EXPLORATION OF DISACCHARIDE CONFORMATIONS BY MOLECULAR MECHANICS

Conformational analyses by molecular mechanics of two-bond linked, glu cose-glucose and some glucose fructose disaccharides are reviewed. Usu ally, the conformations found by diffraction studies of crystals of re levant molecules were in low-energy regions on phi, psi surfaces compu ted with MM3. However, several sucrosyl linkages had energies high abo ve the global minimum. Also, the distributions of experimental conform ations did not conform well to the energy surfaces for sucrose and, to a lesser extent, maltose. A new map for maltose, made with MM3(92), h ad less low-energy area wherein crystal structures were not found, and a miniature crystal model accounts for the remaining distortions of p hi and psi as well as the short O4...O1 distances in crystalline malto se. An internally consistent calculation of the lattice energy (-76 kc al mol-1) required a dielectric constant of 2.5, and values around 3 a lso worked well for various intramolecular and intermolecular paramete rs and the phi, psi map of maltose. The problems with sucrose were not resolved despite further study. The high molecular mechanics energy f or some of the sucrosyl linkages may arise from failure to account for an overlapping exo-anomeric effect.