The transient nature of the diffusion controlled component of the electrochemistry of cytochrome c at ‘bare’ gold electrodes: an explanation based on a self-blocking mechanism

Abstract

The majority of electrochemical studies on metalloproteins have reported the use of chemically modified electrodes or deliberate use of adsorbed species to obtain persistent well defined and stable voltammetric responses. This present study confirms that a diffusion controlled reduction response may be observed for cytochrome c at an unmodified (‘bare’) gold disc electrode in many electrolytes if voltammograms are recorded immediately after the electrode is placed in contact with the solution. However, the current observed at potentials near the reversible value for native cytochrome c rapidly decreases in magnitude in chloride or fluoride electrolytes as the electrode contact time with the solution increases until the response is indistinguishable from that observed due to the background electrolyte. In cacodylate and phosphate buffered media, the response also decays with time, but at a much slower rate. However, in all electrolytes, a concomitant change from a peak-shaped (linear diffusion dominant) to a sigmoidal-shaped (radial diffusion dominant) response is observed as the current decreases in magnitude. The transient behaviour is explicable in terms of a ‘self-blocking’ model in which highly positively-charged but electroinactive cytochrome c is adsorbed on the seconds time scale and blocks the electrode at the reversible potential, leaving only an array of microscopically small electroactive sites available for the diffusion controlled voltammetry of native cytochrome c. Thus, the adsorption of cytochrome c effectively changes the dominant mode of mass transport for electroactive bulk solution native cytochrome c from linear to radial diffusion as surface blockage increases, thereby explaining both the time dependent current magnitude and the change in curve shape from peak to sigmoidal. This unusual form of transient behaviour is postulated to be a consequence of the very high overall positive charges associated with both the blocking (adsorbed) and bulk solution (native) forms of cytochrome c. The proposed mechanism also rationalises how specific and non-specific forms of interaction with the electrolyte lead to a highly electrolyte dependent response.