Abstract. 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 co-hydroxyalkanethiols. Cytochrome c and b s (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.

Changes in backbone dynamics occurring upon oxidation of rat cytochrome b 5 have been examined through model free analyses of 15N-relaxation rates of both oxidation states of the protein. Based on the observed changes, an upper bound for the contribution of backbone dynamics to the entropy change associated with oxidation has been calculated. The magnitude of this backbone contribution, 70 ± 7 J/K·mol, is strikingly similar to the total entropy change associated with oxidation of the protein determined through an analysis of the temperature dependence of the reduction potential. Origins of the differences in dynamic behavior of the oxidized and reduced proteins can be attributed to redox linked changes in hydrogen bond strengths based on large-scale differences in amide proton exchange rates observed between the oxidation states. Based on these observations the magnitude and possible significance of entropic contributions to the electromotive force are discussed. Analysis of the 15N-relaxation rates included modeling of anisotropic diffusional behavior which was expected based on the distinct physical asymmetry of the protein. An axially symmetric diffusion tensor model was found to fit the rotational reorientational properties of the protein in both oxidation states. The contribution of paramagnetic relaxation to the 15N-relaxation rates of the oxidized protein was calculated based on a set of modified Solomon−Bloembergen equations. The determination of the electronic correlation time of the paramagnetic center was based on fits to the proton relaxation rate enhancements of protons in close proximity to the paramagnetic center. Analyses of the dynamic properties of the oxidized cytochrome b 5 were based on multiple field (i.e., 500 and 750 MHz) NMR measurements of 15N T 1 and T 2 relaxation times.

Voltammetric measurements on solutions of human hemoglobin using gold electrodes modified with omega-hydroxyalkanethiols have yielded the first direct measure of the reorganization energy of the protein. The value obtained based on extrapolation of the experimentally measured currents, 0.76 eV, is independent of pH (i.e., over the physiologically relevant rage, pH 6.8-7.4) and is remarkably similar to values obtained for myoglobin. This result is perhaps surprising given the marked dependence of the measured reduction potential of hemoglobin on pH (i.e., the redox Bohr effect). Electron transfer rates from the electrode to hemoglobin were also measured. Using similarly measured heterogeneous electron-transfer rates for cytochrome b(5), it is possible to predict the magnitude of the homogeneous electron-transfer rate from cytochrome b(5) to methemoglobin using a formalism developed by Marcus. These predicted rates are in reasonable agreement with reported rates of this physiological reaction based on stopped-flow kinetics experiments. These results suggest that the intrinsic electroreactivity of these heme proteins is sufficient to account for physiologically observed rates. Residual differences between homogeneous phase kinetics and those predicted by heterogeneous phase reactions are suggested to be due to small reductions in the outer-sphere reorganization energy of both component proteins which arise due to solvent exclusion at the interface between the two proteins in complex.

Using surface-modified electrodes composed of omega-hydroxyalkanethiols, an experimentally based value for the inner-sphere reorganization energy of the bis(imidazole)iron porphyrin system has been obtained by examining the solvent dependence of the reorganization energy of bis(N-methylimidazole)meso-tetraphenyl iron porphyrin. The value obtained (0.41 +/- 0.06 eV) is remarkably similar to values we have recently reported for the reorganization energy of cytochrome b(5) (0.43 +/- 0.02 eV) and cytochrome c (0.58 +/- 0.06 eV). This strongly suggests that the protein matrix mimics the behavior of a low dielectric solvent and effectively shields the heme from the solvent. The effect of the orientation of the heme relative to the electrode was also explored by sytematically varying the steric bulk of the axial ligands. On the basis of a good linear correlation between the electronic coupling and the cosine of the angle between the heme plane and the surface of the electrode, it is suggested that a parallel orientation of the heme yields a maximum in the electronic coupling. Relevance to interheme protein electron transfer is discussed.

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