"We don't even really know if he has an eye socket on the left to be able to hold a conformer," she says.
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Republicans who see an opportunity to fulfill their agenda through Trump regardless of his bad deeds have remained silent, and like a conformer, Democratic House Speaker Nancy PelosiNancy PelosiWhistleblower complaint declassified on eve of high-stakes testimony Ocasio-Cortez on impeachment: 'I think the ground has shifted' Democrats ask Pentagon to probe delayed Ukraine aid MORE fails to act out of fear that her caucus could lose seats in the election.
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Wiley, 2001. Another case is trans-1,2 difluorocyclohexane, which has a larger preference for the di-equatorial conformer, rather than the anti-diaxial conformer, in more polar solvents.
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Product ratios from direct and triplet-sensitized photolysis have been used as evidence for proposals that claim that concerted products arise from the s-cis-conformer and stepwise products occur through the s-trans-conformer.
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Conformer is a clear acrylic shell fitted after an enucleation if the final artificial eye is not available at the time of surgery to hold the shape of the eye socket and allow the eyelids to blink over the shell without rubbing the suture line. The conformer shell holds the shape ready for the artificial eye. The conformer shell will be worn for six to eight weeks after surgery. Some ocularists will make a temporary artificial eye which can be worn as the conformer shell.
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One category of reactions under Curtin–Hammett control includes transformations in which the more stable conformer reacts more quickly. This occurs when the transition state from the major intermediate to its respective product is lower in energy than the transition state from the minor intermediate to the other possible product. The major product is then derived from the major conformer, and the product distribution does not mirror the equilibrium conformer distribution.
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The conformer of methylcyclohexane with equatorial methyl is favored by relative to the conformer where methyl is axial. In cyclohexane, the two chair conformations have the same energy. The situation is more complex is substituted derivatives. In methylcyclohexane the two chair conformers are not isoenergetic.
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The all-trans arrangement is predicted by Hartree-Fock computational methods to be the most stable conformer.
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This geometry is a local energy maximum, so in contrast to the s-trans geometry, it is not a conformer. The gauche geometry, in which the double bonds of the s-cis geometry are twisted to give a dihedral angle of around 38°, is a second conformer that is around 12.0 kJ/mol (2.9 kcal/mol) higher in energy than the s-trans conformer. Overall, there is a barrier of 24.8 kJ/mol (5.9 kcal/mol) for isomerization between the two conformers.
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There are also a variety of stable conformers due to the rotation of single bonds in the N-pyridylmethylamino group. The E-conformer is more stable than the Z-conformer and assumed to be the active form. In solution, two different E-conformers exist which slowly change into each other.
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As the child's face grows and develops, the conformer will need to be changed. An expander may also be needed in anophthalmia to expand the socket that is present. The conformer is changed every few weeks the first two years of life. After that, a painted prosthetic eye can be fitted for the child's socket.
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Using computers it is possible to calculate the lowest energy state conformer and thus most populated and best representing the molecule. This state is referred to as the global minimum. The global minimum for some simple molecules can be discovered quite easily with certainty. Such as for decamethonium the straight line conformer is clearly the lowest energy state.
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Two chair conformations of cyclohexane. The entirety of axial positions become equatorial, and vice versa, upon ring flip. In organic chemistry, a ring flip (also known as a ring inversion or ring reversal) is the interconversion of cyclic conformers that have equivalent ring shapes (e.g., from a chair conformer to another chair conformer) that results in the exchange of nonequivalent substituent positions.
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The dynamics of conformational (and other kinds of) isomerism can be monitored by NMR spectroscopy at varying temperatures. The technique applies to barriers of 8–14 kcal/mol, and species exhibiting such dynamics are often called "fluxional". Besides NMR spectroscopy, IR spectroscopy is used to measure conformer ratios. For the axial and equatorial conformer of bromocyclohexane, νCBr differs by almost 50 cm−1.
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A second category of reactions under Curtin–Hammett control includes those in which the less stable conformer reacts more quickly. In this case, despite an energetic preference for the less reactive species, the major product is derived from the higher- energy species. An important implication is that the product of a reaction can be derived from a conformer that is at sufficiently low concentration as to be unobservable in the ground state.
