text
stringlengths 105
4.44k
| label
int64 0
9
| label_text
stringclasses 10
values |
|---|---|---|
The basic building blocks of carotenoids are isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). These two isoprene isomers are used to create various compounds depending on the biological pathway used to synthesize the isomers. Plants are known to use two different pathways for IPP production: the cytosolic mevalonic acid pathway (MVA) and the plastidic methylerythritol 4-phosphate (MEP). In animals, the production of cholesterol starts by creating IPP and DMAPP using the MVA. For carotenoid production plants use MEP to generate IPP and DMAPP. The MEP pathway results in a 5:1 mixture of IPP:DMAPP. IPP and DMAPP undergo several reactions, resulting in the major carotenoid precursor, geranylgeranyl diphosphate (GGPP). GGPP can be converted into carotenes or xanthophylls by undergoing a number of different steps within the carotenoid biosynthetic pathway. | 5 | Photochemistry |
The specific feature of German silver overlay is the degree of purity – 925 for Sterling silver or 999 for fine silver. The quality of silver used for German overlay can be seen by identifying the impressed 1000 mark (usually on the base or side of an item). The purity and thickness of the silver overlay ensures the beauty of the item is maintained without any loss to the silver even after many years of cleaning. Although other kinds of silver, such as nickel silver, can be sued for silver overlay, the effect is not as vibrant.
Bohemian and Venetian Murano glass are often described as silver overlay, but the silver is so thin that it looks as though it has been painted onto the surface, producing a flat effect.
Friedrich Deusch invented a way to combine silver and a non-conductive surface such as porcelain or glass with galvanization. He achieved this with a special conductive fluid (a type of flux) which was fixed permanently on the prepared form. The particular objects (such as a vase) were first roughened by engraving or using hydrofluoric acid to etch a design. This implies that a very exact covering of the surfaces had to be achieved to prevent any damage to areas which were not to be overlaid. Maybe they used a masking lacquer which could withstand the following baking in the kiln which was used to fuse the flux with the surface of the item. The next step was to galvanize the item with the purest silver (1000).
It was very important to monitor items being overlaid with silver – waiting too long resulted in visible dark spots and slight roughness where the cathode or anode were fixed. The cathode and anode were used to charge the item electronically and this allowed the fusion of silver to the area painted with the silver flux. The thickness of silver desired on the finished item determined how long the item needed to be left in the silver bath; this could be more than 30 hours. Finally, if the masking lacquer (discussed earlier) did not burn in the kiln, it must have been removed later (probably with chemical fluids).
When colored enamels were to be used on the finished product, they had to be fired prior to the final stage of the silver overlay process. Engraving the silver was the last, and sometimes most laborious, work; it was brought to the highest level by Friedrich Wilhelm Spahr and his workers. | 8 | Metallurgy |
Acyl azides are carboxylic acid derivatives with the general formula RCON. These compounds, which are a subclass of organic azides, are generally colorless. | 0 | Organic Chemistry |
In chemistry, a NONOate is a compound having the chemical formula RRN−(NO)−N=O, where R and R are alkyl groups. One example for this is 1,1-diethyl-2-hydroxy-2-nitrosohydrazine, or diethylamine dinitric oxide. These compounds are unusual in having three sequential nitrogen atoms: an amine functional group, a bridging NO group, and a terminal nitrosyl group. In contact with water, these compounds release NO (nitric oxide). | 0 | Organic Chemistry |
The Caspian turtle (Mauremys caspica), although found in neighboring areas, is a wholly freshwater species. The zebra mussel is native to the Caspian and Black Sea basins, but has become an invasive species elsewhere, when introduced. The area has given its name to several species, including the Caspian gull and the Caspian tern. The Caspian seal (Pusa caspica) is the only aquatic mammal endemic to the Caspian Sea, being one of very few seal species that live in inland waters, but it is different from those inhabiting freshwaters due to the hydrological environment of the sea. A century ago the Caspian was home to more than one million seals. Today, fewer than 10% remain.
Archeological studies of Gobustan Rock Art have identified what may be oceanic species including cetaceans from baleen whales to dolphins, and auks most likely Brunnich's Guillemot, although the rock art on Kichikdash Mountain which is assumed to depict either a beaked whale or a dolphin, it may represent the famous beluga sturgeon instead due to its size (430 cm in length). These petroglyphs may suggest potential presences of oceanic faunas in the Caspian Sea presumably until the Quaternary or even the last glacial period or antiquity due to historic marine inflow between the current Caspian Sea and either the Arctic Ocean or North Sea, or the Black Sea. This is supported by the existences of current endemic, oceanic species such as lagoon cockles which was genetically identified to originate in the Caspian and Black Seas regions.
The seas basin (including associated waters such as rivers) has 160 native species and subspecies of fish in more than 60 genera. About 62% of the species and subspecies are endemic, as are 4–6 genera (depending on taxonomic treatment). The lake proper has 115 natives, including 73 endemics (63.5%). Among the more than 50 genera in the lake proper, 3–4 are endemic: Anatirostrum, Caspiomyzon, Chasar (often included in Ponticola) and Hyrcanogobius. By far the most numerous families in the lake proper are gobies (35 species and subspecies), cyprinids (32) and clupeids (22). Two particularly rich genera are Alosa with 18 endemic species/subspecies and Benthophilus with 16 endemic species. Other examples of endemics are four species of Clupeonella, Gobio volgensis, two Rutilus, three Sabanejewia, Stenodus leucichthys, two Salmo, two Mesogobius and three Neogobius'. Most non-endemic natives are either shared with the Black Sea basin or widespread Palearctic species such as crucian carp, Prussian carp, common carp, common bream, common bleak, asp, white bream, sunbleak, common dace, common roach, common rudd, European chub, sichel, tench, European weatherfish, wels catfish, northern pike, burbot, European perch and zander. Almost 30 non-indigenous, introduced fish species have been reported from the Caspian Sea, but only a few have become established.
Six sturgeon species, the Russian, bastard, Persian, sterlet, starry and beluga, are native to the Caspian Sea. The last of these is arguably the largest freshwater fish in the world. The sturgeon yield roe (eggs) that are processed into caviar. Overfishing has depleted a number of the historic fisheries. In recent years, overfishing has threatened the sturgeon population to the point that environmentalists advocate banning sturgeon fishing completely until the population recovers. The high price of sturgeon caviar – more than 1,500 Azerbaijani manats (US$880 ) per kilo – allows fishermen to afford bribes to ensure the authorities look the other way, making regulations in many locations ineffective. Caviar harvesting further endangers the fish stocks, since it targets reproductive females.
Reptiles native to the region include the spur-thighed tortoise (Testudo graeca buxtoni) and Horsfield's tortoise.
*The Asiatic cheetah used to occur in the Trans-Caucasus and Central Asia, but is today restricted to Iran.
*The Asiatic lion used to occur in the Trans-Caucasus, Iran, and possibly the southern part of Turkestan.
*The Caspian tiger used to occur in northern Iran, the Caucasus and Central Asia.
*The endangered Persian leopard is found in Iran, the Caucasus and Central Asia. | 2 | Environmental Chemistry |
Compared to ketones and aldehydes, esters are relatively resistant to reduction. The introduction of catalytic hydrogenation in the early part of the 20th century was a breakthrough; esters of fatty acids are hydrogenated to fatty alcohols.
A typical catalyst is copper chromite. Prior to the development of catalytic hydrogenation, esters were reduced on a large scale using the Bouveault–Blanc reduction. This method, which is largely obsolete, uses sodium in the presence of proton sources.
Especially for fine chemical syntheses, lithium aluminium hydride is used to reduce esters to two primary alcohols. The related reagent sodium borohydride is slow in this reaction. DIBAH reduces esters to aldehydes.
Direct reduction to give the corresponding ether is difficult as the intermediate hemiacetal tends to decompose to give an alcohol and an aldehyde (which is rapidly reduced to give a second alcohol). The reaction can be achieved using triethylsilane with a variety of Lewis acids. | 0 | Organic Chemistry |
In 1972, Friedmann and Roblin authored a paper in Science titled "Gene therapy for human genetic disease?". Rogers (1970) was cited for proposing that exogenous good DNA be used to replace the defective DNA in those with genetic defects. | 1 | Biochemistry |
Amines serve as nucleophiles in attacking the sulfonyl chloride electrophile, displacing chloride. The sulfonamides resulting from primary and secondary amines are poorly soluble and precipitate as solids from solution.
For primary amines (R' = H), the initially formed sulfonamide is deprotonated by base to give a water-soluble sulfonamide salt (Na[PhSONR]).
Tertiary amines promote hydrolysis of the sulfonyl chloride functional group, which affords water-soluble sulfonate salts. | 3 | Analytical Chemistry |
Most well-nourished people in industrialized countries have 4 to 5 grams of iron in their bodies (~38 mg iron/kg body weight for women and ~50 mg iron/kg body for men). Of this, about is contained in the hemoglobin needed to carry oxygen through the blood (around 0.5 mg of iron per mL of blood), and most of the rest (approximately 2 grams in adult men, and somewhat less in women of childbearing age) is contained in ferritin complexes that are present in all cells, but most common in bone marrow, liver, and spleen. The liver stores of ferritin are the primary physiologic source of reserve iron in the body. The reserves of iron in industrialized countries tend to be lower in children and women of child-bearing age than in men and in the elderly. Women who must use their stores to compensate for iron lost through menstruation, pregnancy or lactation have lower non-hemoglobin body stores, which may consist of , or even less.
Of the body's total iron content, about is devoted to cellular proteins that use iron for important cellular processes like storing oxygen (myoglobin) or performing energy-producing redox reactions (cytochromes). A relatively small amount (3–4 mg) circulates through the plasma, bound to transferrin. Because of its toxicity, free soluble iron is kept in low concentration in the body.
Iron deficiency first affects the storage of iron in the body, and depletion of these stores is thought to be relatively asymptomatic, although some vague and non-specific symptoms have been associated with it. Since iron is primarily required for hemoglobin, iron deficiency anemia is the primary clinical manifestation of iron deficiency. Iron-deficient people will suffer or die from organ damage well before their cells run out of the iron needed for intracellular processes like electron transport.
Macrophages of the reticuloendothelial system store iron as part of the process of breaking down and processing hemoglobin from engulfed red blood cells. Iron is also stored as a pigment called hemosiderin, which is an ill-defined deposit of protein and iron, created by macrophages where excess iron is present, either locally or systemically, e.g., among people with iron overload due to frequent blood cell destruction and the necessary transfusions their condition calls for. If systemic iron overload is corrected, over time the hemosiderin is slowly resorbed by the macrophages. | 1 | Biochemistry |
In thermodynamics, a departure function is defined for any thermodynamic property as the difference between the property as computed for an ideal gas and the property of the species as it exists in the real world, for a specified temperature T and pressure P. Common departure functions include those for enthalpy, entropy, and internal energy.
Departure functions are used to calculate real fluid extensive properties (i.e. properties which are computed as a difference between two states). A departure function gives the difference between the real state, at a finite volume or non-zero pressure and temperature, and the ideal state, usually at zero pressure or infinite volume and temperature.
