A QM/MM Conformational Analysis of Crystalline Sucrose Moieties

French, A. D.; Kelterer, A.-M.; Cramer, C. J.; Johnson, G. P.; Dowd, M. K.
Carbohydr. Res. 2000, 326, 305.

Both ab initio quantum mechanics (QM) and molecular mechanics (MM) were used to produce a hybrid energy surface for sucrose that simultaneously provides low eneries for conformations that are observed in crystal structures and high energies for most unobserved structures. HF/6-31G* QM energies were calculated for an analog based on tetrahydropyran (THP) and tetrahydrofuran (THF). Remaining contributions to the potential energy of sucrose were calculated with MM. To do this, the MM surface for the analog was subtracted from the MM surface for the disaccharide and the QM surface for the analog was added. Prediction of the distribution of observable geometries was enhanced by reducing the strength of hydrogen bonding. Reduced hydrogen bonding strength is probably useful because many crystalline sucrose moieties do not have intramolecular hydrogen bonds between the fructose and glucose residues. Therefore, hydrogen bonding does not play a large role in determining the molecular conformation. On the hybrid energy surface that was constructed with a dielectric constant of 3.5, the average potential energy of 23 sucrose moieties from crystal structures is 1.16 kcal/mol, and the population of observed structures drops off exponentially as the energy increases.

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