An improved model for calculating the optical rotation of simple saccharides

A calculational model for the optical rotation (OR) at the sodium D-line of simple saccharides has been developed that eliminates deficiencies of a previous model. Conformational conclusions based on the earlier model are not affected, as established by a recalculation of the OR phi,psi-map of methyl 3-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside. The model relocates the strong long-wavelength sigma-sigma* circular dichroism (CD) component, which is mainly responsible for the Na-D OR from 160 to below 130 nm, where it is now known to occur. That correction allows future modeling of CD bands of different origins that appear in the 150-190 nm region. In order to demonstrate the utility of the revised model, it was applied to calculating the OR of methyl 3-O-(alpha-L-rhamnopyranosyl)-alpha-L-rhamnopyranoside. The results provide experimental support for conformational conclusions derived from a molecular mechanics study of that molecule. (C) 2000 Elsevier Science Ltd. All rights reserved.