beta-lactose in the view of a CFF-optimized force field

A recently developed force field PEF95SAC, based on Consistent Force Field (CFF) optimized potential energy parameters for alcohols and most of the naturally occurring carbohydrates, is applied to and tested on beta-lactose. The properties of the potential energy surface of this disaccharide are compared to X-ray structures, NMR coupling constants and optical rotation data. The overall performance indicates good extrapolative power for the modeling of oligo-and polysaccharide structures. A new glycosidic linkage geometry region is proposed for beta-lactose as being important in both solid state and water solutions. This finding is supported by calculated J(H,C) coupling constants and calculated optical rotation values. In relation to the spectral calculations on beta-lactose, the error of the use of relative energies (Delta E) in place of the Gibbs free energy (Delta G) as the basis for calculating Boltzmann distributed properties is demonstrated. In the beta-lactose case it is shown that the conformational entropy is neither negligible nor uniformly distributed over the potential energy surface.