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Passive daytime radiative cooling can be used in various power plant condensers, including thermoelectric power plants and concentrated solar plants (CSP) to cool water for effective use within the heat exchanger. A generalized study of "a covered pond with radiative cooler revealed that 150 W/m2 flux could be achieved without loss of water." PDRC application for power plant condensers can reduce high water use and thermal pollution caused by water cooling.
For a thermoelectric power plant condenser, one study found that supplementing the air-cooled condenser for radiative cooling panels "get a 4096 kWhth/day cooling effect with a pump energy consumption of 11 kWh/day." For a concentrated solar plant (CSP) "on the supercritical cycle at 550ᵒC can be improved in 5% net output over an air-cooled system by integration with 14 m2 /kWe capacity radiative cooler." | 7 | Physical Chemistry |
Phosphomevalonate kinase is an enzyme () in the mevalonate pathway that in humans is encoded by the PMVK gene. | 1 | Biochemistry |
There are two simple derivations of the equation that are commonly used to generate the hyperbolic curve. The first assumes photosynthetic rate increases with increasing light intensity until Pmax is reached and continues to photosynthesize at the maximum rate thereafter.
:: P = P[I] / (KI + [I])
:*P = photosynthetic rate at a given light intensity
:**Commonly denoted in units such as (mg C m-3 h-1) or (µg C µg Chl-a-1 h-1)
:*Pmax = the maximum potential photosynthetic rate per individual
:*[I] = a given light intensity
:**Commonly denoted in units such as (µMol photons m-2 s-1 or (Watts m-2 h-1)
:*KI = half-saturation constant; the light intensity at which the photosynthetic rate proceeds at ½ Pmax
:**Units reflect those used for [I]
Both Pmax and the initial slope of the curve, ΔP/ΔI, are species-specific, and are influenced by a variety of factors, such as nutrient concentration, temperature and the physiological capabilities of the individual. Light intensity is influenced by latitudinal position and undergo daily and seasonal fluxes which will also affect the overall photosynthetic capacity of the individual. These three parameters are predictable and can be used to predetermine the general PI curve a population should follow.
As can be seen in the graph, two species can have different responses to the same incremental changes in light intensity. Population A (in blue) has an initial rate higher than that of Population B (in red) and also exhibits a stronger rate change to increased light intensities at lower irradiance. Therefore, Population A will dominate in an environment with lower light availability. Although Population B has a slower photosynthetic response to increases in light intensity its Pmax is higher than that of Population A. This allows for eventual population dominance at greater light intensities. There are many determining factors influencing population success; using the PI curve to elicit predictions of rate flux to environmental changes is useful for monitoring phytoplankton bloom dynamics and ecosystem stability.
The second equation accounts for the phenomenon of photoinhibition. In the upper few meters of the ocean, phytoplankton may be subjected to irradiance levels that damage the chlorophyll-a pigment inside the cell, subsequently decreasing photosynthetic rate. The response curve depicts photoinhibition as a decrease in photosynthetic rate at light intensities stronger than those necessary for achievement of Pmax.
Terms not included in the above equation are:
:* βI = light intensity at the start of photoinhibition
:* αI = a given light intensity | 5 | Photochemistry |
Like many other databases that store protein association knowledge, STRING imports data from experimentally derived protein–protein interactions through literature curation. Furthermore, STRING also store computationally predicted interactions from: (i) text mining of scientific texts, (ii) interactions computed from genomic features, and
(iii) interactions transferred from model organisms based on orthology.
All predicted or imported interactions are benchmarked against a common reference of functional partnership as annotated by KEGG (Kyoto Encyclopedia of Genes and Genomes). | 1 | Biochemistry |
TSS of a water or wastewater sample is determined by pouring a carefully measured volume of water (typically one litre; but less if the particulate density is high, or as much as two or three litres for very clean water) through a pre-weighed filter of a specified pore size, then weighing the filter again after the drying process that removes all water on the filter. Filters for TSS measurements are typically composed of glass fibres. The gain in weight is a dry weight measure of the particulates present in the water sample expressed in units derived or calculated from the volume of water filtered (typically milligrams per litre or mg/L).
If the water contains an appreciable amount of dissolved substances (as certainly would be the case when measuring TSS in seawater), these will add to the weight of the filter as it is dried. Therefore, it is necessary to "wash" the filter and sample with deionized water after filtering the sample and before drying the filter. Failure to add this step is a fairly common mistake made by inexperienced laboratory technicians working with sea water samples, and will completely invalidate the results as the weight of salts left on the filter during drying can easily exceed that of the suspended particulate matter.
Although turbidity purports to measure approximately the same water quality property as TSS, the latter is preferred when available as it provides an actual weight of the particulate material present in the sample. In water quality monitoring situations, a series of more labor-intensive TSS measurements will be paired with relatively quick and easy turbidity measurements to develop a site-specific correlation. Once satisfactorily established, the correlation can be used to estimate TSS from more frequently made turbidity measurements, saving time and effort. Because turbidity readings are somewhat dependent on particle size, shape, and color, this approach requires calculating a correlation equation for each location. Further, situations or conditions that tend to suspend larger particles through water motion (e.g., increase in a stream current or wave action) can produce higher values of TSS not necessarily accompanied by a corresponding increase in turbidity. This is because particles above a certain size (essentially anything larger than silt) are not measured by a bench turbidity meter (they settle out before the reading is taken), but contribute substantially to the TSS value. | 3 | Analytical Chemistry |
A guanylate cyclase activator (or "GUCA") is one of group of proteins which upregulates guanylate cyclase. It is also known as guanylate cyclase-activating protein, with the abbreviation "GCAP". Mutations can be associated with vision defects.
There are five genes involved: | 1 | Biochemistry |
HIVE is a massively parallel distributed computing environment where the distributed storage library and the distributed computational powerhouse are linked seamlessly. The system is both robust and flexible due to maintaining both storage and the metadata database on the same network. The distributed storage layer of software is the key component for file and archive management and is the backbone for the deposition pipeline. The data deposition back-end allows automatic uploads and downloads of external datasets into HIVE data repositories. The metadata database can be used to maintain specific information about extremely large files ingested into the system (big data) as well as metadata related to computations run on the system. This metadata then allows details of a computational pipeline to be brought up easily in the future in order to validate or replicate experiments. Since the metadata is associated with the computation, it stores the parameters of any computation in the system eliminating manual record keeping.
Differentiating HIVE from other object oriented databases is that HIVE implements a set of unified APIs to search, view, and manipulate data of all types. The system also facilitates a highly secure hierarchical access control and permission system, allowing determination of data access privileges in a finely granular manner without creating a multiplicity of rules in the security subsystem. The security model, designed for sensitive data, provides comprehensive control and auditing functionality in compliance with HIVE's designation as a FISMA Moderate system. | 1 | Biochemistry |
Paritaprevir (previously known as ABT-450) is an acylsulfonamide inhibitor of the NS3-4A serine protease manufactured by Abbott Laboratories that shows promising results as a treatment of hepatitis C. When given in combination with ritonavir and ribavirin for 12 weeks, the rate of sustained virologic response at 24 weeks after treatment has been estimated to be 95% for those with hepatitis C virus genotype 1. Resistance to treatment with paritaprevir is uncommon, because it targets the binding site, but has been seen to arise due to mutations at positions 155 and 168 in NS3.
Paritaprevir was a component of Viekira Pak and Technivie. In May 2018, the FDA announced that Technivie and Viekira were to be discontinued. The discontinuation was voluntary and not related to the safety, quality, or efficacy of the medicine. It was estimated that both medications would be available until January 1, 2019. | 1 | Biochemistry |
For females:
* 0 to 5 years: < 0.1 mg/24 hours
* 6 to 9 years: < 0.3 mg/24 hours
* 10 to 15 years: 0.1 to 0.6 mg/24 hours
* 16 years and older: 0 to 1.4 mg/ 24 hours.
For males:
* 0 to 5 years: < 0.1 mg/24 hours
* 6 to 9 years: < 0.3 mg/24 hours
* 10 to 15 years: 0.2 to 0.6 mg/24 hours
* 16 years and older: 0.2 to 2 mg/ 24 hours | 1 | Biochemistry |
The dyes are immobilized on the column matrix effectively, since usually the dyes link to a monochlorotriazine or dichlorotriazine ring (triazine dye). This type of dyes works especially well on a support matrix with hydroxyl group. The commonly used supporting matrix would be cross-linked agarose (sepharose), sephadex, polyacrylamide, and silica.
An example for triazine linkage immobilization is Blue Sepharose, resulting from Cibacron blue FG3-A with monochlorotriazine covalently coupled with OH group of sepharose. This reaction form an ether linkage and also hydrogen chloride.
CHClNOS + CHO → CHNOS + HCl
Cibacron Blue FG3-A + Sepharose → Blue Sepharose + HCl | 3 | Analytical Chemistry |
When two or more types of monomers undergo addition polymerization, the resulting polymer is an addition copolymer. Saran wrap, formed from polymerization of vinyl chloride and vinylidene chloride, is an addition copolymer. | 7 | Physical Chemistry |
Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light. | 5 | Photochemistry |
Archaea do not generally have an F-ATPase. Instead, they synthesize ATP using the A-ATPase/synthase, a rotary machine structurally similar to the V-ATPase but mainly functioning as an ATP synthase. Like the bacteria F-ATPase, it is believed to also function as an ATPase. | 5 | Photochemistry |
Base Number (BN) is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). BN is an important measurement in petroleum products, and the value varies depending on its application. BN generally ranges from 6–8 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general internal combustion engine use and 10–15 mg KOH/g for diesel engine operations. BN is typically higher for marine grade lubricants, approximately 15-80 mg KOH/g, as the higher BN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement. | 3 | Analytical Chemistry |
In chemistry, an ideal solution or ideal mixture is a solution that exhibits thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of mixing is zero as is the volume change on mixing by definition; the closer to zero the enthalpy of mixing is, the more "ideal" the behavior of the solution becomes. The vapor pressures of the solvent and solute obey Raoults law and Henrys law, respectively, and the activity coefficient (which measures deviation from ideality) is equal to one for each component.
