Cytochrome electrodes

It has been reported that a reversible one-electron transformation of cytochrome c occurs on an indium oxide electrode. Cytochrome c yields quasi-reversible maxima at EhO 0. 25 V on a gold electrode in the presence of 4,4 -bipyridine. 

A.2.2 Solid electrodes (gold, carbon, metal oxide electrodes). Cytochrome c is also adsorbed irreversibly at solid electrodes, which mostly results in irreversible kinetices of the electrode reaction. A nearly reversible reaction of strongly adsorbed cytochrome c is, however, observed at electrodes of metal oxides (doped metal oxide semiconductor electrodes, tin-doped indium oxide electrode) [202, 203]. 

In this report we address cytochrome c, which is the most well-understood electron transfer protein. It has occupied a prominent role in interfacial electrochemical investigations due to its high degree of structural and reactivity characterization and its ready availability and purification. Cytochrome c has been found to react in a reproducible, quasi-reversible manner at a number of solid electrode surfaces. Electrode surfaces which have been most successful in this regard are metal oxides and chemically modified metal electrodes . Cytochrome c also has the potential to react readily at unmodified metal electrodes, as exemplified by the recent report of a stable, quasi-reversible reaction at bare silver . 

FIG. 26 Cyclic voltammograms of 40 monolayers of Langmuir-Schaefer films of cytochrome P450SCC on indium-tin oxide glass plate (ITO) in 10 mM phosphate buffer at a scan rate of 20 mV/s between 0.4 and —0.4 V vs. Ag/AgCl. LS films on ITO worked as the working electrode, platinum as the counter, and Ag/AgCl as the reference electrode. Cholesterol dissolved in X-triton 100 was added 50 p.1 at a time (1) with cholesterol, (2) 50 p.1 of cholesterol, (3) 100 p.1 cholesterol, and (4) 150 p.1 of cholesterol. 

Armstrong FA, Bond AM, Buchi FN, Hanmett A, Hill HAO, Lannon AM, Lettington OC, Zoski CG. 1993. Electrocatalytic reduction of hydrogen-peroxide at a stationary pyrol3ftic-graphite electrode surface in the presence of cytochrome-c peroxidase— A description based on a microelectrode array model for adsorbed enzyme molecules. Analyst 118 973-978. 

Mayer D, Ataka K, Heberle J, Offenhaeusser A. 2005. Scanning probe microscopic studies of the oriented attachment and membrane reconstitution of cytochrome c oxidase to a gold electrode. Langmuir 21 8580-8583. 

A redox reaction is a special case of the equilibrium reaction of A + B in Equation 13.1 B is now a reducible group in a biomolecule with an EPR spectrum either in its oxidized or in its reduced state (or both), and A is now an electron or a pair of electrons, that is, reducing equivalents provided by a natural redox partner (a reductive substrate, a coenzyme such as NADH, a protein partner such as cytochrome c), or by a chemical reductant (dithionite), or even by a solid electrode ... 

Figure 3.89 Cyclic voltammograms of 500 pm cytochrome c at a gold electrode modified by (a) 2-mercaptopyridine, (b> 2-mercaptosuccinic acid, 4,4 -dithiobis(butanoic acid), (d) 4-mercaploaniline. pH 7.0 phosphate buffer +0.1 M NaC104. Scan rale 50mVs . From Allen...

The results clearly showed the importance of directionality. 4-lhiopyridine is an excellent promoter, while the 2 isomer shows no activity. After adsorption the 4 isomer has the pyridine nitrogen direction out into solution while the 2 isomer points the N back towards the electrode where it is available for adsorption to the gold rather than the cytochrome c. 

Figures 3.91(a) and (b) show cyclic voltammograms of the SERS electrode in aqueous solutions of the SSBipy and PySH. In the potential range —0.3 V to 0.3 V vs. SCE, which is the range of interest for the reversible reduction of cytochrome c, no notable faradaic currents were observed for either of these species. However, at potentials < —0.4 V SSBipy is reduced to PySH and the PySH so formed is re-oxidised to SSBipy at potentials >0.1 V. Similarly, PySH is oxidised to SSBipy at potentials >0.1 V and this product re-reduced at potentials < —0.4 V.

Figure 3.92 shows SERS spectra of adsorbed SSBipy and PySH at 0 V in the absence of the solution species, together with the Raman spectra of PySH in solution and crystalline SSBipy. The activities of the modified electrodes were first confirmed in solution containing cytochrome c. 

At this point it was clear how SSBipy was adsorbed on the electrode and the timescale over which this occurred, as well as the role of the concentration of the initial solution. However, the actual mode of action of the adsorbed species still remained somewhat obscure. An important insight into this was provided by the work of Hill et al. in 1987 who studied the effect of partial substitution of the layer of adsorbed promoter on the electrochemistry of cytochrome c. 

Figure 3.96 The effect of increasing time of exposure (as indicated) of a gold electrode once-modified with SSBipy to thiophenol on the cyclic voltammetry of horse heart cytochrome t (0.4mM). 20 mM sodium phosphate/0.1 M NaCI04 pH 7.0. Scan rate 20mVs l. From Hill...

F. Lisdat, B. Ge, E. Ehrentreich-Forster, R. Reszka, and F.W. Scheller, Superoxide dismutase activity measurement using cytochrome c-modified electrode. Anal. Chem. 71,1359—1365 (1999). 

K. Tammeveski, T. Tenno, A.A. Mashirin, E.W. Hillhouse, P. Manning, and CJ. McNeil, Superoxide electrode based on covalently immobilized cytochrome c modeling studies. Free Radical Biol. Med. 25, 973-978 (1998). 

K.V. Gobi and F. Mizutani, Efficient mediatorless superoxide sensors using cytochrome c-modified electrodes. Surface nano-organization for selectivity and controlled peroxidase activity. J. Electroanal. Chem. 484, 172-181 (2000). 

J.C. Cooper, G. Thompson, and C.J. McNeil, Direct electron transfer between immobilized cytochrome c and gold electrodes. Mol. Cryst. Liq. Cryst. 235, 127-132(1993). 

M.K. Beissenhirtz, F.W. Scheller, and F. Fisdat, A superoxide sensor based on a multilayer cytochrome c electrode. Anal. Chem. 76, 4665-4671 (2004). 

K.D. Gleria, H.A.O. Hill, V.J. Lowe, and D.J. Page, Direct electrochemistry of horse-heart cytochrome c at amino acid-modified gold electrodes. J. Electroanal. Chem. 213, 333-338 (1986). 

J. Wang, M. Li, Z. Shi, N. Li, and Z. Gu, Direct electrochemistry of cytochrome c at a glassy carbon electrode modified with single-wall carbon nanotubes. Anal. Chem. 74, 1993-1997 (2002). 

G.C. Zhao, Z.Z. Yin, L. Zhang, and X.W. Wei, Direct electrochemistry of cytochrome c on a multi-walled carbon nanotube modified electrode and its electrocatalytic activity for the reduction of H2O2. Electrochem. Commun. 7, 256-260 (2005). 

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