For example, to evaluate enthalpy change between two points h(v,T) and h(v,T) we first compute the enthalpy departure function between volume v and infinite volume at T = T, then add to that the ideal gas enthalpy change due to the temperature change from T to T, then subtract the departure function value between v and infinite volume.
Departure functions are computed by integrating a function which depends on an equation of state and its derivative. | 7 | Physical Chemistry |
Vitamin D overdose causes hypercalcemia, which is a strong indication of vitamin D toxicity – this can be noted with an increase in urination and thirst. If hypercalcemia is not treated, it results in excess deposits of calcium in soft tissues and organs such as the kidneys, liver, and heart, resulting in pain and organ damage.
The main symptoms of vitamin D overdose are hypercalcemia including anorexia, nausea, and vomiting. These may be followed by polyuria, polydipsia, weakness, insomnia, nervousness, pruritus and ultimately kidney failure. Furthermore, proteinuria, urinary casts, azotemia, and metastatic calcification (especially in the kidneys) may develop. Other symptoms of vitamin D toxicity include intellectual disability in young children, abnormal bone growth and formation, diarrhea, irritability, weight loss, and severe depression.
Vitamin D toxicity is treated by discontinuing vitamin D supplementation and restricting calcium intake. Kidney damage may be irreversible. Exposure to sunlight for extended periods of time does not normally cause vitamin D toxicity. The concentrations of vitamin D precursors produced in the skin reach an equilibrium, and any further vitamin D produced is degraded. | 1 | Biochemistry |
Cefalexin is the International Nonproprietary Name (INN) and the Australian Approved Name (AAN), while cephalexin is the British Approved Name (BAN) and the United States Adopted Name (USAN). Brand names for cefalexin include Keflex, Acfex, Cephalex, Ceporex, L-Xahl, Medoxine, Ospexin, Torlasporin, Bio-Cef, Panixine DisperDose, and Novo-Lexin. | 4 | Stereochemistry |
Some of the simplest reactive transport problems can be solved analytically. Where equilibrium sorption is described by a linear distribution coefficient, for example, the sorbing solute's velocity is retarded relative to that of a nonreactive tracer; the relative velocities can be described with a retardation factor. Analytical solutions are exact solutions of the governing equations.
Complex reactive transport problems are more commonly solved numerically. In this case, the governing equations are approximated so that they can be solved by computer algorithms. The governing equations, including both reaction and transport terms, can be solved simultaneously using a one-step or global implicit simulator. This technique is straightforward conceptually, but computationally very difficult.
Instead of solving all the relevant equations together, the transport and chemical reaction equations can be solved separately. Operator splitting, as this technique is known, uses appropriate numerical techniques to solve the reaction and transport equations at each time step. Various methods exist, including the sequential non-iterative approach (SNIA), Strang splitting, and sequential iterative approach (SIA). Since the reaction and transport terms are handled separately, separate programs for batch reaction and transport can be linked together. Cross-linkable re-entrant software objects designed for this purpose readily enable construction of reactive transport models of any flow configuration. | 9 | Geochemistry |
Analogously to the in situ IR experiments described above, in situ UV-visible absorbance spectroscopy may be used to monitor the course of a reaction, provided a reagent or product shows distinctive absorbance in the UV spectral region. The rate of reactant consumption and/or product formation may be abstracted from the change of absorbance over time (by application of Beer's Law), again leading to classification as an integral technique. Due to the spectral region utilized, UV-vis techniques are more commonly utilized on inorganic or organometallic systems than on purely organic reactions, and examples include exploration of the samarium Barbier reaction. | 7 | Physical Chemistry |
For discrete probability distributions and defined on the same sample space, the relative entropy from to is defined to be
which is equivalent to
In other words, it is the expectation of the logarithmic difference between the probabilities and , where the expectation is taken using the probabilities .
Relative entropy is only defined in this way if, for all , implies (absolute continuity). Otherwise, it is often defined as but the value is possible even if everywhere, provided that is infinite in extent. Analogous comments apply to the continuous and general measure cases defined below.
Whenever is zero the contribution of the corresponding term is interpreted as zero because
For distributions and of a continuous random variable, relative entropy is defined to be the integral
where and denote the probability densities of and .
More generally, if and are probability measures on a measurable space and is absolutely continuous with respect to , then the relative entropy from to is defined as
where is the Radon–Nikodym derivative of with respect to , i.e. the unique almost everywhere defined function on such that which exists because is absolutely continuous with respect to . Also we assume the expression on the right-hand side exists. Equivalently (by the chain rule), this can be written as
which is the entropy of relative to . Continuing in this case, if is any measure on for which densities and with and exist (meaning that and are both absolutely continuous with respect to ), then the relative entropy from to is given as
Note that such a measure for which densities can be defined always exists, since one can take although in practice it will usually be one that in the context like counting measure for discrete distributions, or Lebesgue measure or a convenient variant thereof like Gaussian measure or the uniform measure on the sphere, Haar measure on a Lie group etc. for continuous distributions.
The logarithms in these formulae are usually taken to base 2 if information is measured in units of bits, or to base if information is measured in nats. Most formulas involving relative entropy hold regardless of the base of the logarithm.
Various conventions exist for referring to in words. Often it is referred to as the divergence between and , but this fails to convey the fundamental asymmetry in the relation. Sometimes, as in this article, it may be described as the divergence of from or as the divergence from to . This reflects the asymmetry in Bayesian inference, which starts from a prior and updates to the posterior . Another common way to refer to is as the relative entropy of with respect to or the information gain from over . | 7 | Physical Chemistry |
When comparing a polar and nonpolar molecule with similar molar masses, the polar molecule in general has a higher boiling point, because the dipole–dipole interaction between polar molecules results in stronger intermolecular attractions. One common form of polar interaction is the hydrogen bond, which is also known as the H-bond. For example, water forms H-bonds and has a molar mass M = 18 and a boiling point of +100 °C, compared to nonpolar methane with M = 16 and a boiling point of –161 °C. | 7 | Physical Chemistry |
Murray was the author or co-author of scientific publications about organic chemistry and lichens. Some of his most significant were:
* James Murray (1959) [https://paperspast.natlib.govt.nz/periodicals/TPRSNZ1960-88.2.6.5 Studies of New Zealand Lichens. I—The Coniocarpineae] Transactions of the Royal Society of New Zealand 88 (2) 177 - 195
* James Murray (1960) [https://paperspast.natlib.govt.nz/periodicals/TPRSNZ1960-88.2.6.6 Studies of New Zealand Lichens. II—The Teloschistaceae] Transactions of the Royal Society of New Zealand 88 (2) 197 - 210
* James Murray (1960) [https://paperspast.natlib.govt.nz/periodicals/TPRSNZ1960-88.2.8.4 Studies on New Zealand Lichens Part III.–The Family Peltigeraceae] Transactions of the Royal Society of New Zealand 88 (3) 381 - 399
* James Murray (1962) [https://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio10Tuat03-t1-body-d2.html Keys to New Zealand lichens. Part 1.] Tuatara 10 (3) 120 - 128 (published posthumously)
* James Murray (1963) [https://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio11Tuat01-t1-body-d8.html Keys to New Zealand lichens. Part 2.] Tuatara 11 (1) 46 - 56 (published posthumously)
* James Murray (1963) [https://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio11Tuat02-t1-body-d9.html Keys to New Zealand lichens. Part 2.] Tuatara 11 (2) 98 - 109 (published posthumously) | 0 | Organic Chemistry |
Effector triggered immunity (ETI) is activated by the presence of pathogen effectors. The ETI response is reliant on R genes, and is activated by specific pathogen strains. Plant ETI often causes an apoptotic hypersensitive response. | 1 | Biochemistry |
The third group of chemokines is known as the C chemokines (or γ chemokines), and is unlike all other chemokines in that it has only two cysteines; one N-terminal cysteine and one cysteine downstream. Two chemokines have been described for this subgroup and are called XCL1 (lymphotactin-α) and XCL2 (lymphotactin-β). | 1 | Biochemistry |
Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), is an analytical technique used for the detection of chemical elements. It is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The plasma is a high temperature source of ionised source gas (often argon). The plasma is sustained and maintained by inductive coupling from electrical coils at megahertz frequencies. The source temperature is in the range from 6000 to 10,000 K. The intensity of the emissions from various wavelengths of light are proportional to the concentrations of the elements within the sample. | 3 | Analytical Chemistry |
A sacrificial metal is a metal used as a sacrificial anode in cathodic protection that corrodes to prevent a primary metal from corrosion or rusting. It may also be used for galvanization. | 7 | Physical Chemistry |
The fluorescence quantum yield is defined as the ratio of the number of photons emitted to the number of photons absorbed.
Fluorescence quantum yield is measured on a scale from 0 to 1.0, but is often represented as a percentage. A quantum yield of 1.0 (100%) describes a process where each photon absorbed results in a photon emitted. Substances with the largest quantum yields, such as rhodamines, display the brightest emissions; however, compounds with quantum yields of 0.10 are still considered quite fluorescent.
Quantum yield is defined by the fraction of excited state fluorophores that decay through fluorescence:
where
* is the fluorescence quantum yield,
* is the rate constant for radiative relaxation (fluorescence),
* is the rate constant for all non-radiative relaxation processes.
Non-radiative processes are excited state decay mechanisms other than photon emission, which include: Förster resonance energy transfer, internal conversion, external conversion, and intersystem crossing. Thus, the fluorescence quantum yield is affected if the rate of any non-radiative pathway changes. The quantum yield can be close to unity if the non-radiative decay rate is much smaller than the rate of radiative decay, that is .
Fluorescence quantum yields are measured by comparison to a standard of known quantum yield. The quinine salt quinine sulfate in a sulfuric acid solution was regarded as the most common fluorescence standard, however, a recent study revealed that the fluorescence quantum yield of this solution is strongly affected by the temperature, and should no longer be used as the standard solution. The quinine in 0.1M perchloric acid ( 0.60) shows no temperature dependence up to 45 °C, therefore it can be considered as a reliable standard solution.
Experimentally, relative fluorescence quantum yields can be determined by measuring fluorescence of a fluorophore of known quantum yield with the same experimental parameters (excitation wavelength, slit widths, photomultiplier voltage etc.) as the substance in question. The quantum yield is then calculated by:
where
* is the quantum yield,
* is the area under the emission peak (on a wavelength scale),
* is absorbance (also called "optical density") at the excitation wavelength,
* is the refractive index of the solvent.
The subscript denotes the respective values of the reference substance. The determination of fluorescence quantum yields in scattering media requires additional considerations and corrections. | 7 | Physical Chemistry |
The structures of Co(SCN) and its hydrate Co(SCN)(HO) have been determined using X-ray crystallography. Co(SCN) forms infinite 2D sheets in the mercury(II) thiocyanate structure type, where as Co(SCN)(HO) consists of isolated tetrahedral Co(SCN)(HO) centers and one equivalent of water of crystallization.
The hydrate may be prepared by the salt metathesis reactions, such as the reaction of aqueous cobalt(II) sulfate and barium thiocyanate to produce a barium sulfate precipitate, leaving the hydrate of Co(SCN) in solution:
:CoSO + Ba(SCN) → BaSO + Co(SCN)
or the reaction of the hexakisacetonitrile cobalt(II) tetrafluoroborate and potassium thiocyanate, precipitating KBF
:[Co(NCMe)](BF) + 2KSCN → 2KBF + Co(SCN).
