Do the solvent properties affect the propensity for self-association of alpha-cyclodextrin? Insights from NMR self-diffusion

摘要

Whether cyclodextrins (CDs) self-aggregate or not in solution is an interesting and timely scientific question. Yet, we are far from a scientific consensus as regards the nature of the phenomenon. Those who are in favor of the formation of CD aggregates justify the phenomenon by the presence of H-bonds, occurring between the hydroxyl groups located at both rims of cyclodextrins. To gain further insight with regard to the importance of these intermolecular CD-CD interactions, a systematic study of H-1 NMR self-diffusion of alpha-cyclodextrin in binary mixtures of deuterated DMSO and water, at different mole fractions, has been carried out. The choice of these solvents was made based on their different degree of polarity and the fact that they are miscible at any molar ratio. The experimental alpha-CD diffusion coefficients scale with the solution viscosity. Moreover, hydrodynamic radii of alpha-CD are independent of the solvent composition. No evidence of alpha-cyclodextrin aggregation at any DMSO mole fraction is found. Finally, experimental diffusion data are compared with those obtained from atomic level hydrodynamic calculations.