Isothermal titration calorimetry was used to measure the 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 were calculated, and its dependence on the concentration of alcohol, was utilized to elucidate the enthalpy of the alcohol-protein interactions. Results show that at low alcohol concentrations, alcohol-protein interactions are unfavorable in terms of enthalpy (endothermic), while at high concentrations they are favorable (exothermic). The change from endothermic to exothermic interactions happened over a narrow concentration interval, and was found to occur concurrently with denaturation of the protein in some but not all the cases. Comparison of the present results with previous investigations of simple binary and tertiary aqueous solutions suggests that the change of sign in the interaction enthalpy is related to the water-water hydrogen bonding properties in the alcohol mixtures. It was argued that long ranged modifications of the percolated hydrogen bond network induced by the alcohol govern the enthalpy of alcohol-protein interactions in the most water rich samples. This suggests that occupancy by the alcohol of a binding site of the surface of the protein may not be necessary for the alcohol to affect the properties of the protein. At higher alcohol concentration, possibly when long range connectivity of the hydrogen bond network of water is lost, the observed interactions enthalpies are dominated by direct (~intrinsic~) effects of protein-alcohol interactions, which are exothermic.
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