Stability of proteins in the presence of polyols estimated from their guanidinium chloride-induced transition curves at different pH values and 25 °C

We have recently concluded from the heat-induced denaturation studies that polyols do not affect ΔGD° (the Gibbs free energy change (ΔGD) at 25 °C) of ribonuclease-A and lysozyme at physiological pH and temperature, and their stabilizing effect increases with decrease in pH. Since the estimation of ΔGD° of proteins from heat-induced denaturation curves requires a large extrapolation, the reliability of this procedure for the estimation of ΔGD° is always questionable, and so are conclusions drawn from such studies. This led us to measure ΔGD° of ribonuclease-A and lysozyme using a more accurate method, i.e., from their isothermal (25 °C) guanidinium chloride (GdmCl)-induced denaturations. We show that our earlier conclusions drawn from heat-induced denaturation studies are correct. Since the extent of unfolding of heat- and GdmCl-induced denatured states of these proteins is not identical, the extent of stabilization of the proteins by polyols against heat and GdmCl denaturations may also differ. We report that in spite of the differences in the structural nature of the heat- and GdmCl-denatured states of each protein, the extent of stabilization by a polyol is same. We also report that the functional dependence of ΔGD of proteins in the presence of polyols on denaturant concentration is linear through the full denaturant concentration range. Furthermore, polyols do not affect the secondary and tertiary structures of the native and GdmCl-denatured states.