Study of thermal denaturation of lysozyme and other glubular proteins by light-scattering spectroscopy

The technique of intensity correlation light-scattering spectroscopy has been used to accurately determine the extent of physical swelling of lysozyme, ribonuclease, and chymotrypsinogen produced by thermal denaturation. The change in hydrodynamic radius is deduced from direct measurements of the diffusion coefficient, carried out in the temperature range 20° to 70°C at various values of pH in the range 1.0 to 3.0 at ionic strengths of from 0.01 M to 0.2 M. An average radius increase of 18% is observed for lysozyme and ribonuclease, with an estimate of 26% for chymotrypsinogen. Analysis of the pH dependence of the transition temperature leads to the conclusion that the lysozyme charge increases by approximately +2e during unfolding. We have applied this value of the charge increase along with the 18% average radius increase to estimate the electrostatic contribution to the free-energy change for denaturation of lysozyme. The results are consistent with the experimental observation that the transition temperature is nearly independent of ionic strength.