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

The majority of electrochemical studies on metalloproteins have report ed the use of chemically modified electrodes or deliberate use of adso rbed species to obtain persistent well defined and stable voltammetric responses. This present study confirms that a diffusion controlled re duction response may be observed for cytochrome c at an unmodified ('b are') gold disc electrode in many electrolytes if voltammograms are re corded immediately after the electrode is placed in contact with the s olution. However, the current observed at potentials near the reversib le value for native cytochrome c rapidly decreases in magnitude in chl oride or fluoride electrolytes as the electrode contact time with the solution increases until the response is indistinguishable from that o bserved due to the background electrolyte. In cacodylate and phosphate buffered media, the response also decays with time, but at a much slo wer rate. However, in all electrolytes, a concomitant change from a pe ak-shaped (linear diffusion dominant) to a sigmoidal-shaped (radial di ffusion dominant) response is observed as the current decreases in mag nitude. The transient behaviour is explicable in terms of a 'self-bloc king' model in which highly positively-charged but electroinactive cyt ochrome c is adsorbed on the seconds time scale and blocks the electro de at the reversible potential, leaving only an array of microscopical ly small electroactive sites available for the diffusion controlled vo ltammetry of native cytochrome c. Thus, the adsorption of cytochrome c effectively changes the dominant mode of mass transport for electroac tive bulk solution native cytochrome c from linear to radial diffusion as surface blockage increases, thereby explaining both the time depen dent current magnitude and the change in curve shape from peak to sigm oidal. 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 cytoc hrome c. The proposed mechanism also rationalises how specific and non -specific forms of interaction with the electrolyte lead to a highly e lectrolyte dependent response. (C) 1997 Elsevier Science S.A.