The anhydrate can then be prepared via addition of diethylether as an antisolvent. | 3 | Analytical Chemistry |
Linked-read sequencing can facilitate de novo genome assembly, which involves reconstructing a genome from scratch without any prior reference. Linked-read sequencing enables assembly of large genomic regions, and helps improve the completeness and contiguity of the resulting genome. This can be particularly useful for studying organisms that lack a high-quality reference genome, such as non-model organisms or organisms with complex genomes. Many scientists have been using linked-read sequencing technology for de novo genome assembly recently in a variety of organisms, including humans, plants, and animals. For example, Dr. Evan Eichler and his research group used linked-read sequencing to assemble genome of orangutan, which had previously been difficult to study due to its complex genome. The resulting genome assembly helped scientists to study new insights into the evolutionary history of primates and the genetic basis of human diseases. Also, the aligned or assembled reads can be used for other genetic investigations or downstream analysis, such as haplotype phasing. | 1 | Biochemistry |
In negative-sense MVM genomes, the left-end hairpin is 121 nucleotides in length and exists in a single flip sequence orientation. This telomere is Y-shaped and contains small internal palindromes that fold into the "ears" of the Y, a duplex stem region 43 nucleotides in length that is interrupted by an asymmetric thymidine residue, and a mismatched "bubble" sequence in which the 5′-GAA-3′ sequence on the inboard arm is opposite of 5′-GA-3′ in the outboard strand. Sequences in this hairpin are involved in both replication and regulation of transcription. The elements involved in these two functions separate the two arms of the hairpin.
The left-end telomere of MVM, and likely of all heterotelomeric parvoviruses, cannot function as a replication origin in its hairpin configuration. Instead, a single origin on the lower strand is created when the hairpin is unfolded, extended, and copied to form a duplex basepaired sequence that spans adjacent genomes in the dimer RF. Within this structure, the sequence from the outboard arm that surrounds a GA/TC dinucleotide serves as an origin, OriL. The equivalent GAA/TTC sequence on the inboard arm that contains the bubble trinucleotide, called OriL, does not serve as an origin. The inboard arm and hairpin configuration of the terminus instead appear to function as upstream control elements for the viral transcriptional promoter P4. Additionally, the ability to segregate one arm from nicking appears essential for replication.
The minimal linear left-end origin is about 50 basepairs long and extends from two 5′-ACGT-3′ motifs, spaced five nucleotides apart at one end, to a position seven basepairs beyond the nick site. The bubble's GA sequence itself is relatively unimportant, but the space that it occupies is necessary for the origin to function. Within the origin, there are three recognition sequences: an NS1-binding site that orients the NS1 complex over the nick site 5′-CTWWTCA-3′, which is located 17 nucleotides downstream (toward the 3′-end), and the two ACGT motifs. These motifs bind a heterodimeric cellular factor called either parvovirus initiation factor (PIF) or glucocorticoid modulating element-binding protein (GMEB).
PIF is a site-specific DNA-binding heterodimeric complex that contains two subunits, p96 and p79, and functions as a transcription modulator in the host cell. It binds DNA via a KDWK fold and recognizes two ACGT half-sites. The spacing between these sites can vary significantly for PIF, from one to nine nucleotides, with an optimal spacing of six. PIF stabilizes the binding of NS1 on the active form of the left-end origin, OriL, but not on the inactive form, OriL, because the two complexes are able to establish contact over the bubble binucleotide. The left-end hairpin of all other species in the Protoparvovirus genus, of which MVM belongs, have bubble asymmetries and PIF-binding sites, though with slight variation in spacing. This suggests that they all share a similar origin segregation mechanism. | 1 | Biochemistry |
When an electron beam is polarized, an unbalance between spin-up, , and spin-down electrons, , exists. The unbalance can be evaluated through the polarization defined as
It is known that, when an electron collides against a nucleus, the scattering event is governed by Coulomb interaction. This is the leading term in the Hamiltonian, but a correction due to spin-orbit coupling can be taken into account and the effect on the Hamiltonian can be evaluated with the perturbation theory. Spin orbit interaction can be evaluated, in the rest reference frame of the electron, as the result of the interaction of the spin magnetic moment of the electron
with the magnetic field that the electron sees, due to its orbital motion around the nucleus, whose expression in the non-relativistic limit is:
In these expressions is the spin angular-momentum, is the Bohr magneton, is the g-factor, is the reduced Planck constant, is the electron mass, is the elementary charge, is the speed of light, is the potential energy of the electron and is the angular momentum.
Due to spin orbit coupling, a new term will appear in the Hamiltonian, whose expression is
Due to this effect, electrons will be scattered with different probabilities at different angles. Since the spin-orbit coupling is enhanced when the involved nuclei possess a high atomic number Z, the target is usually made of heavy metals, such as mercury, gold and thorium. | 7 | Physical Chemistry |
The reactions best for pressure (geobarometers) are ones that have a large change in molar volume during the reaction. Higher pressures cause the reaction to decrease in total volume, and lighter pressures allow reaction to increase in total volume. Therefore, based on the proportion of minerals that have larger volumes versus the proportion of minerals that have smaller volumes, the pressure of the environment during the reaction can be calculated, as a function of temperature. Experiments must be done to calibrate each reaction and determine the rate at which the volume changes with changes in pressure. | 9 | Geochemistry |
The typical SAF setup consists of a laser line (typically 450-633 nm), which is reflected into the aspheric lens by a dichroic mirror. The lens focuses the laser beam in the sample, causing the particles to fluoresce. The fluorescent light then passes through a parabolic lens before reaching a detector, typically a photomultiplier tube or avalanche photodiode detector. It is also possible to arrange SAF elements as arrays, and image the output onto a CCD, allowing the detection of multiple analytes. | 3 | Analytical Chemistry |
Glycans and glycan-based molecules have been used as drugs themselves. The two main functions of these drugs are to either bind protein or inhibit glycosyl degradation. For example, engineered glycans, such as Zanamivir and Oseltamivir have been designed to bind to viral sialidases, which are enzymes that play key roles in viral replication cycles, such as for influenza. With these sialidases inhibited, viral budding and entry into host cells is inhibited. Other drugs, such as Miglitol and Acarbose, serve as therapeutic drugs to people with Type 2 diabetes, as these engineered glycan derivatives bind to glucosidases and amylases to help control patient's blood sugar level. | 1 | Biochemistry |
The program motivates companies to disclose and reduce their environmental impacts by using the power of investors and companies. | 2 | Environmental Chemistry |
Indications of thermal rearrangements of aromatic hydrocarbons were first noted in the early 20th century by natural products chemists who were working with sesquiterpenes. At the time, they noticed the automerization of a substituted azulene shown below, but no further structural or mechanistic investigations were made.
The oldest characterized thermal rearrangement of an aromatic compound was that of the isomerization of azulene to naphthalene by Heilbronner et al. in 1947. Since then, many other isomerizations have been recorded, however the rearrangement of azulene to naphthalene has received the most attention. Likewise, since the characterization of the automerization of naphthalene by Scott in 1977, similar atom scramblings of other aromatic hydrocarbons such as pyrene, azulene, [[Benz(a)anthracene|benz[a]anthracene]] and even benzene have been described. While the existence of these reactions has been confirmed, the isomerization and automerization mechanisms remain unknown. | 5 | Photochemistry |
Original ATM techniques involve rotating the sample at the focal point of a linearly polarized THz beam using a mechanically rotated sample mount. For this reason, the configuration is typically a far-field instrument in which a balanced detector (sensitive to infrared light) is placed a considerable distance from the sample. In the terahertz time-domain spectroscopy configuration, both the infrared and THz beams are transmitted through an electro-optic (EO) crystal like ZnTe or GaP. Here, the infrared beam detects the change in birefringence of the EO crystal due to the THz beam. When a sample is placed in the THz beam, the polarized THz beam is perturbed and the resulting degree of birefringence in the EO crystal is changed. The resulting perturbation of the infrared beam is sensed at the balanced detector.
Rotated sample ATM is very useful for large samples (0.1 to 1 cm). However, when measuring samples such as protein crystals that must be isolated inside a hydration chamber, for example, the sample cannot be easily rotated. Additionally, it is challenging to maintain the same location of a rotated sample at the precise focal point of a THz beam. | 7 | Physical Chemistry |
The cAMP signal transduction contains five main characters: stimulative hormone receptor (Rs) or inhibitory hormone receptor (Ri); stimulative regulative G-protein (Gs) or inhibitory regulative G-protein (Gi); adenylyl cyclase; protein kinase A (PKA); and cAMP phosphodiesterase.
Stimulative hormone receptor (Rs) is a receptor that can bind with stimulative signal molecules, while inhibitory hormone receptor (Ri) is a receptor that can bind with inhibitory signal molecules.
Stimulative regulative G-protein is a G-protein linked to stimulative hormone receptor (Rs), and its α subunit upon activation could stimulate the activity of an enzyme or other intracellular metabolism. On the contrary, inhibitory regulative G-protein is linked to an inhibitory hormone receptor, and its α subunit upon activation could inhibit the activity of an enzyme or other intracellular metabolism.
Adenylyl cyclase is a 12-transmembrane glycoprotein that catalyzes the conversion of ATP to cAMP with the help of cofactor Mg or Mn. The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator of protein kinase A.
Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylating specific committed enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. The protein enzyme contains two catalytic subunits and two regulatory subunits. When there is no cAMP，the complex is inactive. When cAMP binds to the regulatory subunits, their conformation is altered, causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects.
These signals then can be terminated by cAMP phosphodiesterase, which is an enzyme that degrades cAMP to 5'-AMP and inactivates protein kinase A. | 1 | Biochemistry |
A number of these anions known. Some have attracted interest as components in fast ion conductors for use in solid state batteries. The binary thiophosphates do not exhibit the extensive diversity of the analogous oxyanions but contain similar structural features, for example P is 4 coordinate, P−S−P links form and there are P−P bonds. One difference is that ions may include polysulfide fragments of 2 or more S atoms whereas in the P−O anions there is only the reactive −O−O−, peroxo, unit.
* is the analogue of the nitrate ion, (there is no analogue); it was isolated as the yellow tetraphenylarsonium salt
* is the sulfur analogue of , and like is tetrahedral.
* the pyrothiophosphate ion consisting of two corner sharing PS tetrahedra, analogous to the pyrophosphates.
* An ion which can be visualised either as two PS tetrahedra joined by a disulfide link or a pyrothiophosphate where the bridging −S− is replaced by −S−.
* edge-shared bitetrahedral structure. The structure is therefore similar to the isoelectronic AlCl dimer. The oxygen analogue, dimetaphosphate , in contrast, is not known, the metaphosphates favour polymeric structures of chains or rings.
* and are related to but their two bridging −S− atoms are replaced by −S−S− in and by an −S−S−S− bridge in .
*PS These form water-stable salts. The anion has an ethane-like structure and contains a P−P bond. The formal oxidation state of phosphorus is +4. The oxygen analogue is the hypodiphosphate anion, hypophosphoric acid|.
* contains a six-membered PS ring. The ammonium salt is produced by reaction of PS in liquid ammonia. Another way of visualising the structure is that it is the PS adamantane (PO) structure with a PS vertex removed.
