The Circular Dichroism of Carbohydrates

This chapter discusses the use of circular dichroism (CD) spectroscopy in the analysis of carbohydrates. The CD spectroscopy measures the difference in absorption between left- and right-circularly polarized light by an asymmetric molecule. The spectrum results from the interaction among neighboring groups, and is thus extremely sensitive to the conformation of a molecule. Commercial instrumentation measures the CD of electronic absorption bands in the range of 1000 to 190 nm. Nucleic acids and proteins both have absorption bands in this region, and CD has been used extensively to study these molecules. Most sugars are transparent in this region and so they have been relatively neglected. However, the diverse biological activities of sugars undoubtedly depend on their conformations. Thus, improvements in CD instrumentation for the short-wavelength region have stimulated interest in using this powerful technique for investigating the stereochemistry of sugar monomers, the configuration of intersaccharide linkages, the secondary structure of the polymers, and the interaction of sugars with themselves and with other biological molecules. This chapter details the ways in which modern CD instrumentation has been used to solve structural problems involving sugars. The discussion is limited to substituted and unsubstituted pyranoses and does not cover complexes that can be formed with various chromophores.