DFT Conformational Studies of a-Maltotriose

Recent DFT optimization studies on a-maltose improved our understanding of the preferred conformations of a-maltose. The present study extends these studies to a-maltotriose with three a-D-glucopyranose residues linked by two a-[1?4] bridges, denoted herein as DP-3’s. Combinations of gg, gt, and tg hydroxymethyl groups are included for both ‘‘c’’ and ‘‘r’’ hydroxyl rotamers. When the hydroxymethyl groups are for example, gg-gg-gg, and the hydroxyl groups are rotated from all clockwise, ‘‘c’’, to all counterclockwise, ‘‘r’’, the minimum energy positions of the bridging dihedral angles (/H and wH) move from the region of conformational space of (2, 2), relative to (08, 08), to a new position defined by (1, 1). Further, it was found previously that the relative energies of a-maltose gg-gg-c and ‘‘r’’ conformations were very close to one another; however, the DP-3’s relative energies between hydroxyl ‘‘c’’ or ‘‘r’’ rotamers differ by more than one kcal/mol, in favor of the ‘‘c’’ form, even though the lowest energy DP-3 conformations have glycosidic dihedral angles similar to those found in the a-maltose study. Preliminary solvation studies using COSMO, a dielectric solvation method, point to important solvent contributions that reverse the energy profiles, showing an energy preference for the ‘‘r’’ forms. Only structures in which the rings are in the chair conformation are presented here. q 2007 Wiley Periodicals, Inc. J Comput Chem 29: 1103–1112, 2008