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The chair conformation is the most stable conformer. At 25 °C, 99.99% of all molecules in a cyclohexane solution adopt this conformation. The symmetry is D3d. All carbon centers are equivalent.
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Paraldehyde is produced and used as a mixture of two diastereomers, known as cis- and trans- paraldehyde. For each diastereomer, two chair conformers are possible. The structures (1), (4) and (2), (3) are conformers of cis- and trans-paraldehyde, respectively. The structures (3) (a conformer of (2)) and (4) (a conformer of (1)) are high energy conformers on steric grounds (1,3-diaxial interactions are present) and do not exist to any appreciable extent in a sample of paraldehyde.
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The gauche effect is very sensitive to solvent effects, due to the large difference in polarity between the two conformers. For example, 2,3-dinitro-2,3-dimetylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride it prefers the anti conformer by a ratio of 58:42.Smith, Michael. B.; March, J. March's Advanced Organic Chemistry, 5th edition.
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Three main classes of reactions can be explained by the Curtin–Hammett principle: either the more or less stable conformer may react more quickly, or they may both react at the same rate.
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When the α-diazo ketone is in the s-cis conformation, the leaving group (N2) and the migrating group (R1) are antiperiplanar, which favors a concerted mechanism, in which nitrogen extrusion occurs concurrently with 1,2-alkyl shift. There is evidence this mechanism occurs in both thermolytic and photolytic methods, when the s-cis- conformer is strongly favored. Concerted mechanism from the s-cis conformation. CIDNP studies show that photochemical rearrangement of diazoacetone, which largely exists in the s-cis-conformer, is concerted.
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A clear example of the syn-pentane interaction is apparent in the diaxial versus diequatorial heats of formation of cis 1,3-dialkyl cyclohexanes. Relative to the diequatorial conformer, the diaxial conformer is 2-3 kcal/mol higher in energy than the value that would be expected based on gauche interactions alone. Pentane interference helps explain molecular geometries in many chemical compounds, product ratios, and purported transition states. One specific type of syn-pentane interaction is known as 1,3 allylic strain or (A1,3 strain).
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When ground state energies are different but transition state energies are similar, selectivity will be degraded in the transition state, and poor overall selectivity may be observed. For instance, high selectivity for one ground state conformer is observed in the following radical methylation reaction. 500px The conformer in which A(1,3) strain is minimized is at an energy minimum, giving 99:1 selectivity in the ground state. However, transition state energies depend both on the presence of A(1,3) strain and on steric hindrance associated with the incoming methyl radical.
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The coalescence temperature at 60 MHz is ca. –60 °C. The conformational changes that occur in a cyclohexane ring flip take place over several stages. Structure D (10.8 kcal/mol) is the highest energy transition state of the process. The molecular motions involved in a chair flip are detailed in the figure on the right: The half-chair conformation (D, 10.8 kcal/mol, C2 symmetry) is the energy maximum when proceeding from the chair conformer (A, 0 kcal/mol reference, D3d symmetry) to the higher energy twist-boat conformer (B, 5.5 kcal/mol, D2 symmetry).
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RNA Holliday junctions assume an antiparallel stacked conformation at high magnesium concentrations, a perpendicular stacked conformation at moderate concentrations, and rotate into a parallel stacked conformation at low concentrations, while even small calcium ion concentrations favor the antiparallel conformer.
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Works in this field were done both in the Cina and Bar groups. Each conformer is associated with a slightly different SRS spectral signature. Detection of these different landscapes is an indication of the different conformational structures of the same molecule.
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Exogenous antigen binds to a structure on the surface of activated cells; this structure is composed of HLA class I open conformer and HLA-F; the peptide-binding point of contact is a specific HLA class I epitope on the exogenous antigen.
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Classic HLA class I molecules interact with HLA-F through their heavy chain. However, HLA class I molecules only interact with HLA-F when they are in the form of an open conformer (free of peptide). Thus, HLA-F is expressed independently of bound peptide.
