Testing the modified hydration-shell hydrogen-bond model of hydrophobic effects using molecular dynamics simulation

Molecular dynamics simulations are used to test the central hypothesis of Muller's ''modified hydration-shell hydrogen-bond'' model for the molecular explanation of the hydrophobic effect in order to provide an objective assessment of the model that would be difficult to achieve experimentally. The fraction of broken water-water hydrogen bonds within the first solvation shell around simple hydrocarbons is compared with that found within the bulk for ''infinitely dilute'' aqueous simulations of n-butane, n-hexane, n-octane, and benzene. Simulations confirm Muller's hypothesis that a larger fraction of the hydrogen bonds within the first solvation shell are broken in comparison to the bulk solution. Simulations are also used to test Muller's extension of this model to aqueous-urea solutions, which predict further decreases in the fraction of hydrogen bonds intact in the hydration shell. This predicted decrease is observed in the simulation.