Hydration thermodynamics beyond the linear response approximation

The solvation energetics associated with the transformation of a solute molecule at infinite dilution in water from an initial state A to a final state B is reconsidered. The two solute states have different potentials energies of interaction, -A and -B, with the solvent environment. Throughout the A → B transformation of the solute, the solvation system is described by a Hamiltonian H(⊂) that changes linearly with the coupling parameter ⊂. By focusing on the characterization of the probability density Ψ ( y) that the dimensionless perturbational solute solvent interaction energy Y = β (Ψ^BΨ^A) has numerical value y when the coupling parameter is ⊂ we derive a hierarchy of differential equation relations between the ⊂-dependent cumulant functions of various orders in the expansion of the appropriate cumulant generating function. On the basis of this theoretical framework we then introduce an inherently nonlinear salvation model for which we are able to find analytical results for both ℘ ⊂ ( y) and for the salvation thermodynamic functions. The solvation model is based on the premise that there is an upper or a lower bound (depending on the nature of the interactions considered) to the amplitude of the fluctuations of Y in the solution system at equilibrium. The results reveal essential differences in behavior for the model when compared with the linear response approximation to solvation, particularly with regards to the probability density ℘ ( y). The analytical expressions for the solvation properties show, however, that the linear response behavior is recovered from the new model when the room for the thermal fluctuations in Y is not restricted by the existence of a nearby bound. We compare the predictions of the model with the results from molecular dynamics computer simulations for aqueous solvation, in which either (1) the solutesolvent electrostatic interactions, or (2) the shorter-range attractive interactions are switched-on in the A →B process.