, at the maximum absorbance frequency (A) of the blue dye (which is 595 nm). In this case, the greater the absorbance, the higher the protein concentration.
Data for known concentrations of protein are used to make the standard curve, plotting concentration on the X axis, and the assay measurement on the Y axis. The same assay is then performed with samples of unknown concentration. To analyze the data, one locates the measurement on the Y-axis that corresponds to the assay measurement of the unknown substance and follows a line to intersect the standard curve. The corresponding value on the X-axis is the concentration of substance in the unknown sample. | 1 | Biochemistry |
In tissues, many different cell types interact with one another. In the brain, for example, neurons, astrocytes, and oligodendrocytes (specialized cells of the neural tissue, each with specific functions) interact with one another as well as with cells that comprise blood vessels. All these different cell types may interact with all others by the production of ligands that may activate receptors on the cell surface of other cell types. Understanding the way these different cell types interact with one another will allow to predict ways of activating eNSCs. For example, because eNSCs are found in close proximity with blood vessels, it has been hypothesized that signals (e.g., ligands) from cells comprising the blood vessel act on receptors found on the cell surface of eNSCs.
Endogenous neural stem cells are often in close physical proximity to blood vessels. Signals from blood vessels regulate their interaction with stem cells and contribute to the cytoarchitecture of the tissue. The STAT3-Ser/Hes3 signaling axis operating in Hes3+ cells is a convergence point for several of these signals (e.g. Delta4, Angiopoietin 2). Hes3, in turn, by regulating the expression of Shh and potentially other factors, can also exert an effect on blood vessels and other cells comprising their microenvironment. | 1 | Biochemistry |
*Chloride channels: This superfamily of channels consists of approximately 13 members. They include ClCs, CLICs, Bestrophins and CFTRs. These channels are non-selective for small anions; however chloride is the most abundant anion, and hence they are known as chloride channels.
*Potassium channels
**Voltage-gated potassium channels e.g., Kvs, Kirs etc.
**Calcium-activated potassium channels e.g., BKCa or MaxiK, SK, etc.
**Inward-rectifier potassium channels
**Two-pore-domain potassium channels: This family of 15 members form what is known as leak channels, and they display Goldman-Hodgkin-Katz (open) rectification.
*Sodium channels
** Voltage-gated sodium channels (NaVs)
** Epithelial sodium channels (ENaCs)
*Calcium channels (CaVs)
*Proton channels
**Voltage-gated proton channels
*Non-selective cation channels: These non-selectively allow many types of cations, mainly Na, K and Ca, through the channel.
**Most transient receptor potential channels | 1 | Biochemistry |
When conservation of number of particles holds true during scattering, it leads to a unitary condition for the scattering amplitude. In the general case, we have
Optical theorem follows from here by setting
In the centrally symmetric field, the unitary condition becomes
where and are the angles between and and some direction . This condition puts a constraint on the allowed form for , i.e., the real and imaginary part of the scattering amplitude are not independent in this case. For example, if in is known (say, from the measurement of the cross section), then can be determined such that is uniquely determined within the alternative . | 7 | Physical Chemistry |
FeMoco ( cofactor) is the primary cofactor of nitrogenase. Nitrogenase is the enzyme that catalyzes the conversion of atmospheric nitrogen molecules N into ammonia (NH) through the process known as nitrogen fixation. Studying FeMoco's role in the reaction mechanism for nitrogen fixation is a potential use case for quantum computers. Even limited quantum computers could enable better simulations of the reaction mechanism. Because it contains iron and molybdenum, the cofactor is called FeMoco. Its stoichiometry is FeMoSC. | 7 | Physical Chemistry |
Bipolar electrochemistry is a phenomenon in electrochemistry based on the polarization of conducting objects in electric fields. Indeed, this polarization generates a potential difference between the two extremities of the substrate that is equal to the electric field value multiplied by the size of the object. If this potential difference is important enough, then redox reactions can be generated at the extremities of the object, oxidations will occur at one extremity coupled simultaneously to reductions at the other extremity. In a simple experimental setup consisting of a platinum wire in a weighing boat containing a pH indicator solution, a 30 V voltage across two electrodes will cause water reduction at one end of the wire (the cathode) and a pH increase (OH formation) and water oxidation at the anodic end and a pH decrease. The poles of the bipolar electrode also align themselves with the applied electric field. | 7 | Physical Chemistry |
Only few naturally occurring compounds exhibit the isocyanide functionality. The first was discovered in 1957 in an extract of the mold Penicillium notatum. The compound xanthocillin later was used as an antibiotic. Since then numerous other isocyanides have been isolated. Most of the marine isocyanides are terpenoid, while some of the terrestrial isocyanides originate from α-aminoacids. | 0 | Organic Chemistry |
Between 1995 and 2004 in the United States, an average of 1560 cold-related emergency department visits occurred per year and in the years 1999 to 2004, an average of 647 people died per year due to hypothermia. Of deaths reported between 1999 and 2002 in the US, 49% of those affected were 65 years or older and two-thirds were male. Most deaths were not work related (63%) and 23% of affected people were at home. Hypothermia was most common during the autumn and winter months of October through March. In the United Kingdom, an estimated 300 deaths per year are due to hypothermia, whereas the annual incidence of hypothermia-related deaths in Canada is 8000. | 1 | Biochemistry |
Interferome offers many ways of searching and retrieving data from the database:
*Identify interferon regulated gene signatures in microarray data;
*Gene Ontology analysis and annotation;
*Normal tissue expression of interferon regulated genes;
*Regulatory analysis of interferon regulated genes;
*BLAST (Basic Local Alignment Search Tool) analysis and orthologue sequence download; | 1 | Biochemistry |
Cu is strongly cycled in the surface and deep ocean. In the deep ocean, Cu concentrations are ~5 nM in the Pacific and ~1.5 nM in the Atlantic. The deep/surface ratio of Cu in the ocean is typically <10, and vertical concentration profiles for Cu are roughly linear due to biological recycling and scavenging processes as well as adsorption to particles.
