AB INITIO COMPUTATIONAL STUDY OF BETA-D-CELLOBIOSE CONFORMERS USING B3LYP/6-311++G**

Authors: STRATI, GINA - FORMER USDA-ARS; Willett, Julious; Momany, Frank

Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2002
Publication Date: 9/30/2002
Citation: STRATI, G., WILLETT, J.L., MOMANY, F.A. AB INITIO COMPUTATIONAL STUDY OF BETA-D-CELLOBIOSE CONFORMERS USING B3LYP/6-311++G**. CARBOHYDRATE RESEARCH. 2002. v. 337. p. 1833-1849.

Interpretive Summary: Cellulose is one of the most important biological carbohydrate molecules known, in addition to being one of the largest renewable plant materials on the planet. Cellobiose is the disaccharide building unit of cellulose with two glucose rings linked through an ether bridge. The 3-dimensional structure of cellobiose was studied computationally at the electronic molecular level in order to explain some anomalous conformational features of this basic cellulosic unit. For example, our scientific investigations into the electronic nature of cellobiose have led to the realization that when this molecule is alone it energetically prefers a 3-dimensional structure that is different than those structures observed experimentally. In this paper we present results on this subject using powerful computer methods utilized in our laboratory for carbohydrate studies. With these computational tools, we can relate previous experimental information obtained by other researchers to details on the basic structure of many different carbohydrates whose commercial utility is well known. This work has allowed us to better understand the flexibility and electronic organization of the D-glucose components of cellobiose and thus cellulose and will lead to the design of new chemical modifications of cellulose materials. Such chemical modifications will result in the production of biodegradable polymers with new physical properties useful for commercial applications.

Technical Abstract: The molecular structure of 27 conformers of beta-D-cellobiose were studied in vacuo through gradient geometry optimization using B3LYP density functionals and the 6-311++G** basis set. The conformationally dependent geometry changes and energies were explored as well as the hydrogen-bonding network. The lowest electronic energy structures found were not those suggested from available crystallographic and NMR solution data, where the glycosidic dihedral angles fall in the region (Phi, Psi)(40 degrees, -20 degrees). Rather, "flipped" conformations in which the dihedral angles are in the range (Phi, Psi)(180 degrees, 0 degrees) are energetically more stable by 2.5 kcal/mol over the "experimentally accepted" structure. Further, when the vibrational free energy, delta G, obtained from the calculated frequencies, is compared throughout the series, structures with (Phi, Psi) in the experimentally observed range still have higher free energy (2.0 kcal/mol) than "flipped" forms. The range of bridging dihedral angles of the "normal" conformers as a result of variance in the Chi dihedral is larger than that found in the "flipped" forms. Due to this large flat energy surface for the normal conformations, we surmise that the summation of populations of these conformations will favor the "normal" conformations although solution effects may play the dominant role. Even though some empirical studies previously found the "flipped" conformations to be lowest in energy, these studies have been generally discredited because they were in disagreement with experimental results. Most of the DFT/ab initio conformations reported here have not been reported previously in the ab initio literature, in part because the use of less rigorous theoretical methods i.e. 6-31G, have given results in general agreement with experimental data, that is, they did not favor the "flipped" forms, and in part because of the length and difficulty of these high level calculations.