MODELING OF ALDOPYRANOSYL RING PUCKERING WITH MM3(92)

The molecular mechanics algorithm MM3 was used to compute energy surfaces for aldopyranosyl rings having a full range of shapes. Energies were plotted against the Phi-Theta puckering coordinates of Cremer and Pople. The C-4(1) conformations of the model pyranosyl rings are dominant for both anomers of D-allose, D-galactose, D-glucose, D-mannose, and D-talose, as are the C-4(1) conformations of beta-D-altropyranose, beta-D-gulopyranose, and beta-D-idopyranose. alpha-D-Altropyranose is predicted to exist as an equilibrium of C-1(4) and C-4(1), alpha-D-idopyranose as an equilibrium among S-0(2), C-1(4), and C-4(1), and alpha-D-gulopyranose is predominately C-4(1) but has some contribution from C-1(4) (18%) and S-0(2) (9%). The calculated and measured hydrogen-hydrogen coupling constants agree well, although the energies for the beta anomers in water are systematically low by an average of 0.4 kcal/mol. Because the errors in the predicted anomeric ratios are small and are similar for the eight hexoses, and because the only concession to the solvent was a dielectric constant of 3.0, specific solvent effects are apparently small.