Due to equilibrium and biological processes that fractionate Cu isotopes in the marine environment, the bulk copper isotopic composition (δCu = +0.6 to +1.5‰) is different from the δCu values of the riverine input (δCu = +0.02 to +1.45‰, with discharge-weighted average δCu = +0.68‰) to the oceans. δCu values of the surface layers of FeMn-nodules are fairly homogenous throughout the oceans (average = 0.31‰), suggesting low biological demand for Cu in the marine environment compared to that of Fe or Zn. Additionally, δCu values in the Atlantic ocean do not markedly vary with depth, ranging from +0.56 to +0.72‰. However, Cu isotope compositions of material collected on sediment traps at depths of 1,000 and 2,500 m in the central Atlantic ocean show seasonal variation with heaviest δCu values in the spring and summer seasons suggesting seasonal preferential uptake of Cu by biological processes.
Equilibrium processes that fractionate Cu isotopes include high temperature ion exchange and redox speciation between mineral phases, and low temperature ion exchange between aqueous species or redox speciation between inorganic species. In riverine and marine environments, Cu/Cu ratios are driven by preferential adsorption of Cu to particulate matter and preferential binding of Cu to organic complexes. As a net result, ocean sediments tend to be depleted in Cu relative to the bulk ocean. For example, the downcore δCu values of a 760 cm sedimentary core taken from the Central Pacific ocean varied from -0.94 to -2.83‰, significantly lighter than the bulk ocean. | 9 | Geochemistry |
One application of molecular imprinting technology is in affinity-based separations for biomedical, environmental, and food analysis. Sample preconcentration and treatment can be carried out by removing targeted trace amounts of analytes in samples using MIPs. The feasibility of MIPs in solid-phase extraction, solid-phase microextraction, and stir bar sorption extraction has been studied in several publications. Moreover, chromatography techniques such as HPLC and TLC can make use of MIPs as packing materials and stationary phases for the separation of template analytes. The kinetics of noncovalently imprinted materials were observed to be faster than materials prepared by the covalent approach, so noncovalent MIPs are more commonly used in chromatography.
Another application is the use of molecularly imprinted materials as chemical and biological sensors. They have been developed to target herbicides, sugars, drugs, toxins, and vapors. MIP-based sensors not only have high selectivity and high sensitivity, but they can also generate output signals (electrochemical, optical, or piezoelectric) for detection. This allows them to be utilized in fluorescence sensing, electrochemical sensing, chemiluminescence sensing, and UV-Vis sensing. Forensic applications that delve into detections of illicit drugs, banned sport drugs, toxins, and chemical warfare agents are also an area of growing interest.
Molecular imprinting has steadily been emerging in fields like drug delivery and biotechnology. The selective interaction between template and polymer matrix can be utilized in preparation of artificial antibodies. In the biopharmaceutical market, separation of amino acids, chiral compounds, hemoglobin, and hormones can be achieved with MIP adsorbents. Methods to utilize molecular imprinting techniques for mimicking linear and polyanionic molecules, such as DNA, proteins, and carbohydrates have been researched. An area of challenges is protein imprinting. Large, water-soluble biological macromolecules have posed a difficulty for molecular imprinting because their conformational integrity cannot be ensured in synthetic environments. Current methods to navigate this include immobilizing template molecules at the surface of solid substrates, thereby minimizing aggregation and controlling the template molecules to locate at the surface of imprinted materials. However, a critical review of molecular imprinting of proteins by scientists from Utrecht University found that further testing is required.