The concept of an ideal solution is fundamental to chemical thermodynamics and its applications, such as the explanation of colligative properties. | 7 | Physical Chemistry |
* Metalorganic vapour phase epitaxy
* Electrostatic spray assisted vapour deposition (ESAVD)
* Sherardizing
* Some forms of Epitaxy
** Molecular beam epitaxy | 8 | Metallurgy |
Optical molasses is a laser cooling technique that can cool neutral atoms to as low as a few microkelvin, depending on the atomic species. An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and an MOT is the absence of magnetic field in the former. Therefore, unlike a MOT, an optical molasses provides only cooling and no trapping. | 7 | Physical Chemistry |
Broadly neutralizing HIV-1 antibodies (bNAbs) are neutralizing antibodies which neutralize multiple HIV-1 viral strains. bNAbs are unique in that they target conserved epitopes of the virus, meaning the virus may mutate, but the targeted epitopes will still exist. In contrast, non-bNAbs are specific for individual viral strains with unique epitopes. The discovery of bNAbs has led to an important area of research, namely, discovery of a vaccine, not only limited to HIV, but also other rapidly mutating viruses like influenza. | 1 | Biochemistry |
The capnellane group became a focal point for synthesis in the 1970s and 80’s. Scientists believed that these compounds had antimicrobial properties, based on an earlier discovery of antimicrobial activity in gorgonian soft corals and a later study of antimicrobial terpenoid compounds in alcyonarians. It was also postulated that capnellenes also protect the soft coral by preventing larval settlement.
Capnella imbricata is a rich source of many non-isoprenoid sesquiterpenes, which all share the ring system. Consequently, the first known isolation of a capnellane derivative was not capnellene but a capnellanol. As part of an ongoing search for terpenoids from marine sources, Kaisin et al. (1974) characterized the most abundant terpenoid, Δ-capnellene-3β,8β,10α-triol, from colonies of Capnella imbricata. The structure and absolute configuration of the triol were determined by nuclear magnetic resonance (NMR) spectroscopy and later confirmed by x-ray crystallography.
Kaisin et al. (1974) coined the name “capnellane” for the hydrocarbon skeleton on which the molecule was based. However, Shiekh et al. (1976) also claim to have originated the name. The first isolation of the hydrocarbon form, Δ9-capnellene, was achieved in 1978. Since then, numerous groups have isolated both Δ-capnellene and its alcohol derivatives. | 0 | Organic Chemistry |
The Bjerrum length (after Danish chemist Niels Bjerrum 1879–1958 ) is the separation at which the electrostatic interaction between two elementary charges is comparable in magnitude to the thermal energy scale, , where is the Boltzmann constant and is the absolute temperature in kelvins. This length scale arises naturally in discussions of electrostatic, electrodynamic and electrokinetic phenomena in electrolytes, polyelectrolytes and colloidal dispersions.
In standard units, the Bjerrum length is given by
where is the elementary charge, is the relative dielectric constant of the medium and is the vacuum permittivity.
For water at room temperature , so that
In Gaussian units, and the Bjerrum length has the simpler form
The relative permittivity ε of water decreases so strongly with temperature that the product (ε·T) decreases. Therefore, in spite of the (1/T) relation, the Bjerrum length λ increases with temperature, as shown in the graph. | 7 | Physical Chemistry |
Methane functionalization is the process of converting methane in its gaseous state to another molecule with a functional group, typically methanol or acetic acid, through the use of transition metal catalysts.
In the realm of carbon-hydrogen bond activation and functionalization (C-H activation/functionalization), many recent efforts have been made in order to catalytically functionalize the C-H bonds in methane. The large abundance of methane in natural gas or shale gas deposits presents a large potential for its use as a feedstock in modern chemistry. However, given its gaseous natural state, it is quite difficult to transport economically. Its ideal use would be as a raw starting material for methanol or acetic acid synthesis, with plants built at the source to eliminate the issue of transportation. Methanol, in particular, would be of great use as a potential fuel source, and many efforts have been applied to researching the feasibilities of a methanol economy.
The challenges of C-H activation and functionalization present themselves when several factors are taken into consideration. Firstly, the C-H bond is extremely inert and non-polar, with a high bond dissociation energy, making methane a relatively unreactive starting material. Secondly, any products formed from methane would likely be more reactive than the starting product, which would be detrimental to the selectivity and yield of the reaction.
The main strategy currently used to increase the reactivity of methane uses transition metal complexes to activate the carbon-hydrogen bonds. In a typical C-H activation mechanism, a transition metal catalyst coordinates to the C-H bond to cleave it, and convert it into a bond with a lower bond dissociation energy. By doing so, the product can be used in further downstream reactions, since it will usually have a new functional group attached to the carbon. It is also important to note the difference between the terms "activation" and "functionalization," since both terms are often used interchangeably, but should be held distinct from each other. Activation refers to the coordination of a metal center to the C-H bond, whereas functionalization occurs when the coordinated metal complex is further reacted with a group "X" to result in the functionalized product. | 0 | Organic Chemistry |
Other functions of coatings include:
* Anti-fouling coatings
* Anti-microbial coatings.
* Anti-reflective coatings for example on spectacles.
* Coatings that alter or have magnetic, electrical or electronic properties.
* Flame retardant coatings. Flame-retardant materials and coatings are being developed that are phosphorus and bio-based. These include coatings with intumescent functionality.
* Non-stick PTFE coated cooking pots/pans.
* Optical coatings are available that alter optical properties of a material or object.
* UV coatings | 8 | Metallurgy |
The methylylidyne group can exhibit both Lewis acidic and Lewis basic character. Such behavior is only of theoretical interest since it is not possible to produce methylidyne. | 0 | Organic Chemistry |
Permanent coatings are often more expensive than sacrificial coatings, but if used appropriately only have to be applied once. These work by creating a protective surface that spray paint cannot bond to. After the surface has been vandalized, often all that is needed to remove the paint is a simple solvent (toluene) and some manual labor. The underlying surface and the protective coating will remain undamaged.
Some of the types of permanent coatings include those based on polyurethanes, nano-particles, fluorinated hydrocarbons, or siloxanes. Polyurethane coatings are useful because of their barrier properties. High chain stiffness and high crosslinking density reduces the ability of the polymer to swell and absorb graffiti paint.
Fluorinated coatings are some of the most effective in the field of graffiti prevention. Fluorine is the most electronegative element, meaning that it shows very little affinity for the electrons of other elements. When fluorine is attached to a surface it will decrease surface energy at the interface, minimizing the contact with the graffiti paint. For the same reason that a Teflon-coated pan repels both water and oil, a fluorinated coating will repel water and oil-based paints. These coatings also have the added benefit of being chemically inert as well as very durable. They are also expensive and can be difficult to apply.
Silicon based coatings are hydrophobic, which means the surface repels water. This reduces the effects of photo-oxidation of surfaces.
One of the newer additions to this ever growing market are nanoparticle based coatings. Silica particles are formed using the sol-gel method. The resulting silica particles have both reactive (Si-OH) and nonreactive (Si=O) groups on the surface. The reactive groups provide locations for further chemical processing, which allow you to change the surface properties of the nanoparticles. For anti-graffiti coatings, hydrophobic and oleophobic (oil-fearing) ligands are grafted onto the silica nanoparticles. Hydrophobic ligands are non-polar molecules such as hydrocarbon chains. Oleophobic ligands consist of polar molecules. Normally these two different types of molecules would phase separate in solution, for the same reason that water and oil do not mix. By chemically grafting the ligands onto the silica particles, this effect is counteracted. The effect is a coating that shows an equal dislike for both water-based and oil-based paints. | 7 | Physical Chemistry |
To analyze the transport of ions in the channel, behaviors of system in electrochemistry as well as fluid mechanics need to be considered. The Poisson–Nernst–Planck (PNP) equations are utilized to describe ionic current flowing through a channel, and the Navier–Stokes (NS) equations are used to represent the fluid dynamics in the channel.
The PNP equations consist of the Poisson equation:
and the Nernst–Planck equations, which gives the particle flux of ion species due to a concentration gradient and electric potential gradient:
where is the electrostatic potential, is the unit charge of electron, is the permittivity in vacuum, and is the dielectric constant of solution; , and are the diffusivity, the number density of ions, and the valence of ion species .
The solution in steady-state satisfies the continuity equation. To describe fluid velocity field in the channel, using Navier–Stokes equations:
where , , , and are pressure, velocity vector, viscosity, and density of fluid, respectively. The equations above are usually solved with numerical algorithm to determine the velocity, pressure, electric potential, and ionic concentration in the fluid, as well as the electric current flow through the channel. | 7 | Physical Chemistry |
Since only one of the stereoisomers has the desired effect, the separation of a racemic mixture of hydroxy N- methyl morphinan using tartaric acid and subsequent methylation of the hydroxyl group is a suitable method. By using (D)-tartrate, the (+)-isomer remains as the product.
This synthetic pathway was patented by Roche in 1950. | 4 | Stereochemistry |
A catabolic pathway is a series of reactions that bring about a net release of energy in the form of a high energy phosphate bond formed with the energy carriers adenosine diphosphate (ADP) and guanosine diphosphate (GDP) to produce adenosine triphosphate (ATP) and guanosine triphosphate (GTP), respectively. The net reaction is, therefore, thermodynamically favorable, for it results in a lower free energy for the final products. A catabolic pathway is an exergonic system that produces chemical energy in the form of ATP, GTP, NADH, NADPH, FADH2, etc. from energy containing sources such as carbohydrates, fats, and proteins. The end products are often carbon dioxide, water, and ammonia. Coupled with an endergonic reaction of anabolism, the cell can synthesize new macromolecules using the original precursors of the anabolic pathway. An example of a coupled reaction is the phosphorylation of fructose-6-phosphate to form the intermediate fructose-1,6-bisphosphate by the enzyme phosphofructokinase accompanied by the hydrolysis of ATP in the pathway of glycolysis. The resulting chemical reaction within the metabolic pathway is highly thermodynamically favorable and, as a result, irreversible in the cell. | 1 | Biochemistry |
Benzyltrimethylammonium fluoride is a quaternary ammonium salt. It is commercially available as the hydrate. The compound is a source of organic-soluble fluoride to removal of silyl ether protecting groups. As is the case for tetra-n-butylammonium fluoride and most other quaternary ammonium fluorides, the compound cannot be obtained in anhydrous form. | 0 | Organic Chemistry |
*The science fiction novel Mother of Storms by John Barnes offers a fictional example of catastrophic climate change caused by methane clathrate release.
