EXPERIMENTAL CHIROPTICAL VERIFICATION OF LINKAGE FLEXIBILITY IN METHYL -O(ALPHA-D-MANNOPYRANOSYL)-ALPHA-D-MANNOPYRANOSIDE

Vacuum UV CD spectra of methyl O-(alpha-D-mannopyranosyl)-alpha-D-mann opyranoside in D2O and as a cast film were obtained in the 145-200 nM region. The disaccharide solution CD per residue is nearly identical t o that of the monosaccharide solution CD, and to the monosaccharide fi lm CD. Conversely, the disaccharide film spectrum exhibits a strong po sitive CD linkage contribution in the 160-170 nm range, which is consi stent with the known crystal conformation under the aegis of previousl y determined sector rules. The close similarity between the monosaccha ride and disaccharide solution spectra, therefore, reflects conformati onal averaging in which the net linkage contribution is approximately zero. The present observation of significant solution linkage flexibil ity confirms previous conclusions based on optical rotation, as well a s conclusions of others based on nmr data. Moreover, when combined wit h those earlier results, the present work demonstrates the population of at least three distinct potential energy wells on the disaccharide phi,psi potential energy surface. (C) 1996 John Wiley & Sons, Inc.