DOE OSTI2018
Amongst all intermolecular interactions that bring molecules, biomolecules, and even molecular fragments together, dispersion interactions are the omnipresent ones. Nevertheless, their contribution to solution-phase phenomena has recently become a matter of debate. In aqueous solution, in particular, they compete with the hydrophobic effect, which also drives associative processes, and when macrocyclic cavities are involved, the release of high-energy water has been identified as an extra driving force. We have now studied the binding of noble gases (He, Ne, Ar, Kr, Xe) with the macrocyclic receptor cucurbit[5]uril (CB5) in aqueous solution through a displacement strategy by using methane and ethane as 1H NMR probes. We dissected the hydration free energies of the noble gases into an attractive dispersive contribution and a repulsive one for cavity formation. This allowed us to fine-grain the components that contribute to host-guest binding and we reach the conclusion that the binding process in solution is driven by differential cavitation energies rather than dispersion interactions. This means that the free energy required to create a cavity to accept the noble gas inside CB5 is much lower (actually zero, since the cavity of the host is non-solvated) than that to create a similarly sized cavity in bulk water, and it is the recovery of the latter cavitation energy, which drives the overall process. The concept of differential cavitation energies merges seamlessly two previous approaches for predicting supramolecular thermodynamics, namely that of cohesive energy density (which applies to bulk solvent) and that of high-energy water (which becomes important for discrete concave binding sites). The dominance of differential cavitation energies in driving guest binding to poorly hydrated, concave binding sites (V < 100 Å3) has implications for the refinement of gas-storage materials and the understanding of biological receptors.