Pharmaceutical applications include selective drug delivery and control drug release systems, which make use of MIPs’ stable conformations, fast equilibrium release, and resistance to enzymatic and chemical stress. Intelligent drug release, the release of a therapeutic agent as a result of a specific stimuli, has also been explored. Molecularly imprinted materials of insulin and other drugs at the nanoscale were shown to exhibit high adsorption capacity for their respective targets, showing huge potential for newfound drug delivery systems. In comparison with natural receptors, MIPs also have higher chemical and physical stability, easier availability, and lower cost. MIPs could especially be used for stabilization of proteins, particularly selective protection of proteins against denaturation from heat. | 6 | Supramolecular Chemistry |
Partial molar properties are useful because chemical mixtures are often maintained at constant temperature and pressure and under these conditions, the value of any extensive property can be obtained from its partial molar property. They are especially useful when considering specific properties of pure substances (that is, properties of one mole of pure substance) and properties of mixing (such as the heat of mixing or entropy of mixing). By definition, properties of mixing are related to those of the pure substances by:
Here denotes a pure substance, the mixing property, and corresponds to the specific property under consideration. From the definition of partial molar properties,
substitution yields:
So from knowledge of the partial molar properties, deviation of properties of mixing from single components can be calculated. | 7 | Physical Chemistry |
This form of corrosion occurs principally in metal alloys. The less noble metal of the alloy, is selectively leached from the alloy. Removal of zinc from brass is a more common example. | 8 | Metallurgy |
There were three EXPOSE experiments completed between 2008 and 2015: EXPOSE-E, EXPOSE-R and EXPOSE-R2.
The EXPOSE-E was launched on 7 February 2008 on board the Space Shuttle Atlantis and was mounted on the ISS European module Columbus to the European Technology Exposure Facility (EuTEF). EXPOSE-R was launched to the ISS on 26 November 2008 from Baikonur Cosmodrome in Kazakhstan on board of a Progress capsule and was mounted on the ISS Russian module Zevzda. EXPOSE-E provided accommodation in three exposure trays for a variety of astrobiological test samples that were exposed to selected space conditions: either to space vacuum, solar electromagnetic radiation at >110 nm and cosmic radiation (trays 1 and 3) or to simulated Martian surface conditions (tray 2). The different experiments consisted in exposing solid molecules, gas mixtures or biological samples to the solar ultraviolet (UV) radiation, cosmic rays, vacuum and temperature fluctuations of outer space as the ISS repeatedly passed between areas of direct sunlight and the cold darkness of Earth's shadow.
At the end of the exposition period, EXPOSE-E was brought back to the ground in September 2009 as part of the Space Shuttle Discovery mission STS-128. EXPOSE-R was brought back in 2011 by a Soyuz spacecraft. From the landing site in Kazakhstan, the trays were returned via Moscow and distributed to scientists for further analysis in their laboratories. EXPOSE-R2 was launched on 24 July 2014, exposure was finished in April 2015, and was returned to Earth in early 2016 where it is still undergoing analyses. | 1 | Biochemistry |
Monitoring of pesticide residues in the UK began in the 1950s. From 1977 to 2000 the work was carried out by the Working Party on Pesticide Residues (WPPR), until in 2000 the work was taken over by the Pesticide Residue Committee (PRC). The PRC advise the government through the Pesticides Safety Directorate and the Food Standards Agency (FSA). | 2 | Environmental Chemistry |
Barluenga was born in Tardienta (Huesca), Spain, where he spent his childhood and attended primary school.
He studied chemistry at the University of Zaragoza (B.Sc., 1963; Ph.D. 1966) with Professor V. Gómez Aranda. In 1967, he moved to Germany and, after a postdoctoral appointment at the Max-Planck Institut für Kohlenforschung in Mülheim an der Ruhr (1967–1970, Professor H. Hoberg), he returned to Spain to hold research positions at the Spanish Council for Scientific Research in Zaragoza (1970–1972) and University of Zaragoza (1972–1975). | 0 | Organic Chemistry |
1,2,3-Cyclohexatriene is an unstable chemical compound with the molecular formula . It is an unusual isomer of benzene in which the three double bonds are cumulated.
In 1990, 1,2,3-cyclohexatriene was first prepared by reacting a cyclohexadiene derivative with cesium fluoride. The product was too reactive to be isolated on its own, so its existence was confirmed by trapping via a cycloaddition reaction.
