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 Delta H-m(van), the enthalpy change on denaturation at T-m, the midpoint of denaturation, is significantly less than Delta(cal)(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 pre- and post-transition baselines; and (b) that there exists an excellent agreement between Delta H-m(van) and Delta H-m(cal) values of proteins, if the analysis of thermal denaturation curves assumes that the temperature-dependence of pre- and post-transition baselines is described by a parabolic function. The latter analysis is supported by our observations that the temperature-dependencies of the absorption and circular dichroism properties of protein groups are indeed nonlinear. It is observed that the estimate of Delta C-p, the constant-pressure heat capacity change is independent of the model used to describe the temperature-dependence of the pre- and 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.