Electrochemical origin of hysteresis in the electron-transfer reactions of adsorbed proteins: Contrasting behavior of the "blue" copper protein, azurin, adsorbed on pyrolytic graphite and modified gold electrodes

Azurin and other small redox proteins exhibit fast electron transfer when adsorbed on a pyrolytic graphite "edge" electrode, but close examination reveals unusual electrochemical behavior that is not predicted by simple models. Cyclic voltammetry over a wide range of scan rates (up to 1000 V s(-1)) shows that the apparent reduction potential depends on the scan rate and initial polarization potential, and that a small finite peak separation persists in the slowest experiments (1 mV s(-1)). To determine the origin of these effects, the voltammetric behavior of azurin adsorbed at PGE has been compared with results obtained using gold electrodes modified with a self-assembled monolayer (SAM) of hexanethiol or decanethiol. The contrastingly simple results that are obtained with the SAM electrodes show that the complexities stem from properties of the graphite surface or its interface with the protein. Fast scan cyclic voltammetry, initiated after prepolarizing the graphite electrode over a range of potentials, reveals that rapid electron exchange with the "blue" Cu center is perturbed by further processes that are relatively slow. Moreover, similar effects are observed for a ferredoxin that has two Fe-S clusters, each with a much more negative reduction potential. These slow processes are responsible for the complex hysteresis behavior that is observed with the PGE electrode. Two models are proposed and compared: in the first, redox-active surface groups on the graphite surface modulate the reduction potential of the adsorbed protein. In the second model, the change in redox state of the active site is sensed by the electrode-protein interface, which adjusts to a new state.