1,2,3-Cyclohexatriene and its derivatives undergo a variety of reactions including cycloadditions, nucleophilic additions, and σ-bond insertions, and therefore they can be versatile reagents for organic synthesis. | 4 | Stereochemistry |
Zeldovich mechanism is a chemical mechanism that describes the oxidation of nitrogen and NO formation, first proposed by the Russian physicist Yakov Borisovich Zeldovich in 1946. The reaction mechanisms read as
where and are the reaction rate constants in Arrhenius law. The overall global reaction is given by
The overall reaction rate is mostly governed by the first reaction (i.e., rate-determining reaction), since the second reaction is much faster than the first reaction and occurs immediately following the first reaction. At fuel-rich conditions, due to lack of oxygen, reaction 2 becomes weak, hence, a third reaction is included in the mechanism, also known as extended Zel'dovich mechanism (with all three reactions),
Assuming the initial concentration of NO is low and the reverse reactions can therefore be ignored, the forward rate constants of the reactions are given by
where the pre-exponential factor is measured in units of cm, mol, s and K (these units are incorrect), temperature in kelvins, and the activation energy in cal/mol; R is the universal gas constant. | 7 | Physical Chemistry |
Iron isotopes have been applied for a number of purposes on planetary bodies. Their variations have been measured to more precisely determine the processes that occurred during planetary accretion. In the future, the comparison of observed biological fractionation of iron on Earth to fractionation on other planetary bodies may have astrobiological implications. | 9 | Geochemistry |
In chemistry, ammonolysis (/am·mo·nol·y·sis/) is the process of splitting ammonia into . Ammonolysis reactions can be conducted with organic compounds to produce amines (molecules containing a nitrogen atom with a lone pair, :N), or with inorganic compounds to produce nitrides. This reaction is analogous to hydrolysis in which water molecules are split. Similar to water, liquid ammonia also undergoes auto-ionization, , where the rate constant is k = 1.9 × 10.
Organic compounds such as alkyl halides, hydroxyls (hydroxyl nitriles and carbohydrates), carbonyl (aldehydes/ketones/esters/alcohols), and sulfur (sulfonyl derivatives) can all undergo ammonolysis in liquid ammonia. | 1 | Biochemistry |
Pseudoephedrine (PSE) is a sympathomimetic drug of the phenethylamine and amphetamine chemical classes. It may be used as a nasal/sinus decongestant, as a stimulant, or as a wakefulness-promoting agent in higher doses.
It was first characterized in 1889 by the German chemists Ladenburg and Oelschlägel, who used a sample that had been isolated from Ephedra vulgaris by the Merck pharmaceutical corporation of Darmstadt, Germany. The salts pseudoephedrine hydrochloride and pseudoephedrine sulfate are found in over-the-counter preparations, either as a single ingredient or (more commonly) in a fixed-dose combination with one or more additional active ingredients such as antihistamines, guaifenesin, dextromethorphan, paracetamol (acetaminophen), or an NSAID (such as aspirin or ibuprofen). | 4 | Stereochemistry |
The methylene blue value is defined as the number of milliliter's standard methylene value solution decolorized 0.1 g of activated carbon (dry basis).
Methylene blue value reflects the amount of clay minerals in aggregate samples. In materials science, methylene blue solution is successively added to fine aggregate which is being agitated in water. The presence of free dye solution can be checked with stain test on a filter paper. | 3 | Analytical Chemistry |
It was concluded that bacterial deposition mainly occurred in a secondary energy minimum by using DLVO theory. DLVO calculation predicted an energy barrier of 140kT at 31.6 mM ionic strength to over 2000kT at 1mM ionic strength. This data was not in agreement with the experimental data, which showed increasing deposition with increasing ionic strength. Therefore, the deposit could occur at secondary minimum having the energy from 0.09kT to 8.1kT at 1mM and 31.6 mM ionic strength, respectively. The conclusion was further proven by the partial release of deposited bacteria when the ionic strength decreased. Because the amount of released bacteria was less than 100%, it was suggested that bacteria could deposit at the primary minimum due to the heterogeneity of the surface collector or bacterial surface. This fact was not covered in classical DLVO theory.<br />
The presence of divalent electrolytes (Ca) can neutralize the charge surface of bacteria by the binding between Ca and the functional group on the oocyst surface. This resulted in an observable bacterial deposition despite the very high electrostatic repulsive energy from the DLVO prediction.
The motility of bacteria also has a significant effect on the bacterial adhesion. Nonmotile and motile bacteria showed different behavior in deposition experiments. At the same ionic strength, motile bacteria showed greater adhesion to the surface than nonmotile bacteria and motile bacteria can attach to the surface of the collector at high repulsive electrostatic force. It was suggested that the swimming energy of the cells could overcome the repulsive energy or they can adhere to regions of heterogeneity on the surface. The swimming capacity increase with the ionic strength and 100mM is the optimal concentration for the rotation of flagella.
