Electrochemical Comparison of Heme Proteins by Insulated Electrode Voltammetry

Relatively simple electrochemical probes of redox active proteins can yield a wealth of diverse information concerning their kinetic and thermodynamic reactivity, charge state, and transport rates. In this report, the electrochemical properties of three heme‐containing proteins are probed using Au electrodes derivatized with self‐assembled monolayers of ω‐hydroxyalkanethiols. Cytochrome c and b5 (wild type from horse and rat, respectively) display low reorganization energies (0.58 eV and 0.44 eV, respectively) and electronic coupling terms which are quite comparable to small redox molecule models. In contrast, metmyoglobin reduction is characterized by a somewhat higher reorganization energy (0.76 eV) and an order of magnitude reduction in its electronic coupling to the electrode. Upon binding oxygen, the reduced oxymyoglobin becomes electroinactive. This radical decrease in the oxymyoglobin redox reactivity is associated with structural and electronic differences caused by the spin‐state change induced by oxygen binding. The reactivity differences between these heme‐containing proteins are discussed in light of their biochemical function, evolutionary pressures, and structural differences.