CONFORMATIONS OF DISACCHARIDES BY EMPIRICAL FORCE-FIELD CALCULATIONS .1. BETA-MALTOSE

Conformations of β-maltose have been studied by using convergent energy minimisation in a simple force field. Parameters for the force field were taken from similar studies on α- and β-d-glucopyranose. Four local minima are found on the φ,ψ-map; the free enthalpy differences are 2.8, 3.4, and 5.6 kJ.mol-1 above the lowest, corresponding to a distribution of 59:19:15:7. These minima are surrounded by a manifold of minimum conformers differing only in exocyclic torsions. Conformations of the two lowest minima are close to X-ray structures, and (φ,ψ) of the lowest minima are very close to what can be derived from n.m.r. experiments. The path on the φ,ψ-map of conformational interchange between the two lowest minima has been investigated, and barrier heights and rates of conversion estimated. Conversion is fast on the n.m.r. time-scale. The distribution-weighted average of a crucial H---H distance agrees with n.m.r. measurements. The possible existence of several minima for maltose can explain known irregularities in some amylose conformations. © 1979.