Despite the electrostatic repulsion energy from DLVO calculation between the bacteria and surface collector, the deposition could occur due to other interactions such as the steric impact of the presence of flagella on the cell environment and the strong hydrophobicity of the cell. | 7 | Physical Chemistry |
Solid material in wastewater may be dissolved, suspended, or settled. Total dissolved solids or TDS (sometimes called filterable residue) is measured as the mass of residue remaining when a measured volume of filtered water is evaporated. The mass of dried solids remaining on the filter is called total suspended solids (TSS) or nonfilterable residue. Settleable solids are measured as the visible volume accumulated at the bottom of an Imhoff cone after water has settled for one hour. Turbidity is a measure of the light scattering ability of suspended matter in the water. Salinity measures water density or conductivity changes caused by dissolved materials. | 3 | Analytical Chemistry |
* BS 1881:204 Testing concrete. Recommendations on the use of electromagnetic covermeters
* DGZfP:B2: Guideline “für Bewehrungsnachweis und Überdeckungsmessung bei Stahl- und Spannbeton”
* DIN 1045: Guideline Concrete, reinforced and prestressed concrete structures
* ACI Concrete Practices Non Destructive testing 228.2R-2.51: Covermeters | 8 | Metallurgy |
Like carbon, silicon can form four stable bonds with itself and other elements, and long chemical chains known as silane polymers, which are very similar to the hydrocarbons essential to life on Earth. Silicon is more reactive than carbon, which could make it optimal for extremely cold environments. However, silanes spontaneously burn in the presence of oxygen at relatively low temperatures, so an oxygen atmosphere may be deadly to silicon-based life. On the other hand, it is worth considering that alkanes are as a rule quite flammable, but carbon-based life on Earth does not store energy directly as alkanes, but as sugars, lipids, alcohols, and other hydrocarbon compounds with very different properties. Water as a solvent would also react with silanes, but again, this only matters if for some reason silanes are used or mass-produced by such organisms.
Silicon lacks an important property of carbon: single, double, and triple carbon-carbon bonds are all relatively stable. Aromatic carbon structures underpin DNA, which could not exist without this property of carbon. By comparison, compounds containing silene double bonds (such as silabenzene, an unstable analogue of benzene) exhibit far lower stability than the equivalent carbon compound. A pair of silane single bonds have significantly greater total enthalpy than a single silene double bond, so simple disilenes readily autopolymerise, and silicon favors the formation of linear chains of single bonds (see the double bond rule).
Hydrocarbons and organic compounds are abundant in meteorites, comets, and interstellar clouds, while their silicon analogs have never been observed in nature. Silicon does, however, form complex one-, two- and three-dimensional polymers in which oxygen atoms form bridges between silicon atoms. These are termed silicates. They are both stable and abundant under terrestrial conditions, and have been proposed as a basis for a pre-organic form of evolution on Earth (see clay hypothesis). | 1 | Biochemistry |
Alloying a metal is done by combining it with one or more other elements. The most common and oldest alloying process is performed by heating the base metal beyond its melting point and then dissolving the solutes into the molten liquid, which may be possible even if the melting point of the solute is far greater than that of the base. For example, in its liquid state, titanium is a very strong solvent capable of dissolving most metals and elements. In addition, it readily absorbs gases like oxygen and burns in the presence of nitrogen. This increases the chance of contamination from any contacting surface, and so must be melted in vacuum induction-heating and special, water-cooled, copper crucibles. However, some metals and solutes, such as iron and carbon, have very high melting-points and were impossible for ancient people to melt. Thus, alloying (in particular, interstitial alloying) may also be performed with one or more constituents in a gaseous state, such as found in a blast furnace to make pig iron (liquid-gas), nitriding, carbonitriding or other forms of case hardening (solid-gas), or the cementation process used to make blister steel (solid-gas). It may also be done with one, more, or all of the constituents in the solid state, such as found in ancient methods of pattern welding (solid-solid), shear steel (solid-solid), or crucible steel production (solid-liquid), mixing the elements via solid-state diffusion.
By adding another element to a metal, differences in the size of the atoms create internal stresses in the lattice of the metallic crystals; stresses that often enhance its properties. For example, the combination of carbon with iron produces steel, which is stronger than iron, its primary element. The electrical and thermal conductivity of alloys is usually lower than that of the pure metals. The physical properties, such as density, reactivity, Young's modulus of an alloy may not differ greatly from those of its base element, but engineering properties such as tensile strength, ductility, and shear strength may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the atoms in the alloy, because larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element are present. For example, impurities in semiconducting ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.
Unlike pure metals, most alloys do not have a single melting point, but a melting range during which the material is a mixture of solid and liquid phases (a slush). The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. For many alloys there is a particular alloy proportion (in some cases more than one), called either a eutectic mixture or a peritectic composition, which gives the alloy a unique and low melting point, and no liquid/solid slush transition. | 8 | Metallurgy |
A dot blot is a special case of any of the above blots where the analyte is added directly to the blotting matrix (and appears as a "dot") as opposed to separating the sample by electrophoresis prior to blotting. | 1 | Biochemistry |
Homogeneity testing for a candidate reference material typically involves replicated measurements on multiple units or subsamples of the material.
Homogeneity tests for CRMs follow planned experimental designs. Because the experiment is intended to test for (or estimate the size of) variation in value between different CRM units, the designs are chosen to allow separation of variation in results due to random measurement error and variation due to differences between units of the CRM. Among the simplest designs recommended for this purpose is a simple balanced nested design (see schematic). Typically 10-30 CRM units are taken from the batch at random; stratified random sampling is recommended so that the selected units are spread across the batch. An equal number of subsamples (usually two or three) is then taken from each CRM unit and measured. Subsamples are measured in random order. Other designs, such as randomized block designs, have also been used for CRM certification.
