Thermodynamic Studies of Protein Stabilities

Experimental studies of protein stabilities are numerous and there are still points of serious disagreement concerning the conclusions to be drawn from these studies. Areas of discord include the extent to which thermal denaturation corresponds to denaturation by chemical agents and the extent to which hydrophobic, van der Waals, and/or hydrogen bonds stabilize the native state. In this discussion, we will focus on the work of Peter L. Privalov.k Privalov has developed much of the microcalorimetric instrumentation that has made the calorimetric studies of proteins feasible. He has also published numerous review articles that summarize experimental data and formulate general observations concerning protein denaturation. His 1995 paper in Advances in Protein Chemistry9 presents a recent, comprehensive, review of the experimental results 

We will present only a brief description of Privalov s work here. We begin with the derivation of some additional techniques for the analysis of the calorimetric data that Privalov and others have developed and applied to the study of protein unfolding. We end with a summary of the observations and conclusions that have resulted from his work. 

The Calorimetrically Obtained van t Hoff Enthalpy In a manner analogous to that used to obtain the van t Hoff enthalpy from the fractional change in the optical absorbance, one can use the temperature dependence of the fractional enthalpy as a function of temperature to determine an effective enthalpy. We will adopt the notation A Hto represent the total enthalpy associated with the denaturation transition. It can be obtained from an integration of the excess heat capacity, corrected for the baselines, as discussed before  

The calorimetric heat capacity measurements have an advantage over spectroscopic measurements in the application of equations (16.18) or (16.19). In the analysis of spectroscopic data, the temperature derivative must be determined numerically from a differentiation of the a versus T curve. The heat capacity measurements provide this derivative directly as we show below  

the derivative can be evaluated at any temperature by dividing the baseline-corrected excess heat capacity, ACP(T), by the integrated enthalpy A H. By substituting this result into equation (16.19) one obtains 

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