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RTI-352 is a phenyltropane that is used as a radiolabeling ligand for the DAT. RTI-352 is a geometric isomer of RTI-55 (β-CIT). Based on X-ray crystallography, this compound is in a tautomeric equilibrium residing mostly on the side of the boat-shaped conformer.
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Comparison of butadiene (s-trans conformer) and ethylene. The most stable conformer of 1,3-butadiene is the s-trans conformation, in which the molecule is planar, with the two pairs of double bonds facing opposite directions. This conformation is most stable because orbital overlap between double bonds is maximized, allowing for maximum conjugation, while steric effects are minimized. Conventionally, the s-trans conformation is considered to have a C2-C3 dihedral angle of 180°. In contrast, the s-cis conformation, in which the dihedral angle is 0°, with the pair of double bonds facing the same direction is approximately 16.5 kJ/mol (3.9 kcal/mol) higher in energy, due to steric hindrance.
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Theoretical analyisis implies a total of 16 conformational and configurational isomers, all chiral, forming 8 enantiomer pairs. Its most stable cis stereoisomer can adopt various conformations, the most stable one being shaped like a ribbon; The most stable trans-conformer is shaped like the 8-carbon equivalent chair conformation of cyclohexane.
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Xenosaurus grandis is a diurnally active thermal conformer, meaning that its body temperature correlates with substrate and air temperatures. Its average body temperature is 22.7 C.Ballinger RE, Lemos-Espinal J, Sanoja-Sarabia S, Coady NR (1995). "Ecological observations of the lizard, Xenosaurus grandis in Cuautlapan, Veracruz, Mexico". Biotropica 27: 128-132.
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The two possible stacked forms differ in which pairs of the arms are stacked with each other; which of the two dominates is highly dependent on the base sequences nearest to the junction. Some sequences result in an equilibrium between the two conformers, while others strongly prefer a single conformer. In particular, junctions containing the sequence A-CC bridging the junction point appear to strongly prefer the conformer that allows a hydrogen bond to form between the second cytosine and one of the phosphates at the junction point. While most studies have focused on the identities of the four bases nearest to the junction on each arm, it is evident that bases farther out can also affect the observed stacking conformations.
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Native-like conformations are then selected from these decoys using scoring functions as well as conformer clustering. High-resolution refinement is sometimes used as a final step to fine-tune native-like structures. There are two major classes of scoring functions. Physics-based functions are based on mathematical models describing aspects of the known physics of molecular interaction.
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A molecular entity, or chemical entity, is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity". A molecular entity is any singular entity, irrespective of its nature, used to concisely express any type of chemical particle that can exemplify some process: for example, atoms, molecules, ions, etc. can all undergo a chemical reaction.
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A boat transition state is disfavored, but rearrangements occur via this path to an appreciable extent as well, resulting in the production of diastereomeric mixtures. 496x496px Steric effects can be significant. Rearrangements for which a chair transition state is geometrically impossible nonetheless occur. In fact, enolate formation provides enough of a driving force to overcome the energetic barrier associated with both dearomatization and the boat conformer.
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Two models provide an explanation of their structure, the classical and the non-classical view. The classical view (Figure 1.a) involves a fast- equilibrating system in which a hydrogen atom rapidly shifts between two adjacent carbon atoms. In this model, fast equilibrium results from a low energy barrier between the two conformations of the molecule, and each conformer has a localized positive charge.
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Heterocyclic analogs of cyclohexane are pervasive in sugars, piperidines, dioxanes, etc. They exist generally follow the trends seen for cyclohexane, i.e. the chair conformer being most stable. The axial- equatorial equilibria (A values) are however strongly affected by the replacement of a methylene by O or NH. Illustrative are the conformations of the glucosides. 1,2,4,5-Tetrathiane ((SCH2)3) lacks the unfavorable 1,3-diaxial interactions of cyclohexane.
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The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group. In a concerted step, the migrating R group attacks the α-carbonyl group forming another alkoxide with concomitant formation of a keto-group at the other carbon. This migration step is rate-determining. This sequence resembles a nucleophilic acyl substitution.