Data processing for homogeneity tests usually involves a statistical significance test for evidence of differences between units of the candidate CRM. For the simple balanced design above, this typically uses an F test following ANOVA. A check for trends with production order is also recommended.
This approach is not taken in ISO Guide 35:2017; rather, emphasis is placed on deciding whether the between-unit standard deviation is sufficiently small for the intended end use. If statistical tests are used, however, the homogeneity experiment should be capable of detecting important heterogeneity, ISO Guide 35:2017 in turn requiring a sufficient combination of precision of the measurement procedure, number of RM units and number of replicates per unit. Statistical power calculations can assist in ensuring a sufficiently effective test .
In extreme cases, such as microanalysis, materials must be checked for homogeneity on sub-micron scales; this may involve much larger numbers of observations and adjustments to statistical analysis. | 3 | Analytical Chemistry |
In 2003, Paterson and co-workers reported a strategy that relies heavily on substrate-derived stereocontrol. Instead of a reagent-controlled aldol reaction in Paterson first-generation synthesis, a dicyclohexylboron-mediated anti-aldol was used to connect C(5)-C(6), which leads to a significant increase in diastereoselectivity from 4:1 to 92:8. Notably, Still–Gennari modified Horner–Wadsworth–Emmons reaction was also used to construct the C(13)-C(14) trisubstituted olefin in early stage of this synthesis. The Paterson second-generation synthesis of (+)-discodermolide has an overall yield of 7.8% with a longest linear sequence of 24 steps and 35 total steps.
In 2004, Paterson and his co-workers disclosed the third-generation total synthesis of (+)-discodermolide. The stepwise method used in previous generations to incorporate the C(1)–C(8) subunit was replaced by a late-stage Still-Gennari olefination, which leads to a notable improvement in convergence. The Paterson third-generation synthesis of (+)-discodermolide has an overall yield of 11.1% (highest reported to date) with a longest linear sequence of 21 steps and 37 total steps. | 0 | Organic Chemistry |
Evolution of microbial resistance to vancomycin is a growing problem, in particular, within healthcare facilities such as hospitals. While newer alternatives to vancomycin exist, such as linezolid (2000) and daptomycin (2003), the widespread use of vancomycin makes resistance to the drug a significant worry, especially for individual patients if resistant infections are not quickly identified and the patient continues the ineffective treatment. Vancomycin-resistant Enterococcus emerged in 1986. Vancomycin resistance evolved in more common pathogenic organisms during the 1990s and 2000s, including vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA). Agricultural use of avoparcin, another similar glycopeptide antibiotic, may have contributed to the evolution of vancomycin-resistant organisms.
One mechanism of resistance to vancomycin involves the alteration to the terminal amino acid residues of the NAM/NAG-peptide subunits, under normal conditions, -alanyl--alanine, to which vancomycin binds. The -alanyl--lactate variation results in the loss of one hydrogen-bonding interaction (4, as opposed to 5 for -alanyl--alanine) possible between vancomycin and the peptide. This loss of just one point of interaction results in a 1000-fold decrease in affinity. The -alanyl--serine variation causes a six-fold loss of affinity between vancomycin and the peptide, likely due to steric hindrance.
In enterococci, this modification appears to be due to the expression of an enzyme that alters the terminal residue. Three main resistance variants have been characterised to date among resistant Enterococcus faecium and E. faecalis populations:
* VanA - enterococcal resistance to vancomycin and teicoplanin; inducible on exposure to these agents
* VanB - lower-level enterococcal resistance; inducible by vancomycin, but strains may remain susceptible to teicoplanin
* VanC - least clinically important; enterococci resistant only to vancomycin; constitutive resistance
Variant of vancomycin has been tested that binds to the resistant D-lactic acid variation in vancomycin-resistant bacterial cell walls, and also binds well to the original target (vancomycin-susceptible bacteria). | 0 | Organic Chemistry |
The veterinary doctor Maurice Crowther Hall (1881-1938) discovered in 1921 that carbon tetrachloride was incredibly effective as an anthelminthic in eradicating hookworm via ingestion. In one of the clinical trials of carbon tetrachloride, it was tested on criminals to determine its safety for use in human beings. Beginning in 1922, capsules of pure carbon tetrachloride were marketed by Merck under the name Necatorina (variants include Neo-necatorina and Necatorine). Necatorina was used as a medication against parasitic diseases in humans. This medication was most prevalently used in Latin American countries. Its toxicity was not well understood at the time and toxic effects were attributed to impurities in the capsules rather than carbon tetrachloride itself. Due to carbon tetrachloride's toxicity, tetrachloroethylene (which was also investigated by Hall in 1925) replaced its use as an anthelmintic by the 1940s. | 2 | Environmental Chemistry |
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams. Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier, phototube and image intensifier. | 7 | Physical Chemistry |
