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 comp utation of free energy profiles for the approach of urea and guanidini um ion (Gdm+) to benzene and of urea to naphthalene in water. The calc ulations feature Monte Carlo statistical mechanics simulations at 25-d egrees-C with explicit representation of the solutes and solvent via O PLS potential functions and the TIP4P water model. The reaction coordi nate (r(c)) was defined as the distance between the center of mass of the arene and the chaotrope's central atom, and statistical perturbati on theory (SPT) afforded the free energy profiles over the 3-8-angstro m 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 wate r are discussed and show differences from the optimal gas-phase struct ures. 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 prof ile.