|French, Alfred - Al|
Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2002
Publication Date: 8/1/2002
Citation: Thibodeaux, D.P., Johnson, G.P., Stevens, E.D., French, A.D. 2002. CRYSTAL STRUCTURE OF PENTA-O-ACETYL-BETA-D-GALACTOPYRANOSE WITH MODELING OF THE CONFORMATION OF THE ACETATE GROUPS. Carbohydrate Research. 337:2301-2310. Interpretive Summary: The ability to alter agriculturally produced carbohydrates such as cotton cellulose and corn starch is essential for the development of new uses and markets for these products and ultimately lessening U.S. dependence on foreign oil. The properties of these modified materials will depend on the structure of their molecules. To develop a relationship between a material's molecular structure and its chemical and physical functions, a more complete 3-dimensional picture of the molecule is needed. This work provides a complete structure of a common carbohydrate (galactose) that was modified by the addition of acetate groups to form the molecule galactose penta-acetate. This structure was determined by experiments with the interaction (diffraction) of x-rays with a crystalline specimen. Other agricultural carbohydrates do not crystallize well enough to do such experiments, so theoretical modeling methods are used to learn about their structures. Those methods were applied to the freshly determined structure to test the ability of those methods. The theoretical (modeling) part of the work showed that very time-consuming computer calculations were needed to get a totally satisfactory description of the structure of the molecule. Acetate groups are frequently used to modify carbohydrates, and the results of this study should be useful for understanding many other acetylated carbohydrates. This work also calls attention to the surprising fact that these more-complicated molecules are actually simpler to model than their parent structures. This work is primarily of interest to chemists working with modeling methods and those trying to develop new materials from agricultural products.
Technical Abstract: The crystal structure of beta-D-galactose penta-acetate (1) was determined with MoKalpha radiation at 150K to R = 0.029. The space group is P212121 and the unit cell dimensions are: a - 8.348, b - 9/021 and c - 25.418 A. The ring has the usual chair shape and 06 is in the tg position as frequently observed for sugars having the axial glacto configuration at 04. Conformations of the acetate groups were compared with those from the literature. Nearly eclipsed, "Z" conformations are found for the ester moiety and the torsion angles for the sequence involving the ring hydrogen, carbon, alkoxy oxygen and carbonyl carbon for both 1 and related compounds are eclipsed-to-gauche. Orientations and conformations of the acetate substituents were modeled with both M3 molecular mechanics and various levels of quantum mechanics theory. Higher levels of theory and more complete models provided better prediction of the experimental observations.