Activation and reaction volumes for redox reactions of horse-heart cytochrome c with inorganic reagents

Abstract

Negative activation volumes have generally been considered as a criterion for outer-sphere redox reactions. Marcus and Sutin have, however, emphasized the effect of the overall enthalpy and entropy changes of the reaction on the activation enthalpy and entropy. The effects are expected to be even more pronounced for the activation volumes in view of the large electrostriction accompanying cross-redox reactions. This is reflected in the large overall volume changes. We show that the activation volumes for outer-sphere redox reactions of cytochrome c with positive and negative inorganic complexes can be either positive or negative, depending on the overall volume change of the reaction.