Proteins in aqueous solutions. calorimetric studies and thermodynamic characterization

Abstract

Calorimetric techniques applied to the study of the thermal denaturation of proteins, and of their interaction with various ligands, have permitted, over the last two decades, a better insight to be gained into our understanding of complex biological phenomena. Improvements in the design of calorimetric equipment have enabled the detection of very small thermal effects associated with the formation or disruption of non-covalent bonds in a macromolecule. Only through differential scanning calorimetry has it become possible to assess the two-state nature of the thermal unfolding of many globular proteins in aqueous solution. It is mainly by means of this technique that we are now able to deal satisfactorily with a phenomenon known for a long time, but experimentally difficult to study: the “cold” denaturation of proteins. Why does a protein unfold at a temperature so much different from that of the more usual “heat” denaturation? What are the forces controlling this phenomenon? The thermodynamic stability of a protein conformation is the result of several non-covalent interactions which may occur intramolecularly or with the solvent. The importance of the role of water in biochemical processes has long been recognized, starting with the analysis of the solvent effect made by Kauzmann [l]. This is the reason why it is impossible to discuss the thermodynamic stability of proteins without considering studies of the thermodynamics of model systems in aqueous solution. Besides, the ability of proteins to interact with small and large ligands (for instance, with other proteins, as in self-association) is the basis of many biological phenomena, such as specific binding, cooperative binding, catalysis, and ligand-induced structural changes. Isothermal calorimetry acquires special significance, because it provides all the thermodynamic parameters characterizing the association process.