ADVANCED STARCH-BASED MATERIALS FOR NON-FOOD APPLICATIONS
Location: Plant Polymer Research
Title: DFT CONFORMATIONAL STUDIES OF ALPHA-MALTOTRIOSE
Submitted to: Journal of Computational Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 10, 2007
Publication Date: May 1, 2008
Citation: Schnupf, U., Willett, J.L., Bosma, W., Momany, F.A. 2008. DFT conformational studies of alpha-maltotriose. Journal of Computational Chemistry. 29(7):1103-1112.
Interpretive Summary: Alpha-maltotriose is the second most abundant fermentable sugar in brewer's wort and, due to incomplete fermentation, causes quality and economic problems in the beer and wine industry. This sugar is of structural interest being a repeating unit of pullulan, a linear homopolysaccharide of glucose made up of linked maltotriose units. Cutting edge density functional ab initio (DFT) studies of maltotriose were carried out in order to determine the preferred conformational structures of the molecule. That is, we wish to know the hydroxymethyl rotamer energetic preferences as well as the preferred directions of the hydroxyl groups, and the effect these parameters have upon the overall structure as might be found in starch materials. Many different structures were studied by our powerful DFT methods. The results of this extensive study suggest that water plays an important role in the structures of the molecules, reversing the energetic ordering of conformations as determined in vacuo.
Recent DFT optimization studies on alpha-maltose improved our understanding of the preferred conformations of alpha-maltose and the present study extends these studies to alpha-maltotriose with three alpha-D-glucopyranose residues linked by two alpha-[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 hydroxylmethyl 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 (Phi and Psi) move from the region of conformational space of (-,-), relative to (0 deg C, 0 deg C), to a new position defined by (+,+). Further, it was found previously that the relative energies of alpha-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 differentiate 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 alpha-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.