’Intermolecular interactions’ in aqueous solutions of three components including lysozyme

Excess partial molar enthalpies were measured in ternary aqueous solutions, tert-butyl alcohol (TBA)DMSO-H2O, and lysozyme (L)-alcohols (A)-H2O. The solute-solute interactions were evaluated as the derivatives of these data with respect to the mole fraction of a solute. In the water-rich region, where the so-called mixing scheme I is operating in binary aqueous solutions of TEA, DMSO or alcohols, two solutes in TBA-DMSO-H2O seem to modify the hydrogen bond network of H2O in an additive manner. Here, mixing scheme I refers to the way in which a solute (hydrophobic) modifies the molecular organization of H2O. Specifically, a solute enhances the hydrogen bond strength of H2O in its immediate vicinity. On the other hand, the probability of hydrogen bond in the bulk away from a solute is reduced, and it is still high enough, however, to keep the hydrogen bond network connected throughout the entire macroscopic: system. As the composition of solute increases, the hydrogen bond probability in the bulk decreases to the point at which the macroscopic bond connectivity is no longer possible. At this point, a new mixing scheme, II, sets in. For L-A-H2O mixtures at infinite dilution of L, the L-A interaction changes drastically from repulsive: to attractive at the boundary between I and II for binary aqueous alcohols. Moreover, a denatured lysozyme interacts with alcohols more strongly than the native form, and is more repulsive in mixing scheme I and more attractive in IT. This behaviour is shown to be consistent with the alcohol-dependent enthalpy of denaturation of lysozyme found by Velicelebi and Sturtevant [Velicelebi and Sturtevant, Biochem. 18 (1979) 1188-86]. (C) 1997 Elsevier Science B.V.