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Annie Collins is a film editor from New Zealand, best known for her work on The Return of the King. She was a film conformer on The Fellowship of the Ring, and moved up to assistant editor on The Two Towers. Her work with Jamie Selkirk (as an "additional editor") helped earn The Return of the King an Oscar for Best Editing in 2004.
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Driguez, P. Duchaussoy, P. Sizun, J.-P. Herault, J.-M. Herbert, M. Petitou, P. Sinaÿ, « Synthesis of conformationally locked L- iduronic acid derivatives : direct evidence for a critical role of the skew-boat 2S0 conformer in the activation of antithrombin by heparin », Chemistry- A European Journal, 2001, 7, p. 4821-34 Pierre Sinaÿ has also discovered and developed a whole series of conceptually new reactions.
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Another approach uses selection algorithms such as ENSEMBLE and ASTEROIDS. Calculation procedures first generate a pool of random conformers (initial pool) so that they sufficiently sample the conformation space. The selection algorithms start by choosing a smaller set of conformers (an ensemble) from the initial pool. Experimental parameters (NMR/SAXS) are calculated (usually by some theoretical prediction methods) for each conformer of chosen ensemble and averaged over ensemble.
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However, intramolecular hydrogen bonding can be an example of a stabilizing 1,3-diaxial interaction. Dipoles also play a role in conformer stability, aligned dipoles lead to an increase in energy while opposed dipoles lead to a lowering of energy hence a stabilizing effect, this can be complicated by solvent effects. Polar solvents tend to stabilize aligned dipoles. All interaction must be taken into account when determining a preferred conformation.
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Greenwood, 658 Two forms (α-, β-) of cyclo-S9 are known, one of which has been characterized.Steudel, 8 Two forms of cyclo-S18 are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other cyclo-Sn compounds refer to different packings of essentially the same conformer, these two conformers have been termed endo- and exo-.
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O. vulgaris has a pH- independent venous reserve that represents the amount of oxygen that remains bound to the respiratory pigment at constant pressure of oxygen. This reserve allows the octopus to tolerate a wide range of pH related to temperature. As a temperature conformer,Noyola, J., Caamal-Monsreal, C., Díaz, F., Re, D., Sánchez, A., & Rosas, C. (2013). Thermopreference, tolerance and metabolic rate of early stages juvenile Octopus maya acclimated to different temperatures.
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Rotation about single bond of butane to interconvert one conformation to another. The gauche conformation on the right is a conformer, while the eclipsed conformation on the left is a transition state between conformers. Above: Newman projection; below: depiction of spatial orientation. In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation).
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Although noncovalent interactions are known to be relatively weak interactions, small stabilizing interactions can add up to make an important contribution to the overall stability of a conformer. Not only are salt bridges found in proteins, but they can also be found in supramolecular chemistry. The thermodynamics of each are explored through experimental procedures to access the free energy contribution of the salt bridge to the overall free energy of the state.
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Mutations or buffer conditions known to improve aggregation strongly increase the population of the beta conformer, thus suggesting this could be a conformation related to pathogenic aggregation. One theory is that the majority of alpha-synuclein aggregates are located in the presynapse as smaller deposits which causes synaptic dysfunction. Among the strategies for treating synucleinopathies are compounds that inhibit aggregation of alpha-synuclein. It has been shown that the small molecule cuminaldehyde inhibits fibrillation of alpha-synuclein.
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Anatoly M Belostotskii (2015): "Conformer and conformation", chapter 2 of Conformational Concept For Synthetic Chemist's Use: Principles And In Lab Exploitation. 580 pages. .) Interactions with other molecules of the same or different compounds (for example, through hydrogen bonds) can significantly change the energy of conformations of a molecule. Therefore, the possible isomers of a compound in solution or in its liquid and solid phases many be very different from those of an isolated molecule in vacuum.
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Chemical Entities of Biological Interest, also known as ChEBI, is a database and ontology of molecular entities focused on 'small' chemical compounds, that is part of the Open Biomedical Ontologies effort. The term "molecular entity" refers to any "constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity". The molecular entities in question are either products of nature or synthetic products which have potential bioactivity.
