Dehydration of beta-cyclodextrin: facts and opinions.

The thermodynamics of dehydration of beta-cyclodextrins, beta-CD*nH,O, is discussed. Despite the presence of different types of water, the enthalpies of dehydration per mole of beta-CD, (with n > 2) depend linearly upon n, yielding a slope of 23.5 kJ (mol H,O)- 1. This value is substantially smaller than the isosteric heat of dehydration, as determined through DSC data, and estimated from dehydration isotherms at two different temperatures. This implies that the B-CD substrate contributes to dehydration by relaxing to an energetically favoured state; this relaxation ends with an endothermic transformation to a different crystalline structure, which is primarily driven by the loss of water, and occurs near II = 2. The enthalpies of dissolution, measured in a recent paper by Bilal et al., also have a linear dependence upon n. However, this linear law apparently covers also the range of the structural transformation and its slope of 10.50 kJ (mol H2O)- 1 is not consistent with our data. The possible reasons for the discrepancy are discussed.