*In The Life Lottery by Ian Irvine unprecedented seismic activity triggers a release of methane hydrate, reversing global cooling.
*The hypothesis is the basis of an experiment in the PlayStation 2 game Death By Degrees.
*In Transcendent by Stephen Baxter, averting such a crisis is a major plotline.
*The novel The Black Silent by author David Dun features this idea as a key scientific point.
*In the anime Ergo Proxy, a string of explosions in the methane hydrate reserves wipes out 85% of species on Earth.
*The novel The Far Shore of Time by Frederik Pohl features an alien race attempting to destroy humanity by bombing the methane clathrate reserves, thus releasing the gas into the atmosphere.
*The novel The Swarm by Frank Schätzing features what first appear to be freak events related to the world's oceans.
*In Charles Stross Laundry Files' universe, an intentionally triggered clathrate gun scenario is viewed as a possible retaliatory strategy that could be utilized by Blue Hades in response to terminal violation of the Benthic Treaty. | 6 | Supramolecular Chemistry |
Immunological methods using monoclonal antibodies can be used to detect indicator bacteria in water samples. Precultivation in select medium must preface detection to avoid detection of dead cells. ELISA antibody technology has been developed to allow for readable detection by the naked eye for rapid identification of coliform microcolonies. Other uses of antibodies in detection use magnetic beads coated with antibodies for the concentration and separation of the oocysts and cysts as described below for immunomagnetic separation (IMS) methods. | 3 | Analytical Chemistry |
For fusion to take place, it has to overcome huge repulsive forces due to the strong hydration repulsion between hydrophilic lipid head groups. However, it has been hard to exactly determine the connection between adhesion, fusion and interbilayer forces. The forces that promote cell adhesion are not the same as the ones that promote membrane fusion. Studies show that by creating a stress on the interacting bilayers, fusion can be achieved without disrupting the interbilayer interactions. It has also been suggested that membrane fusion takes place through a sequence of structural rearrangements that help to overcome the barrier that prevents fusion. Thus, interbilayer fusion takes place through
* local approach of membrane
* structural rearrangements causing hydration repulsion forces to be overcome
* complete merging to form a single entity | 6 | Supramolecular Chemistry |
When exposure to a carcinogenic substance is suspected, the cause/effect relationship on any given case can never be ascertained. Lung cancer occurs spontaneously, and there is no difference between a "natural" cancer and another one caused by radon (or smoking). Furthermore, it takes years for a cancer to develop, so that determining the past exposure of a case is usually very approximative. The health effect of radon can only be demonstrated through theory and statistical observation.
The study design for epidemiological methods may be of three kinds:
* The best proofs come from observations of cohorts (predetermined populations with known exposures and exhaustive follow-up), such as those on miners, or on Hiroshima and Nagasaki survivors. Such studies are efficient, but very costly when the population needs to be a large one. Such studies can only be used when the effect is strong enough, hence, for high exposures.
* Alternate proofs are case-control studies (the environment factors of a "case" population is individually determined, and compared to that of a "control″ population, to see what the difference might have been, and which factors may be significant), like the ones that have been used to demonstrate the link between lung cancer and smoking. Such studies can identify key factors when the signal/noise ratio is strong enough, but are very sensitive to selection bias, and prone to the existence of confounding factors.
* Lastly, ecological studies may be used (where the global environment variables and their global effect on two different populations are compared). Such studies are "cheap and dirty": they can be easily conducted on very large populations (the whole USA, in Dr Cohen's study), but are prone to the existence of confounding factors, and exposed to the ecological fallacy problem.
Furthermore, theory and observation must confirm each other for a relationship to be accepted as fully proven. Even when a statistical link between factor and effect appears significant, it must be backed by a theoretical explanation; and a theory is not accepted as factual unless confirmed by observations. | 2 | Environmental Chemistry |
Under pre-equilibrium conditions, the catalyst and substrate undergo rapid and reversible association prior to a relatively slow step leading to product formation and release. Under these conditions, the system can be described by a "one-plus" rate law where the numerator consists of all rate constants and species required to go from starting material to product, and the denominator consists of a sum of terms describing each of the states in which the catalyst exists (and 1 corresponds to the free catalyst). For the simplest case where one substrate goes to one product through a single intermediate:
In the slightly more complex situation where two substrates bind in sequence followed by product release:
In the case of the simple pre-equilibrium conditions described above, the catalyst resting state is either entirely or partially (depending on the magnitude of the equilibrium constant) the substrate bound complex. | 7 | Physical Chemistry |
Twenty-three out of 25 patients showed high expression of SFRP1 mRNA in leiomyoma than the matched normal myometrium. During the menstrual cycle, the level of SFRP1 mRNA in leiomyoma was highest in the follicular phase. Gonadotropin releasing hormone analogue (GnRHa) decreases estrogen secretion from the ovary. Patients treated with (GnRHa) presurgically showed the lowest expression of SFRP1 in both myometrial and leiomyoma tissues. These findings suggest that SFRP1 could be under the control of estrogen. Gene expression of estrogen receptors in leiomyomas is stronger than that in the myometrium. This suggests that leiomyoma possess increased sensitivity to E2 (estradiol, a form of estrogen) and the estrogen-dependent expression of SFRP1 in leiomyoma could be associated with the growth and pathogenesis of leiomyoma. | 1 | Biochemistry |
These techniques employ electric currents to drive or enhance sintering. English engineer A. G. Bloxam registered in 1906 the first patent on sintering powders using direct current in vacuum. The primary purpose of his inventions was the industrial scale production of filaments for incandescent lamps by compacting tungsten or molybdenum particles. The applied current was particularly effective in reducing surface oxides that increased the emissivity of the filaments.
In 1913, Weintraub and Rush patented a modified sintering method which combined electric current with pressure. The benefits of this method were proved for the sintering of refractory metals as well as conductive carbide or nitride powders. The starting boron–carbon or silicon–carbon powders were placed in an electrically insulating tube and compressed by two rods which also served as electrodes for the current. The estimated sintering temperature was 2000 °C.
In the United States, sintering was first patented by Duval d'Adrian in 1922. His three-step process aimed at producing heat-resistant blocks from such oxide materials as zirconia, thoria or tantalia. The steps were: (i) molding the powder; (ii) annealing it at about 2500 °C to make it conducting; (iii) applying current-pressure sintering as in the method by Weintraub and Rush.
Sintering that uses an arc produced via a capacitance discharge to eliminate oxides before direct current heating, was patented by G. F. Taylor in 1932. This originated sintering methods employing pulsed or alternating current, eventually superimposed to a direct current. Those techniques have been developed over many decades and summarized in more than 640 patents.
Of these technologies the most well known is resistance sintering (also called hot pressing) and spark plasma sintering, while electro sinter forging is the latest advancement in this field. | 8 | Metallurgy |
Some pairs of minerals that are not related structurally or compositionally may also exhibit epitaxy. A common example is rutile TiO on hematite FeO. Rutile is tetragonal and hematite is trigonal, but there are directions of similar spacing between the atoms in the (100) plane of rutile (perpendicular to the a axis) and the (001) plane of hematite (perpendicular to the c axis). In epitaxy these directions tend to line up with each other, resulting in the axis of the rutile overgrowth being parallel to the c axis of hematite, and the c axis of rutile being parallel to one of the axes of hematite. | 3 | Analytical Chemistry |
The stereochemical result of a given reaction on a macrocycle capable of adopting several conformations can be modeled by a Curtin-Hammett scenario. In the diagram below, the two ground state conformations exist in an equilibrium, with some difference in their ground state energies. Conformation B is lower in energy than conformation A, and while possessing a similar energy barrier to its transition state in a hypothetical reaction, thus the product formed is predominantly product B (P B) arising from conformation B via transition state B (TS B). The inherent preference of a ring to exist in one conformation over another provides a tool for stereoselective control of reactions by biasing the ring into a given configuration in the ground state. The energy differences, ΔΔG and ΔG are significant considerations in this scenario. The preference for one conformation over another can be characterized by ΔG, the free energy difference, which can, at some level, be estimated from conformational analysis. The free energy difference between the two transition states of each conformation on its path to product formation is given by ΔΔG. The value of ΔG between not just one, but many accessible conformations is the underlying energetic impetus for reactions occurring from the most stable ground state conformation and is the crux of the peripheral attack model outlined below. | 0 | Organic Chemistry |
As the equation originated with Henri, not with Michaelis and Menten, it is more accurate to call it the Henri–Michaelis–Menten equation, though it was Michaelis and Menten who realized that analysing reactions in terms of initial rates would be simpler, and as a result more productive, than analysing the time course of reaction, as Henri had attempted. Although Henri derived the equation he made no attempt to apply it. In addition, Michaelis and Menten understood the need for buffers to control the pH, but Henri did not. | 7 | Physical Chemistry |
Eukaryotic transcription is the elaborate process that eukaryotic cells use to copy genetic information stored in DNA into units of transportable complementary RNA replica. Gene transcription occurs in both eukaryotic and prokaryotic cells. Unlike prokaryotic RNA polymerase that initiates the transcription of all different types of RNA, RNA polymerase in eukaryotes (including humans) comes in three variations, each translating a different type of gene. A eukaryotic cell has a nucleus that separates the processes of transcription and translation. Eukaryotic transcription occurs within the nucleus where DNA is packaged into nucleosomes and higher order chromatin structures. The complexity of the eukaryotic genome necessitates a great variety and complexity of gene expression control.
