A QM/MM analysis of the conformations of crystalline sucrose moieties

Both ab initio quantum mechanics (QM) and molecular mechanics (MM) were use d to produce a hybrid energy surface for sucrose that simultaneously provid es low energies for conformations that are observed in crystal structures a nd high energies for most unobserved structures. HF/6-31G* QM energies were calculated for an analogue based on tetrahydropyran (THP) and tetrahydrofu ran (THF). Remaining contributions to the potential energy of sucrose were calculated with MM. To do this, the MM surface for the analogue was subtrac ted from the MM surface for the disaccharide, and the QM surface for the an alogue was added. Prediction of the distribution of observable geometries w as enhanced by reducing the strength of the hydrogen bonding. Reduced hydro gen-bonding strength is probably useful because many crystalline sucrose mo ieties do not have intramolecular hydrogen bonds between the fructose and g lucose residues. Therefore, hydrogen bonding does not play a large role in determining the molecular conformation. On the hybrid energy surface that w as constructed with a dielectric constant of 3.5, the average potential ene rgy of 23 sucrose moieties from crystal structures is 1.16 kcal/mol, and th e population of observed structures drops off exponentially as the energy i ncreases. (C) 2000 Elsevier Science Ltd. All rights reserved.