Translational Diffusion of Dilute Aqueous Solutions of Sugars as Probed by NMR and Hydrodynamic Theory

The translational diffusion coefficients of dilute aqueous solutions of a series of carbohydrates that differ in size and branching pattern have been measured with pulsed field gradient (PFG) NMR experiments. Accurate data have been obtained by calibrating the PFG with respect to several carbohydrates of known translational diffusion. The most satisfactory results were observed with the stimulated echo experiments when the pulse sequence was kept as short as possible and transverse relaxation was accounted for. The molecular dimensions of several of these sugars were determined from optimized theoretical models previously reported in exhaustive conformational searches and compared to those evaluated from hydrodynamic theory. It was demonstrated that cylindrically shaped oligosaccharides with maximum extension about 10 times that of the water radius display classical Stokes behavior. In such cases, a combined approach including analysis of translational diffusion coefficients and molecular modeling can reveal average molecular shape. These measurements are very rapid, and such a strategy is complementary to studies based on much more time-consuming NMR probes of rotational diffusion (homo- and heteronuclear relaxation data).