Eukaryotic transcription proceeds in three sequential stages: initiation, elongation, and termination.
The RNAs transcribed serve diverse functions. For example, structural components of the ribosome are transcribed by RNA polymerase I. Protein coding genes are transcribed by RNA polymerase II into messenger RNAs (mRNAs) that carry the information from DNA to the site of protein synthesis. More abundantly made are the so-called non-coding RNAs account for the large majority of the transcriptional output of a cell. These non-coding RNAs perform a variety of important cellular functions. | 1 | Biochemistry |
microRNA mediated repression occurs in two ways, either by translational repression or stimulating mRNA decay. miRNA recruit the RISC complex to the mRNA to which they are bound. The link to P-bodies comes by the fact that many, if not most, of the proteins necessary for miRNA gene silencing are localized to P-bodies, as reviewed by Kulkarni et al. (2010). These proteins include, but are not limited to, the scaffold protein GW182, Argonaute (Ago), decapping enzymes and RNA helicases.
The current evidence points toward P-bodies as being scaffolding centers of miRNA function, especially due to the evidence that a knock down of GW182 disrupts P-body formation. However, there remain many unanswered questions about P-bodies and their relationship to miRNA activity. Specifically, it is unknown whether there is a context dependent (stress state versus normal) specificity to the P-body's mechanism of action. Based on the evidence that P-bodies sometimes are the site of mRNA decay and sometimes the mRNA can exit the P-bodies and re-initiate translation, the question remains of what controls this switch. Another ambiguous point to be addressed is whether the proteins that localize to P-bodies are actively functioning in the miRNA gene silencing process or whether they are merely on standby. | 1 | Biochemistry |
This is when a mesocrystal is formed by filling organic matrix compartments with crystalline matter. This crystalline matter would be oriented by the organic matrix. This is the process of biomineralization and this is how mesocrystals are produced in nature. | 7 | Physical Chemistry |
LTR retrotransposons are class I transposable element characterized by the presence of long terminal repeats (LTRs) directly flanking an internal coding region. As retrotransposons, they mobilize through reverse transcription of their mRNA and integration of the newly created cDNA into another location. Their mechanism of retrotransposition is shared with retroviruses, with the difference that most LTR-retrotransposons do not form infectious particles that leave the cells and therefore only replicate inside their genome of origin. | 1 | Biochemistry |
The volumetric titration is based on the same principles as the coulometric titration, except that the anode solution above now is used as the titrant solution. The titrant consists of an alcohol (ROH), base (B), and a known concentration of . Pyridine has been used as the base in this case.
One mole of is consumed for each mole of . The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above. | 3 | Analytical Chemistry |
The malate shuttle allows the mitochondria to move electrons from NADH without the consumption of metabolites and it uses two antiporters to transport metabolites and keep balance within the mitochondrial matrix and cytoplasm.
On the cytoplasmic side a transaminase enzyme is used to remove an amino group from aspartate which is converted into oxaloacetate, then malate dehydrogenase enzyme uses an NADH cofactor to reduce oxaloacetate to malate which can be transported across the membrane because of the presence of a transporter.
Once the malate is inside the matrix its converted back to oxaloacetate, which is converted to aspartate and can be transported back outside the mitochondria to allow the cycle to continue. The movement of oxaloacetate across the membrane transports electrons and is known as the outer ring. The inner ring primary function is not to move electrons but regenerate the metabolites. | 1 | Biochemistry |
TFA is prepared industrially by the electrofluorination of acetyl chloride or acetic anhydride, followed by hydrolysis of the resulting trifluoroacetyl fluoride:
Where desired, this compound may be dried by addition of trifluoroacetic anhydride.
An older route to TFA proceeds via the oxidation of 1,1,1-trifluoro-2,3,3-trichloropropene with potassium permanganate. The trifluorotrichloropropene can be prepared by Swarts fluorination of hexachloropropene. | 0 | Organic Chemistry |
Unlike a classical ideal gas, whose pressure is proportional to its temperature
where P is pressure, k is Boltzmanns constant, N is the number of particles—typically atoms or molecules—, T is temperature, and V' is the volume, the pressure exerted by degenerate matter depends only weakly on its temperature. In particular, the pressure remains nonzero even at absolute zero temperature. At relatively low densities, the pressure of a fully degenerate gas can be derived by treating the system as an ideal Fermi gas, in this way
where m is the mass of the individual particles making up the gas. At very high densities, where most of the particles are forced into quantum states with relativistic energies, the pressure is given by
where K is another proportionality constant depending on the properties of the particles making up the gas.
All matter experiences both normal thermal pressure and degeneracy pressure, but in commonly encountered gases, thermal pressure dominates so much that degeneracy pressure can be ignored. Likewise, degenerate matter still has normal thermal pressure; the degeneracy pressure dominates to the point that temperature has a negligible effect on the total pressure. The adjacent figure shows the thermal pressure (red line) and total pressure (blue line) in a Fermi gas, with the difference between the two being the degeneracy pressure. As the temperature falls, the density and the degeneracy pressure increase, until the degeneracy pressure contributes most of the total pressure.
While degeneracy pressure usually dominates at extremely high densities, it is the ratio between degenerate pressure and thermal pressure which determines degeneracy. Given a sufficiently drastic increase in temperature (such as during a red giant star's helium flash), matter can become non-degenerate without reducing its density.
Degeneracy pressure contributes to the pressure of conventional solids, but these are not usually considered to be degenerate matter because a significant contribution to their pressure is provided by electrical repulsion of atomic nuclei and the screening of nuclei from each other by electrons. The free electron model of metals derives their physical properties by considering the conduction electrons alone as a degenerate gas, while the majority of the electrons are regarded as occupying bound quantum states. This solid state contrasts with degenerate matter that forms the body of a white dwarf, where most of the electrons would be treated as occupying free particle momentum states.
Exotic examples of degenerate matter include neutron degenerate matter, strange matter, metallic hydrogen and white dwarf matter. | 7 | Physical Chemistry |
N-Oxoammonium salts are a class of organic compounds with the formula [RR=O]X. The cation [RR=O] is of interest for the dehydrogenation of alcohols. Oxoammonium salts are diamagnetic, whereas the nitroxide has a doublet ground state. A prominent N-oxoammonium salt is prepared by oxidation of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, commonly referred to as [TEMPO]. A less expensive analogue is Bobbitt's salt. | 0 | Organic Chemistry |
The total acid number (TAN) is a measurement of acidity that is determined by the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil. It is an important quality measurement of crude oil.
The TAN value indicates to the crude oil refinery the potential of corrosion problems. It is usually the naphthenic acids in the crude oil that cause corrosion problems. This type of corrosion is referred to as naphthenic acid corrosion (NAC).
TAN values may also be useful in other industries where oils are used as lubricants to determine oxidation and the subsequent corrosion risk to machinery.
TAN value can be deduced by various methods, including
* Potentiometric titration: The sample is normally dissolved in toluene and propanol with a little water and titrated with alcoholic potassium hydroxide (if sample is acidic). A glass electrode and reference electrode is immersed in the sample and connected to a voltmeter/potentiometer. The meter reading (in millivolts) is plotted against the volume of titrant. The end point is taken at the distinct inflection of the resulting titration curve corresponding to the basic buffer solution.
* Color indicating titration: An appropriate pH color indicator e.g. phenolphthalein, is used. Titrant is added to the sample by means of a burette. The volume of titrant used to cause a permanent color change in the sample is recorded and used to calculate the TAN value.
* Spectroscopic methods: as with many chemical parameters, spectroscopy can be used to make fast, accurate measurements once calibrated by a reference method. Mid and near infrared spectroscopy are most commonly used for this purpose. Spectroscopic methods are valuable as they can also be used to simultaneously measure a number of other parameters and do away with the need for wet chemistry. | 3 | Analytical Chemistry |
The mass concentration is defined as the mass of a constituent divided by the volume of the mixture :
The SI unit is kg/m (equal to g/L). | 3 | Analytical Chemistry |
The RNA Characterization of Secondary Structure Motifs database (RNA CoSSMos) is a repository of three-dimensional nucleic acid PDB structures containing secondary structure motifs ( loops, hairpin loops ...). | 4 | Stereochemistry |
Double ionization is a process of formation of doubly charged ions when laser radiation is exerted on neutral atoms or molecules. Double ionization is usually less probable than single-electron ionization. Two types of double ionization are distinguished: sequential and non-sequential. | 7 | Physical Chemistry |
Catalytic hydrogenation yields sulfolane, a solvent used in the petrochemical industry for the extraction of aromatics from hydrocarbon streams. The hydrogenation of 3-sulfolene over Raney nickel at approx. 20 bar and 60 °C gives sulfolane in yields of up to 65% only because of the poisoning of the catalyst by sulfur compounds. | 0 | Organic Chemistry |
Oligonucleotides are chemically synthesized using building blocks called nucleoside phosphoramidites. These can be normal or modified nucleosides which have protecting groups to prevent their amines, hydroxyl groups and phosphate groups from interacting incorrectly. One phosphoramidite is added at a time, the 5 hydroxyl group is deprotected and a new base is added and so on. The chain grows in the 3 to 5' direction, which is backwards relative to biosynthesis. At the end, all the protecting groups are removed. Nevertheless, being a chemical process, several incorrect interactions occur leading to some defective products. The longer the oligonucleotide sequence that is being synthesized, the more defects there are, thus this process is only practical for producing short sequences of nucleotides. The current practical limit is about 200 bp (base pairs) for an oligonucleotide with sufficient quality to be used directly for a biological application. HPLC can be used to isolate products with the proper sequence. Meanwhile, a large number of oligos can be synthesized in parallel on gene chips. For optimal performance in subsequent gene synthesis procedures they should be prepared individually and in larger scales. | 1 | Biochemistry |
RACE can be used to amplify unknown 5 (5-RACE) or 3 (3-RACE) parts of RNA molecules where part of the RNA sequence is known and targeted by a gene-specific primer. Combined with high-throughput sequencing for characterization of these amplified RACE products, it is possible to apply the approach to characterize any types of coding or non-coding RNA-molecules.