* contains a square P ring, contains a P ring and a P ring. These cyclic anions contain P with an oxidation state +3. Note they are not trigonal as arsenic(III) is in arsenites, but are tetrahedral with two bonds to other phosphorus atoms and two to sulfur. The anion is analogous to the ring anion.
* An unusual butterfly-shaped ion, SP(P)PS, which can be visualised as a P molecule where two P−S bonds replace one P−P bond.
* is a sulfido heptaphosphane cluster anion. | 0 | Organic Chemistry |
Venton received her BS in Chemistry from University of Delaware in 1998 and her PhD in Chemistry from University of North Carolina, Chapel Hill in 2003. She was an NIH postdoctoral fellow at University of Michigan from 2003 to 2005. | 3 | Analytical Chemistry |
The Golden Gate assembly protocol was defined by Engler et al. 2008 to define a DNA assembly method that would give a final construct without a scar sequence, while also lacking the original restriction sites. This allows the protein to be expressed without containing unwanted protein sequences which could negatively affect protein folding or expression. By using the BsaI restriction enzyme that produces a 4 base pair overhang, up to 240 unique, non-palindromic sequences can be used for assembly.
Plasmid design and assembly
In Golden Gate cloning, each DNA fragment to be assembled is placed in a plasmid, flanked by inward facing BsaI restriction sites containing the programmed overhang sequences. For each DNA fragment, the 3 overhang sequence is complementary to the 5 overhang of the next downstream DNA fragment. For the first fragment, the 5 overhang is complementary to the 5 overhang of the destination plasmid, while the 3 overhang of the final fragment is complementary to the 3 overhang of the destination plasmid. Such a design allows for all DNA fragments to be assembled in a one-pot reaction (where all reactants are mixed together), with all fragments arranged in the correct sequence. Successfully assembled constructs are selected by detecting the loss of function of a screening cassette that was originally in the destination plasmid.
MoClo and Golden Braid
The original Golden Gate Assembly only allows for a single construct to be made in the destination vector . To enable this construct to be used in a subsequent reaction as an entry vector, the MoClo and Golden Braid standards were designed.
The MoClo standard involves defining multiple tiers of DNA assembly:
* Tier 1: Tier 1 assembly is the standard Golden Gate assembly, and genes are assembled from their components parts (DNA parts coding for genetic elements like UTRs, promoters, ribosome binding sites or terminator sequences). Flanking the insertion site of the tier 1 destination vectors are a pair of inward cutting BpiI restriction sites. This allows these plasmids to be used as entry vectors for tier two destination vectors.
* Tier 2: Tier 2 assembly involves further assembling the genes assembled in tier 1 assembly into multi-gene constructs. If there is a need for further, higher tier assembly, inward cutting BsaI restriction sites can be added to flank the insertion sites. These vectors can then be used as entry vectors for higher tier constructs.
Each assembly tier alternates the use of BsaI and BpiI restriction sites to minimise the number of forbidden sites, and sequential assembly for each tier is achieved by following the Golden Gate plasmid design. Overall, the MoClo standard allows for the assembly of a construct that contains multiple transcription units, all assembled from different DNA parts, by a series of one-pot Golden Gate reactions. However, one drawback of the MoClo standard is that it requires the use of dummy parts with no biological function, if the final construct requires less than four component parts. The Golden Braid standard on the other hand introduced a pairwise Golden Gate assembly standard.
The Golden Braid standard uses the same tiered assembly as MoClo, but each tier only involves the assembly of two DNA fragments, i.e. a pairwise approach. Hence in each tier, pairs of genes are cloned into a destination fragment in the desired sequence, and these are subsequently assembled two at a time in successive tiers. Like MoClo, the Golden Braid standard alternates the BsaI and BpiI restriction enzymes between each tier.
The development of the Golden Gate assembly methods and its variants has allowed researchers to design tool-kits to speed up the synthetic biology workflow. For example, EcoFlex was developed as a toolkit for E. Coli that uses the MoClo standard for its DNA parts, while a similar toolkit has also been developed for engineering the Chlamydomonas reinhardtii microalgae. | 1 | Biochemistry |
Over recent years, the genome-wide CRISPR screen has emerged as a powerful tool for studying the intricate networks of cellular signaling. Cellular signaling is essential for a number of fundamental biological processes, including cell growth, proliferation, differentiation, and apoptosis.
One practical example is the identification of genes required for proliferative signaling in cancer cells. Cells are transduced with a CRISPR sgRNA library, and studied for growth over time. By comparing sgRNA abundance in selected cells to a control, one can identify which sgRNAs become depleted and in turn which genes may be responsible for the proliferation defect. Such screens have been used to identify cancer-essential genes in acute myeloid leukemia and neuroblastoma, and to describe tumor-specific differences between cancer cell lines. | 1 | Biochemistry |
In crystals, the electronic energy spectrum has a band structure . Near the minimum of an isotropic energy band, electron energy can be expanded in powers of as where is the electron effective mass. It can be shown that it satisfies the equation
Here the sum runs over all bands with . Therefore, the ratio of the free electron mass to its effective mass in a crystal can be considered as the oscillator strength for the transition of an electron from the quantum state at the bottom of the band into the same state. | 7 | Physical Chemistry |
In July 1984, Rosenkranz' wife Edith was kidnapped at the summer North American Bridge Championships in Washington, D.C., by Glenn I. Wright and Dennis Moss, and ransomed for one million dollars. The FBI and the District of Columbia police captured Wright and Moss, and she was returned safely. The ransom money was later recovered and the two kidnappers were later convicted and sentenced, as was a third defendant, Orland D. Tolden. Wright was released in 1999, Moss in 1994, and Tolden in 1989. | 0 | Organic Chemistry |
Oxidised copper ores include carbonates such as azurite and malachite, the silicate chrysocolla, and sulfates such as atacamite. In some cases, sulfide ores are allowed to degrade to oxides. Such ores are amenable to hydrometallurgy. Specifically, such oxide ores are usually extracted into aqueous sulfuric acid, usually in a heap leaching or dump leaching. The resulting pregnant leach solution is purified by solvent extraction (SX). It is treated with an organic solvent and an organic chelators. The chelators bind the copper ions (and no other ions, ideally), the resulting complexes dissolve in the organic phase. This organic solvent is evaporated, leaving a residue of the copper complexes. The copper ions are liberated from the residue with sulfuric acid. The barred (denuded) sulfuric acid recycled back on to the heaps. The organic ligands are recovered and recycled as well. Alternatively, the copper can be precipitated out of the pregnant solution by contacting it with scrap iron; a process called cementation. Cement copper is normally less pure than SX-EW copper. | 8 | Metallurgy |
As the mandelic acid salt (methenamine mandelate) or the hippuric acid salt (methenamine hippurate), it is used for the treatment of urinary tract infections. In an acidic environment, methenamine is believed to act as an antimicrobial by converting to formaldehyde. A systematic review of its use for this purpose in adult women found there was insufficient evidence of benefit and further research was needed. A UK study showed that methenamine is as effective as daily low-dose antibiotics at preventing UTIs among women who experience recurrent UTIs. As methenamine is an antiseptic, it may avoid the issue of antibiotic resistance.
Methenamine acts as an over-the-counter antiperspirant due to the astringent property of formaldehyde. Specifically, methenamine is used to minimize perspiration in the sockets of prosthetic devices. | 0 | Organic Chemistry |
Phycobilisomes can be used in [https://store-7fikt.mybigcommerce.com/product_images/uploaded_images/J_Fluorescence_PBXL_Detection.pdf prompt fluorescence], flow cytometry, Western blotting and protein microarrays. Some phycobilisomes have an absorption and emission profile similar to Cy5, allowing them to be used in many of the same applications. They can also be up to 200 times brighter and with a larger Stokes shift, providing a larger signal per binding event. This property allows the detection of low-level target molecules or rare events. | 5 | Photochemistry |
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break (chain scission), resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics. In textiles the process is called phototendering.
Technologies have been developed to both accelerate and inhibit this process. For example, plastic building components like doors, window frames and gutters are expected to last for decades, requiring the use of advanced UV-polymer stabilizers. Conversely, single-use plastics can be treated with biodegradable additives to accelerate their fragmentation.
Many pigments and dyes can similarly have effects due to their ability to absorb UV-energy. | 5 | Photochemistry |
In some transitions a number of atoms occupying crystallographic positions that were originally equivalent will move away slightly from their ideal positions according to a certain pattern. This pattern or repeat motif may span multiple unit cells. The cause of this phenomenon is the small changes in chemical bonding that favor formations of semi-regular and larger clusters of atoms. Although having the undistorted substructure, these materials are typically unsaturated in the sense that one of the bands in the band structure is only partially filled. The distortion changes the band structure, in part splitting the bands up into smaller bands that can be more completely filled or emptied to lower the energy of the system. This process may not go to completion, however, because the substructure only allows for a certain amount of distortion. Superstructures of this type are often incommensurate. A good example is found in the structural transitions of 1T-TaS, a compound with a partially filled, narrow d band (Ta(IV) has a d configuration). | 3 | Analytical Chemistry |
Assay offices did and do exist in the U.S., but they are affiliated with the governments coinage mints and serve only the governments purposes in that field. They are not involved in hallmarking, as there has never been a hallmarking scheme in the U.S.
In the 1800s, the functions of assay offices in the U.S. included receiving bullion deposits from the public and from mining prospectors in the various American territories. The assay offices that still operate today function solely within national coining system (including bullion coinage for sales to investors). | 3 | Analytical Chemistry |
Ortho effect is an organic chemistry phenomenon where the presence of an chemical group at the at ortho position or the 1 and 2 position of a phenyl ring, relative to the carboxylic compound changes the chemical properties of the compound. This is caused by steric effects and bonding interactions along with polar effects caused by the various substituents which are in a given molecule, resulting in changes in its chemical and physical properties. The ortho effect is associated with substituted benzene compounds.
There are three main ortho effects in substituted benzene compounds:
* Steric hindrance forces cause substitution of a chemical group in the ortho position of benzoic acids become stronger acids.
* Steric Inhibition of Protonation caused by substitution of anilines to become weaker bases, compared to substitution of isomers in the meta and para position.
* Electrophilic aromatic substitution of disubstituted benzene compounds causes steric effects which determines the regioselectivity of an incoming electrophile in disubstituted benzene compounds | 4 | Stereochemistry |
The first known use of metals in the Southern Levant is during the Chalcolithic period (end of 5th–most of the 4th millennium BCE). More than 500 metal objects were found, mainly in hoards, burials, and habitation
remains. Most of the metals originate from sites in the southern part of Israel and Jordan; very rarely do they occur beyond the center of Israel and north of Wadi Qana. The metal findings from this period were separated into three groups; most of them belong to the following first two groups:
Prestige/cult-elaborated and complex-shaped objects made of copper (Cu) alloyed (a deliberate
choice of complex minerals that could be reduced to a mixture of metals with specific recognizable
and desirable properties, totally different from unalloyed copper) with distinct amounts of antimony
(Sb) or nickel (Ni) and arsenic (As). They were cast using a “lost wax” technique into single closed clay moulds and then polished into their final shining gray or gold-like colors depending on the amount of antimony or nickel and arsenic in the copper. The Nahal Mishmar hoard was the biggest hoard (416 metal objects comprising mainly artistically complex-shaped objects), found hidden in a cave by Nahal Mishmar, Judean Desert, Israel.
