Population of conformers

In the furanose series, O-inethylation a- and /3-shifts of +9 to +11 p.p.m. and —1 to —3 p.p.m., respectively, were observed, with smaller shifts occurring on O-substitution with isopropyl, which was suggested, as just mentioned, as a more-accurate, model substituent for gauging O-glycosylation shifts.15 The O-isopropylation displacements were found to be independent of steric distortion, or variation of population of conformational components, or both. 

The majority of enzyme substrates contain flexible moieties. As the 3D structure of the substrate to be analyzed (the conformation) has a reasonable impact on the outcome of the method, a precise protocol is used to build it. Each substrate is subjected to a conformational search followed by energy minimization by means of a software developed at Molecular Discovery Ltd. The population of conformers is... 

The populations of the rotamers are affected by steric effects. Karabatsos and Taller (77) examined the populations of unsymmetrical nitrosamines and found that in methyl-terr-butylnitrosamine only one isomer was observable. Nelsen and associates (78) similarly found that the populations of conformers of 2-nitroso-2-azabicyclo[2.2.2]octanes (40) are affected by the substituent in the 3-position. 

Spectral nonequivalence in diastereomeric solvates results from the population of conformations that place at least one set of chemically equivalent nuclei in different positions with respect to a CSA substituent capable of perturbing their magnetic environment. Having three points of interaction or reference is a general requirement for chiral recognition (26). The manner in which this requirement is met is readily understood in terms of complexes 6 and 7 through... 

The present calculations on po1y di-/j-hexylsilylene) are consistent with the hypothesis that the origin of thermochromlc behavior in selected polysilane polymers resides in a change in population of conformational states along the silicon backbone with temperature. 

Two empirical increment systems (Table 4.22) derived from experimental data as collected in Table 4.23 permit prediction of alkanol carbon-13 shifts. One is related to the shift value B of the hydrocarbon R —H and involves, as usual, addition of the increments Z, = <5,(r oh) — r)(- R > according to eq. (4.8 a) [268]. The other employs a linear equation (4.8 b), correlating the shifts of an alkanol R — OH and the corresponding methyl-alkane R —CH3 by a constant bk and a slope ak, which is 0.7-0.8 for a and about 1 for ji and y positions [269]. Specific parameter sets characterize primary, secondary, and tertiary alcohols (Table 4.22). The magnitudes of Z) increments in eq. (4.8 a) decrease successively from primary to tertiary alcohols (Table 4.22), obviously as a result of reduced populations of conformers with yliauche interactions in the conformational equilibrium when the degree of alkylation increases. 

An analysis of character and frequencies of normal vibrations and scan of relaxed potential energy surface represents two complementary approaches to investigation of conformational flexibility of pyrimidine rings in nucleic acid bases. A combined application of these approaches allows estimating population of conformations with nonplanar rings for each molecule. 

The shape of a conformation can be characterized by various manipulations of the principal moments. The asymmetry of any one of the conformations is characterized by the dimensionless ratios 1 > L lL > L /L2 > OJ34,35 Spherical symmetry requires L lL2 = Lh)l = 1. Averaging of the corresponding principal moments over all conformations permits discussion of the asymmetry of the population of conformations in terms of (L2)I(Li) and (L2)/(L2). Examples of these dimensionless ratios are presented in Table 1.2 for four types of macromolecules, unperturbed by long-range interactions. In none of the four cases do the conformations have spherical symmetry. 

Recently, a notable temperature-related effect was reported for site-selectivity (double-bond selectivity or chemoselectivity) in the PB reaction of unsymmetrically substituted furans (Scheme 7.14) [30]. For example, the selective formation of the more substituted oxetane, OX1, was observed during the PB reaction of 2-methyl-furan with benzophenone at a high temperature (61 °C). However, a 58 42 mixture of the oxetanes, 0X1 and 0X2, was reported at low temperature (—77 °C). This notable effect of temperature could be explained by the relative population of conformers of the intermediary triplet 1,4-biradicals, T-BR1 andT-BR2. The excited benzophenone was considered to attack the double bonds equally so as to produce a mixture of the conformers of T-BR1 and T-BR2 however, at low temperature the conformational change was suppressed. Thus, the site-random formation of oxetanes 0X1 and 0X2 was observed after the ISC process. Nonetheless, at high... 

Nevertheless, the authors do not comment on the different endo/exo selectivity observed in reactions with furan (the endo adduct is predominant, Scheme 55) and cyclopentadiene (only exo adducts are obtained, Scheme 54) under kinetic control conditions. They explain the changes in the facial diastereoselec-tivity, observed in the absence of ZnCl2, as a consequence of the increase in population of conformation B (Fig. 9), attributed to dipole-dipole repulsion. However they pay no attention to the role of the diene, which could be mainly responsible for the observed differences. With furan, the de decreased, but the major adduct remains the same, whereas with cyclopentadiene, the 7r-facial... 

The dipole moment effects of halogen atoms are sizeable (though very similar because their electronegativity varies inversely with size) and they differ from those of such other simple groups as CH3, CF3 and CN (see Table 5). Dipole moment effects are important among those that influence the relative populations of conformers of a substance as a function of its physical state (solid, liquid, gas) or the presence of a solvent. 

Barriers to Rotation and Population of Conformers in Thiazole Derivatives"... 

As a result of static disorder, often alternate positions of atoms can be resolved in electron density maps. In macromolecular crystals, some of the flexible regions possess variable conformers and as a consequence none of the individual ones can be detected in electron density maps. Thus, dynamics between several energetically similar states and/or larger amplitude makes to vanish more mobile sequential subunits. In practice, more than two or three conformer makes detection and/ or assignment impossible. In other words lower than 25-33% of relative population of conformers is unseen by diffraction methods. 

A large enhancement of H2 from H3 would not have been expected for conformation a because H2 is much closer to Hy. So there must also be a large population of conformation b. We cannot easily quantify the relative populations from this NOE difference spectrum because H2 and H4 are in such different proton environments ( 6.5.3). Equilibration between the two conformations is obviously fast on the chemical shift timescale. The appropriate experiment here is separate irradiation of H2 and H4 and measurement of the relative enhancements at H3 and H . This experiment is illustrated below, and the resulting spectra are shown on the next page. 

In this study we use REMD [67,75] to study the thermodynamic equilibrium between the conformations of the P450 BM-3/NPG complex in which the terminal carbon atoms of NPG is distant from the heme iron as in the low temperature X-ray crystal structure [147] (the distal state, see Fig. 5.2a) and conformations with the terminal carbon atoms of NPG proximal to the heme iron as in the conformation proposed by Jovanovic et al. [151] (the proximal state, see Fig. 5.2b). REMD is ideally suited for this problem not only because it improves conformational sampling but also because it yields the populations of conformational states over a range of temperatures. 

The alditol crystal structures are conformationally homogeneous. The cocrystallization of different conformers of the same molecule is not common, but sufficient examples are known, especially in hydrogen-bonded crystal structures, for it to be a phenomenon with which crystallographers are familiar. It occurs when there is a population of conformers in solution, all of which have similar energies. A classical example is that of 2,3-dimethyl-2,3-butanediol (pinacol)... 

When modeling multichain systems, it is necessary to start with a configuration in which the population of conformers and the global characteristics of... 

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