Molecular Dynamics of Cyclodextrins in Water Solutions from NMR Deuterium Relaxation: Implications for Cyclodextrin Aggregation

The aggregation of the most common natural cyclodextrins (alpha-, beta-, and gamma-) in aqueous solutions is addressed by studying the CD-CD interactions using deuterium relaxation rates for deuterium labeled CDs. Relaxation times (T-1) and their corresponding relaxation rates (R-1 = 1/T-1) provide information about the rotational correlation times of CDs and serve as a proxy for solute-solute interactions. Measured T-1's for alpha-, beta-, and gamma-CD at the lowest CD concentrations were in agreement with predictions of a hydrodynamic model for toroids, in particular with regard to the dependence of T-1 on CD size. On the other hand,, the dependence Of T-1's with respect to the increase in CD concentration could not be explained by hydrodynamic or direct interaction between CD molecules, and it is suggested that there is an equilibrium between monomeric and dimeric CD to account for the observed concentration dependence. No evidence in favor of large aggregates of CDs involving a non-negligible fraction was found for the investigated CDs.