They were wrapped in a straw mat (e.g., Shalev; Tadmor). Carbon-14 dating of the reed mat in which the objects were wrapped suggests that it dates to at least 3500 B.C.
The origin of the complex source material for the production of these objects is currently unknown. The nearest suitable ore is in Trans-Caucasus and Azerbaijan — more than 1500 km from the finding sites of the objects. Several clay and stone cores and clay
mould remains were petrographically analyzed and the results point to a possible local
production in the area of the Judean Desert, within the metals distribution zone in Israel, which is concentrated mainly in the southern part of the country: between Giv’at Oranit and Wadi Qana (east of
modern Tel Aviv) in the north and the Be’er Sheva valley sites in the south. Currently, no production
remains or production sites of these prestige/cult objects were found.
Unalloyed copper tools comprising mainly relatively thick- and short-bladed objects (axes, adzes,
and chisels) and points (awls and/or drills) made from a smelted copper ore, cast into an open mould and then hammered and annealed into their final shape. The copper tools were produced in the Chalcolithic villages on the banks of the Be’er Sheva valley where slag fragments, clay crucibles, some possible furnace lining pieces, copper prills, and amorphous lumps were found, in addition to high-grade carbonated copper ore (cuprite). The ore was collected and selected in the area of Feinan in Trans-Jordan and transported to northern Negev villages some 150 km to the north, to be smelted for the local production of these copper objects.
A third group of eight gold (Au) and electrum (Au + up to 30% Ag) solid rings was found in Wadi Qanah cave.
This unique find, with no dated parallels, is attributed by the excavators to the Chalcolithic period based on local stratigraphic and geological evidence and 14C dating of ground samples from the vicinity of the finds in the cave. Surface analyses of these objects revealed a surface
gold enrichment caused by the depletion of silver and the copper traces. This effect could be caused naturally by deposition but could have been achieved intentionally at the time of production in
order to achieve a yellow color for the electrum rings rich in silver, as well. During the Chalcolithic
(copper and stone) era at least two, if not three distinct industries of different metals were operating
and their products were found in the Southern Levant. | 8 | Metallurgy |
Rebecca Jane Miriam Goss is a professor of organic chemistry at the University of St. Andrews who won the 2006 Royal Society of Chemistry Meldola Medal. She is known for combining synthetic biology and chemistry for medicinal purposes. | 0 | Organic Chemistry |
The steady state approximation, occasionally called the stationary-state approximation or Bodensteins quasi-steady state approximation', involves setting the rate of change of a reaction intermediate in a reaction mechanism equal to zero so that the kinetic equations can be simplified by setting the rate of formation of the intermediate equal to the rate of its destruction.
In practice it is sufficient that the rates of formation and destruction are approximately equal, which means that the net rate of variation of the concentration of the intermediate is small compared to the formation and destruction, and the concentration of the intermediate varies only slowly, similar to the reactants and products (see the equations and the green traces in the figures below).
Its use facilitates the resolution of the differential equations that arise from rate equations, which lack an analytical solution for most mechanisms beyond the simplest ones. The steady state approximation is applied, for example, in Michaelis-Menten kinetics.
As an example, the steady state approximation will be applied to two consecutive, irreversible, homogeneous first order reactions in a closed system. (For heterogeneous reactions, see reactions on surfaces.) This model corresponds, for example, to a series of nuclear decompositions like .
If the rate constants for the following reaction are and ; , combining the rate equations with a mass balance for the system yields three coupled differential equations: | 7 | Physical Chemistry |
In order to ensure growth of only transformed bacteria (which carry the desired plasmids to be harvested), a marker gene is used in the destination vector for selection. Typical marker genes are for antibiotic resistance or nutrient biosynthesis. So, for example, the "marker gene" could be for resistance to the antibiotic ampicillin. If the bacteria that were supposed to pick up the desired plasmid had picked up the desired gene then they would also contain the "marker gene". Now the bacteria that picked up the plasmid would be able to grow in ampicillin whereas the bacteria that did not pick up the desired plasmid would still be vulnerable to destruction by the ampicillin. Therefore, successfully transformed bacteria would be "selected." | 1 | Biochemistry |
DNA molecules often have a preferred direction to bend, i.e., anisotropic bending. This is, again, due to the properties of the bases which make up the DNA sequence - a random sequence will have no preferred bend direction, i.e., isotropic bending.
Preferred DNA bend direction is determined by the stability of stacking each base on top of the next. If unstable base stacking steps are always found on one side of the DNA helix then the DNA will preferentially bend away from that direction. As bend angle increases then steric hindrances and ability to roll the residues relative to each other also play a role, especially in the minor groove. A and T residues will be preferentially be found in the minor grooves on the inside of bends. This effect is particularly seen in DNA-protein binding where tight DNA bending is induced, such as in nucleosome particles. See base step distortions above.
DNA molecules with exceptional bending preference can become intrinsically bent. This was first observed in trypanosomatid kinetoplast DNA. Typical sequences which cause this contain stretches of 4-6 T and A residues separated by G and C rich sections which keep the A and T residues in phase with the minor groove on one side of the molecule. For example:
The intrinsically bent structure is induced by the propeller twist of base pairs relative to each other allowing unusual bifurcated Hydrogen-bonds between base steps. At higher temperatures this structure is denatured, and so the intrinsic bend is lost.
All DNA which bends anisotropically has, on average, a longer persistence length and greater axial stiffness. This increased rigidity is required to prevent random bending which would make the molecule act isotropically. | 4 | Stereochemistry |
Phosphinous acids exist mainly as minor tautomers of secondary phosphine oxides. For example diphenylphosphinous acid, which is not detectable directly, is invoked as the tautomer of diphenylphosphine oxide.
Highly electron-withdrawing substituents stabilize the phosphinous acid tautomer as illustrated by (CF)POH. | 0 | Organic Chemistry |
;Pre-2004
*Organometallic C–H Bond Activation: An Introduction Alan S. Goldman and Karen I. Goldberg ACS Symposium Series 885, Activation and Functionalization of C–H Bonds, 2004, 1–43 | 0 | Organic Chemistry |
Animal products such as meat, fish, shellfish, fowl, eggs, and dairy contain zinc. The concentration of zinc in plants varies with the level in the soil. With adequate zinc in the soil, the food plants that contain the most zinc are wheat (germ and bran) and various seeds, including sesame, poppy, alfalfa, celery, and mustard. Zinc is also found in beans, nuts, almonds, whole grains, pumpkin seeds, sunflower seeds, and blackcurrant.
Other sources include fortified food and dietary supplements in various forms. A 1998 review concluded that zinc oxide, one of the most common supplements in the United States, and zinc carbonate are nearly insoluble and poorly absorbed in the body. This review cited studies that found lower plasma zinc concentrations in the subjects who consumed zinc oxide and zinc carbonate than in those who took zinc acetate and sulfate salts. For fortification, however, a 2003 review recommended cereals (containing zinc oxide) as a cheap, stable source that is as easily absorbed as the more expensive forms. A 2005 study found that various compounds of zinc, including oxide and sulfate, did not show statistically significant differences in absorption when added as fortificants to maize tortillas. | 1 | Biochemistry |
Ekimov was born in the Soviet Union. In 1967, he graduated from the Faculty of Physics, Leningrad State University. He went on to receive his PhD in physics at the Ioffe Institute of the Russian Academy of Sciences in 1974. | 7 | Physical Chemistry |
# Cholesterol + oxygen --(enzyme cholesterol oxidase)--> cholestenone + hydrogen peroxide
# Hydrogen peroxide + 4-aminophenazone + phenol --(enzyme peroxidase)--> colored complex + water | 3 | Analytical Chemistry |
The idea of a quantum harmonic oscillator and its associated energy can apply to either an atom or a subatomic particle. In ordinary atomic physics, the zero-point energy is the energy associated with the ground state of the system. The professional physics literature tends to measure frequency, as denoted by above, using angular frequency, denoted with and defined by . This leads to a convention of writing the Planck constant with a bar through its top () to denote the quantity . In these terms, the most famous such example of zero-point energy is the above associated with the ground state of the quantum harmonic oscillator. In quantum mechanical terms, the zero-point energy is the expectation value of the Hamiltonian of the system in the ground state.
If more than one ground state exists, they are said to be degenerate. Many systems have degenerate ground states. Degeneracy occurs whenever there exists a unitary operator which acts non-trivially on a ground state and commutes with the Hamiltonian of the system.
According to the third law of thermodynamics, a system at absolute zero temperature exists in its ground state; thus, its entropy is determined by the degeneracy of the ground state. Many systems, such as a perfect crystal lattice, have a unique ground state and therefore have zero entropy at absolute zero. It is also possible for the highest excited state to have absolute zero temperature for systems that exhibit negative temperature.
The wave function of the ground state of a particle in a one-dimensional well is a half-period sine wave which goes to zero at the two edges of the well. The energy of the particle is given by:
where is the Planck constant, is the mass of the particle, is the energy state ( corresponds to the ground-state energy), and is the width of the well. | 7 | Physical Chemistry |
The basic relationship for heat transfer by convection is:
where is the heat transferred per unit time, A is the area of the object, h is the heat transfer coefficient, T is the objects surface temperature, and T' is the fluid temperature.
The convective heat transfer coefficient is dependent upon the physical properties of the fluid and the physical situation. Values of h have been measured and tabulated for commonly encountered fluids and flow situations. | 7 | Physical Chemistry |
The concept of DNA computing came about as a way to address storage density issues because of the exploding volumes of data information. Theoretically, a gram of single-stranded DNA is capable of storing over 400 exabytes (order of 10 bytes) of data at a density of two bits per nucleotide. Leonard Adleman is credited with having established the field in 1994. Recently, molecular logic gate systems have been utilized in DNA computing models.
