Thermodynamic Contribution of Water in Cryptophane Host-Guest Binding Reaction

A detailed examination of binding thermodynamics is undertaken for the interaction between rubidium ion and a water-soluble cryptophane molecule using isothermal titration calorimetry. The equilibrium-binding quotient for this host-guest pair decreases with increasing product formation. When analyzed with a thermodynamic framework that considers water explicitly in the governing equation, the shift in equilibrium is ascribed to an unfavorable change in the free energy of solvation upon formation of the inclusion complex. A van't Hoff analysis of the binding data, as well as an observation of aggregation between inclusion complexes, suggests that charge-charge interactions between rubidium ion and the phenolate groups of the cryptophane host provide the driving force for association in water that overcomes a large and unfavorable change in solvent enthalpy.