Conformational analysis of beta-D-fructofuranosyl-(2-->6)-beta-D-glucopyranoside by molecular mechanics (MM2) calculations.

Conformational energies for models of the disaccharide beta-D-fructofuranosyl-(2-->6)-beta-D-glucopyranoside were computed by molecular mechanics using MM2(87). An initial investigation of staggered forms examined the linkage bonds characterized by the torsion angles phi, psi, and omega, and subsequently the fructose hydroxymethyl side groups, characterized by the torsion angles chi-1 and chi-6. Then, in our major search of conformational space, the torsion angles of two linkage bonds, phi and omega, were driven through 360 degrees in 20 degree increments at all staggered side group combinations. From these results, the low-energy forms were minimized without the driver restrictions to generate the global minimum structure found and herein reported. This conformer was then used to map the conformational space of phi and omega by driving only those torsion angles through 360 degrees. Both the 4(3)T (northern) conformer (Cremer-Pople puckering phase angle of phi 2 = 265 degrees) and the 3(4)T, (southern) conformer (phi 2 = 80 degrees) of the fructofuranose ring were tested for comparison, and both were shown to be significant contributors of populated forms. As these two conformers had different minima for a number of important torsion angles, experimental studies may reveal different properties than those expected solely from the preferred northern conformer.