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In 3-methyl-1-butene, the interactions between the hydrogen and the two methyl groups in the allylic system cause a change in enthalpy equal to 2 kcal/mol. As expected, with an increase in substituent size, the equilibrium enthalpies between rotamers also increases. For example, when examining 4-methyl-2-pentene which contains an additional allylic methyl group compared to 3-methyl-1-butene, the enthalpy of rotation for the highest energy conformer increases from 2 kcal/mol to 4 kcal/mol.
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It is unclear whether other serpins can adopt this conformer, and whether this conformation has a functional role, but it is speculated that the δ-conformation may be adopted by Thyroxine-binding globulin during thyroxine release. The non-inhibitory proteins related to serpins can also cause diseases when mutated. For example, mutations in SERPINF1 cause osteogenesis imperfecta type VI in humans. In the absence of a required serpin, the protease that it normally would regulate is over-active, leading to pathologies.
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Note that the twist-boat (D2) conformer and the half-chair (C2) transition state are in chiral point groups and are therefore chiral molecules. In the figure, the two depictions of B and two depictions of D are pairs of enantiomers. As a consequence of the chair flip, the axially- substituted and equatorially-substituted conformers of a molecule like chlorocyclohexane cannot be isolated at room temperature. However, in some cases, the isolation of individual conformers of substituted cyclohexane derivatives has been achieved at low temperatures (–150 °C).
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Various strain interactions shown in red. (Other hydrogens left off for simplicity) Nonbonded 1,3-diaxial interaction energies are commonly used to approximate strain energy in cyclic molecules, as values for these interactions are available. By taking the difference in nonbonded interactions for each conformer, the equilibrium enthalpy can be estimated. The strain energy for methylidenecyclohexane has been calculated to be 4.5 kcalmol−1 using estimations for 1,3-diaxial strain (0.9 kcalmol−1), methyl/hydrogen allylic strain (1.3kcalmol−1), and methyl/methyl allylic strain (7.6 kcalmol−1) values.
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Allylic strain in an olefin. Allylic strain (also known as A1,3 strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin with an allylic substituent on the other end.Eric V. Anslyn and Dennis A. Dougherty Modern Physical Organic Chemistry University Science Books, 2006. If the substituents (R and R') are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other.
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When one is considering allylic strain, one needs to consider the possible conformers and the possible stereoelectronic demand of the reaction. For example, in the conformation of (Z)-4-methylpent-2-ene, the molecule isn't frozen in the favored conformer but rotates in the dihedral angle around 30° at <1kcal/mol cost. In stereoselective reactions, there are 2 effects of allylic strain on the reaction which is the sterics effect and the electronic effects. The sterics effect is where the largest group prefer to be the farthest from the alkene.
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The chair conformation of six-membered rings have a dihedral angle of 60° between adjacent substituents thus usually making it the most stable conformer. Since there are two possible chair conformation steric and stereoelectronic effects such as the anomeric effect, 1,3-diaxial interactions, dipoles and intramolecular hydrogen bonding must be taken into consideration when looking at relative energies. Conformations with 1,3-diaxial interactions are usually disfavored due to steric congestion and can shift equilibrium to the other chair form (example: 1C4 to 4C1). The size of the substituents greatly affects this equilibrium.
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After the deprotonation, the imine turns into an azaenolate with lithium cation chelating both the nitrogen and oxygen. There are two possible options for lithium chelation. One is that lithium is antiperiplanar to the C=C bond (blue colored), leading to the conformation of ZC-N; the other one is that lithium and the C=C bond are at the same side of the C-N bond (red colored), leading to the EC-N conformer. There are also two available orientations for the chelating nitrogen and R2 group, being either EC=C or ZC=C.
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A tetrameric form has also been described, under different catalytic conditions. The synthesis of tetrameric acetone peroxide has been disputed.Primary Explosives - Robert Matyáš, Jiří Pachman (auth.), p.275Matya´sˇ, R., Pachman, J.: Study of TATP: Influence of reaction conditions on product composition. Propellants Explosives Pyrotechnics 35, 31–37 (2010) This substance is reported to be tetraacetone tetraperoxide; 3,3,6,6,9,9,12,12-octamethyl-1,2,4,5,7,8,10,11-octaoxacyclododecane (TeATeP)by Schulte-Ladbeck et al. [20], Pena Quevedo et al. [21] or as a structural conformer of TATP by Widmer et al. [22]. We did not analyze this side product further.