Massey and coworkers constructed photonic DNA molecular logic circuits using cascades of AND, OR, NAND, and NOR molecular logic gates. They used lanthanide complexes as fluorescent markers, and their luminescent outputs were detected by FRET-based devices at the terminals of DNA strands. Works by Campbell et al. on demonstrating NOT, AND, OR, and XNOR logic systems based on DNA crossover tiles, Bader and co. on manipulating the DNA G-quadruplex structure to realize YES, AND, and OR logic operations, and Chatterjee and coworkers on constructing logic gates using reactive DNA hairpins on DNA origami surfaces are some examples that mark the progress in the field of logic gate-based DNA computing. | 6 | Supramolecular Chemistry |
Important aldehydes and related compounds. The aldehyde group (or formyl group) is colored red. From the left: (1) formaldehyde and (2) its trimer 1,3,5-trioxane, (3) acetaldehyde and (4) its enol vinyl alcohol, (5) glucose (pyranose form as α--glucopyranose), (6) the flavorant cinnamaldehyde, (7) retinal, which forms with opsins photoreceptors, and (8) the vitamin pyridoxal. | 0 | Organic Chemistry |
Grid spectroscopy is an extension of force spectroscopy described above. In grid spectroscopy multiple force spectra are taken in a grid over a surface, to build up a three-dimensional force map above the surface. These experiments can take a considerable time, often over 24 hours, thus the microscope is usually cooled with liquid helium or an atom tracking method is employed to correct for drift. | 6 | Supramolecular Chemistry |
Heatwork is the combined effect of temperature and time. It is important to several industries:
*Ceramics
*Glass and metal annealing
*Metal heat treating
Pyrometric devices can be used to gauge heat work as they deform or contract due to heatwork to produce temperature equivalents. Within tolerances, firing can be undertaken at lower temperatures for a longer period to achieve comparable results. When the amount of heatwork of two firings is the same, the pieces may look identical, but there may be differences not visible, such as mechanical strength and microstructure. Heatwork is taught in material science courses, but is not a precise measurement or a valid scientific concept. | 8 | Metallurgy |
Plant sRNA pathways are understood to be important components of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). Bacteria‐induced microRNAs (miRNAs) in Arabidopsis have been shown to influence hormonal signalling including auxin, abscisic acid (ABA), jasmonic acid (JA) and salicylic acid (SA). Advances in genome‐wide studies revealed a massive adaptation of host miRNA expression patterns after infection by fungal pathogens Fusarium virguliforme, Erysiphe graminis, Verticillium dahliae, and Cronartium quercuum, and the oomycete Phytophthora sojae. Changes to sRNA expression in response to fungal pathogens indicate that gene silencing may be involved in this defense pathway. However, there is also evidence that the antifungal defense response to Colletotrichum spp. infection in maize is not entirely regulated by specific miRNA induction, but may instead act to fine-tune the balance between genetic and metabolic components upon infection.
Transport of sRNAs during infection is likely facilitated by extracellular vesicles (EVs) and multivesicular bodies (MVBs). The composition of RNA in plant EVs has not been fully evaluated, but it is likely that they are, in part, responsible for trafficking RNA. Plants can transport viral RNAs, mRNAs, miRNAs and small interfering RNAs (siRNAs) systemically through the phloem. This process is thought to occur through the plasmodesmata and involves RNA-binding proteins that assist RNA localization in mesophyll cells. Although they have been identified in the phloem with mRNA, there is no determinate evidence that they mediate long-distant transport of RNAs. EVs may therefore contribute to an alternate pathway of RNA loading into the phloem, or could possibly transport RNA through the apoplast. There is also evidence that plant EVs can allow for interspecies transfer of sRNAs by RNA interference such as Host-Induced Gene Silencing (HIGS). The transport of RNA between plants and fungi seems to be bidirectional as sRNAs from the fungal pathogen Botrytis cinerea have been shown to target host defense genes in Arabidopsis and tomato. | 1 | Biochemistry |
For most stable isotopes, the magnitude of fractionation from kinetic and equilibrium fractionation is very small; for this reason, enrichments are typically reported in "per mil" (‰, parts per thousand). These enrichments (δ) represent the ratio of heavy isotope to light isotope in the sample over the ratio of a standard. That is, | 9 | Geochemistry |
There was only one proven polymorph Form I of aspirin, though the existence of another polymorph was debated since the 1960s, and one report from 1981 reported that when crystallized in the presence of aspirin anhydride, the diffractogram of aspirin has weak additional peaks. Though at the time it was dismissed as mere impurity, it was, in retrospect, Form II aspirin.
Form II was reported in 2005, found after attempted co-crystallization of aspirin and levetiracetam from hot acetonitrile.
In form I, pairs of aspirin molecules form centrosymmetric dimers through the acetyl groups with the (acidic) methyl proton to carbonyl hydrogen bonds. In form II, each aspirin molecule forms the same hydrogen bonds, but with two neighbouring molecules instead of one. With respect to the hydrogen bonds formed by the carboxylic acid groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described as polytypes.
Pure Form II aspirin could be prepared by seeding the batch with aspirin anhydrate in 15% weight. | 3 | Analytical Chemistry |
Black light is commonly used to authenticate oil paintings, antiques and banknotes. Black lights can be used to differentiate real currency from counterfeit notes because, in many countries, legal banknotes have fluorescent symbols on them that only show under a black light. In addition, the paper used for printing money does not contain any of the brightening agents which cause commercially available papers to fluoresce under black light. Both of these features make illegal notes easier to detect and more difficult to successfully counterfeit. The same security features can be applied to identification cards such as passports or driver's licenses.
Other security applications include the use of pens containing a fluorescent ink, generally with a soft tip, that can be used to "invisibly" mark items. If the objects that are so marked are subsequently stolen, a black light can be used to search for these security markings. At some amusement parks, nightclubs and at other, day-long (or night-long) events, a fluorescent mark is rubber stamped onto the wrist of a guest who can then exercise the option of leaving and being able to return again without paying another admission fee. | 5 | Photochemistry |
Due to its size and complexity, the 43S PIC has eluded high resolution structural characterization. However, combined approaches including cryo-EM, cross-linking, and the structural characterization of individual components, has led to models for the complex organization. | 1 | Biochemistry |
The names of esters that are formed from an alcohol and an acid, are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplest carboxylic acids are commonly named according to the more traditional, so-called "trivial names" e.g. as formate, acetate, propionate, and butyrate, as opposed to the IUPAC nomenclature methanoate, ethanoate, propanoate, and butanoate. Esters derived from more complex carboxylic acids are, on the other hand, more frequently named using the systematic IUPAC name, based on the name for the acid followed by the suffix -oate. For example, the ester hexyl octanoate, also known under the trivial name hexyl caprylate, has the formula .
The chemical formulas of organic esters formed from carboxylic acids and alcohols usually take the form or RCOOR, where R and R are the organyl parts of the carboxylic acid and the alcohol, respectively, and R can be a hydrogen in the case of esters of formic acid. For example, butyl acetate (systematically butyl ethanoate), derived from butanol and acetic acid (systematically ethanoic acid) would be written . Alternative presentations are common including BuOAc and .
Cyclic esters are called lactones, regardless of whether they are derived from an organic or inorganic acid. One example of an organic lactone is γ-valerolactone. | 0 | Organic Chemistry |
Slack was born on 22 April 1937 in Ashton-under-Lyne, Lancashire, England; the first and only child of Albert and Eva Slack. He studied biochemistry at the University of Nottingham, where he graduated with a Bachelor of Science (Honours) in 1958, and a PhD in 1962. He married Pam Shaw in March 1963, and had two children.
From 1962, Slack worked as a biochemist at the David North Plant Research Centre in Brisbane, Queensland, Australia (funded by the Colonial Sugar Refining Co. Ltd). In 1970, he joined the Department of Scientific and Industrial Research in New Zealand. From 1989 until his retirement in 2000, Slack was a senior scientist at the newly formed Crown Research Institute for Crop & Food Research in Palmerston North.
Slack died in Palmerston North in 2016. | 1 | Biochemistry |
In polymer chemistry, gelation (gel transition) is the formation of a gel from a system with polymers. Branched polymers can form links between the chains, which lead to progressively larger polymers. As the linking continues, larger branched polymers are obtained and at a certain extent of the reaction, links between the polymer result in the formation of a single macroscopic molecule. At that point in the reaction, which is defined as gel point, the system loses fluidity and viscosity becomes very large. The onset of gelation, or gel point, is accompanied by a sudden increase in viscosity. This "infinite" sized polymer is called the gel or network, which does not dissolve in the solvent, but can swell in it. | 7 | Physical Chemistry |
When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.
The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such, the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle needs a large mass flow rate and is bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often used nowadays in terrestrial cooling devices. However, the air cycle machine is very common on gas turbine-powered jet aircraft as cooling and ventilation units, because compressed air is readily available from the engines' compressor sections. Such units also serve the purpose of pressurizing the aircraft. | 7 | Physical Chemistry |
After phase II reactions, the xenobiotic conjugates may be further metabolized. A common example is the processing of glutathione conjugates to acetylcysteine (mercapturic acid) conjugates. Here, the γ-glutamate and glycine residues in the glutathione molecule are removed by gamma-glutamyl transpeptidase and dipeptidases. In the final step, the cysteine residue in the conjugate is acetylated.
Conjugates and their metabolites can be excreted from cells in phase III of their metabolism, with the anionic groups acting as affinity tags for a variety of membrane transporters of the multidrug resistance protein (MRP) family. These proteins are members of the family of ATP-binding cassette transporters and can catalyse the ATP-dependent transport of a huge variety of hydrophobic anions, and thus act to remove phase II products to the extracellular medium, where they may be further metabolized or excreted. | 1 | Biochemistry |
Strictly speaking, the term "catalysis" should not be used unless it can be shown that the number of product molecules produced per number of active sites is greater than one; this is difficult to do in practice, although it is often assumed to be true if there is no loss in the photoactivity of the catalyst for an extended period of time. Reactions that are not strictly catalytic may be designated "assisted photoreactions". Furthermore, phenomena that involve complex mixtures of compounds (e.g. soil) may be hard to classify unless complete reactions (not just individual reactants or products) can be identified. | 5 | Photochemistry |
Mohamed researches diabetes due to a family history of suffering from the disease. His father, mother and brother are diabetics and his concern for the growing number of diabetics worldwide prompted his invention. He developed a remote monitoring and control system for diabetes symptoms. He set about creating an artificial pancreas and a remote system to monitor the stability of glucose levels in diabetics. The device, which can be linked to a hospital database system as well as family and friends, enables an immediate response if a medical situation arises. | 7 | Physical Chemistry |
In a one-dimensional system at a given mode the vibration will have nodes, or places where the displacement is always zero. These nodes correspond to points in the mode shape where the mode shape is zero. Since the vibration of a system is given by the mode shape multiplied by a time function, the displacement of the node points remain zero at all times.
When expanded to a two dimensional system, these nodes become lines where the displacement is always zero. If you watch the animation above you will see two circles (one about halfway between the edge and center, and the other on the edge itself) and a straight line bisecting the disk, where the displacement is close to zero. In an idealized system these lines equal zero exactly, as shown to the right. | 7 | Physical Chemistry |
Denote the two ground states and the excited state of the electron and , respectively. The atom also has overall momentum, so the overall state of the atom is a product state of its internal state and its momentum, as shown in the figure. In the presence of counter-propagating beams of opposite polarization, the internal states experience the atom-light interaction Hamiltonian
where is the Rabi frequency, approximated to be the same for both transitions. Using the definition of the translation operator in momentum space,
the effect of on the state is
This suggests the dressed state that couples to is a more convenient basis state of the two ground states. The orthogonal basis state defined below does not couple to at all.
The action of on these states is
Thus, and undergo Sisyphus-like cooling, identifying the former as the bright state. is optically inaccessible and constitutes the dark state. However, and are not eigenstates of the momentum operator, and thus motionally couple to one another via the kinetic energy term of the unperturbed Hamiltonian:
As a result of this coupling, the dark state evolves into the bright state with frequency proportional to the momentum, effectively selecting hotter atoms to re-enter the Sisyphus cooling cycle. This nonadiabatic coupling occurs predominantly at the potential minima of the light-shifted coupling state. Over time, atoms cool until they lack the momentum to traverse the sinusoidal light shift of the bright state and instead populate the dark state. | 7 | Physical Chemistry |
Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions. The center of each grain, which is the first part to freeze, is rich in the high-melting element (e.g., nickel for this Cu–Ni system), whereas the concentration of the low-melting element increases with position from this region to the grain boundary. This is termed a cored structure, which gives rise to less than the optimal properties. The distribution of the two elements within the grains is nonuniform, a phenomenon termed segregation; that is, concentration gradients are established across the grains.
