Model binding experiments with cucurbit[7]uril and p-sulfonatocalix[4]arene support use of explicit solvation term in governing equation for binding equilibria

Abstract

ABSTRACT The thermodynamics of model host–guest-binding reactions is examined in depth using isothermal titration calorimetry. In conflict with classical thermodynamics, the results indicate that the equilibrium-binding quotient, K, is not a constant for all pairings. This outcome is predicted by an equation for binding equilibria that includes an explicit term for the change in solvation free energy that accompanies the formation of a binary complex. Application of this framework to the experimentally observed concentration dependence of K allows one to obtain the energetic contribution of the solvent, a linked equilibrium denoted here as ΔG H2O. The estimated values of ΔG H2O are large and unfavourable for the binding of selected guest molecules to two hosts, cucurbit[7]uril and p-sulfonatocalix[4]arene. Intriguingly, the estimated values of ΔG H2O are near zero for the binding of two hydrophobic guest molecules to β-cyclodextrin, leading to a thought-provoking discussion on the driving force behind the hydrophobic effect.