Redox thermodynamics of the Fe3+/Fe2+ couple in horseradish peroxidase and its cyanide complex

The thermodynamics of Fe3+ to Fe2+ reduction for the five-coordinate high-spin native form of horseradish peroxidase and for its six-coordinate low-spin cyanide adduct have been determined from variable-temperature UV-vis spectroelectrochemical experiments. In both cases, the ΔH°′rc and ΔS°′rc values are positive. Hence, the negative reduction potentials turn out to be the result of two opposing and partially compensating contributions: a large enthalpic term, which is the determinant of the negative E°′ values for both species, and a smaller, yet relevant, entropic contribution. The decrease in E°′ of the Fe3+/Fe2+couple on cyanide binding turns out to be a fully entropic effect, unequivocally demonstrating the importance of entropic effects in determining the E°′ values of redox metal centers. Copyright © 2002 American Chemical Society.