Calorimetric study of the interaction of lysozyme with aqueous 1-propanol

The enthalpies of transfer of hen egg white lysozyme from water to aqueous solutions of 1-propanol were determined by isothermal flow calorimetry at 10, 17, 25, and 40 degrees C in 0.04 M, pH 2 glycine buffer. Alcohol concentrations up to 3.4 M were employed. Four regions in the dependence of the enthalpy of transfer on alcohol concentration can be discerned: a region of linear increase observable at 10, 17, and 25 degrees C, an inflection region observed at 17 and 25 degrees C, a second linear region observable at 17, 25, and 40 degrees C, and a region of decreasing enthalpies seen at 40 degree C. Combination of differential scanning calorimetric data on lysozyme in PrOH-H2O mixtures [Velicelebi, G., & Sturtevant, J. M. (1979) Biochemistry 18, 1180-1186] with the transfer enthalpies reported here shows that the enthalpy in the system can be regarded as a state function and that the apparent specific heat is at first slightly decreased and then strongly increased by the addition of 1-propanol. Comparison of the results of the interaction with guanidine hydrochloride [Pfeil, W., & Privalov, P. L. (1976) Biophys. Chem. 4, 23-50] indicates that the denaturing effects of these two reagents involve very different mechanisms.