The idea of combining RACE with high-throughput sequencing was first introduced in 2009 as Deep-RACE to perform mapping of Transcription start sites (TSS) of 17 genes in a single cell-line. For example, In a study from 2014 to accurately map cleavage sites of target RNA directed by synthetic siRNAs, the approach was first named RACE-seq. Further, the methodology was used to characterize full-length unknown parts of novel transcripts and fusion transcripts in colorectal cancer.
In another study aiming to characterize unknown transcript structures of lncRNAs, RACE was used in combination with semi-long 454 sequencing. | 1 | Biochemistry |
Phosphates are the naturally occurring form of the element phosphorus, found in many phosphate minerals. In mineralogy and geology, phosphate refers to a rock or ore containing phosphate ions. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry.
The largest global producer and exporter of phosphates is Morocco. Within North America, the largest deposits lie in the Bone Valley region of central Florida, the Soda Springs region of southeastern Idaho, and the coast of North Carolina. Smaller deposits are located in Montana, Tennessee, Georgia, and South Carolina. The small island nation of Nauru and its neighbor Banaba Island, which used to have massive phosphate deposits of the best quality, have been mined excessively. Rock phosphate can also be found in Egypt, Israel, Palestine, Western Sahara, Navassa Island, Tunisia, Togo, and Jordan, countries that have large phosphate-mining industries.
Phosphorite mines are primarily found in:
* North America: United States, especially Florida, with lesser deposits in North Carolina, Idaho, and Tennessee
* Africa: Morocco, Algeria, Egypt, Niger, Senegal, Togo, Tunisia, Mauritania
* Middle East: Saudi Arabia, Jordan, Israel, Syria, Iran and Iraq, at the town of Akashat, near the Jordanian border.
* Central Asia: Kazakhstan
* Oceania: Australia, Makatea, Nauru, and Banaba Island
In 2007, at the current rate of consumption, the supply of phosphorus was estimated to run out in 345 years. However, some scientists thought that a "peak phosphorus" would occur in 30 years and Dana Cordell from Institute for Sustainable Futures said -ton crust, albeit at predominantly lower concentration than the deposits counted as reserves, which are inventoried and cheaper to extract. If it is assumed that the phosphate minerals in phosphate rock are mainly hydroxyapatite and fluoroapatite, phosphate minerals contain roughly 18.5% phosphorus by weight. If phosphate rock contains around 20% of these minerals, the average phosphate rock has roughly 3.7% phosphorus by weight.
Some phosphate rock deposits, such as Mulberry in Florida, are notable for their inclusion of significant quantities of radioactive uranium isotopes. This is a concern because radioactivity can be released into surface waters from application of the resulting phosphate fertilizer.
In December 2012, Cominco Resources announced an updated JORC compliant resource of their Hinda project in Congo-Brazzaville of 531 million tons, making it the largest measured and indicated phosphate deposit in the world.
Around 2018, Norway discovered phosphate deposits almost equal to those in the rest of Earth combined.
In July 2022 China announced quotas on phosphate exportation.
The largest importers in millions of metric tons of phosphate are Brazil 3.2, India 2.9 and the USA 1.6. | 0 | Organic Chemistry |
Theories about the origin of life require knowledge of chemical pathways that permit formation of lifes key building blocks under plausible prebiotic conditions. The RNA world hypothesis holds that in the primordial soup there existed free-floating ribonucleotides, the fundamental molecules that combine in series to form RNA. Complex molecules like RNA must have arisen from small molecules whose reactivity was governed by physico-chemical processes. RNA is composed of purine and pyrimidine nucleotides, both of which are necessary for reliable information transfer, and thus Darwinian evolution. Becker et al. showed how pyrimidine nucleosides can be synthesized from small molecules and ribose, driven solely by wet-dry cycles. Purine nucleosides can be synthesized by a similar pathway. 5-mono- and di-phosphates also form selectively from phosphate-containing minerals, allowing concurrent formation of polyribonucleotides with both the purine and pyrimidine bases. Thus a reaction network towards the purine and pyrimidine RNA building blocks can be established starting from simple atmospheric or volcanic molecules. | 1 | Biochemistry |
Through the dissimilatory sulfate reduction pathway, sulfate can be reduced either bacterially (bacterial sulfate reduction) or inorganically (thermochemical sulfate reduction). This pathway involves the reduction of sulfate by organic compounds to produce hydrogen sulfide, which occurs in both processes.
The main products and reactants of bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) are very similar. For both, various organic compounds and dissolved sulfate are the reactants, and the products or by-products are as follows: HS, CO, carbonates, elemental sulfur and metal sulfides. However, the reactive organic compounds differ for BSR and TSR because of the mutually exclusive temperature regimes. Organic acids are the main organic reactants for BSR and branched/n-alkanes are the main organic reactants for TSR. The inorganic reaction products in BSR and TSR are HS (HS) and (CO).
These processes occur because there are two very different thermal regimes in which sulfate is reduced, particularly in low-temperature and high-temperature environments. BSR usually occurs at lower temperatures from 0−80 °C, while TSR happens at much higher temperatures around 100–140 °C. Temperatures for TSR are not as well defined; the lowest confirmed temperature is 127 °C and the highest temperatures occur in settings around 160−180 °C. These two different regimes appear because at higher temperatures most sulfate-reducing microbes can no longer metabolize due to the denaturation of proteins or deactivation of enzymes, so TSR takes over. However, in hot sediments around hydrothermal vents BSR can happen at temperatures up to 110 °C.
BSR and TSR occur at different depths. BSR takes place in low-temperature environments, which are shallower settings such as oil and gas fields. BSR can also take place in modern marine sedimentary environments such as stratified inland seas, continental shelves, organic-rich deltas, and hydrothermal sediments which have intense microbial sulfate reduction because of the high concentration of dissolved sulfate in the seawater. Additionally, the high amounts of hydrogen sulfide found in oil and gas fields is thought to arise from the oxidation of petroleum hydrocarbons by sulfate. Such reactions are known to occur by microbial processes but it is generally accepted that TSR is responsible for the bulk of these reactions, especially in deep or hot reservoirs. Thus, TSR occurs in deep reservoirs where the temperatures are much higher. BSR is geologically instantaneous in most geologic settings, while TSR occurs at rates in the order of hundreds of thousands of years. Although much slower than BSR, even TSR appears to be a geologically fairly fast process.
BSR in shallow environments and TSR in deep reservoirs are key processes in the oceanic sulfur cycle. Approximately, 10% (of the total gas) of HS is produced in BSR settings, whereas 90% of the HS is produced in TSR settings. If there is more than a few percent of HS in any deep reservoir, then it is assumed that TSR has taken over. This is due to the fact that thermal cracking of hydrocarbons doesn't provide more than 3% of HS. The amount of HS is affected by several factors such as, the availability of organic reactants and sulfate and the presence/availability of base and transition metals. | 9 | Geochemistry |
Because of sputtering occurring at the cathode, the colors emitted from regions near the cathode are quite different from the anode. Particles sputtered from the cathode are excited and emit radiation from the metals and oxides that make up the cathode. The radiation from these particles combines with radiation from excited carrier gas, giving the cathode region a white or blue color, while in the rest of the tube, radiation is only from the carrier gas and tends to be more monochromatic.
Electrons near the cathode are less energetic than the rest of the tube. Surrounding the cathode is a negative field, which slows electrons as they are ejected from the surface. Only those electrons with the highest velocity are able to escape this field, and those without enough kinetic energy are pulled back into the cathode. Once outside the negative field, the attraction from the positive field begins to accelerate these electrons toward the anode. During this acceleration electrons are deflected and slowed down by positive ions speeding toward the cathode, which, in turn, produces bright blue-white bremsstrahlung radiation in the negative glow region. | 3 | Analytical Chemistry |
Protein labeling use a short tag to minimize disruption of protein folding and function. Transition metals are used to link specific residues in the tags to site-specific targets such as the N-termini, C-termini, or internal sites within the protein. Examples of tags used for protein labeling include biarsenical tags, Histidine tags, and FLAG tags. | 1 | Biochemistry |
In response to the criticism that the Gaia hypothesis seemingly required unrealistic group selection and cooperation between organisms, James Lovelock and Andrew Watson developed a mathematical model, Daisyworld, in which ecological competition underpinned planetary temperature regulation.
Daisyworld examines the energy budget of a planet populated by two different types of plants, black daisies and white daisies, which are assumed to occupy a significant portion of the surface. The colour of the daisies influences the albedo of the planet such that black daisies absorb more light and warm the planet, while white daisies reflect more light and cool the planet. The black daisies are assumed to grow and reproduce best at a lower temperature, while the white daisies are assumed to thrive best at a higher temperature. As the temperature rises closer to the value the white daisies like, the white daisies outreproduce the black daisies, leading to a larger percentage of white surface, and more sunlight is reflected, reducing the heat input and eventually cooling the planet. Conversely, as the temperature falls, the black daisies outreproduce the white daisies, absorbing more sunlight and warming the planet. The temperature will thus converge to the value at which the reproductive rates of the plants are equal.
Lovelock and Watson showed that, over a limited range of conditions, this negative feedback due to competition can stabilize the planets temperature at a value which supports life, if the energy output of the Sun changes, while a planet without life would show wide temperature changes. The percentage of white and black daisies will continually change to keep the temperature at the value at which the plants reproductive rates are equal, allowing both life forms to thrive.
It has been suggested that the results were predictable because Lovelock and Watson selected examples that produced the responses they desired. | 9 | Geochemistry |
In the case of antiques, a range of views are held on the value of patination and its replacement if damaged, known as repatination.