In the case of esterification with acetic anhydrides the currently accepted mechanism involves three steps. First, DMAP and acetic anhydride react in a pre-equilibrium reaction to form an ion pair of acetate and the acetylpyridinium ion. In the second step the alcohol adds to the acetylpyridinium, and elimination of pyridine forms an ester. Here the acetate acts as a base to remove the proton from the alcohol as it nucleophilically adds to the activated acylpyridinium. The bond from the acetyl group to the catalyst gets cleaved to generate the catalyst and the ester. The described bond formation and breaking process runs synchronous concerted without the appearance of a tetrahedral intermediate. The acetic acid formed will then protonate the DMAP. In the last step of the catalytic cycle the auxiliary base (usually triethylamine or pyridine) deprotonates the protonated DMAP, reforming the catalyst. The reaction runs through the described nucleophilic reaction pathway irrespective of the anhydride used, but the mechanism changes with the pKa value of the alcohol used. For example, the reaction runs through a base-catalyzed reaction pathway in the case of a phenol. In this case, DMAP acts as a base and deprotonates the phenol, and the resulting phenolate ion adds to the anhydride. | 0 | Organic Chemistry |
The interior of cells is a crowded environment. For example, an Escherichia coli cell is only about 2 micrometres (μm) long and 0.5 μm in diameter, with a cell volume of 0.6 - 0.7 μm. However, E. coli can contain up to 4,288 different types of proteins, and about 1,000 of these types are produced at a high enough level to be easily detected. Added to this mix are various forms of RNA and the cells DNA chromosome, giving a total concentration of macromolecules of between 300 and 400 mg/ml. In eukaryotes the cells interior is further crowded by the protein filaments that make up the cytoskeleton, this meshwork divides the cytosol into a network of narrow pores.
These high concentrations of macromolecules occupy a large proportion of the volume of the cell, which reduces the volume of solvent that is available for other macromolecules. This excluded volume effect increases the effective concentration of macromolecules (increasing their chemical activity), which in turn alters the rates and equilibrium constants of their reactions. In particular this effect alters dissociation constants by favoring the association of macromolecules, such as when multiple proteins come together to form protein complexes, or when DNA-binding proteins bind to their targets in the genome. Crowding may also affect enzyme reactions involving small molecules if the reaction involves a large change in the shape of the enzyme.
The size of the crowding effect depends on both the molecular mass and shape of the molecule involved, although mass seems to be the major factor – with the effect being stronger with larger molecules. Notably, the size of the effect is non-linear, so macromolecules are much more strongly affected than are small molecules such as amino acids or simple sugars. Macromolecular crowding is therefore an effect exerted by large molecules on the properties of other large molecules. | 7 | Physical Chemistry |
Operando spectroscopy is an analytical methodology wherein the spectroscopic characterization of materials undergoing reaction is coupled simultaneously with measurement of catalytic activity and selectivity. The primary concern of this methodology is to establish structure-reactivity/selectivity relationships of catalysts and thereby yield information about mechanisms. Other uses include those in engineering improvements to existing catalytic materials and processes and in developing new ones. | 7 | Physical Chemistry |
In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.
The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to concentrate the valuable minerals, or hard rock mining, which pulverizes the rock containing the ore and then relies on chemical reactions to concentrate the sought-after material. In the latter, the extraction of minerals from ore requires comminution, i.e., grinding the ore into fine particles to facilitate extraction of the target element(s). Because of this comminution, tailings consist of a slurry of fine particles, ranging from the size of a grain of sand to a few micrometres. Mine tailings are usually produced from the mill in slurry form, which is a mixture of fine mineral particles and water.
Tailings are likely to be dangerous sources of toxic chemicals such as heavy metals, sulfides and radioactive content. These chemicals are especially dangerous when stored in water in ponds behind tailings dams. These ponds are also vulnerable to major breaches or leaks from the dams, causing environmental disasters, such as the Mount Polley disaster in British Columbia. Because of these and other environmental concerns such as groundwater leakage, toxic emissions and bird death, tailing piles and ponds have received more scrutiny, especially in first world countries, but the first UN-level standard for tailing management was only established 2020.
There are a wide range of methods for recovering economic value, containing or otherwise mitigating the impacts of tailings. However, internationally, these practices are poor, sometimes violating human rights. | 8 | Metallurgy |
The development of chemoenzymatic strategies to generate large libraries of non-native sugar nucleotides has enabled a process referred to as glycorandomization where these sugar nucleotide libraries serve as donors for permissive glycosyltransferases to afford differential glycosylation of a wide range of pharmaceuticals and complex natural product-based leads. | 0 | Organic Chemistry |
FRELEX was developed in 2016 by [https://neoaptamers.com/ NeoVentures Biotechnology Inc] to allow the selection of aptamers without immobilizing the target or the oligonucleotide library. Immobilization is a necessary component of SELEX; however, it has the potential to inhibit key epitopes, and thus weaken the likelihood of successful binding, particularly when working with small molecules. FRELEX follows a similar overall methodology to SELEX; however, instead of immobilizing the target, the researcher introduces a series of random and blocker oligonucleotides to an immobilization field before introduction to the target. This allows the researcher to better target small molecules that may be lost during partitioning. It also can be used in some circumstances to select an aptamer library without knowing the target.
