Solvent molecular dynamics in regions of hydrophobic hydration

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Abstract
We present a dynamical analysis of the solvent in an aqueous solution of two nonpolar atomic solutes which are constrained to an interatomic distance corresponding to a solvent-separated free energy minimum. The results are obtained from a molecular dynamics simulation using ST2 model water. Molecular mobility for solvent near the solutes is seen to be retarded, as evidenced in translational diffusion and rotational reorientation. These slower net motions are analogous to pure solvent dynamics at a temperature reduced by 10–15 °C. An analysis of intermolecular hydrogen bonding reveals that solvation shell molecules have correspondingly longer bond half-lives compared to bulk molecules, by a factor of 1.5–2.0. The spectral densities for intermolecular vibrations are computed from translational and rotational velocity autocorrelation functions for shell and bulk motions. These densities are seen to correlate well with the local binding energy distributions.