Intermolecular interactions of lysozyme and small alcohols: A calorimetric investigation

Isothermal titration calorimetry was used to measure transfer enthalpies of hen egg white lysozyme from water to aqueous solutions of methanol, ethanol, 1-propanol, glycerol, and 2,2,2-trifluoroethanol. Excess partial molar enthalpies of lysozyme at infinite dilution in the alcoholic solvents, H-infinity(L)E, were calculated, and the dependence of H-infinity(L)E on the concentration of the alcohol, was utilized to elucidate the enthalpy of alcohol-protein interactions. Results show that, at low alcohol concentrations, alcohol-protein interactions are unfavorable in terms of enthalpy (endothermic), while at higher concentrations they are favorable (exothermic). The change from endothermic to exothermic interactions happened sharply over a narrow alcohol concentration interval, and was found to occur concurrently with denaturation of the protein in some but not all cases. Comparison of the present results with previous investigations of simple binary and ternary aqueous solutions suggests that the change of sign of the interaction enthalpy is related to the water-water hydrogen bonding properties in the alcohol mixtures. It is argued that modifications by the alcohol of the percolated hydrogen bond network govern the enthalpy of alcohol-lysozyme interactions in the most water-rich samples. This suggests that occupancy by the alcohol of a binding site on the surface of the protein may not be necessary for the alcohol to affect the properties of the protein. At higher alcohol concentrations the observed interaction enthalpies are dominated by direct (’’intrinsic’’) effects of protein-alcohol interactions, which are exothermic.