Preserving a piece's look and character is important and removal or reduction may dramatically reduce its value. If patination has flaked off, repatination may be recommended. Appraiser Reyne Haines notes that a repatinated metal piece will be worth more than one with major imperfections in the patina, but less than a piece still with its original finish. | 8 | Metallurgy |
In chemistry, equivalent weight (also known as gram equivalent or equivalent mass) is the mass of one equivalent, that is the mass of a given substance which will combine with or displace a fixed quantity of another substance. The equivalent weight of an element is the mass which combines with or displaces 1.008 gram of hydrogen or 15.99 grams of oxygen or 35.5 grams of chlorine. These values correspond to the atomic weight divided by the usual valence; for oxygen gas as example that is 31.98 g.
For acid–base reactions, the equivalent weight of an acid or base is the mass which supplies or reacts with one mole of hydrogen cations (). For redox reactions, the equivalent weight of each reactant supplies or reacts with one mole of electrons (e) in a redox reaction.
Equivalent weight has the units of mass, unlike atomic weight, which is now used as a synonym for relative atomic mass and is dimensionless. Equivalent weights were originally determined by experiment, but (insofar as they are still used) are now derived from molar masses. The equivalent weight of a compound can also be calculated by dividing the molecular mass by the number of positive or negative electrical charges that result from the dissolution of the compound. | 7 | Physical Chemistry |
Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important role in the cellular metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. Urea is Neo-Latin, , , itself from Proto-Indo-European *h₂worsom.
It is a colorless, odorless solid, highly soluble in water, and practically non-toxic ( is 15 g/kg for rats). Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules () with a carbon dioxide () molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.
In 1828, Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, which was an important conceptual milestone in chemistry. This showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that only living organisms could produce the chemicals of life. | 0 | Organic Chemistry |
The liquid junction potential interferes with the exact measurement of the electromotive force of a chemical cell, so its effect should be minimized as much as possible for accurate measurement. The most common method of eliminating the liquid junction potential is to place a salt bridge consisting of a saturated solution of potassium chloride (KCl) and ammonium nitrate (NHNO) with lithium acetate (CHCOOLi) between the two solutions constituting the junction. When such a bridge is used, the ions in the bridge are present in large excess at the junction and they carry almost the whole of the current across the boundary. The efficiency of KCl/NHNO is connected with the fact that in these salts, the transport numbers of anions and cations are the same. | 7 | Physical Chemistry |
In the 1960s and 1970s various groups reported the ring-opening polymerization of norbornene catalyzed by hydrated trichlorides of ruthenium and other late transition metals in polar, protic solvents. This prompted Robert H. Grubbs and coworkers to search for well-defined, functional group tolerant catalysts based on ruthenium. The Grubbs group successfully polymerized the 7-oxo norbornene derivative using ruthenium trichloride, osmium trichloride as well as tungsten alkylidenes. They identified a Ru(II) carbene as an effective metal center and in 1992 published the first well-defined, ruthenium-based olefin metathesis catalyst, (PPh)ClRu=CHCH=CPh:
The corresponding tricyclohexylphosphine complex (PCy)ClRu=CHCH=CPh was also shown to be active. This work culminated in the now commercially available 1st generation Grubbs catalyst. | 0 | Organic Chemistry |
Calcein, also known as fluorexon, fluorescein complex, is a fluorescent dye with excitation and emission wavelengths of 495 and 515 nm, respectively, and has the appearance of orange crystals. Calcein self-quenches at concentrations above 70 mM and is commonly used as an indicator of lipid vesicle leakage. It has also been traditionally used as a complexometric indicator for titration of calcium ions with EDTA, and for fluorometric determination of calcium. | 3 | Analytical Chemistry |
Particulate inorganic carbon (PIC) usually takes the form of calcium carbonate (CaCO), and plays a key part in the ocean carbon cycle. This biologically fixed carbon is used as a protective coating for many planktonic species (coccolithophores, foraminifera) as well as larger marine organisms (mollusk shells). Calcium carbonate is also excreted at high rates during osmoregulation by fish, and can form in whiting events. While this form of carbon is not directly taken from the atmospheric budget, it is formed from dissolved forms of carbonate which are in equilibrium with CO and then responsible for removing this carbon via sequestration.
:CO + HO → HCO → H + HCO
:Ca + 2HCO → CaCO + CO + HO
While this process does manage to fix a large amount of carbon, two units of alkalinity are sequestered for every unit of sequestered carbon. The formation and sinking of CaCO therefore drives a surface to deep alkalinity gradient which serves to raise the pH of surface waters, shifting the speciation of dissolved carbon to raise the partial pressure of dissolved CO in surface waters, which actually raises atmospheric levels. In addition, the burial of CaCO in sediments serves to lower overall oceanic alkalinity, tending to raise pH and thereby atmospheric CO levels if not counterbalanced by the new input of alkalinity from weathering. The portion of carbon that is permanently buried at the sea floor becomes part of the geologic record. Calcium carbonate often forms remarkable deposits that can then be raised onto land through tectonic motion as in the case with the White Cliffs of Dover in Southern England. These cliffs are made almost entirely of the plates of buried coccolithophores. | 9 | Geochemistry |
Wallach's rule (first proposed by Otto Wallach) states that racemic crystals tend to be denser than their chiral counterparts. This rule has been substantiated by crystallographic database analysis. | 4 | Stereochemistry |
Fmoc carbamate is frequently used as a protecting group for amines, where the Fmoc group can be introduced by reacting the amine with fluorenylmethyloxycarbonyl chloride (Fmoc-Cl), e.g.:
The other common method for introducing the Fmoc group is through 9-fluorenylmethylsuccinimidyl carbonate (Fmoc-OSu), which may itself be obtained by the reaction of Fmoc-Cl with the dicyclohexylammonium salt of N-hydroxysuccinimide.
Reacting with 9-fluorenylmethyloxycarbonyl azide (itself made by reacting Fmoc-Cl with sodium azide) in sodium bicarbonate and aqueous dioxane is also a method to install Fmoc group.
Because the fluorenyl group is highly fluorescent, certain UV-inactive compounds may be reacted to give the Fmoc derivatives, suitable for analysis by reversed phase HPLC. Analytical uses of Fmoc-Cl that do not use chromatography may be limited by the requirement that excess Fmoc-Cl be removed before an analysis of fluorescence. | 1 | Biochemistry |
Hypothiocyanite occurs naturally in the antimicrobial immune system of the human respiratory tract in a redox reaction catalyzed by the enzyme lactoperoxidase. It has been researched extensively for its capabilities as an alternative antibiotic as it is harmless to human body cells while being cytotoxic to bacteria. The exact processes for making hypothiocyanite have been patented as such an effective antimicrobial has many commercial applications. | 1 | Biochemistry |
Cyanobacteria can be found in almost every terrestrial and aquatic habitat – oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil, and even Antarctic rocks. They can occur as planktonic cells or form phototrophic biofilms. They are found inside stones and shells (in endolithic ecosystems). A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage.
Aquatic cyanobacteria are known for their extensive and highly visible blooms that can form in both freshwater and marine environments. The blooms can have the appearance of blue-green paint or scum. These blooms can be toxic, and frequently lead to the closure of recreational waters when spotted. Marine bacteriophages are significant parasites of unicellular marine cyanobacteria.
Cyanobacterial growth is favoured in ponds and lakes where waters are calm and have little turbulent mixing. Their lifecycles are disrupted when the water naturally or artificially mixes from churning currents caused by the flowing water of streams or the churning water of fountains. For this reason blooms of cyanobacteria seldom occur in rivers unless the water is flowing slowly. Growth is also favoured at higher temperatures which enable Microcystis species to outcompete diatoms and green algae, and potentially allow development of toxins.
Based on environmental trends, models and observations suggest cyanobacteria will likely increase their dominance in aquatic environments. This can lead to serious consequences, particularly the contamination of sources of drinking water. Researchers including Linda Lawton at Robert Gordon University, have developed techniques to study these. Cyanobacteria can interfere with water treatment in various ways, primarily by plugging filters (often large beds of sand and similar media) and by producing cyanotoxins, which have the potential to cause serious illness if consumed. Consequences may also lie within fisheries and waste management practices. Anthropogenic eutrophication, rising temperatures, vertical stratification and increased atmospheric carbon dioxide are contributors to cyanobacteria increasing dominance of aquatic ecosystems.
Cyanobacteria have been found to play an important role in terrestrial habitats and organism communities. It has been widely reported that cyanobacteria soil crusts help to stabilize soil to prevent erosion and retain water. An example of a cyanobacterial species that does so is Microcoleus vaginatus. M. vaginatus stabilizes soil using a polysaccharide sheath that binds to sand particles and absorbs water. M. vaginatus also makes a significant contribution to the cohesion of biological soil crust.
Some of these organisms contribute significantly to global ecology and the oxygen cycle. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean. Circadian rhythms were once thought to only exist in eukaryotic cells but many cyanobacteria display a bacterial circadian rhythm. | 5 | Photochemistry |
E. coli use fermentation pathways as a final option for energy metabolism, as they produce very little energy in comparison to respiration.
Mixed acid fermentation in E. coli occurs in two stages. These stages are outlined by the biological database for E. coli, EcoCyc.
The first of these two stages is a glycolysis reaction. Under anaerobic conditions, a glycolysis reaction takes place where glucose is converted into pyruvate:
glucose → 2 pyruvate
There is a net production of 2 ATP and 2 NADH molecules per molecule of glucose converted. ATP is generated by substrate-level phosphorylation. NADH is formed from the reduction of NAD.
In the second stage, pyruvate produced by glycolysis is converted to one or more end products via the following reactions. In each case, both of the NADH molecules generated by glycolysis are reoxidized to NAD. Each alternative pathway requires a different key enzyme in E. coli. After the variable amounts of different end products are formed by these pathways, they are secreted from the cell. | 1 | Biochemistry |
This system is the best understood of the plasmid partition system.
It is composed of an actin-like ATPAse, ParM, and a CBP called ParR. The centromere like site, parC contains two sets of five 11 base pair direct repeats separated by the parMR promoter.
The amino-acid sequence identity can go down to 15% between ParM and other actin-like ATPase.
