Conformational analysis of C-disaccharides using molecular mechanics calculations.

Relaxed-residue energy maps based on the MM3 force field were computed for the methyl glycosides of eight C-linked D-glucosyl disaccharides: the two-b ond axial-equatorial linked disaccharides beta -kojibioside [(1-->2)alpha-] , beta -nigeroside [(1-->3)alpha-] and beta -maltose C(1-->4)alpha-], the t wo-bond equatorial-equatorial linked disaccharides beta -sophoroside [(1--> 2)beta-], beta -laminarabioside [(1-->3)beta-], beta -cellobioside [(1-->4) beta-] and the three-bond-linked (1-->6) disacharides C-isomaltoside and C- gentiobioside, Optimized structures were calculated on a 20 degrees grid sp acing of the torsional angles about the C-glycosidic bonds and the final is oenergy surfaces were based on 11664 conformations, for the two-bond-linked disaccharides and 69984 conformations for the three-bond-linked disacchari des. Boltzmann-weighted (3)J coupling constants were calculated and compare d to the experimental values. They are satisfactory except for maltose wher e hydrogen bonds cause an over-estimation of the energy differences between the conformers. The energy maps are similar to maps of the corresponding O -disaccharides, but there are differences in the locations and the relative energies of the minima. The preferred conformations of the C-glycosidic bo nds are as if they were conforming to the exo-anomeric effect but are close r to staggered conformations than shown by the MM3 results for the O-linkag es.