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 for a number of charge transfer reactions of redox mediators were used to evaluate the corresponding charge transfer entropies in Tris-HCl (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 couple [(cytc)ox/(cytc)red] in Tris-HCl (pH 8) buffer and were found to be equal to -16 cal/degrees K mol. Reversible electrode potentials 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 vary from -13.8 to -29.7 cal/degrees K mol for a closely related series 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 entropies of protein redox couples which cannot be studied by direct electrochemical methods is outlined. The factors contributing to the magnitude of the charge transfer entropies are discussed.