O. Hill

Protein electrochemistry at carbon nanotube electrodes

Abstract The application of the carbon nanotube as an electrode material is demonstrated. Redox proteins on and within these tubes give reproducible, well-behaved voltammetric responses.

Surface modifiers for the promotion of direct electrochemistry of cytochrome c

Fifty-four bifunctional organic compounds were studied to assess their ability to promote the direct electrochemistry of horse heart cytochrome c at a modified gold electrode. From the results of the survey it was possible to identify those features important for successful promotion of the electrochemical activity. It is suggested that it is necessary to provide groups on the electrode surface which can hydrogen bond or form salt bridges to the positively charged lysine side chain groups around the heme crevice on cytochrome c. The functional groups, Y, may be anionic or weakly basic and can be attached to the electrode using a bifunctional compound X ∼ Y. The group, X, adsorbs or binds to the gold surface through nitrogen, phosphorus, or sulphur. A “pre-activation” step for the adsorption of some surface modifiers has been discovered. The molecular structure of a compound which promotes cytochrome c electrochemistry can be either conformationally rigid or flexible, aromatic or aliphatic, but it should direct Y out from the electrode. The length of the molecule does not appear to affect the rate of electron transfer. The presence of a hydrophobic zone in the structure is neither necessary nor sufficient for successful promotion of cytochrome c electrochemistry.

The direct electrochemistry of redox proteins at metal oxide electrodes

Abstract The dc cyclic voltammetry of several redox proteins at thin film ruthenium dioxide electrodes is described. Quasi-reversible voltammetric responses were obtained for cytochrome c , azurin, ferredoxin, rubredoxin and plastocyanin at this electrode with solution conditions, pH 8.0 (5 m M Tricine) and 100 m M NaCl as supporting electrolyte. The values of E 1/2 are in good agreement with those in the literature. On varying the pH and solution ionic strength striking differences in response emerge. For the negatively charged proteins, ferredoxin, rubredoxin and plastocyanin, no measurable faradaic response is observed at lower ionic strength (pH 8.0, 1 m M N2Cl). The electrochemistry can, however, be promoted by multivalent cations (Cr(NH 3 ) 3+ 6 or Mg 2+ ) giving rise to a well-defined quasi-reversible response. For cytochrome c , variation of the dc cyclic voltammetric response with pH correlates with the (acid-base) protonation properties of the electrode surface. The electrochemistry of azurin does not vary appreciably with changes of pH or ionic strength. The results found highlight the importance of electrostatics at the electrode/solution interface. Preliminary results using IrO 2 , Na 0.7 WO 3 and single-crystal RuO 2 electrodes are reported. Subtle differences in behaviour are found at the single-crystal and thin-film RuO 2 electrodes.

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.

Direct electrochemistry, at modified gold electrodes, of redox proteins having negatively-charged binding domains: spinach plastocyanin and a multi-substituted carboxydinitrophenyl derivative of horse heart cytochrome c

Abstract The direct electrochemistry of spinach plastocyanin and a multi-substituted carboxydinitrophenyl (CDNP) derivative of horse heart cytochrome c at gold electrodes modified with 2-aminoethanethiol, 2,2′-dithiobisethanamine and (pyridinylmethylene)hydrazinecarbothioamides, is described. A comparison is made with the electrochemistry of native horse heart cytochrome c . The results are interpreted in terms of interaction of negatively-charged binding domains on the surface of plastocyanin and the multi-CDNP cytochrome c with the modified electrode surface.

The aqueous electrochemistry of C60 and methanofullerene films

Abstract C 60 and methanofullerene films give an unexpected electrochemical response in aqueous solution—sharply different from the, now well characterised, responses in acetonitrile. A comparison is made between the film and solution responses of carboxylate and ethyl ester adducts. The large differences observed may be accounted for by a specific ordering of the acid functions on the gold electrode surface.

Promotion of the electrochemical response of some negatively charged proteins at an edge-plane graphite electrode by various redox inert cations : an electrochemical manifestation of Frumkin adsorption

Promotion of the cyclic voltammetric response of several small negatively charged metalloproteins P at an edge-plane graphite (EPG) electrode E by various inorganic cations M at pH 8 has been interpreted in terms of two equilibria: P + M K→ PM E + PM KM*→ EPM The first equilibrium, where a 1 : 1 protein-cation complex PM is formed, occurs in the bulk, while the second takes place at the electrode surface. The value of the equilibrium constant K depends on the cation charge and the charge on the cation binding site of the protein. A simple model for understanding the interactions in the inorganic ion pairs in an aqueous medium has been developed. The model is extended to compare the association of the inorganic ions with various proteins in an aqueous medium and is used to estimate K. The adsorption of the protein-metal ion complex on the electrode surface (the second equilibrium) is assumed to be weak (i.e. the equilibrium constant KM* is small) and reversible. It is found that only EPM, the PM species adsorbed on the electrode surface, is electroactive. The experimental consequences of the two equilibria are discussed. The adsorption pattern is found to obey the Frumkin isotherm. The standard Gibbs energies of adsorption ΔGads0 have been calculated specifically for 2[4Fe-4S] ferredoxin in the presence of various cations. The magnitude of the ΔGads0 values, which are found to lie within the range −13.4 to 22.2 kJ mol −1 for 2[4Fe-4S] ferredoxin, suggest physical adsorption of the protein. The ΔGads0 values for the various cations are found to vary linearly with z2/r where z is the charge on the cation r is its ionic radius. It is concluded that the role of the cation is to induce a reversible weak adsorption of a negatively charged protein suitable for electron transfer on an EPG surface at pH 8. Speculations are made on the nature of the adsorption.

Electrochemical investigations of a novel ferrocene surfactant

Abstract A novel ferrocene surfactant, N-ferrocenoyl-hexadecamide-O-sulphato-L-serine sodium salt (FS), has been synthesized. Its sulphate head group and chirality make it distinctively different from the ferrocene surfactants reported so far. Its critical micelle concentration (CMC) has been determined using the surface tension method and FS has been extensively electrochemically characterized in aqueous solution. A change in the electrochemical behaviour of FS around its CMC has led to a new method for the determination of the CMC in electrolyte solutions by using cyclic voltammetry. This method has been tested with a positive ferrocene surfactant, dodecy(ferrocenyl-methyl)dimethylammonium bromide (C12) which has been described earlier. The results obtained for both ferrocene surfactants were found to be in fair agreement with values obtained by surface tension measurements.

The coupling of heterogeneous electron transfer to photosystem 1

Abstract Direct electrochemistry of cytochrome c at a surface-modified gold electrode was used as the source of reducing equivalents for a membrane fraction enriched in photosystem 1. The system was studied using the techniques of bulk electrolysis and “long-time” chronoamperometry. Digital simulation of the latter experiment gave a good fit between experimental and calculated results using a rate constant of 3.1 × 104 M−1 s−1 for photo-oxidation of cytochrome(II) c by P700.