AB-INITIO MOLECULAR-ORBITAL CALCULATION OF CARBOHYDRATE MODEL COMPOUNDS .4. FLEXIBILITY OF PSI-TYPE GLYCOSIDIC BONDS IN CARBOHYDRATES

An ab initio study of the conformational behavior of aglycon O-C glyco sidic bonds and rotameric distribution in O-methylated carbohydrates h as been carried out on axial and equatorial 1,2-, 1,3- and 1,4-dimetho xytetrahydropyran as models. The geometry of the conformers about the C(aglycon)-O(glycosidic) bond (Psi-type) in 12 models was determined b y gradient optimization at the SCF level using split valence 6-31G ba sis sets. The potential of rotation has been calculated using 6-31G a nd 631 + G basis sets. At all levels of theory, both axial and equato rial forms prefer the GT or TG conformers around the C-O glycosidic bo nd over the GG conformer. Exceptions are models of (1 --> 2) linkages with the equatorial anomeric methoxyl group (E2A and E2E), where the T G conformer is not present. Calculated potential energy profiles show high flexibility within a 1.5 kcal mol(-1) low energy region that is 1 80 degrees wide. The glycosidic bond angle Cl-Oi-Ci depends on the tor sion angle Psi and assumes values in the interval from 115 degrees to 125 degrees. (C) 1997 Elsevier Science B.V.