Hydrophobic interactions in urea - Trimethylamine-N-oxide solutions

The influence of osmolytes urea and trimethylemine-N-oxide (TMAO) on hydrophobic interactions is investigated employing molecular dynamics simulations. These osmolytes are of interest because of their opposing influence on proteins in solution; the denaturing effect of urea can be countered with TMAO. A neopentane pair is used to model typical nonpolar entities. Binary water-urea and water-TMAO as well as ternary water-urea-TMAO systems are considered. Neopentane-neopentane potentials of mean force, neopentane-water, and neopentane-osmolyte distribution functions are reported. Urea is found to have only modest influence on the neopentane-neopentane potential of mean force, but the hydrophobic attraction is completely destroyed by the presence of TMAO. It is shown that TMAO tends to act as a simple "surfactant" displacing water and urea (if it is present) from the first solvation shell of neopentane. It is likely the surfactant-like influence of TMAO that accounts for the elimination of the hydrophobic attraction. The implications of Our results for explanations of the action of TMAO in protein solutions are discussed.