Topic
Conformational isomerism
About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.
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TL;DR: A fluoride-ammonium ion gauche effect constitutes a partial solution to the long-standing problem of governing internal rotations in cinchonium-based catalysts relying solely on a fluorine conformational effect.
Abstract: The C9 position of cinchona alkaloids functions as a molecular hinge, with internal rotations around the C8 C9 (tau(1)) and C9 C4' (tau(2)) bonds giving rise to four low energy conformers (1; anti-closed, anti-open, syn-closed, and syn-open). By substituting the C9 carbinol centre by a configurationally defined fluorine substituent, a fluorine-ammonium ion gauche effect (sigma(C-H) -> sigma(C-F)*; F-delta-...N+) encodes for two out of the four possible conformers (2). This constitutes a partial solution to the long-standing problem of governing internal rotations in cinchonium-based catalysts relying solely on a fluorine conformational effect.
68 citations
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TL;DR: Overall, hydrophobic residues are most precisely characterized and agree most closely in the two environments and there are correlations between amino acid types, local precision of the structure determination by both techniques and local differences between the structures in the crystals and in solution.
68 citations
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TL;DR: It is shown that, besides the steric repulsion model, the conformational energies of the compounds studied are dictated by hyperconjugative interactions involving mainly the methine hydrogen.
Abstract: Full geometry optimizations were carried out at the HF/6-31G** and B3LYP/6-31G** levels for methylcyclohexane, 2-, 3-, and 4-methyltetrahydropyran, 2-, 3-, and 4-methylpiperidine, 2-, 3-, and 4-methylthiane, 2-, 4-, and 5-methyl-1,3-dioxane, and 2-, 4-, and 5-methyl-1,3-dithiane and also for S-methyl thianium. Constrained geometry optimizations were carried out for methylcyclohexane, 2-methyl-1,3-dioxane, and the axial conformers of 2- and 3-methyltetrahydropyran and 2- and 3-methylpiperidine. The steric repulsion model, which is believed to account for the conformational energies of the cited compounds, was tested by stretching bonds and bending angles so that the axial methyl group is either forced to approach the ring gamma methylenes or get farther away from them. The calculated energies show that the energy costs of these perturbations are not dependent on the distances between the axial methyl group and the ring gamma methylenes and are not dependent on whether the methyl is axial or equatorial. It is shown that, besides the steric repulsion model, the conformational energies of the compounds studied are dictated by hyperconjugative interactions involving mainly the methine hydrogen. The C[bond]C lengths of the axial and equatorial conformers of methylcyclohexane are shown to be related to hyperconjugation.
68 citations
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TL;DR: Ion mobility and spectroscopic data combined with density functional theory (DFT) based molecular dynamics simulations confirm the presence of one major conformer per family, which arises from cis/trans isomerization about the proline residue.
68 citations
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TL;DR: It is demonstrated, using a set of labeled peptide analogues, that simple analysis of peak separation in their (13)C NMR spectra at slow and fast exchange allows an accurate value for the ratio of helical conformers to be obtained.
Abstract: While an unequal population of rapidly interconverting left- and right-handed conformers of a helical oligomer can be detected by circular dichroism, precise quantification of a conformer ratio has not previously been achieved. We demonstrate, using a set of labeled peptide analogues, that simple analysis of peak separation in their (13)C NMR spectra at slow and fast exchange allows an accurate value for the ratio of helical conformers to be obtained. The method reports the ratio of conformers at the site of the label and can therefore be used to investigate local variations in helical conformational control.
68 citations