As a casting having a cored structure is reheated, grain boundary regions will melt first in as much as they are richer in the low-melting component. This produces a sudden loss in mechanical integrity due to the thin liquid film that separates the grains. Furthermore, this melting may begin at a temperature below the equilibrium solidus temperature of the alloy. Coring may be eliminated by a homogenization heat treatment carried out at a temperature below the solidus point for the particular alloy composition. During this process, atomic diffusion occurs, which produces compositionally homogeneous grains.
Coring is predominantly observed in alloys having a marked difference between liquidus and solidus temperatures. It is often being removed by subsequent annealing and/or hot-working. It is exploited in zone refining techniques to produce high purity metals. Coring was first discovered by Aubrey Tang. | 8 | Metallurgy |
Molecular recognition plays an important role in biological systems and is observed in between receptor-ligand, antigen-antibody, DNA-protein, sugar-lectin, RNA-ribosome, etc. An important example of molecular recognition is the antibiotic vancomycin that selectively binds with the peptides with terminal D-alanyl-D-alanine in bacterial cells through five hydrogen bonds. The vancomycin is lethal to the bacteria since once it has bound to these particular peptides they are unable to be used to construct the bacteria's cell wall. | 6 | Supramolecular Chemistry |
As aromatic compounds have been exhausted to a large extent as building blocks for life science products, N-heterocyclic structures prevail nowadays. They are found in many natural products, such as chlorophyll, hemoglobin, and the vitamins biotin, folic acid, niacin (PP), pyridoxine (vitamin B), riboflavin (vitamin B), and thiamine (vitamin B). In synthetic life science products, N-heterocyclic moieties are widely used in both pharmaceuticals and agrochemicals.
Thus, β-lactams are structural elements of penicillin and cephalosporin antibiotics, imidazoles are found both in modern herbicides, e.g. Arsenal (imazapyr) and pharmaceuticals, e.g. the antiulcerants Tagamet (cimetidine. see above) and Nexium (omeprazole), the antimycotics Daktarin (miconazole), Fungarest (ketoconazole) and Travogen (isoconazole). Tetrazoles and tetrazolidines are pivotal parts of the "sartan" class of hypertensives, e.g. Candesartan cilexetil (candesartan), Avapro (irbesartan), Cozaar (losartan) and Diovan (valsartan).
A vast array of pharmaceuticals and agrochemicals are based on pyrimidines, such as Vitamin B1 (thiamine), the sulfonamide antibiotics, e.g. Madribon (sulfadimethoxime) and –half a century later– the sulfonyl urea herbicides, e.g. Eagle (amidosulfuron) and Londax (bensulfuron-methyl). Benzodiazepine derivatives are the pivotal structural elements of breakthrough CNS Drugs, such as Librium (chlordiazepoxide) and Valium (diazepam). Pyridine derivatives are found in both well-known Diquat and Chlorpyrifos herbicides, and in modern nicotinoid insecticides, such as Imidacloprid.
Even modern pigments, such as diphenylpyrazolopyrazoles, quinacridones, and engineering plastics, such as polybenzimidazoles, polyimides, and triazine resins, exhibit an N-heterocyclic structure. | 0 | Organic Chemistry |
(born July 16, 1943) is a prominent organic chemist and currently a member of the faculty at the University of Chicago and professor of Chubu University. | 0 | Organic Chemistry |
#The sample is dissolved, if it is not already in solution.
#The solution may be treated to adjust the pH (so that the proper precipitate is formed, or to suppress the formation of other precipitates). If it is known that species are present which interfere (by also forming precipitates under the same conditions as the analyte), the sample might require treatment with a different reagent to remove these interferents.
#The precipitating reagent is added at a concentration that favors the formation of a "good" precipitate (see below). This may require low concentration, extensive heating (often described as "digestion"), or careful control of the pH. Digestion can help reduce the amount of coprecipitation.
#After the precipitate has formed and been allowed to "digest", the solution is carefully filtered. The filter is used to collect the precipitate; smaller particles are more difficult to filter.
#*Depending on the procedure followed, the filter might be a piece of ashless filter paper in a fluted funnel, or a filter crucible. Filter paper is convenient because it does not typically require cleaning before use; however, filter paper can be chemically attacked by some solutions (such as concentrated acid or base), and may tear during the filtration of large volumes of solution.
#*The alternative is a crucible whose bottom is made of some porous material, such as sintered glass, porcelain or sometimes metal. These are chemically inert and mechanically stable, even at elevated temperatures. However, they must be carefully cleaned to minimize contamination or carryover(cross-contamination). Crucibles are often used with a mat of glass or asbestos fibers to trap small particles.
#*After the solution has been filtered, it should be tested to make sure that the analyte has been completely precipitated. This is easily done by adding a few drops of the precipitating reagent; if a precipitate is observed, the precipitation is incomplete.
#After filtration, the precipitate – including the filter paper or crucible – is heated, or charred. This accomplishes the following:
#*The remaining moisture is removed (drying).
#*Secondly, the precipitate is converted to a more chemically stable form. For instance, calcium ion might be precipitated using oxalate ion, to produce calcium oxalate (CaCO); it might then be heated to convert it into the oxide (CaO). It is vital that the empirical formula of the weighed precipitate be known, and that the precipitate be pure; if two forms are present, the results will be inaccurate.
#*The precipitate cannot be weighed with the necessary accuracy in place on the filter paper; nor can the precipitate be completely removed from the filter paper to weigh it. The precipitate can be carefully heated in a crucible until the filter paper has burned away; this leaves only the precipitate. (As the name suggests, "ashless" paper is used so that the precipitate is not contaminated with ash.)
#After the precipitate is allowed to cool (preferably in a desiccator to keep it from absorbing moisture), it is weighed (in the crucible). To calculate the final mass of the analyte, the starting mass of the empty crucible is subtracted from the final mass of the crucible containing the sample. Since the composition of the precipitate is known, it is simple to calculate the mass of analyte in the original sample. | 3 | Analytical Chemistry |
The organic matter in soil derives from plants, animals and microorganisms. In a forest, for example, leaf litter and woody material falls to the forest floor. This is sometimes referred to as organic material. When it decays to the point in which it is no longer recognizable, it is called soil organic matter. When the organic matter has broken down into a stable substance that resist further decomposition it is called humus. Thus soil organic matter comprises all of the organic matter in the soil exclusive of the material that has not decayed.
An important property of soil organic matter is that it improves the capacity of a soil to hold water and nutrients, and allows their slow release, thereby improving the conditions for plant growth. Another advantage of humus is that it helps the soil to stick together which allows nematodes, or microscopic bacteria, to easily decay the nutrients in the soil.
There are several ways to quickly increase the amount of humus. Combining compost, plant or animal materials/waste, or green manure with soil will increase the amount of humus in the soil.
# Compost: decomposed organic material.
# Plant and animal material and waste: dead plants or plant waste such as leaves or bush and tree trimmings, or animal manure.
# Green manure: plants or plant material that is grown for the sole purpose of being incorporated with soil.
These three materials supply nematodes and bacteria with nutrients for them to thrive and produce more humus, which will give plants enough nutrients to survive and grow.
Soil organic matter is crucial to all ecology and to all agriculture, but it is especially emphasized in organic farming, where it is relied upon especially heavily. | 0 | Organic Chemistry |
Until 1986, physicists had believed that BCS theory forbade superconductivity at temperatures above about 30 K. In that year, Bednorz and Müller discovered superconductivity in lanthanum barium copper oxide (LBCO), a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987). It was soon found that replacing the lanthanum with yttrium (i.e., making YBCO) raised the critical temperature above 90 K.
This temperature jump is of particular engineering significance, since it allows liquid nitrogen as a refrigerant, replacing liquid helium. Liquid nitrogen can be produced relatively cheaply, even on-site. The higher temperatures additionally help to avoid some of the problems that arise at liquid helium temperatures, such as the formation of plugs of frozen air that can block cryogenic lines and cause unanticipated and potentially hazardous pressure buildup.
Many other cuprate superconductors have since been discovered, and the theory of superconductivity in these materials is one of the major outstanding challenges of theoretical condensed matter physics. There are currently two main hypotheses – the resonating-valence-bond theory, and spin fluctuation which has the most support in the research community. The second hypothesis proposed that electron pairing in high-temperature superconductors is mediated by short-range spin waves known as paramagnons.
In 2008, holographic superconductivity, which uses holographic duality or AdS/CFT correspondence theory, was proposed by Gubser, Hartnoll, Herzog, and Horowitz, as a possible explanation of high-temperature superconductivity in certain materials.
From about 1993, the highest-temperature superconductor known was a ceramic material consisting of mercury, barium, calcium, copper and oxygen (HgBaCaCuO) with T = 133–138 K.
In February 2008, an iron-based family of high-temperature superconductors was discovered. Hideo Hosono, of the Tokyo Institute of Technology, and colleagues found lanthanum oxygen fluorine iron arsenide (LaOFFeAs), an oxypnictide that superconducts below 26 K. Replacing the lanthanum in LaOFFeAs with samarium leads to superconductors that work at 55 K.
In 2014 and 2015, hydrogen sulfide () at extremely high pressures (around 150 gigapascals) was first predicted and then confirmed to be a high-temperature superconductor with a transition temperature of 80 K. Additionally, in 2019 it was discovered that lanthanum hydride () becomes a superconductor at 250 K under a pressure of 170 gigapascals.
In 2018, a research team from the Department of Physics, Massachusetts Institute of Technology, discovered superconductivity in bilayer graphene with one layer twisted at an angle of approximately 1.1 degrees with cooling and applying a small electric charge. Even if the experiments were not carried out in a high-temperature environment, the results are correlated less to classical but high temperature superconductors, given that no foreign atoms need to be introduced. The superconductivity effect came about as a result of electrons twisted into a vortex between the graphene layers, called "skyrmions". These act as a single particle and can pair up across the graphene's layers, leading to the basic conditions required for superconductivity.
In 2020, a room-temperature superconductor (critical temperature 288 K) made from hydrogen, carbon and sulfur under pressures of around 270 gigapascals was described in a paper in Nature. However, in 2022 the article was retracted by the editors because the validity of background subtraction procedures had been called into question. All nine authors maintain that the raw data strongly support the main claims of the paper.
On 31 December 2023 "Global Room-Temperature Superconductivity in Graphite" was published in the journal "Advanced Quantum Technologies" claiming to demonstrate superconductivity at room temperature and ambient pressure in Highly oriented pyrolytic graphite with dense arrays of nearly parallel line defects. | 7 | Physical Chemistry |
The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. n-type doping produces mobile electrons (leaving behind positively charged donors) while p-type doping produces mobile holes (and negatively charged acceptors) In practice, p-n junctions of silicon solar cells are not made in this way, but rather by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).