Most modern aptamer selection methods strive to improve the conventional SELEX aptamer search method. Despite the publication of various methods aimed at increasing the affinity and specificity of aptamers, experimental approaches face limitations in the number and variety of sequences that can be examined and selected. Library capacity for SELEX experiments is practically limited to 10 candidates, whereas, assuming there is a 4-monomeric repertoire from which pools can be created, there are ~1.6 × 10 unique sequences in sequence space limited to a 100-residue matrix, which is clearly beyond experimental capabilities. The library of oligonucleotides must be extremely diverse and not contain linear, incapable of providing a stable spatial arrangement, and double-stranded structures; due to these limitations, oligonucleotide libraries can cover the diversity of only ~10 sequences. This means that existing aptamers may not fully cover the diversity of target molecules or may not have optimal properties due to limitations of the underlying method. To yield the best possible aptamers one must maximize the effectiveness of the discovery process and the library itself.
RNA and DNA secondary structure prediction by dynamic programming algorithms such as RNAfold (ViennaRNA) and by machine learning models such as SPOT-RNA, MXfold2 provides the opportunity to assess the ability of sequences in the primary library to fold into complex structures, allowing for the selection of only the most promising sequences from the entire pool. However, these algorithms are low-performance, making them poorly suited for this task. For this reason, algorithms like Ufold from the University of California and AliNA from Nanobiorobots Inc. have been developed, which demonstrate a significant increase in computational speed due to their faster architecture, and can be applied for preliminary in silico analysis of these libraries. | 1 | Biochemistry |
The term vaporization has also been used in a colloquial or hyperbolic way to refer to the physical destruction of an object that is exposed to intense heat or explosive force, where the object is actually blasted into small pieces rather than literally converted to gaseous form. Examples of this usage include the "vaporization" of the uninhabited Marshall Island of Elugelab in the 1952 Ivy Mike thermonuclear test. Many other examples can be found throughout the various MythBusters episodes that have involved explosives, chief among them being Cement Mix-Up, where they "vaporized" a cement truck with ANFO.
At the moment of a large enough meteor or comet impact, bolide detonation, a nuclear fission, thermonuclear fusion, or theoretical antimatter weapon detonation, a flux of so many gamma ray, x-ray, ultraviolet, visual light and heat photons strikes matter in a such brief amount of time (a great number of high-energy photons, many overlapping in the same physical space) that all molecules lose their atomic bonds and "fly apart". All atoms lose their electron shells and become positively charged ions, in turn emitting photons of a slightly lower energy than they had absorbed. All such matter becomes a gas of nuclei and electrons which rise into the air due to the extremely high temperature or bond to each other as they cool. The matter vaporized this way is immediately a plasma in a state of maximum entropy and this state steadily reduces via the factor of passing time due to natural processes in the biosphere and the effects of physics at normal temperatures and pressures.
A similar process occurs during ultrashort pulse laser ablation, where the high flux of incoming electromagnetic radiation strips the target material's surface of electrons, leaving positively charged atoms which undergo a coulomb explosion. | 7 | Physical Chemistry |
The word "titration" descends from the French word titrer (1543), meaning the proportion of gold or silver in coins or in works of gold or silver; i.e., a measure of fineness or purity. Tiltre became titre, which thus came to mean the "fineness of alloyed gold", and then the "concentration of a substance in a given sample". In 1828, the French chemist Joseph Louis Gay-Lussac first used titre as a verb (titrer), meaning "to determine the concentration of a substance in a given sample".
Volumetric analysis originated in late 18th-century France. François-Antoine-Henri Descroizilles (fr) developed the first burette (which was similar to a graduated cylinder) in 1791. Gay-Lussac developed an improved version of the burette that included a side arm, and invented the terms "pipette" and "burette" in an 1824 paper on the standardization of indigo solutions. The first true burette was invented in 1845 by the French chemist Étienne Ossian Henry (1798–1873). A major improvement of the method and popularization of volumetric analysis was due to Karl Friedrich Mohr, who redesigned the burette into a simple and convenient form, and who wrote the first textbook on the topic, Lehrbuch der chemisch-analytischen Titrirmethode (Textbook of analytical chemistry titration methods), published in 1855. | 3 | Analytical Chemistry |
In 1900 an endowment was made to found the Garton Lectureship in Indian and Colonial Agriculture at the University of Edinburgh. The Garton Lectureship still exists as a biennial award to promising young lecturers in the School of Agriculture but is without emoluments and is no longer tied to colonial agriculture. | 1 | Biochemistry |
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