NMR ANALYSIS OF CARBOHYDRATES WITH MODEL-FREE SPECTRAL DENSITIES - THE DISPERSION RANGE REVISITED

Over the past decade molecular mechanics and molecular dynamics studie s have demonstrated considerable flexibility for carbohydrates. In ord er to interpret the corresponding NMR parameters, which correspond to a time-averaged or 'virtual' conformer, it is necessary to simulate th e experimental data using the averaged geometrical representation obta ined with molecular modelling methods. This structural information can be transformed into theoretical NMR data using empirical Karplus-type equations for the scalar coupling constants and the appropriate forma lism for the relaxation parameters. In the case of relaxation data, th e 'model-free' spectral densities have been widely used in order to ac count for the internal motions in sugars. Several studies have been co nducted with truncated model-free spectral densities based on the assu mption that internal motion is very fast with respect to overall tumbl ing. In this report we present experimental and theoretical evidence t hat suggests that this approach is not justified. Indeed, recent resul ts show that even in the case of moderate-sized carbohydrates internal motions are occurring on the same timescale as molecular reorientatio n. Simulations of relaxation parameters (NOESY volumes, proton cross-r elaxation rates, carbon T-1 and nOe values) in the dispersion range (0 .1 < T-c < 5 ns) show that rates of internal motion can be fairly prec isely defined with respect to overall tumbling. Experimental data for a variety of oligosaccharides clearly indicate similar timescales for internal and overall motion.