Predictions of Sugar Conformation in Aqueous Solution by NMR-2JCH and Ab initio Calculation

Abstract

Two bond distance 13C-1H Spin-Spin coupling constants, which obtained from the NMR analysis, strongly depend on the rotational isomerism of adjacent hydroxyl groups attached to hexapyraosyl ring, so it can be expected to elucidate hydrogen bonding network structure in chemical and biochemical environment and its change concerned with the sugar residue around active site in a protein. In this study, we have performed thorough conformational analysis about several unique conformers of alpha-galactose to identify predominant conformations in aqueous solution. As the result, some conformers which comparatively reproduced the experimental 2JCH values of alpha-galactose were found. However, these conformers were not predominance in evaluation of their distributions based on the Bolzmann law using conformational energies of ab initio calculation with PCM and SMD solvation models. Therefore, we have reminded the difficulty of the prediction of suger conformations in aqueous solution including the direction of the hydroxyl groups, even if using high-level ab initio calculation. The latest result of higher-level calculation suggests that application of IPCM model may give a good prediction for the NMR/2JCH values in comparison with the experimental but unstable convergence of SCF calculation simultaneously. We are now continuously trying to eliminate the causative factor.