OPLS ALL-ATOM FORCE-FIELD FOR CARBOHYDRATES

The OPLS all-atom (AA) force field for organic and biomolecular system s has been expanded to include carbohydrates. Starting with reported n onbonded parameters of alcohols, ethers, and diols, torsional paramete rs were fit to reproduce results from nb initio calculations on the he xopyranoses, alpha,beta-D-glucopyranose, alpha,beta-D-mannopyranose, a lpha,beta-D-galactopyranose, methyl alpha,beta-D-glucopyranoside, and methyl alpha,beta-D-mannopyranoside. in all, geometry optimizations we re carried out for 144 conformers at the restricted Hartree-Fock (RHF) /6-31G level. For the conformers with a relative energy within 3 kcal /mol of the global minima, the effects of electron correlation and bas is-set extension were considered by performing single-point calculatio ns with density functional theory at the B3LYP/6-311 + G* level. The torsional parameters for the OPLS-AA force field were parameterized to reproduce the energies and structures of these 44 conformers. The res ultant force field reproduces the ab initio calculated energies with a n average unsigned error of 0.41 kcal/mol. The alpha/beta ratios as we ll as the relative energies between the isomeric hexopyranoses are in good accord with the ab initio results. The predictive abilities of th e force field were also tested against RHF/6-31G results for D-allopy ranose with excellent success; a surprising discovery is that the lowe st energy conformer of D-allopyranose is a beta anomer. (C) 1997 John Wiley & Sons, Inc.