Circular dichroism of the O-specific polysaccharide of Vibrio cholerae O1 and some related derivatives

The O-specific polysaccharide (O-SP) of Vibrio cholerae O1 is a homopolymer of α-(1 → 2)-linked 4-amino-4,6-dideoxy-d-mannopyranose whose amino group is acylated with 3-deoxy-l-glycero-tetronic acid [N-(3-deoxy-l-glycero-tetronyl)-α-d-perosamine]. The circular dichroism (CD) of the O-SP as well as of a number of N-acyl (formyl, acetyl, 4-hydroxybutyl, 3-deoxy-l and d-glycero-tetronyl) derivatives of methyl α-glycosides of 4-amino-4,6-dideoxy-d-mannopyranose (methyl α-d-perosaminide) has been studied for solutions in water, acetonitrile and 1,1,1-trifluoroethanol. The strong solvent dependence of the sign and intensity of the CD observed for the monosaccharide amides bearing achiral acyl groups is explained by solvent-mediated change of the orientation of the amido group relative to the proximal hydroxyl group at C-3. A change in the population of the nonplanar conformers with a pyramidal arrangement of bonds at the amido nitrogen has also been considered. The effect of solvents upon the CD spectra of compounds bearing chiral N-acyl substituents is less pronounced than that of their counterparts bearing achiral N-acyl substituents. The sign of the CD for the O-SP was found negative in all solvents used. This result is in agreement with the negative sign of the CD of the n → π electron transition observed, independent of solvent, for the monosaccharide derivative containing the l-glycero-3-deoxytetronamido group, and the positive sign found for its d-glycero-counterpart.