If a piece of p-type silicon is placed in close contact with a piece of n-type silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse into the p-type side, each one annihilates a hole, making that side net negatively charged (because now the number of mobile positive holes is now less than the number of negative acceptors). Similarly, holes diffusing to the n-type side make it more positively charged. However (in the absence of an external circuit) this diffusion current of carriers does not go on indefinitely because the charge build up on either side of the junction produces an electric field that opposes further diffusion of more charges. Eventually, an equilibrium is reached where the net current is zero, leaving a region either side of the junction where electrons and holes have diffused across the junction and annihilated each other called the depletion region because it contains practically no mobile charge carriers. It is also known as the space charge region, although space charge extends a bit further in both directions than the depletion region.
Once equilibrium is established, electron-hole pairs generated in the depletion region are separated by the electric field, with the electron attracted to the positive n-type side and holes to the negative p-type side, reducing the charge (and the electric field) built up by the diffusion just described. If the device is unconnected (or the external load is very high) then diffusion current would eventually restore the equilibrium charge by bringing the electron and hole back across the junction, but if the load connected is small enough, the electrons prefer to go around the external circuit in their attempt to restore equilibrium, doing useful work on the way. | 7 | Physical Chemistry |
Cristina Nevado (born 1977) is a Spanish chemist who is a Professor of Organic Chemistry at the University of Zurich. Her research considers chemical synthesis and organometallic reactions. She received the 2021 Margaret Faul Women in Chemistry Award. | 0 | Organic Chemistry |
* Current capacity limited by anode mass and self consumption at low current density.
* Lower driving voltage means the anodes may not work in high-resistivity environments.
* Often requires that the protected structure be electrically isolated from other structures and ground.
* Anodes are heavy and will increase water resistance on moving structures or pipe interiors.
* Where D.C. power is available, electrical energy can be obtained more cheaply than by galvanic anodes.
* Where large arrays are used, wiring is needed due to high current flow and need to keep resistance losses low.
* Anodes must be carefully placed to avoid interfering with water flow into the propeller.
* To retain effectiveness, the anodes must be inspected and/or replaced as part of normal maintenance. | 7 | Physical Chemistry |
Psychrophiles are protected from freezing and the expansion of ice by ice-induced desiccation and vitrification (glass transition), as long as they cool slowly. Free living cells desiccate and vitrify between −10 °C and −26 °C. Cells of multicellular organisms may vitrify at temperatures below −50 °C. The cells may continue to have some metabolic activity in the extracellular fluid down to these temperatures, and they remain viable once restored to normal temperatures.
They must also overcome the stiffening of their lipid cell membrane, as this is important for the survival and functionality of these organisms. To accomplish this, psychrophiles adapt lipid membrane structures that have a high content of short, unsaturated fatty acids. Compared to longer saturated fatty acids, incorporating this type of fatty acid allows for the lipid cell membrane to have a lower melting point, which increases the fluidity of the membranes. In addition, carotenoids are present in the membrane, which help modulate the fluidity of it.
Antifreeze proteins are also synthesized to keep psychrophiles internal space liquid, and to protect their DNA when temperatures drop below waters freezing point. By doing so, the protein prevents any ice formation or recrystallization process from occurring.
The enzymes of these organisms have been hypothesized to engage in an activity-stability-flexibility relationship as a method for adapting to the cold; the flexibility of their enzyme structure will increase as a way to compensate for the freezing effect of their environment.
Certain cryophiles, such as Gram-negative bacteria Vibrio and Aeromonas spp., can transition into a viable but nonculturable (VBNC) state. During VBNC, a micro-organism can respire and use substrates for metabolism – however, it cannot replicate. An advantage of this state is that it is highly reversible. It has been debated whether VBNC is an active survival strategy or if eventually the organism's cells will no longer be able to be revived. There is proof however it may be very effective – Gram positive bacteria Actinobacteria have been shown to have lived about 500,000 years in the permafrost conditions of Antarctica, Canada, and Siberia. | 1 | Biochemistry |
Diborane can be produced in situ by reduction BF with NaBH (see for Flavopiridol). Usually however, borane dimethylsulfide complex BHS(CH) (BMS) is used as a source of BH. It can be obtained in highly concentrated forms.
The adduct BH(THF) is also commercially available as THF solutions wherein it exists as the 1:1 adduct. It degrades with time.
Borane adducts with phosphines and amines are also available, but are not widely used. Borane makes a strong adduct with triethylamine; using this adduct requires harsher conditions in hydroboration. This can be advantageous for cases such as hydroborating trienes to avoid polymerization. More sterically hindered tertiary and silyl amines can deliver borane to alkenes at room temperature. | 0 | Organic Chemistry |
According to their properties, cations are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. This happens because cationic analysis is based on the solubility products of the ions. As the cation gains its optimum concentration needed for precipitation it precipitates and hence allowing us to detect it. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes. | 3 | Analytical Chemistry |
Iron–nickel (Fe–Ni) clusters are metal clusters consisting of iron and nickel, i.e. Fe–Ni structures displaying polyhedral frameworks held together by two or more metal–metal bonds per metal atom, where the metal atoms are located at the vertices of closed, triangulated polyhedra.
Individually, iron (Fe) and nickel (Ni) generally form metal clusters with π-acceptor ligands. Π acceptor ligands are ligands that remove some of the electron density from the metal.
Figure 1 contains pictures of representative cluster shapes. Clusters take the form of closed, triangulated polyhedral.
Corresponding bulk systems of Fe and Ni atoms show a variety of composition-dependent abnormalities and unusual effects. Fe–Ni composites are studied in hopes to understand and utilize these unusual and new properties.
Fe–Ni clusters are used for several main purposes. Fe–Ni clusters ranging from single to hundreds of atoms are used in catalysis, depending on the reaction mechanism. Additionally, Fe–Ni clusters, usually of one or two metal atoms, are used in biological systems. These applications are discussed below. | 7 | Physical Chemistry |
Photovoltaics (PV) use silicon solar cells to convert the energy of sunlight into electricity. Operates under the photoelectric effect which results in the emission of electrons. Concentrated solar power (CSP) Uses lenses or mirrors and tracking devices to focus a large area of sunlight into a small beam. Solar power is anticipated to be the world's largest source of electricity by 2050. Solar power plants, such as Ivanpah Solar Power Facility in the Mojave Desert produces over 392MW of power. Solar projects exceeding 1 GW (1 billion watts) are in development and are anticipated to be the future of solar power in the US. | 7 | Physical Chemistry |
Vigilant caution should be taken while using TRIzol (due to the phenol and chloroform).
TRIzol is labeled as acute oral, dermal, and inhalation toxicity besides skin corrosion/irritation in the manufacturer MDS.
Exposure to TRIzol can be a serious health hazard. Exposure can lead to serious chemical burns, permanent scarring and kidney failure.
Experiments should be performed under a chemical hood, with lab coat, nitrile gloves and a plastic apron.
TRIzol waste should never be mixed with bleach or acids: the guanidinium thiocyanate in TRIzol reacts to form highly toxic gases. | 1 | Biochemistry |
In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type where R is a branched alkyl and n = 3 or 4. Representative examples are the tetraisopropyl derivative and the tris(tert-butyl) derivative . These ligands are so large that their complexes behave differently from the pentamethylcyclopentadienyl analogues. Because they cannot closely approach the metal, these bulky ligands stabilize high spin complexes, such as (CHBu)FeI. These large ligands stabilize highly unsaturated derivatives such as (CHBu)FeN. | 0 | Organic Chemistry |
In genetics, a master regulator gene is a regulator gene at the top of a gene regulation hierarchy, particularly in regulatory pathways related to cell fate and differentiation. | 1 | Biochemistry |
Phosphorochloridites are produced by partial alcoholysis of phosphorus trichloride, which proceeds stepwise:
:PCl + ROH → HCl + (RO)PCl (phosphochloridite)
:(RO)PCl + ROH → HCl + (RO)PCl (phosphodichloridite)
:(RO)PCl + ROH → HCl + (RO)P (phosphite)
These reactions are readily controlled with aromatic diols, such as binaphthol and 2,2'-biphenol.
Phosphorochloridites are precursors to diphosphite ligands. When combined with rhodium precursors such as Rh(acac)(CO), these diphosphite ligands afford catalysts that are used industrially for the hydroformylation of alkenes. it and related ligands have become popular in hydroformylation catalysis. | 0 | Organic Chemistry |
Polymethylhydrosiloxane (PMHS) is a polymer with the general structure . It is used in organic chemistry as a mild and stable reducing agent easily transferring hydrides to metal centers and a number of other reducible functional groups. A variety of related materials are available under the following CAS registry numbers 9004-73-3, 16066-09-4, 63148-57-2, 178873-19-3. These include the tetramer (), copolymers of dimethylsiloxane and methylhydrosiloxane, and trimethylsilyl terminated materials.
This material is prepared by the hydrolysis of monomethyldichlorosilane CAS#: 75-54-7:
The related polymer polydimethylsiloxane (PDMS) is made similarly, but lacking bonds, it exhibits no reducing properties. Dimethyldichlorosilane CAS#: 75-78-5 is then used instead of monomethyldichlorosilane CAS#: 75-54-7.
Illustrative of its use, PMHS is used for in situ conversion of tributyltin oxide to tributyltin hydride: | 0 | Organic Chemistry |
Many mechanisms exist reflecting the myriad types of cross-couplings, including those that do not require metal catalysts. Often, however, cross-coupling refers to a metal-catalyzed reaction of a nucleophilic partner with an electrophilic partner.
In such cases, the mechanism generally involves reductive elimination of R-R from LMR(R) (L = spectator ligand). This intermediate LMR(R) is formed in a two step process from a low valence precursor LM. The oxidative addition of an organic halide (RX) to LM gives LMR(X). Subsequently, the second partner undergoes transmetallation with a source of R. The final step is reductive elimination of the two coupling fragments to regenerate the catalyst and give the organic product. Unsaturated substrates, such as C(sp)−X and C(sp)−X bonds, couple more easily, in part because they add readily to the catalyst. | 0 | Organic Chemistry |
A metal carbido complex is a coordination complex that contains a carbon atom as a ligand. They are analogous to metal nitrido complexes. Carbido complexes are a molecular subclass of carbides, which are prevalent in organometallic and inorganic chemistry. Carbido complexes represent models for intermediates in Fischer–Tropsch synthesis, olefin metathesis, and related catalytic industrial processes. Ruthenium-based carbido complexes are by far the most synthesized and characterized to date. Although, complexes containing chromium, gold, iron, nickel, molybdenum, osmium, rhenium, and tungsten cores are also known. Mixed-metal carbides are also known. | 0 | Organic Chemistry |
When a rock melts to form a liquid, the liquid is known as a primary melt. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature, primary melts are rarely seen. Some leucosomes of migmatites are examples of primary melts. Primary melts derived from the mantle are especially important and are known as primitive melts or primitive magmas. By finding the primitive magma composition of a magma series, it is possible to model the composition of the rock from which a melt was formed, which is important because we have little direct evidence of the Earth's mantle. | 9 | Geochemistry |
Denigés' reagent is used to detect isolefin or tertiary alcohols which can be easily dehydrated to form isoolefin in the presence of acid. Treatment of solutions containing either isolefin or tertiary alcohols with this reagent will result in the formation of a solid yellow or red precipitate. | 3 | Analytical Chemistry |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.