2D-and 3D-potential energy surfaces of β-(1→3)-linked disaccharides calculated with the MM3 force-field

The adiabatic conformational surfaces of sixteen 4',6',6-trideoxy-beta-D-(I --> 3)-linked disaccharides were obtained using the MM3 force-field. Calculations were carried out on disaccharides with different configurations at C2, C4 and C2', which are neighbors to the glycosidic linkage, as well as that of the linked carbon (0). The surfaces were plotted as contour maps and as 2D graphs representing the energy vs. the psi angle. The resulting maps were similar in each case, indicating that the substituents do not play a major role in the conformational features of these disaccharides. However, the number of minima, the preferred minimum conformation and the flexibility depended on the configurations of the mentioned carbons. Vicinal equatorial substituents tend to decrease the overall flexibility, especially those on C2, although cross over effects were found. The relative stabilities of the minimal energy conformations of the 16 compounds were compared with those of their equivalent alpha-linked counterparts. Deviations of the predicted increased stabilities of equatorially substituted compounds over axially substituted ones follow a relationship with their configurations, and consequently can serve to formulate predictive trends.