Ferredoxin electrodes

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). 

Fig. 13. Representative Trumpet Plots for the [3Fe-4S]+/0 couple in native and D15N mutant forms of Azotobacter vinelandii ferredoxin I adsorbed on a PGE electrode. The plots for D15N also show the fits based on k0 t = 2.5 s-1. Note the intermediate region of the plot (pH 5.50) in which an oxidation peak is not observed because ET is gated. Data points shown in red are for the pH values indicated whereas data points shown in blue are for the uncoupled electron-transfer reaction occurring at pH > pffoiuater- Reproduced from Ref. (33) by permission of the Royal Society of Chemistry.

Figure 20 Cyclic voltammograms recorded at an edge-oriented pyrolitic graphite electrode in an aqueous solution (pH 8) of spinach ferredoxin. In the absence (a) and in the presence (b) of [Cr(NH3)6J3 +. Scan rate 0.02 V s ...

Figure 29 Cyclic voltammogram recorded at an edge-orientedpyrolitic graphite electrode in an aqueous solution (pH 6.4) of the 7Fe ferredoxin of Sulfolabus acidocaldarius in the presence of neomicin sulfate...

Figure 4.40 Voltammograms of films of different 7Fe ferredoxins on a pyrolitic graphite (edge) electrode, obtained at 0 °C (a) Azotobacter mnelandii (Fd I), at pH 7.0, with a scan rate of 20 mV s 1 (b) Desulfovibrio africanus (Fd III), at pH 7.0, with a scan rate of 191 mV s 1 (c) Sulfolobus acidocal-darius (Fd), at pH 7.4, with a scan rate of 10 mV s-1. In each case, an electroactive coverage of approximately one monolayer is obtained in the presence of polymyxin as the co-adsorbate. The signals, A, B and C refer to the redox couples [3FE-4S]+/0, [4FE-4S]2+/+ and [3FE-4S]0/,2+, respectively. Reprinted from Electrochim. Acta, 45, F.A. Armstrong and G.S. Wilson, Recent developments in faradaic bioelectrochemistry, 2623-2645, (Copyright) 2000, with permission from Elsevier Science...

CV = cyclic voltammetry Fd = ferredoxin HP = high potential iron-sulfur protein IRP = iron regulatory protein LS3 = l,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(/ -tolylthio)benzene (3-) Rd = rubredoxin SCE = standard calomel electrode tibt = 2,4,6-triisopropylphenyl Tp = tris(pyrazolyl)hydroborate (1-). 

Redox proteins are relatively small molecules. In biological systems they are membrane associated, mobile (soluble) or associated with other proteins. Their molecular structure ensures specific interactions with other proteins or enzymes. In a simplified way this situation is mimicked when electrodes are chemically modified to substitute one of the reaction partners of biological redox pairs. The major classes of soluble redox active proteins are heme proteins, ferredoxins, flavoproteins and copper proteins (Table 2.1). In most cases they do not catalyze specific chemical reactions themselves, but function as biological (natural) electron carriers to or between enzymes catalyzing specific transformations. Also some proteins which are naturally not involved in redox processes but carry redox active sites (e.g., hemoglobin and myoglobin) show reversible electron exchange at proper functionalized electrodes. 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

Robust voltammetry and in situ STM to molecular resolution have been achieved when the Au(lll)-electrode surfaces are modified by linker molecules, Fig. 8-10, prior to protein adsorption. Comprehensive voltammetric data are available for horse heart cyt and P. aeruginosa The latter protein, which we address in the next Section, has in a sense emerged as a paradigm for nanoscale bioelectrochemistry. We address first briefly two other proteins, viz. the electron transfer iron-sulfur protein Pyrococcus furiosus ferredoxin and the redox metalloenz5mie Achromobacter xylosoxidans copper nitrite reductase. 

Figure 8-12. Left Monolayer protein voltammetry of P. furiosus ferredoxin on an Au(lll)-electrode modified by a mercaptopropionic acid (MPA) SAM, cf, Fig. 8-lOC 5 mM phosphate buffer, pH 7.9. Scan rate 5 mV s . Middle In situ STM image of P. furiosus ferredoxin molecules in electron transport action on the same MPA-modified Au(lll)-electrode surface Ar-atmosphere. Working electrode potential -0.35 V (SCE), bias voltage -0.35 V. Tunneling current 0.10 nA. Right Schematic view of the P. furiosus ferredoxin molecule on the MPA-modified Au( 111 )-surface. From ref. 137 with permission.

Electrochemical regeneration of NAD(P)H represents another interesting method 134 361. The system involves electron transfer from the electrode to the electron mediator such as methyl viologen or acetophenone etc., then to the NAD(P)+ (which is catalyzed by an electrocatalyst such as ferredoxin-NADP reductase or alcohol dehydrogenase, etc.) [34l Other methods involve the direct reduction of NAD on the electrode[35). Both one-enzyme systems and two-enzyme systems have been reported. 

Iron-Sulfur Electron Transfer Proteins.31 Ferredoxins. These relatively small proteins (6,000-12,000) which contain non-heme iron, cysteine-sulfur, and so-called inorganic sulfur have redox potentials close to that of the standard hydrogen electrode. They appear to occur in all green plants, including algae, in all photosynthetic bacteria and protozoa and in some fermentative anaerobic bacteria. These molecules play an essential role as electron-transfer agents at the low-potential end of the photosynthetic process, but an exact chemical specification of their activity is still lacking. 

Recently it was proposed that the apparently slow heterogeneous electron-transfer rates for such proteins as cytochrome c, cytochrome b5, plasto-cyanin, and ferredoxin are an artifact of the experimental approach (25). Instead of assuming that protein molecules react at a planar and essentially homogeneous surface, it is assumed instead that movement of the protein occurs predominantly by radial diffusion to very small, specific sites. These sites are presumed to facilitate very rapid electron transfer at the reversible potential while the rest of the surface remains inactive. Thus, the modified electrode surface behaves like an array of microelectrodes. If this theory is used to treat previous data, much higher electron-transfer rate constants are obtained. Although this theory deserves more detailed scrutiny, it may serve... 

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