How Does Solute-Polarization Affect the Hydrophobic Hydration of Methane?

Temperature, Pressure, and Concentration Derivatives of Nonpolar Gas Hydration: Impact on the Heat Capacity, Temperature of Maximum Density, and Speed of Sound of Aqueous Mixtures

The hydrophobic effect is an umbrella term encompassing a number of solvation phenomena associated with solutions of nonpolar species in water, including the following: a meager solubility opposed by entropy at room temperature; large positive hydration heat capacities; positive shifts in the temperature of maximum density of aqueous mixtures; increases in the speed of sound of dilute aqueous mixtures; and negative volumes of association between interacting solutes. Here we present a molecular simulation study of nonpolar gas hydration over the temperature range 273.15-373.15 K and a pressure range -500 to 1000 bar to investigate the interrelationships between distinct hydrophobic phenomena. We develop a new free energy correlation for the solute chemical potentials founded on the Tait equation description of the equation-of-state of liquid water. This analytical correlation is shown to provide a quantitatively accurate description of nonpolar gas hydration over the entire range of thermodynamic state points simulated, with an error of ∼0.02 k_BT or lower in the fitted chemical potentials. Our simulations and the correlation accurately reproduce many of the available experimental results for the hydration of the solutes examined here. Moreover, the correlation reproduces the characteristic entropies of hydration, temperature dependence of the hydration heat capacity, perturbations in the temperature of maximum density, and changes in the speed of sound. While negative volumes of association result from pairwise interactions in solution, beyond the limits of our simulations performed at infinite dilution, we discuss how our correlation could be supplemented with second virial coefficient information to expand to finite concentrations. In total, this work demonstrates that many distinct phenomena associated with the hydrophobic effect can be captured within a single thermodynamically consistent correlation for solute hydration free energies.

Remarkable patterns of surface water ordering around polarized buckminsterfullerene

Accurate description of water structure affects simulation of protein folding, substrate binding, macromolecular recognition, and complex formation. We study the hydration of buckminsterfullerene, the smallest hydrophobic nanosphere, by molecular dynamics simulations using a state-of-the-art quantum mechanical polarizable force field (QMPFF3), derived from quantum mechanical data at the MP2/aug-cc-pVTZ(-hp) level augmented by CCSD(T). QMPFF3 calculation of the hydrophobic effect is compared to that obtained with empirical force fields. Using a novel and highly sensitive method, we see polarization increases ordered water structure so that the imprint of the hydrophobic surface atoms on the surrounding waters is stronger and extends to long-range. We see less water order for empirical force fields. The greater order seen with QMPFF3 will affect biological processes through a stronger hydrophobic effect.

Assessing the thermodynamic signatures of hydrophobic hydration for several common water models

Following the conclusions of an information theory analysis that hydrophobic hydration is dictated by the equation of state of liquid water, we perform simulations of ten different water models to examine the correlation between the fidelity of each model to the experimental density of liquid water and the accuracy of its description of methane hydration. We find that the three- and five-point water models provide an inferior description of both the liquid density and methane solubility compared to the four-point water models. Of the four-point water models, TIP4P/2005 provides the best description of both the aqueous equation-of-state and methane hydration thermodynamics. When the optimized potentials for liquid simulation united-atom description for methane is used, we find that while the entropy and heat capacity of methane hydration are in excellent agreement with experiment, the chemical potential and enthalpy are systematically shifted upwards. We subsequently reoptimize the methane interaction to accurately reproduce the experimental solubilities as a function of temperature by accounting for missing attractive interactions.