Ab initio molecular orbital calculation of carbohydrate model compounds 4. Flexibility of Ψ-type glycosidic bonds in carbohydrates

Abstract

An ab initio study of the conformational behavior of aglycon OC glycosidic bonds and rotameric distribution in O-methylated carbohydrates has been carried out on axial and equatorial 1,2-, 1,3- and 1,4-dimethoxytetrahydropyran as models. The geometry of the conformers about the C(aglycon)O(glycosidic) bond (Ψ-type) in 12 models was determined by gradient optimization at the SCF level using split valence 6-31G ∗ basis sets. The potential of rotation has been calculated using 6-31G ∗ and 631 + G ∗ basis sets. At all levels of theory, both axial and equatorial forms prefer the GT or TG conformers around the CO glycosidic bond over the GG conformer. Exceptions are models of (1 → 2) linkages with the equatorial anomeric methoxyl group ( E2A and E2E), where the TG conformer is not present. Calculated potential energy profiles show high flexibility within a 1.5 kcal mol −1 low energy region that is 180 ° wide. The glycosidic bond angle ClO iC i depends on the torsion angle Ψ and assumes values in the interval from 115 ° to 125 °.