Surface Adsorption and Bulk Aggregation of Cyclodextrins by Computational Molecular Dynamics Simulations as a Function of Temperature: α-CD vs β-CD

The structural simplicity of native cyclodextrins (CDs) contrasts with their complex behavior in the bulk of aqueous solutions, mainly when they are combined with other cosolutes. Many scientific and industrial applications based on these molecules are supported only by empirical information. The lack of fundamental knowledge, which would allow one to rationally optimize many of these applications, is notable mainly at the solution/air interface. Basic information on phenomena such as the spontaneous adsorption of native CDs or on the structure of CD aggregates in the bulk solution is really scarce. In order to fill these gaps, a detailed computational study on the adsorption and aggregation of alpha- and beta-CDs as a function of temperature is presented here. Our simulations reproduce, at atomic resolution, the experimentally observed much higher ability of beta-CD to aggregate compared to that of alpha-CD at 298 K, as well as their dependence on temperature. The adsorption of both individual CDs and small CD aggregates (up to 20 molecules) to the solution/air interface is found to be negligible. 0.8 mu s long trajectories of single CD molecules in aqueous solution reveal that the main differences in the behavior of both CDs are their flexibility, higher for beta-CD, and the occupancy of individual intramolecular hydrogen bonds that is significantly longer for the same cyclodextrin. The aggregation pattern of alpha- and beta-CDs is followed at the hundreds of ns time scale, allowing both the spontaneous self-assembly of cyclodextrins and their redistribution along the aggregates to be observed. This is the first attempt to study the adsorption and aggregation of native cyclodextrins by atomistic molecular dynamics simulations.