On the Estimation of Stability Parameters from Heat-Induced Conformational Transition Curves of Proteins

A method is suggested to determine valid and authentic values of thermodynamic stability parameters of proteins from their heat-induced conformational transition curves. We show (a) that the estimate of ∆Hm, the enthalpy change on denaturation at Tm, the midpoint of denaturation, is significantly less than ∆Hm, the value obtained by the calorimetric measurements, if the analysis of the conformational transition curve uses the conventional method which assumes a linear temperature-dependence of the preand post-transition baselines; and (b) that there exists an excellent agreement between ∆Hm and ∆Hm values of proteins, if the analysis of thermal denaturation curves assumes that the temperature-dependence of preand post-transition baselines is described by a parabolic function. The latter analysis is supported by our observations that the temperaturedependencies of the absorption and circular dichroism properties of protein groups are indeed nonlinear. It is observed that the estimate of ∆Cp, the constant-pressure heat capacity change is independent of the model used to describe the temperaturedependence of the preand post-transition baselines. An important conclusion is that for proteins which exhibit a two-state character, all stability parameters are measured with the same error as that observed with a calorimeter.