MODELING OF ALDOPYRANOSYL RING PUCKERING WITH MM3(92)

The molecular mechanics algorithm MM3 was used to compute energy surfa ces for aldopyranosyl rings having a full range of shapes. Energies we re plotted against the Phi-Theta puckering coordinates of Cremer and P ople. The C-4(1) conformations of the model pyranosyl rings are domina nt for both anomers of D-allose, D-galactose, D-glucose, D-mannose, an d D-talose, as are the C-4(1) conformations of beta-D-altropyranose, b eta-D-gulopyranose, and beta-D-idopyranose. alpha-D-Altropyranose is p redicted to exist as an equilibrium of C-1(4) and C-4(1), alpha-D-idop yranose 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-hyd rogen coupling constants agree well, although the energies for the bet a anomers in water are systematically low by an average of 0.4 kcal/mo l. Because the errors in the predicted anomeric ratios are small and a re similar for the eight hexoses, and because the only concession to t he solvent was a dielectric constant of 3.0, specific solvent effects are apparently small.