Solvation Structure, Thermodynamics, and Molecular Conformational Equilibria for n-Butane in Water Analyzed by Reference Interaction Site Model Theory Using an All-Atom Solute Model

Abstract

For the four thermodynamic states: temperature T = 283.15, 298.15, 313.15, and 328.15 K and the corresponding bulk water density ρ = 0.9997, 0.9970, 0.9922, and 0.9875 g cm-3, for which experimental data are available, we have studied hydration structure, hydration thermodynamics, and molecular conformational equilibria for n-butane in water at infinite dilution, by means of the hypernetted chain closure reference interaction site model (HNC−RISM) theory with an all-atom solute model. The hydration structures of the trans and the gauche conformers of n-butane are presented and analyzed at the atomic level in terms of the atomic solute−solvent radial distribution functions. With these radial distribution functions as input, the n-butane conformational average hydration free energies, energies, enthalpies, and entropies are calculated. At room temperature, the normalized equilibrium distribution of n-butane conformers, the water solvent-induced rotational free energy surface and the trans−gauche and trans−cis cavity thermodynamic properties are calculated. With the optimized nonbonded potential parameters based on the CHARMM96 all-atom model for alkanes (Yin, D.; Mackerell, A. D., Jr. J. Comput. Chem. 1998, 19, 334), n-butane hydration thermodynamics and its conformational equilibria in water are well described by the HNC−RISM theory in comparison with the available experimental and computer simulation results. We also calculated the solute density derivatives of the water−water radial distribution functions δhvv, with the optimized CHARMM96 all-atom model, the united-atom OPLS (optimized potentials for liquid simulations), and the all-atom OPLS models for n-butane, respectively. The δhvv(r) reflect the effect of increased pressure disrupting the hydrogen bonding between water molecules. The all-atom model seems to enhance such an effect due to the well-documented shortcoming of the RISM theory in the treatment of the excluded volume of so-called auxiliary sites.