DO DENATURANTS INTERACT WITH AROMATIC-HYDROCARBONS IN WATER

The possibility of direct interactions between protein denaturants and aromatic hydrocarbons in water has been investigated through the computation of free energy profiles for the approach of urea and guanidinium ion (Gdm+) to benzene and of urea to naphthalene in water. The calculations feature Monte Carlo statistical mechanics simulations at 25-degrees-C with explicit representation of the solutes and solvent via OPLS potential functions and the TIP4P water model. The reaction coordinate (r(c)) was defined as the distance between the center of mass of the arene and the chaotrope's central atom, and statistical perturbation theory (SPT) afforded the free energy profiles over the 3-8-angstrom range. In all cases, single minima were obtained with well depths of 2-3.5 kcal/mol near 4 angstrom. This supports the existence of direct chaotrope-arene interactions. The structures of the complexes in water are discussed and show differences from the optimal gas-phase structures. In view of the interest in cation-pi interactions in host-guest complexes, computations were also carried out for tetramethylammonium ion (TMA) with benzene in water and yielded a similar free energy profile.