INTERMOLECULAR INTERACTIONS OF LYSOZYME AND SMALL ALCOHOLS - A CALORIMETRIC INVESTIGATION

Isothermal titration calorimetry was used to measure transfer enthalpi es of hen egg white lysozyme from water to aqueous solutions of methan ol, ethanol, 1-propanol, glycerol, and 2,2,2-trifluoroethanol. Excess partial molar enthalpies of lysozyme at infinite dilution in the alcoh olic solvents, H-infinity(L)E, were calculated, and the dependence of H-infinity(L)E on the concentration of the alcohol, was utilized to el ucidate the enthalpy of alcohol-protein interactions. Results show tha t, at low alcohol concentrations, alcohol-protein interactions are unf avorable in terms of enthalpy (endothermic), while at higher concentra tions they are favorable (exothermic). The change from endothermic to exothermic interactions happened sharply over a narrow alcohol concent ration interval, and was found to occur concurrently with denaturation of the protein in some but not all cases. Comparison of the present r esults with previous investigations of simple binary and ternary aqueo us solutions suggests that the change of sign of the interaction entha lpy is related to the water-water hydrogen bonding properties in the a lcohol mixtures. It is argued that modifications by the alcohol of the percolated hydrogen bond network govern the enthalpy of alcohol-lysoz yme interactions in the most water-rich samples. This suggests that oc cupancy 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 ent halpies are dominated by direct (''intrinsic'') effects of protein-alc ohol interactions, which are exothermic.