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Bruce faced criticism from House Democratic and Republican leaders in April for distributing campaign fliers critical of Waller to Republican representatives on the House floor. Although not a violation of the law or House rules, legislative leaders denounced the action as a breach of decorum and tradition. Waller also called for Bruce's resignation in April 2008, after Bruce's controversial comments about Mexican guest workers. Bruce, in response, characterised Waller as a "conformer" rather than a "reformer," and stated that his controversial actions were designed to draw attention to inappropriate legislative actions.
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Understanding the stereochemistry of α-diazo ketones is essential in elucidating the mechanism of the Wolff rearrangement. α-diazocarbonyl compounds are generally locally planar, with large rotational barriers (55–65 kJ/mol) due to C-C olefin character between the carbonyl and α-carbon, illustrated in the rightmost resonance structure. Such a large barrier slows molecular rotations sufficiently to lead to an equilibrium between two conformers, an s-trans and s-cis-conformer. s-cis-Conformers are electronically favored due to Coulombic attraction between the oxygen with a partial negative charge and the cationic nitrogen, as seen in the rightmost resonance structure.
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They are useful for visualizing the effective shape and relative dimensions of the molecule, and the shapes of surface a given static conformer might present. On the other hand, these models mask the chemical bonds between the atoms, and make it difficult to see the structure of the molecule that is obscured by the atoms nearest to the viewer in a particular pose. For this reason, such models are of greater utility if they can be used dynamically, especially when used with complex molecules (e.g., see the greater understanding of the molecules shape given when the THC model is clicked on to rotate).
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In junctions with symmetrical sequences, the branchpoint is mobile and can migrate in a random walk process. The rate of branch migration varies dramatically with ion concentration, with single- step times increasing from 0.3−0.4 ms with no ions to 270−300 ms with 10 mM Mg2+. The change in rate is correlated with the formation of the stacked versus the unstacked structures. Holliday junctions with a nick, or break in one of the strands, at the junction point adopt a perpendicular orientation, and always prefer the stacking conformer that places the nick on a crossover strand rather than a helical strand.
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After the autoinhibitory CR1 regulatory domain is released, B-Raf's CR3 kinase domain must change to its ATP-binding active conformer before it can catalyze protein phosphorylation. In the inactive conformation, F595 of the DFG motif blocks the hydrophobic adenine binding pocket while activation loop residues form hydrophobic interactions with the P-loop, stopping ATP from accessing its binding site. When the activation loop is phosphorylated, the negative charge of the phosphate is unstable in the hydrophobic environment of the P-loop. As a result, the activation loop changes conformation, stretching out across the C-lobe of the kinase domain.
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Viral proteins and other exogenous antigens decrease surface HLA-F expression because the exogenous proteins interact with HLA class I molecules at the same sites where HLA-F interacts, producing crosslinking. The exogenous proteins trigger an internal co-localization of both HLA-F and HLA class I molecules. Exogenous proteins with higher affinity will interact more readily with HLA class I molecules triggering a dissociation of HLA class I/HLA-F, thereby reducing the surface levels of HLA-F. HLA-F interacts with the open conformer (OC) of HLA class I and they function together in cross-presentation of exogenous antigen.
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Other work using chorismate mutase from Bacillus subtilis showed evidence that when a cation was aptly placed in the active site, the electrostatic interactions between it and the negatively charged transition state promoted catalysis. Additional studies have been done in order to support the relevance of a near attack conformer (NAC) in the reaction catalyzed by chorismate mutase. This NAC is the reactive conformation of the ground state that is directly converted to the transition state in the enzyme. Using thermodynamic integration (TI) methods, the standard free energies (ΔGN°) for NAC formation were calculated in six different environments.
