The direct electrochemistry of redox proteins at metal oxide electrodes

Enhanced interfacial electron transfer between thylakoids and RuO2 nanosheets for photosynthetic energy harvesting

The harvesting of photosynthetic electrons (PEs) directly from photosynthetic complexes has been demonstrated over the past decade. However, their limited efficiency and stability have hampered further practical development. For example, despite its importance, the interfacial electron transfer between the photosynthetic apparatus and the electrode has received little attention. In this study, we modified electrodes with RuO₂ nanosheets to enhance the extraction of PEs from thylakoids, and the PE transfer was promoted by proton adsorption and surface polarity characteristics. The adsorbed protons maintained the potential of an electrode more positive, and the surface polarity enhanced thylakoid attachment to the electrode in addition to promoting ensemble docking between the redox species and the electrode. The RuO₂ bioanode exhibited a five times larger current density and a four times larger power density than the Au bioanode. Last, the electric calculators were successfully powered by photosynthetic energy using a RuO₂ bioanode.

A comparison of the higher order harmonic components derived from large-amplitude Fourier transformed ac voltammetry of myoglobin and heme in DDAB films at a pyrolytic graphite electrode

A debate as to whether heme remains bound or is released in myoglobin molecules incorporated into a didodecyldimethylammonium bromide (DDAB) film adhered to a pyrolytic graphite electrode has prompted a comparison of their electrochemistry by the highly sensitive large-amplitude Fourier transformed ac voltammetric method. The accessibility of third, fourth, and higher harmonic components that are devoid of background current and the enhanced resolution relative to that available in dc voltammetry have allowed a detailed comparison of the Fe(III)/Fe(II) and Fe(II)/Fe(I) redox processes of myoglobin and heme molecules to be undertaken as a function of buffer composition and pH and in the presence and absence of NaBr in the buffer and/or film. Under most conditions examined, only very subtle differences, in the Fe(III)/Fe(II) process were found, implying this process cannot be used to indicate the intactness or otherwise of myoglobin in myoglobin-DDAB films. In contrast, higher order ac harmonics obtained from myoglobin-DDAB and heme-DDAB films reveal pH dependent differences with respect to the Fe(II)/Fe(I) couple. Analysis of the ac harmonics, and with the hypothesis that the Fe(II)/Fe(I) process reflects the myoglobin state, suggests that the majority of the iron heme is released from myoglobin-DDAB (pH 5.0, no NaBr) films in contact with pH 5.0 (0.1 M sodium acetate) buffer solution devoid of or containing NaBr. However, myoglobin films prepared with pH 5.0 buffer containing NaBr shows significant difference in the higher harmonic shapes and midpoint potentials in the Fe(II)/Fe(I) process relative to the case when heme molecules are used, although as noted in other studies, a significant fraction of the Mb is rendered electroinactive in the presence of NaBr. The voltammetric responses of myoglobin and heme-DDAB (pH 5.0) films in contact with pH 7.0 (0.1 M) phosphate buffer solution also exhibit significant differences in the Fe(II)/Fe(I) redox couple in the higher harmonics in contrast to a report [de Groot, M.T.; Merkx, M.; Koper, M. T. M. J. Am. Chem. Soc. 2005, 127, 16224] that claimed identical midpoint potentials apply to both films under conditions of dc cyclic voltammetry. The FT-ac voltammetric data therefore suggest that a substantial fraction of myoglobin in myoglobin-DDAB (pH 5.0) films in contact with pH 7.0 phosphate buffer solution remains intact. No evidence of a catalytic effect that enhanced the released of heme from myoglobin was found at the pyrolytic graphite electrode surface. In summary, higher harmonic ac voltammetric data indicate that the Fe(II)/Fe(I) process but not the Fe(III)/Fe(II) reflects the state of myoglobin in DDAB films. On this basis, films prepared at pH 5.0 should include NaBr, or else films should be prepared at neutral pH to achieve films with myoglobin remains in its intact near native state when a myoglobin-DDAB film is confined to a graphite electrode surface. Otherwise, the release of heme in myoglobin molecules incorporated into a DDAB film is likely to be a dominant reaction pathway.

