Calorimetric study of the effect of intrachain cross-linking on lysozyme unfolding

Thermodynamics of unfolding of lysozyme cross-linked between Glu 35 and Trp 108 were studied in solutions of various concentrations of 1-propanol (1-PrOH) at pH 3.7 by means of scanning microcalorimetry. The transition temperature for the cross-linked lysozyme increases by 17-19 degrees C due to cross-linking at every concentration of 1-PrOH. This corresponds to the increase in the unfolding Gibbs free energy of about 28 kJ.mol-1, which is independent of the concentration of 1-PrOH. It was found that the unfolding enthalpy of cross-linked lysozyme is only slightly larger than that of intact one, and the unfolding entropy of the cross-linked one is nearly equal to that of the intact one, if both are compared at the same temperature. The stabilization mechanism for the cross-linked lysozyme is discussed on the basis of these calorimetric data.