The mechanism of partition involved here is a pushing mechanism:
# ParR binds to parC and pairs plasmids which form a nucleoprotein complex, or partition complex
# The partition complex serves as nucleation point for the polymerization of ParM; ParM-ATP complex inserts at this point and push plasmids apart
# The insertion leads to hydrolysis of ParM-ATP complex, leading to depolymerization of the filament
# At cell division, plasmids copies are at each cell extremity, and will end up in future daughter cell
The filament of ParM is regulated by the polymerization allowed by the presence the partition complex (ParR-parC), and by the depolymerization controlled by the ATPase activity of ParM. | 1 | Biochemistry |
Urea is a raw material for the manufacture of formaldehyde based resins, such as UF, MUF, and MUPF, used mainly in wood-based panels, for instance, particleboard, fiberboard, OSB, and plywood. | 0 | Organic Chemistry |
Pyruvate is oxidized to acetyl-CoA and CO by the pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. In the conversion of pyruvate to acetyl-CoA, one molecule of NADH and one molecule of CO is formed. | 1 | Biochemistry |
Technical journals published on behalf of ASM include:
* Alloy Digest
* International Materials Reviews (IMR)
* Journal of Failure Analysis & Prevention (JFAP)
* Journal of Materials Engineering and Performance (JMEP)
* Journal of Phase Equilibria and Diffusion (JPED)
* Journal of Thermal Spray Technology (JTST)
* Metallography, Microstructure, and Analysis (MMA)
* Metallurgical and Materials Transactions A and B (MetTransA & MetTransB)
* Shape Memory and Superelasticity. | 8 | Metallurgy |
Trimethylamine N-oxide (MeNO) can be used in the nucleophilic abstraction of carbonyl. There is an nucleophilic attack of MeNO on the carbon of the carbonyl group which pushes electrons on the metal. The reaction then proceeds to kick out CO and NMe.
An article from the Bulletin of Korean Chemical Society journal showed interesting results where one iridium complex undergoes carbonyl abstraction while a very similar iridium complex undergoes hydride extraction. | 0 | Organic Chemistry |
Pitch is a viscoelastic polymer which can be natural or manufactured, derived from petroleum, coal tar, or plants. Pitch produced from petroleum may be called bitumen or asphalt, while plant-derived pitch, a resin, is known as rosin in its solid form. Tar is sometimes used interchangeably with pitch, but generally refers to a more liquid substance derived from coal production, including coal tar, or from plants, as in pine tar. | 7 | Physical Chemistry |
Over the life of Fulmer about 500 people were members of staff. Among these, because of the wide range of projects that Fulmer undertook, investigators and other technical staff had to be able to adapt their specialist skills and to innovate. They were also expected to play a part in attracting the necessary funding from business or Government. | 8 | Metallurgy |
The O−O bond length in peroxides is about 1.45 Å, and the R−O−O angles (R = H, C) are about 110° (water-like). Characteristically, the C−O−O−R (R = H, C) dihedral angles are about 120°. The O−O bond is relatively weak, with a bond dissociation energy of , less than half the strengths of C−C, C−H, and C−O bonds. | 0 | Organic Chemistry |
Ion chromatography has advanced through the accumulation of knowledge over a course of many years. Starting from 1947, Spedding and Powell used displacement ion-exchange chromatography for the separation of the rare earths. Additionally, they showed the ion-exchange separation of 14N and 15N isotopes in ammonia. At the start of the 1950s, Kraus and Nelson demonstrated the use of many analytical methods for metal ions dependent on their separation of their chloride, fluoride, nitrate or sulfate complexes by anion chromatography. Automatic in-line detection was progressively introduced from 1960 to 1980 as well as novel chromatographic methods for metal ion separations. A groundbreaking method by Small, Stevens and Bauman at Dow Chemical Co. unfolded the creation of the modern ion chromatography. Anions and cations could now be separated efficiently by a system of suppressed conductivity detection. In 1979, a method for anion chromatography with non-suppressed conductivity detection was introduced by Gjerde et al. Following it in 1980, was a similar method for cation chromatography.
As a result, a period of extreme competition began within the IC market, with supporters for both suppressed and non-suppressed conductivity detection. This competition led to fast growth of new forms and the fast evolution of IC. A challenge that needs to be overcome in the future development of IC is the preparation of highly efficient monolithic ion-exchange columns and overcoming this challenge would be of great importance to the development of IC.
The boom of Ion exchange chromatography primarily began between 1935 and 1950 during World War II and it was through the "Manhattan project" that applications and IC were significantly extended. Ion chromatography was originally introduced by two English researchers, agricultural Sir Thompson and chemist J T Way. The works of Thompson and Way involved the action of water-soluble fertilizer salts, ammonium sulfate and potassium chloride. These salts could not easily be extracted from the ground due to the rain. They performed ion methods to treat clays with the salts, resulting in the extraction of ammonia in addition to the release of calcium. It was in the fifties and sixties that theoretical models were developed for IC for further understanding and it was not until the seventies that continuous detectors were utilized, paving the path for the development from low-pressure to high-performance chromatography. Not until 1975 was "ion chromatography" established as a name in reference to the techniques, and was thereafter used as a name for marketing purposes. Today IC is important for investigating aqueous systems, such as drinking water. It is a popular method for analyzing anionic elements or complexes that help solve environmentally relevant problems. Likewise, it also has great uses in the semiconductor industry.
Because of the abundant separating columns, elution systems, and detectors available, chromatography has developed into the main method for ion analysis.
When this technique was initially developed, it was primarily used for water treatment. Since 1935, ion exchange chromatography rapidly manifested into one of the most heavily leveraged techniques, with its principles often being applied to majority of fields of chemistry, including distillation, adsorption, and filtration. | 3 | Analytical Chemistry |
Monocrystalline silicon (mono-Si) solar cells feature a single-crystal composition that enables electrons to move more freely than in a multi-crystal configuration. Consequently, monocrystalline solar panels deliver a higher efficiency than their multicrystalline counterparts. The corners of the cells look clipped, like an octagon, because the wafer material is cut from cylindrical ingots, that are typically grown by the Czochralski process. Solar panels using mono-Si cells display a distinctive pattern of small white diamonds. | 7 | Physical Chemistry |
In addition to RTK pathway, fibroblast growth factors can also activate the JAK-STAT signaling pathway. Instead of carrying covalently associated tyrosine kinase domains, Jak-STAT receptors form noncovalent complexes with tyrosine kinases of the Jak (Janus kinase) class. These receptors bind are for erythropoietin (important for erythropoiesis), thrombopoietin (important for platelet formation), and interferon (important for mediating immune cell function).
After dimerization of the cytokine receptors following ligand binding, the JAKs transphosphorylate each other. The resulting phosphotyrosines attract STAT proteins. The STAT proteins dimerize and enter the nucleus to act as transcription factors to alter gene expression. In particular, the STATs transcribe genes that aid in cell proliferation and survival – such as myc.
Phenotype and survival of mice after knockout of some JAK or STAT genes: | 1 | Biochemistry |
* I. Zegkinoglou, A. Zendegani, I. Sinev, S. Kunze, H. Mistry, H. S. Jeon, J. Zhao, M. Hu, E. E. Alp, S. Piontek, M. Smialkowski, U.-P. Apfel, F. Körmann, J. Neugebauer, T. Hickel, B. Roldan Cuenya: Operando phonon studies of the protonation mechanism in highly active hydrogen evolution reaction pentlandite catalysts, JACS 2017, 139, 14360,
* H. Mistry, Y. Choi, A. Bagger, F. Scholten, C. Bonifacio, I. Sinev, N. J. Divins, I. Zegkinoglou, H. Jeon, K. Kisslinger, E. A. Stach, J. C. Yang, J. Rossmeisl, B. Roldan Cuenya: Enhanced carbon dioxide electroreduction to carbon monoxide over defect rich plasma-activated silver catalysts, Angew. Chem. 2017, 56, 11394,
* H. Mistry, A. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B. Roldan Cuenya, Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene, Nature Commun. 2016, 7, 12123, . | 7 | Physical Chemistry |
The colour change from morphine is proposed to be a result of two molecules of morphine and two molecules of formaldehyde condensing to the dimeric product which is protonated to the oxocarbenium salt. | 3 | Analytical Chemistry |
Shock hardening is a process used to strengthen metals and alloys, wherein a shock wave produces atomic-scale defects in the material's crystalline structure. As in cold work, these defects interfere with the normal processes by which metallic materials yield (plasticity), making materials stiffer, but more brittle. When compared to traditional cold work, such an extremely rapid process results in a different class of defect, producing a much harder material for a given change in shape. If the shock wave applies too great a force for too long, however, the rarefaction front that follows it can form voids in the material due to hydrostatic tension, weakening the material and often causing it to spall. Since voids nucleate at large defects, such as oxide inclusions and grain boundaries, high-purity samples with a large grain size (especially single crystals) are able to withstand greater shock without spalling, and can therefore be made much harder.
Shock hardening has been observed in many contexts:
Explosive forging uses the detonation of a high explosive charge to create a shockwave. This effect is used to harden rail track cast components and, coupled with the Misnay-Schardin effect, in the operation of explosively forged penetrators. Greater hardening can be achieved by using a lower quantity of an explosive with greater brisance, so that the force applied is greater but the material spends less time in hydrostatic tension.
Laser shock, similar to inertial confinement fusion, uses the ablation plume caused by a laser pulse to apply force to the laser's target. The rebound from the expelled matter can create very high pressures, and the pulse length of lasers is often quite short, meaning that good hardening can be achieved with little risk of spallation. Surface effects can also be achieved by laser treatment, including amorphization.
