NMR DEVELOPMENTS IN STRUCTURAL STUDIES OF CARBOHYDRATES AND THEIR COMPLEXES

Notable recent developments in NMR methods used to study carbohydrate structure include the application of tailored homo- and heteronuclear (multi-) selective two- and three-dimensional techniques, improved measurements of long-range C-13-H-1 scalar couplings, and the observation of hydroxyl proton signals used to detect intramolecular hydrogen bonds in aqueous solution. In addition, technological advances in C-13 isotopic labeling of carbohydrates are creating new ways of measuring NMR conformational parameters. The detailed study of a given oligosaccharide by NMR spectroscopy typically yields a number of constraints that are not all compatible with the existence of a single rigid structure, implying that oligosaccharides are, to a greater or lesser extent, dynamic with respect to torsional vibrations around each glycosidic bond. Consequently, the past two years have witnessed a vast increase in the number of efforts aimed at the measurement of NMR parameters directly related to the flexibility of carbohydrates, including H-1 and C-13 relaxation rates, homo- and heteronuclear cross-relaxation rates, and global and local correlation times. Improvements in the quality and quantity of obtainable NMR data are continuing to inspire molecular modelers to re-evaluate their previous analyses and interpretations. Furthermore, these NMR methods are now being used to investigate the structures of carbohydrates in complexes with proteins (antibodies, lectins) and the orientation and restrained dynamics of carbohydrates covalently attached to model membranes.