ENTROPIES OF REDOX REACTIONS BETWEEN PROTEINS AND MEDIATORS - THE TEMPERATURE-DEPENDENCE OF REVERSIBLE ELECTRODE-POTENTIALS IN AQUEOUS BUFFERS

The temperature dependencies of the reversible electrode potentials fo r a number of charge transfer reactions of redox mediators were used t o evaluate the corresponding charge transfer entropies in Tris-HCI (pH 8) buffer. The redox mediator thermodynamic data, along with reaction enthalpy data for mediator redox protein electron transfer, were used to evaluate the charge transfer entropy for the cytochrome c redox co uple [(cytc)(ox)/(cytc)(red)] in Tris-HCl (pH 8) buffer and were found , to be equal to -16 cal/degrees K mol. Reversible electrode potential s at 298 degrees K for the redox mediator half-reactions were observed to vary from -528 to +657 mV (vs NHE). Charge transfer entropies were observed to depend upon the structure of the redox mediators and to v ary fs-om -13.8 to -29.7 cal/degrees K mol for a closely related serie s of organic dications (viologens) and a value of -43.6 cal degrees K mol was observed for the [Fe(CN)(6)](3-)/[Fe(CN)(6)](4-) couple under the same conditions. A procedure for determining charge transfer entro pies of protein redox couples which cannot be studied by direct electr ochemical methods is outlined. The factors contributing to the magnitu de of the charge transfer entropies are discussed. (C) 1997 Academic P ress.