`Intermolecular interactions' in aqueous solutions of three components including lysozyme

Abstract

Excess partial molar enthalpies were measured in ternary aqueous solutions, tert-butyl alcohol (TBA)-DMSO-H 2O, and lysozyme (L)–alcohols (A)–H 2O. 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 TBA, DMSO or alcohols, two solutes in TBA–DMSO–H 2O seem to modify the hydrogen bond network of H 2O in an additive manner. Here, mixing scheme I refers to the way in which a solute (hydrophobic) modifies the molecular organization of H 2O. Specifically, a solute enhances the hydrogen bond strength of H 2O 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–H 2O 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 II. 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) 1180–86].