DISACCHARIDE SOLUTION STEREOCHEMISTRY FROM VIBRATIONAL RAMAN OPTICAL-ACTIVITY

The vibrational Raman optical activity (ROA) spectra of D-maltose, D-m altose-O-d(8), D-cellobiose, D-isomaltose, D-gentiobiose, D-trehalose, and alpha-D-cyclodextrin in aqueous solution between similar to 350 a nd 1500 cm(-1) measured in backscattering are reported, which show tha t ROA is an incisive new probe of disaccharide stereochemistry. Some o f the ROA signals in the fingerprint region and two ROA couplets at si milar to 430+/-10 and similar to 917 cm(-1) have been identified to ex hibit contributions from vibrational coordinates of the glycosidic lin k. The sign of the former couplet was found to depend on the configura tion 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 spect ra of D-maltose and D-isomaltose and was attributed from deuteration, studies to a mode involving glycosidic C-O-C stretching coordinates an d 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 t he 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. Comple mentary information is obtained in the CH2 and COH deformations region similar to 1200-1500 cm(-1),where the disaccharide ROA spectra were d emonstrated to be superpositions of the corresponding ROA spectra of t he constituent monosaccharides. Also in this region a positive ROA ban d 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.