Electrochemistry of Cytochromes

Cytochromes are iron-porphyrin based (hemo)proteins ubiquitous both 

Further classifications among cytochromes belonging to the same class are made using symbols related to the order of their participation in electron-transfer chains or to the wavelength of their maximum absorbance. 

The different cytochromes display rather similar redox activity, i.e. the reversible Fe(III) zzt Fe(II) process, but their redox potentials change with the peripheral substituents of the Fe(III)-porphyrin (particularly, as a function of the axially coordinated groups). 

One may deduce that, based on the absence of significant structural changes at the active site, the electron transfer process should be fast (which means electrochemically reversible). 

When using electrochemistry to monitor directly the Fe(III)/Fe(II) reduction it is important to keep in mind that cytochrome c, at pH 7, in the oxidised state has an overall charge of +7.5, whereas in the reduced state it has a charge of + 6.5.  

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They found that they could assign the compounds tested to four main classes depending on the observed electrochemistry of cytochrome c (see Figure 3.89). 

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. 

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). 

H. Allen, O. Hill, N.I. Hunt, and A.M. Bond, 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. J. Electroanal. Chem. 436, 17-25 (1997). 

J.M. Sevilla, T. Pineda, A.J. Roman, R. Madueno, and M. Blazquez, The direct electrochemistry of cytochrome c at a hanging mercury drop electrode modified with 6-mercaptopurine. J. Electroanal. Chem. 451, 89-93 (1998). 

F.A. Armstrong, A.M. Bond, H.A.O. Hill, B.N. Oliver, and I.S.M. Psalti, 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. J. Am. Chem. Soc. 111,91859189 (1989). 

O. Ikeda, M. Ohtani, T. Yamaguchi, and A. Komura, Direct electrochemistry of cytochrome c at a glassy carbon electrode covered with a microporous alumina membrane. Electrochim. Acta 43, 833—839 (1998). 

These arguments were apparently in contradiction with electrochemical results reported by Cruanes et al. (158), according to which the reduction of cytochrome c is accompanied by a volume collapse of 24 cm3 mol-1. This value is so large that it almost represents all of the reaction volume found for the investigated reactions discussed above. A reinvestigation of the electrochemistry of cytochrome c as a function of pressure, using cyclic and differential pulse voltammetric techniques (155), revealed a reaction volume of -14.0 0.5 cm3 mol-1 for the reaction... 

Recently, an inorganic promoter as [Ru(CN)5(Spy)]4 (Spy = 4-thio-pyridine) adsorbed on a gold electrode surface also proved to be very effective in the direct electrochemistry of cytochrome c the tetraanion not... 

Bipyridine resembles nicotine in its pharmacological properties but is not as active. The 3,4 -bipyridine derivative 113 known as amrinone and its relatives are of interest as cardiotonic agents. 4,4 -Bipyridine has been tested as an insecticide, but it is not of practical value.It is used in the study of the electrochemistry of cytochrome c and acts as a polymerization catalyst or hardening agent for various resins. l-Hexyl-4,4 -bipyridinium salts are especially effective as electron carriers in photochemical hydrogen producing systems. l,l -Dimethyl-4,4 -bipyridinium (92 R = R = CHj) and l,l -dibenzyl-4,4 -bipyridinium... 

The electrochemistry of cytochrome c is attracting much attention,690 in particular the development of electrodes for cytochrome c, which either have a redox mediator bound to the surface, or utilize a solution mediator. Examples are a platinum gauze electrode functionalized with 2,3,4,5-tetramethyl-l-(dichlorosilyl)methyl[2]ferrocenophane,691 and 4,4 -bipyridyl, which serves as a bridge between cytochrome c and the electrode.692... 

Subsequently, Taniguchi and co-workers found (13) that bis(4-pyridyl)disulfide (SS-Bipy) adsorbed so strongly on gold that a predip in the SS-Bipy solution for several minutes was sufficient to give excellent electrochemistry of cytochrome c in a promoter-free solution. However, the adsorption behavior of this promoter was interpretated by the Frumkin isotherm rather than by the Langmuir isotherm. As... 

A new group of promoters has recently been employed (22) in achieving the direct electrochemistry of cytochrome c and other proteins. Several cysteine-containing peptides for example, (Cys-Glu)2, proved very successful, as they bind tightly to the gold electrode via the sulfur... 

The opportunity of obtaining direct electrochemistry of cytochrome c and other metalloproteins at various electrode materials such as modified gold and pyrolytic graphite has led to numerous reports of heterogeneous electron transfer rates and mechanisms between the protein and the electrode. In all the reports, Nicholson s method (37) was employed to calculate rate constants, which were typically within the range of 10" -10 cm sec with scan rates varying between 1 and 500 mV sec This method is based on a macroscopic model of the electrode surface that assumes that mass transport of redox-active species to and from the electrode occurs via linear diffusion to a planar disk electrode and that the entire surface is uniformly electroactive, i.e., the heterogeneous electron transfer reaction can take place at any area. 

Another system (77) in which the direct electrochemistry of cytochrome c has been usefully exploited is in driving the turnover of yeast flavocytochrome 62 (L-lactate/cytochrome c oxidoreductase E.C. 1.1.2.3). A second-order rate constant of 5 x 10 M sec" was obtained for turnover of the cytochrome c by the enzyme, which agreed well with the value from conventional spectrophotometric methods. 

Eddowes, M.J., Hill, H.A.O., and Uosaki, K. (1980) The electrochemistry of cytochrome c. Investigation of the mechanism of the 4,4 -bipyridyl surface modified gold electrode. 

CJ McNeil, KA Smith, P Bellavite, JV Bannister. Application of the electrochemistry of cytochrome-c to the measurement of superoxide radical production. Free Radical Res Commun 7 89-96, 1989. 

Hill, H. A. O., Page, D. J., Walton, N. J., Intra-Molecular Hydrogen Bonding in Surface-Modified Gold Electrodes and the Effect of Specific Anions on the Electrochemistry of Cytochrome C , J. Electroanal. Chem. 208 (1986) 395-400. 

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