Non-polar solutes in water and in aqueous solutions of protein denaturants. Modeling of solution and transfer processes

A simple molecular model for the thermodynamic behavior of non-polar solute s in water and in aqueous solutions of protein denaturants is presented. Th ree contributions are considered: (i) combinatorial arising from the mixing process, (ii) interactional characterizing the molecular interactions occu rring in the mixture and (iii) a contribution originating from the structur al changes occurring in the first shell of water molecules around the solut e. The latter is modeled assuming that water molecules in contact with the solute are involved in a chemical equilibrium between two states. The model describes well the temperature and denaturant concentration dependences of the Gibbs energies of solution and transfer for benzene, toluene and alkan es in water and aqueous solutions of urea and guanidine hydrochloride. Mode l parameters are physically meaningful, allowing a discussion of the molecu lar interactions involved. A preferential solvation of the solute by the de naturant is found. However, the non-polar solute-denaturant interaction is not specific, i.e. leading to a distinct chemical entity. Urea and guanidin e hydrochloride are non-polar solubilizing agents because their interaction s with the solute are less unfavorable than those between water and the sol ute. (C) 2001 Elsevier Science B.V. All rights reserved.