Dehydration of the cyclodextrins: A model system for the interactions of biomolecules with water

The thermodynamics of hydration of biomolecules is experimentally studied in the β-cyclodextrin (β-CD), which contains water molecules in a range of configurations and has been proposed as a model system for complex biomolecules. The thermal measurements point to the role of a structural transition from the hydrated β-CD (phase I) to a ‘‘dehydrated’’ form (phase II). We show that dehydration in phase I is assisted by a ‘‘compensation mechanism’’ for which β-CD contributes a constant amount of energy for each H2O mole. Despite the presence of different types of H2O’s, water losses in phase I are accurately described in terms of this energy and the isosteric molar enthalpy of dehydration. Moreover, in going from the fully hydrated to the fully dehydrated form, the contribution of β-CD to dehydration is over all equal to the enthalpy of transition from phase I to phase II. Our analysis yields the changes of an enthalpy associated with the biomolecule alone as a function of the water content. In the case of β-CD, we can sketch a qualitative phase diagram, which assists the interpretation of details of our thermal experiments. The role of kinetic factors in the attainment of the thermodynamic equilibrium is investigated with 2H-NMR in samples recrystallized from heavy water. We find that, over a wide range of hydration levels, water molecules have a liquidlike diffusion, which, together with the compensation mechanism, explains the fast and nearly reversible dehydration of the β-CD.