Light-gas guns have been used to study shock hardening. Although too labor-intensive for widespread industrial application, they do provide a versatile research testbed. They allow precise control of both magnitude and profile of the shock wave through adjustments to the projectile's muzzle velocity and density profile, respectively. Studies of various projectile types have been crucial in overturning a prior theory that spallation occurs at a threshold of pressure, independent of time. Instead, experiments show longer-lasting shocks of a given magnitude produce more material damage. | 8 | Metallurgy |
At atmospheric pressure, three allotropic forms of iron exist, depending on temperature: alpha iron (α-Fe, ferrite), gamma iron (γ-Fe, austenite), and delta iron (δ-Fe). At very high pressure, a fourth form exists, epsilon iron (ε-Fe, hexaferrum). Some controversial experimental evidence suggests the existence of a fifth high-pressure form that is stable at very high pressures and temperatures.
The phases of iron at atmospheric pressure are important because of the differences in solubility of carbon, forming different types of steel. The high-pressure phases of iron are important as models for the solid parts of planetary cores. The inner core of the Earth is generally assumed to consist essentially of a crystalline iron-nickel alloy with ε structure. The outer core surrounding the solid inner core is believed to be composed of liquid iron mixed with nickel and trace amounts of lighter elements. | 8 | Metallurgy |
In anhydrous rubidium rhodizonate , the rhodizonate anions are stacked in parallel columns, as are the rubidium ions. In the plane perpendicular to the columns, these are arranged as two interleaved hexagonal grids. The anions are planar.
Anhydrous potassium rhodizonate has a distinct but similar structure. The anions and cations are arranged in alternate planes. Within each plane, the anions are arranged in a hexagonal grid. Each potassium ion is arranged so that it connects symmetrically to eight oxygens of four anions, two from each adjacent plane. The anions are slightly twisted in the "boat" shape (with 0.108 Å of rms deviation from mean plane). Sodium rhodizonate has the same structure, with slightly more distorted anions (0.113 Å rms)
In solution, the rhodizonate anion is not hydrated. | 3 | Analytical Chemistry |
The thermohaline circulation plays an important role in supplying heat to the polar regions, and thus in regulating the amount of sea ice in these regions, although poleward heat transport outside the tropics is considerably larger in the atmosphere than in the ocean. Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget.
Large influxes of low-density meltwater from Lake Agassiz and deglaciation in North America are thought to have led to a shifting of deep water formation and subsidence in the extreme North Atlantic and caused the climate period in Europe known as the Younger Dryas. | 9 | Geochemistry |
Most artificial plasmas are generated by the application of electric and/or magnetic fields through a gas. Plasma generated in a laboratory setting and for industrial use can be generally categorized by:
*The type of power source used to generate the plasma—DC, AC (typically with radio frequency (RF)) and microwave
*The pressure they operate at—vacuum pressure (< 10 mTorr or 1 Pa), moderate pressure (≈1 Torr or 100 Pa), atmospheric pressure (760 Torr or 100 kPa)
*The degree of ionization within the plasma—fully, partially, or weakly ionized
*The temperature relationships within the plasma—thermal plasma (), non-thermal or "cold" plasma ()
*The electrode configuration used to generate the plasma
*The magnetization of the particles within the plasma—magnetized (both ion and electrons are trapped in Larmor orbits by the magnetic field), partially magnetized (the electrons but not the ions are trapped by the magnetic field), non-magnetized (the magnetic field is too weak to trap the particles in orbits but may generate Lorentz forces) | 7 | Physical Chemistry |
An ion-exchange membrane is a semi-permeable membrane that transports certain dissolved ions, while blocking other ions or neutral molecules.
Ion-exchange membranes are therefore electrically conductive. They are often used in desalination and chemical recovery applications, moving ions from one solution to another with little passage of water.
Important examples of ion-exchange membranes include the proton-exchange membranes, that transport hydrogen| cations, and the anion exchange membranes used in certain alkaline fuel cells to transport Hydroxyl| anions. | 3 | Analytical Chemistry |
The Knudsen effusion cell was developed by Martin Knudsen (1871–1949). A typical Knudsen cell contains a crucible (made of pyrolytic boron nitride, quartz, tungsten or graphite), heating filaments (often made of metal tantalum), water cooling system, heat shields, and an orifice shutter. | 3 | Analytical Chemistry |
A decade after the discovery of RNAi mechanism in 1993, the pharmaceutical sector heavily invested in the research and development of siRNA therapy. There are several advantages that this therapy has over small molecules and antibodies. It can be administered quarterly or every six months. Another advantage is that, unlike small molecule and monoclonal antibodies that need to recognize specific conformation of a protein, siRNA functions by Watson-Crick basepairing with mRNA. Therefore, any target molecule that needs to be treated with high affinity and specificity can be selected if the right nucleotide sequence is available. One of the biggest challenges researchers needed to overcome was the identification and establishment of a delivery system through which the therapies would enter the body. And that the immune system often mistakes the RNAi therapies as remnants of infectious agents, which can trigger an immune response. Animal models did not accurately represent the degree of immune response that was seen in humans and despite the promise in the treatment investors divested away from RNAi.
However, there were a few companies that continued with the development of RNAi therapy for humans. Alnylam Pharmaceuticals, Sirna Therapeutics and Dicerna Pharmaceuticals are few of the companies still working on bringing RNAi therapies to market. It was learned that almost all siRNA therapies administered in the bloodstream accumulated in the liver. That is why most of the early drug targets were diseases that affected the liver. Repeated developmental work also shed light on improving the chemical composition of the RNA molecule to reduce the immune response, subsequently causing little to no side effects. Listed below are some of approved therapies or therapies in pipeline. | 1 | Biochemistry |
Shang et al. discovered the decarboxylative coupling of potassium oxalate monoesters with aryl halides to obtain aryl or alkenyl esters. | 0 | Organic Chemistry |
The mechanism of PEP carboxylase has been well studied. The enzymatic mechanism of forming oxaloacetate is very exothermic and thereby irreversible; the biological Gibbs free energy change (△G°’) is -30kJmol. The substrates and cofactor bind in the following order: metal cofactor (either Co, Mg, or Mn), PEP, bicarbonate (HCO). The mechanism proceeds in two major steps, as described below and shown in figure 2:
# The bicarbonate acts as a nucleophile to attack the phosphate group in PEP. This results in the splitting of PEP into a carboxyphosphate and the (very reactive) enolate form of pyruvate.
# Proton transfer takes place at the carboxyphosphate. This is most likely modulated by a histidine (H138) residue that first deprotonates the carboxy side, and then, as an acid, protonates the phosphate part. The carboxyphosphate then exothermically decomposes into carbon dioxide and inorganic phosphate, at this point making this an irreversible reaction. Finally, after the decomposition, the carbon dioxide is attacked by the enolate to form oxaloacetate.
The metal cofactor is necessary to coordinate the enolate and carbon dioxide intermediates; the CO molecule is only lost 3% of the time. The active site is hydrophobic to exclude water, since the carboxyphosphate intermediate is susceptible to hydrolysis. | 5 | Photochemistry |
This method required the introduction of a very small volume of concentrated tracer at the inlet of the reactor, such that it approaches the Dirac delta function. Although an infinitely short injection cannot be produced, it can be made much smaller than the mean residence time of the vessel. If a mass of tracer, , is introduced into a vessel of volume and an expected residence
time of , the resulting curve of can be transformed into a dimensionless residence time distribution curve by the following relation: | 9 | Geochemistry |
Packings where all spheres are constrained by their neighbours to stay in one location are called rigid or jammed. The strictly jammed (mechanically stable even as a finite system) sphere packing with the lowest known density is a diluted ("tunneled") fcc crystal with a density of only . The loosest known jammed packing has a density of approximately 0.0555. | 3 | Analytical Chemistry |
Unlike traditional steel or titanium, this material dissolves in organisms at a rate of roughly 1 millimeter per month and is replaced with bone tissue. This speed can be adjusted by varying the content of zinc.
Amorphous CaZnMg alloy exhibits extremely poor corrosion resistance. Wang et al. reported that the said amorphous alloy completely disintegrated after no more than 3 hours exposure in biocorrosion environment. In static distilled water at room temperature, Dahlman et al. also reported destructive corrosion reactions of the same material, decomposing into a multiphase powder.
Ca-BMGs with higher Zn contents as reported by Cao et al. showed an elastic modulus in the range of 35–46 GPa, and a hardness of 0.7–1.4 GPa. | 8 | Metallurgy |
The first total synthesis of torreyanic acid was reported by Porco an co-workers in 2000. This total synthesis aimed to employ and confirm the Diels–Alder genesis proposed by Lee et al. To synthesize the monomers required for Diels–Alder dimerization, 1,3-dioxane intermediate 4 was lithiated with BuLi, brominated with BrCFCFBr, and underwent acid hydrolysis to afford benzaldehyde 5. Upon selective methylation of 5 with sulfuric acid, phenol 6 was produced in 52% yield. Phenol 6 first underwent an allylation with allyl bromide, then a borohydride reduction, and finally a protection with a silyl group to furnish 7. Dimethoxyacetal 8 was furnished upon thermal Claisen rearrangement of 7, which afforded an unstable allyl phenol that directly underwent a hypervalent iodine oxidation with in methanol. 8 was then subjected to an acetal exchange with 1,3-propanediol to afford 1,3-dioxane 9, which was smoothly monoepoxidized with Ph3COOH, KHMDS, −78 °C to −20 °C over 6 hours to afford 10. A 2-methyl-2-butenoic acid moiety was installed to afford 11. Intermediate 11 underwent a Stille vinylation with (E)-tributyl-1-heptenyl stannane, subsequently subjected to TBAF/AcOH for silyl removal and acetal hydrolysis to afford quinone epoxide 12. Treatment of 12 with Dess-Martin periodinane initiated a tandem oxidation-6p-electrocyclization-dimerization to afford two dimeric products 13 and 14. Upon treatment of 13 and 14 with TFA to remove the tert-butyl ester, iso-torreyanic acid 15 and torreyanic acid 1 were afforded, respectively. | 0 | Organic Chemistry |
Here the solvent travels up the chromatographic paper. Both descending and ascending paper chromatography are used for the separation of organic and inorganic substances.
The sample and solvent move upward. | 3 | Analytical Chemistry |
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