Electrochemistry of cytochrome c, plastocyanin, and ferredoxin at edge- and basal-plane graphite electrodes interpreted via a model based on electron transfer at electroactive sites of microscopic dimensions in size

Abstract

The electrochemistry of a range of electron-transfer proteins at edge- and basal-plane graphite electrodes has been reconsidered using a microscopic model, which involves fast electron transfer at very small oxygen-containing electroactive surface sites. This model assumes that mass transport to the electrode occurs by radial diffusion when the density of the surface active sites is low (as is generally true in the case of the basal-plane graphite electrode) and by linear diffusion when the density of the active sites is increased sufficiently to cause overlap of the diffusion layers. With this model it is now proposed that the electrochemistry of cytochrome c, plastocyanin, and ferredoxin occurs with a very fast rate of charge transfer (≥1 cm s-1) at both edge- and basal-plane graphite electrodes. Critical factors, such as the mode of surface preparation (including covalent derivatization), the pH, and the presence in the electrolyte of cations such as Mg2+ or Cr(NH3)63+, control the density of surface sites, which result in the electrochemistry of a specific protein. This contrasts with the conclusion that has been reached previously based upon a conventional macroscopic model, which supposes that the rate of electron transfer is subject to enhancement or depression through these factors. The proposal that the electron-transfer process at the protein-graphite electrode interface is very fast over a wide range of conditions is now consistent with homogeneous kinetic studies where electron-transfer reactions of proteins, particularly amongst physiological partners, are also known to be fast. © 1989 American Chemical Society.