Submitted to: Journal of Computational Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 17, 1999
Publication Date: October 5, 2000
Interpretive Summary: Starch is one of the most important biological molecules known, in addition being one of the largest renewable crops on the planet. Unfortunately, the 3-dimensional structure of starch granules has to date defied description. We require an understanding of the electronic structural features of the basic components of starch polymers. In this paper, we present results on the electronic structure of one of these basic components (maltose) using powerful computer methods employed in our laboratory. With these computational tools, we can relate previous information from structural observations obtained by other researchers to details on the basic flexibility and molecular structure of maltose. This work has allowed us to better understand the flexibility and structural organization of other components in starch molecules, as well to more accurately define the role of specific molecular structures which make up the starch granule. This work will lead to more efficient design methods for chemical modifications of starch that will result in biodegrad- able polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: Geometry optimization was carried out on selected conformations of maltose, as well as the model compounds 2-methoxytetrahydropyran, methanol, methanol dimers, dimethyl ether, water and water dimers using the density functional exchange-correlation potential denoted as B3LYP. This functional is combined with the 6-31G* split-valence with polarization basis set to make up the B3LYP/6-31G* procedure. Internal coordinates were fully relaxed an structures were gradient optimized at the B3LYP/6-31G* level of theory. Dimethyl ether was studied to compare calculated ether linkages with experiment. Dimers of methanol were examined to confirm geometry and energy for hydrogen bonding of hydroxyl groups, and water dimers were included to examine basis set dependence upon the interaction geometries and energies between water and the functional groups of maltose. The details of the ab initio optimized geometry are presented here with particular attention given to the positions of the atoms around the anomeric center and the effect of the particular anomer and hydrogen bonding pattern on the maltose ring structures and relative conformational energies. The optimized B3LYP/6-31G* structures gave details of conforma- tionally dependent geometry changes and energies that had previously been unavailable. Three classes of conformations were studied, as defined by the clockwise or counter clockwise direction of the exo-hydroxyl groups, or a flipped conformer in which the psi-dihedral is rotated by ~180 deg. Different combinations of omega side-chain rotations gave energy differences of more than 6 kcal/mol above the lowest energy structure found. The lowest energy structures bear close resemblance to the neutron and X-ray diffraction crystal structures.