A theoretical study of d-glucose, d-galactose, and parent molecules: solvent effect on conformational stabilities and rotational motions of exocyclic groups

d-Glucopyranose and d-galactopyranose, in their α and β forms, together with some of their mono- and di-substituted parent molecules, were investigated theoretically in order to evaluate the solvent effect on the differential stability of various regions of the potential surfaces. In particular, the aim of this study was to evaluate the solvent effect on the conformational stabilities and the possibility of wide oscillations and/or correlated motions of exocyclic groups in the presence of water. Energies of molecules in aqueous solution were calculated by the polarizable continuum model in conjunction with the semiempirical AM1 method. Water as solvent favours the anti conformation of the exocyclic hydroxyl groups (except the anomeric one), mostly reduces the oscillation amplitude, and lowers the barriers to the rotation of the hydroxymethyl group. Concerted motions of exocyclic groups imply barriers which are somewhat higher, especially in galactose.