Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 5, 2006
Publication Date: December 12, 2006
Citation: Schnupf, U., Willett, J.L., Bosma, W.B., Momany, F.A. 2006. DFT study of Alpha- and BETA-allopyranose at the B3LYP/6-311++G* level of theory. Carbohydrate Research. 342:196-216. Interpretive Summary: Allopyranose (D-allose) is an extremely rare sugar in nature but is found in the leaves of the passion fruit plant in the form of benzylic B-D-allopyranosides, a compound used by plants in their defense mechanism. This rare sugar has lately gained attention for a variety of uses namely, as a potential cancer inhibitor, a sweetner and as use as a bulk agent. For example, it was recently shown that allose has a significant inhibitory effect on ovarian cancer cell production. Despite the interest in the biological functions and possible commercial use of D-allose, not much is known about its structural properties. Our study of D-allose is a continuation of high level theoretical studies of a series of simple sugars. In this paper we present a structural characterization of D-allose, an epimer of D-glucose in which the three position hydroxyl group is moved from the equatorial to the axial configuration. 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 of allose. This work has allowed us to better understand the flexibility and structural organization of the building blocks of naturally occurring carbohydrates. 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: One hundred and two conformations of a- and B-D-allopyranose, the C-3 substituted epimer of glucopyranose, were geometry optimized using the density functional, B3LYP, and the basis set, 6-311++G**. Full geometry optimization was performed on different ring geometries and on the hydroxymethyl rotamers (gg/gt/tg). Analytically derived Hessians were used to calculate zero point energy, enthalpy, and entropy. The lowest energy and free energy conformation found is the x-tg(g-)-4C1-c chair conformation, which is only slightly higher in electronic (less than 0.2 kcal/mol) and free energy than the lowest x-D-glucopyranose. The in vacuo calculations showed a small (less than 0.3 kcal/mol) energetic preference for the x- over the B-anomer for allopyranose in the 4C1 chair conformations whereas in the 1C4 chair conformation a considerable (less than 1.6 kcal/mol) energetic preference for the B- over the x-anomer for allopyranose was encountered. Results are compared to previous aldopyranose calculations in vacuo. Boat and skew-boat forms were found that remained stable upon gradient optimization although many starting boat conformations moved to other skew-boat forms upon optimization. As found for glucose, mannose, and galactose the orientation and interaction of the hydroxyl groups make the most significant contributions to the conformation/energy relationship in vacuo.