HF/6-31G* Energy Surfaces for Disaccharide Analogs

French, A. D.; Kelterer, A.-M.; Johnson, G. P.; Dowd, M. K.; Cramer, C. J.
J. Comput. Chem. 2001, 22, 65.

Quantum mechanical HF/6-31G* theory was used to calculate Ramachandran energy surfaces for 12 analogs of disaccharides. The analogs were made by replacing glucose with tetrahydropyran and fructose with 2-methyltetrahydrofuran. Molecules had zero, one or two anomeric carbon atoms, and di-axial, axial-equatorial and di-equatorial linkages. Despite the absence of hydroxyl groups, the surfaces account well for the conformations that are observed in crystals of the related disaccharides. Thus, torsional energy and the simple bulk of ring structures are major factors in determining disaccharide conformation. The contour shapes around the global minima depend on the number of anomeric carbons involved in the linkage, while the propensity for alternative minima that have relative energies less than 4 kcal/mol depends mostly on whether the linkage bond in question is axial or equatorial. Molecules with two adjacent anomeric centers gave exceptions to these rules. Flexibility values related to a partition function show that the di-axial trehalose analog is the most rigid. The di-equatorial pseudo-disaccharide analog with no anomeric centers is most flexible. Reproduction of these surfaces is proposed as a simple test of force fields for modeling carbohydrates. Also, these surfaces can be used in a simple hybrid method for calculating disaccharide energy surfaces.

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