Doctor of Philosophy
Fred M. Hawkridge
The interfacial electrochemistry of horse heart cytochrome c was studied at metal and metal oxide electrodes. Sperm whale myoglobin reactions at metal oxide and surface modified electrodes were also investigated. Emphasis was on the measurement and mechanistic interpretation of the heterogeneous electron transfer kinetic parameters for these electrode reactions.
The motivation for this study stems from the fact that some electron transfer proteins, e.g., cytochrome c, transfer electrons heterogeneously in physiological systems. Specific electrodes may, therefore, serve as suitable models for physiological surfaces. Electrode and membrane interfaces are both electrically charged and both have oriented hydration water layers.
The cyclic voltammetric behavior of cytochrome c at solid electrodes is strongly influenced by the purity of the sample. Following dissolution, commercial samples exhibit a continuous, time-dependent decrease in reversibility until a stable reproducible cyclic voltammogram is obtained after about one hour in quiet solution. The decrease in the formal heterogeneous electron transfer rate constant corresponding to this process is ≥ two orders of magnitude at doped tin oxide and indium oxide electrodes. However, commercial cytochrome c samples purified by ion-exchange chromatography exhibit more reversible responses which do not decay significantly with time.
Formal heterogeneous electron transfer rate constants ( k° ) for purified cytochrome c were determined in pH 7 tris/cacodylate media by analyzing cyclic voltammetric peak separations assuming that a = 0.5. Electrodes and approximate ranges of k° in cm/s: tin doped indium oxide, 10-3 to 10-2; fluorine doped tin oxide, 10-4 to 10-3; gold, 10-3; Pt, 10-3. Kinetics are also affected by electrode pretreatment, cytochrome c concentration, and electrolyte composition. Evidence for cytochrome c adsorption sites of differing affinities on indium oxide is presented. It is proposed that the exposed heme edge of cytochrome c is the electron transfer site and that the rate is controlled by a combination of electrostatic and chemical interfacial forces. A hydrophilic electrode surface along with a suitable surface charge appear to be sufficient conditions for facile electron transfer. Specific chemical and electrostatic interactions between cytochrome c lysine residues and charged oxygen groups on metal oxide surfaces are also likely. Electron tunneling to the exposed heme edge is proposed to be an important rate-controlling process for the metal oxide semiconductors.
Reductive and oxidative kinetics of commercial cytochrome c at tin oxide electrodes were measured using single potential step chronoabsorptometry ( SPS/CA ) and asymmetric double-potential step chronoabsorptometry, respectively. Formal rate constants were near lo-s cm/s for both reactions, but a and (1 - a) values were ca. 0.3 and < ca. 0.1, respectively. The irreversibility of these reactions appears to be due to surface adsorption of an impurity which restricts approach of the cytochrome c molecules to the electrode surface.
Reductive kinetics of myoglobin at tin oxide and at methyl viologen modified gold electrodes were measured using SPS/CA. The reaction was irreversible with marked adsorption effects. Electron transfer to diffusing molecules appeared to proceed through exposed heme groups of adsorbed denatured myoglobin molecules.
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