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The chair-chair conformation is the second most abundant conformation at room temperature, with a ratio of 96:4 chair-boat:chair-chair observed. Substitution positional preferences in the ground state conformer of methyl cyclooctane can be approximated using parameters similar to those for smaller rings. In general, the substituents exhibit preferences for equatorial placement, except for the lowest energy structure (pseudo A-value of -0.3 kcal/mol in figure below) in which axial substitution is favored. The "pseudo A-value" is best treated as the approximate energy difference between placing the methyl substituent in the equatorial or axial positions.
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Thus, they could isolate the optically active 6,5-bicyclic ketol described so far only in the Hajos-Parrish publications. Hajos and Parrish investigated further the exact configuration of the above cis-fused-7a-methyl- 6,5-bicyclic-ketol by circular dichroism, and these results were confirmed by a single-crystal X-ray diffraction study. The centro symmetrical crystal of the corresponding racemic ketol without a heavy atom label has been obtained by the use of racemic proline. It showed by X-ray diffraction an axial orientation of the angular methyl group and an equatorial orientation of the hydroxyl group in the chair conformer of the six-membered ring.
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In creating Andromaque, Racine believes he must "[se] conformer à l'idée que nous avons maintenant de cette princesse".Second Preface of Andromaque. Astyanax, whom Euripides describes (in The Trojan Women and the Andromache) as having been thrown from the walls of Troy and killed, and whose death is foreshadowed in book 24 of the Iliad, is made to survive the capture of Troy and the extinction of his dynasty. In another respect also, Racine departs from the lines laid down by the Andromache, for whereas in the earlier play the heroine fears that the son she has had by Pyrrhus may suffer death if she refuses to marry the father, the later heroine fears for the life of a legitimate son.
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D2R conformers are equilibrated between two full active (D2HighR) and inactive (D2LowR) states, while in complex with an agonist and antagonist ligand, respectively. The monomeric inactive conformer of D2R in binding with risperidone was reported in 2018 (PDB ID: 6CM4). However, the active form which is generally bound to an agonist, is not available yet and in most of the studies the Homology modeling of the structure is implemented. The difference between the active and inactive of G protein-coupled receptor is mainly observed as conformational changes at the cytoplasmic half of the structure, particularly at the transmembrane domains (TM) 5 and 6. The conformational transitions occurred at the cytoplasmic ends are due to the coupling of G protein to the cytoplasmic loop between the TM 5 and 6.
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Initial addition of the ylide results in a betaine with adjacent charges; density functional theory calculations have shown that the rate-limiting step is rotation of the central bond into the conformer necessary for backside attack on the sulfonium. Selectivity in the Johnson–Corey–Chaykovsky reaction The degree of reversibility in the initial step (and therefore the diastereoselectivity) depends on four factors, with greater reversibility corresponding to higher selectivity: # Stability of the substrate with higher stability leading to greater reversibility by favoring the starting material over the betaine. # Stability of the ylide with higher stability similarly leading to greater reversibility. #Steric hindrance in the betaine with greater hindrance leading to greater reversibility by disfavoring formation of the intermediate and slowing the rate-limiting rotation of the central bond.
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While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a relatively long time period as a stable rotational isomer or rotamer (an isomer arising from hindered single-bond rotation).
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MAD1 localizes to unattached kinetochores while binding strongly to MAD2. The localization of MAD2 and BubR1 to the kinetochore may also be dependent on the Aurora B kinase. Cells lacking Aurora B fail to arrest in metaphase even when chromosomes lack microtubule attachment. Unattached kinetochores first bind to a MAD1-C-MAD2-p31comet complex and releases the p31comet through unknown mechanisms. The resulting MAD-C-MAD2 complex recruits the open conformer of Mad2 (O-Mad2) to the kinetochores. This O-Mad2 changes its conformation to closed Mad2 (C-Mad2) and binds Mad1. This Mad1/C-Mad2 complex is responsible for the recruitment of more O-Mad2 to the kinetochores, which changes its conformation to C-Mad2 and binds Cdc20 in an auto-amplification reaction. Since MAD1 and CDC20 both contain a similar MAD2-binding motif, the empty O-MAD2 conformation changes to C-MAD2 while binding to CDC20.
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