Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

DFT conformational study of cysteine in gas phase and aqueous solution

Abstract: Different conformers of cysteine in gas phase are investigated at the DFT B3LYP/6-31G* and B3LYP/6-311 + +G** levels. The effect of the solvent is simulated by using the Onsager and polarizable continuum (PCM) models within the self-consistent reaction field method (SCRF) at the B3LYP/6-31G* level. Specifically, five neutral forms, two anions and one zwitterion were analysed. Both, in gas phase and solution the most stable normal form has the carboxyl group directed toward the amino group. In accord with the experiment, the PCM model predicts that the most stable structure in solution is a zwitterion, a species that does not exist or has a very small stability in gas phase. A major stabilization in solution is also predicted for the zwitterionic form of anionic cysteine. Thus the PCM model renders correct stability order of the different conformers in solution, while the Onsager model does not, which is due to the underestimation of the electrostatic contributions to the solute-solvent interaction for the zwitterions. (C) 2000 Elsevier Science B.V.

Crystalline cellulose: structure and hydrogen bonds

Abstract: The state-of-the-art studies of the cellulose structure, major cellulose polymorphs, the crystal packing, conformers and hydrogen bond systems are analyzed based on publications of the last decade.

Molecular structure and antioxidant properties of delphinidin.

Abstract: Density functional theory calculations were performed to evaluate the antioxidant activity of delphinidin, taking into account its acid/base equilibrium. The conformational behavior of both the isolated and the aqueous solvation species (simulated with the polarizable continuum model) were analyzed at the B3LYP/6-31++G(d,p) level, considering the cationic, neutral, and anionic forms, the latter two forms consisting of diverse tautomers. The analysis of their electron density distributions, using the quantum theory of atoms in molecules, reveals several facts that are not in line with their usual Lewis structures. The prototropic preferences observed in the gas phase and in solution are similar. Thus, in both phases, most stable tautomer of neutral delphinidin is obtained by deprotonating the hydroxyl at C4′, and the most stable tautomer of the anion is obtained by deprotonating the hydroxyls at C4′ and C5. All the planar conformers obtained display an intramolecular hydrogen bond (IHB) between O3 and H6′....

Relative stabilities of tetramethoxycalix[4]arenes: Combined NMR spectroscopy and molecular mechanics studies

Abstract: Relative stabilities of four conformers and thermodynamic parameters for interconversion among four conformers of 25,26,27,28-tetramethoxycalix[4]arene (1a) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene (1b) have been determined by 1H NMR spectroscopy. The relative stability of 1a is in the order partial cone (most stable) > cone > 1,2-alternate and 1,3-alternate (undetected) and that for 1b is in the order partial cone (most stable) > cone > 1,2-alternate > 1,3-alternate (least stable). These orders are reproduced well by MM3, in contrast with MM2. In particular, the energy differences between partial cones and cones computed by MM3 (0.27 kcal mol–1 for 1a and 1.50 kcal mol–1 for 1b) show good agreement with those determined by 1H NMR spectroscopy (0.32 ± 0.13 kcal mol–1 for 1a and 1.2 ± 0.3 kcal mol–1 for 1b). Both the computational and the spectroscopic results suggest that the basic skeletons for cones, 1,2-alternates and 1,3-alternates are relatively rigid (sharp potential energy surfaces) whereas that for partial cones is more or less flexible (flattened potential energy surface). Thus, introduction of the tert-butyl groups into the para-positions destabilizes cones and 1,2-alternates because it is difficult to reduce the increased steric crowding by the conformational change. In 1,3-alternates four phenol units are parallel, so that introduced tert-butyl groups would increase the steric crowding to a lesser extent. The basic skeleton for partial cones changes significantly upon introduction of tert-butyl groups, indicating that the increased steric crowding is relaxed by the conformational change. The finding clearly explains why partial cones frequently appear as the most stable conformer.