DISACCHARIDE SOLUTION STEREOCHEMISTRY FROM VIBRATIONAL RAMAN OPTICAL-ACTIVITY

The vibrational Raman optical activity (ROA) spectra of D-maltose, D-maltose-O-d(8), D-cellobiose, D-isomaltose, D-gentiobiose, D-trehalose, and alpha-D-cyclodextrin in aqueous solution between similar to 350 and 1500 cm(-1) measured in backscattering are reported, which show that ROA is an incisive new probe of disaccharide stereochemistry. Some of the ROA signals in the fingerprint region and two ROA couplets at similar to 430+/-10 and similar to 917 cm(-1) have been identified to exhibit contributions from vibrational coordinates of the glycosidic link. The sign of the former couplet was found to depend on the configuration of the link, and its width was found to depend on the particular linkage type. The second ROA couplet was present only in the ROA spectra of D-maltose and D-isomaltose and was attributed from deuteration, studies to a mode involving glycosidic C-O-C stretching coordinates and C-O-H deformations. Opposite signs for this coupler in D-maltose and D-isomaltose may reflect either the involvement of the CH2 group in the linkage or conformational changes. Sign, intensity, and position of the ROA signals in the fingerprint region similar to 950-1200 cm(-1) appear to be characteristic for a particular disaccharide unit. Complementary information is obtained in the CH2 and COH deformations region similar to 1200-1500 cm(-1),where the disaccharide ROA spectra were demonstrated to be superpositions of the corresponding ROA spectra of the constituent monosaccharides. Also in this region a positive ROA band at similar to 1260 cm(-1) was assigned to the beta-anomeric form and used to estimate the anomeric populations in the reducing residues of some disaccharides.