Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 19, 2007
Publication Date: November 5, 2007
Citation: Schnupf, U., Willett, J.L., Bosma, W., Momany, F.A. 2007. DFT studies of the disaccharide, a-maltose: relaxed isopotential maps. Carbohydrate Research. 342(15):2270-2285. Interpretive Summary: Starch is one of the most important biological materials known, and comes from several of the largest renewable crops on the planet. Even so, the structure of starch granules has to date defied description at the microscopic level, and for this reason, a detailed study of the structural and energetic features of maltose was carried out. Maltose is the basic disaccharide component of starch polymers, containing one glucosidic bond between two glucose residues. In this paper we examine the energetic and structural behavior of maltose via calculated energy and geometry maps showing where regions of low energy occur in the psi and phi glycosidic bonds torsional space. These studies are carried out using powerful computer methods employed in our laboratory. With these cutting edge computational tools, we can relate previous information from structural observations obtained by other researchers to details on the basic structure and energetics of maltose. This work has allowed us to better understand the flexibility and structural organization of the building blocks of large naturally occurring amylose or starch materials. These studies will lead to more efficient design methods for chemical modifications of starch or polymer blends integrated with starch that will ultimately result in new biodegradable polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: The disaccharide, alpha-maltose, forms the molecular basis for the analysis of the structure of starch, and determining the conformational energy landscape as the molecule oscillates around the glycosidic bonds is of importance. Thus, it is of interest to determine, using density functionals and a medium size basis set, an isopotential adiabatic contour map plotted as a function of the psi and phi dihedral angles. The technical aspects include the method of choosing the starting conformations, the choice of scanning step size, the method of constraining the specific dihedral angles, and the fitting of data to obtain well defined contour maps. Maps were calculated at the B3LYP/6-31+G* level of theory in 5 deg C intervals around the (psi, phi)=(0 deg C, 0 deg C) position, out to less than +/-30 deg C or greater, for gg-gg'-c, gg-gg'-r, gt, gt'-r, tg-tg'-r conformers, as well as one split gg(c)-gg'(r) conformer. The results show that the preferred conformation of alpha-maltose in vacuo depends strongly upon the hydroxyl group orientations ('c'/'r'), but the energy landscape moving away from the minimum energy position is generally shallow and transitions between conformational positions can occur without addition of significant energy. Mapped deviations of selected parameters such as the dipole moment, the C1-01-C4', H1-C1-01 and H4'-C4'01 bond angles, and deviations in hydroxylmethyl rotamers, 05-C5-C6-06, 05'-C5'C6'-06', C5-C6-06-H, and C5'-C6-06'-H', are presented. These allow visualization of the structural and energetic changes that occur upon rotation about the glycosidic bonds. Interactions across the bridge are visualized by deviations in (02)H---03',(03')H---02 and H1---H4'distances and the (02)H-02-C2-C1 and (03')H'-03'-C3'-C2' hydroxyl rotamers dihedrals.