HF/6-31G*energy surfaces for disaccharide analogs

The HF/6-31G* lever of theory was used to calculate relaxed potential energ y surfaces for 12 analogs of disaccharides. The analogs were made by replac ing 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 gro ups, the surfaces account well for conformations that are observed in cryst als of the parent disaccharides. Thus, torsional energy and the simple bulk of ring structures are major factors in determining disaccharide conformat ion. The contour shapes around the global minima depend on the number of an omeric carbons involved in the linkage, while the presence of alternative m inima that have relative energies less than 4 kcal/mol mostly requires equa torial bonds. However, molecules with two adjacent anomeric centers gave ex ceptions to these rules. Flexibility values related to a partition function show that the di-axial trehalose analog is the most rigid. The di-equatori al pseudodisaccharide analog with no anomeric centers is most flexible. Rep roduction of these surfaces is proposed as a simple test of force fields fo r modeling carbohydrates. Also, these surfaces can be used in a simple hybr id method for calculating disaccharide energy surfaces. (C) 2000 John Wiley & Sons, Inc.