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 NaD 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.