THE ENTHALPY OF TRANSFER OF UNFOLDED PROTEINS INTO SOLUTIONS OF UREA AND GUANIDINIUM CHLORIDE

G. Makhatadze and P. Privalov [J. Mol. Biol., 226 (1995) 491] have rec ently measured the enthalpy of transfer of three proteins into urea an d guanidinium chloride solutions as a function of concentration and te mperature. The present paper applies the solvent-exchange model [J.A. Schellman, Biopolymers, (1994)] to the data and compares it with the b inding model utilized in the original publication. Both calculations a ssume identical binding sites. It is found that the data may be fit to lerably well using either procedure, but that the parameters describin g the binding vary considerably. Consideration of the transfer propert ies of amino acid moieties and small peptides leads to the conclusion that solvation sites are heterogeneous and that the quantities determi ned by both methods are statistical averages. The parameters describe an identical-site system that has (approximately) the same properties as the real heterogeneous system. The results have mainly heuristic an d mechanistic value. One quantity determined with these simplified iso therms, Sigma k(j) Delta h(j), is a property of the real system and ca n serve as a measure of a thermal binding capacity for a protein. The appendices contain a resume of the solution theory required for the ex change model of solvation as well as the development of a number of em pirical equations for the thermodynamic properties of urea and guanidi nium chloride solutions.