Metadynamics modelling of the solvent effect on primary hydroxyl rotamer equilibria in hexopyranosides

Accurate modelling of rotamer equilibria for the primary hydroxyl groups of monosaccharides continues to be a great challenge of computational glycochemistry. The metadynamics technique was applied to study the conformational free energy surfaces of methyl alpha-D-glucopyranoside and methyl Of-D-galactopyranoside, employing the GLYCAM06 force field. For both molecules, seven to eight conformational free-energy minima, differing in the omega (0-5-C-5-C-6-O-6) and chi (C-3-C-4-O-4-HO-4) dihedral angles, were identified in vacuum or in a water environment. The calculated rotamer equilibrium of the primary hydroxyl group is significantly different in vacuum than in water. The major effect of a water environment is the destabilisation of a hydrogen bond between O-4-HO-4 and O-6-HO-6 groups. It was possible to calculate the free-energy differences of individual rotamers with an accuracy of better than 2 kJ/mol. The calculated gg, gt and tg rotamer populations in water are in close agreement with experimental measurements, and therefore support the theoretical background of metadynamics. (C) 2009 Elsevier Ltd. All rights reserved.