Syntheses of fully sulfonated polyaniline nano-networks and its application to the direct electrochemistry of cytochrome c

Fully sulfonated polyaniline nano-particles, nano-fibrils and nano-networks have been achieved for the first time by electrochemical homopolymerization of orthanilic acid using a three-step electrochemical deposition procedure in a mixed solvent of acetonitrile (ACN) and water. The diameter of the uniform nano-particles is about 60 nm, and the nano-fibrils can be organized in two-dimensional (2D) or three-dimensional (3D) non-periodic networks with good electrical contact. Average distance between contacts is about 850 and 600nm for a 2D and 3D system, respectively. The details of the poly(orthanilic acid) (POA) nano-structure were examined with a field emission scanning electron microscope (SEM). The structure and properties of POA were characterized with FTIR, UV-vis and electrochemical methods. The 3D POA nano-networks coated platinum electrode gave a direct electrochemical behavior of horse heart cytochrome c (Cyt c) immobilized on this electrode surface, a pair of well-defined redox waves with formal potential (E( degrees ')) of -0.032 V (versus Ag/AgCl) was achieved. The interaction between Cyt c and POA makes the formal potential shift negatively compared to that of Cyt c in solution. Spectrophotometric and electrochemical methods were used to investigate the interaction of Cyt c with POA. The immobilized Cyt c in the nano-networks POA film maintained its activity, showing a surface-controlled electrode process with the electron transfer rate constant (k(s)) of 21s(-1) and a of 0.53, and could be used for the electrocatalytic reduction of hydrogen peroxide. The quantitative determination of Cyt c by differential pulse voltammetry (DPV) using the fully sulfonated 3D POA nano-networks film coated platinum electrode was also studied.

Determination of myoglobin based on its enzymatic activity by stopped-flow spectrophotometry

A new method has been developed for the determination of myoglobin (Mb) based on its enzymatic activity for the oxidation of o-phenylenediamine (OPDA) with hydrogen peroxide. Stopped-flow spectrophotometry was used to study the kinetic behavior of the oxidation reaction. The catalytic activity of Mb was compared to other three kinds of catalyst. The time dependent absorbance of the reaction product, 2,3-diamimophenazine (DAPN), at a wavelength of 426 nm was recorded. The initial reaction rate obtained at 40 degrees C was found to be proportional to the concentration of Mb in the range of 1.0 x 10(-6) to 4.0 x 10(-9)mol L(-1). The detection limit of Mb was found to be 9.93 x 10(-10)mol L(-1). The relative standard deviations were within 5% for the determination of different concentrations of Mb. Excess of bovine serum albumin (BSA), Ca(II), Mg(II), Cu(II), glucose, caffeine, lactose and uric acid did not interfere.

Hydrogen peroxide sensor based on horseradish peroxidase immobilized on a silver nanoparticles/cysteamine/gold electrode

A third-generation hydrogen peroxide biosensor was prepared by immobilizing horseradish peroxidase (HRP) on a gold electrode modified with silver nanoparticles. A freshly-cleaned gold electrode was first immersed in a cysteamine-ethanol solution, and then silver nanoparticles were immobilized on the cysteamine monolayer, and finally HRP was adsorbed onto the surfaces of the silver nanoparticles. This self-assemble process was examined via atomic force microscopy (AFM). The immobilized horseradish peroxidase exhibited an excellent electrocatalytic response toward the reduction of hydrogen peroxide. The linear range of the biosensor was 3.3 microM to 9.4 mM, and the detection limit was estimated to be 0.78 microM. Moreover, the biosensor exhibited a fast response, high sensitivity, good reproducibility, and long-term stability.

Direct electrochemistry of the Desulfovibrio gigas aldehyde oxidoreductase

This work reports on the direct electrochemistry of the Desulfovibrio gigas aldehyde oxidoreductase (DgAOR), a molybdenum enzyme of the xanthine oxidase family that contains three redox-active cofactors: two [2Fe-2S] centers and a molybdopterin cytosine dinucleotide cofactor. The voltammetric behavior of the enzyme was analyzed at gold and carbon (pyrolytic graphite and glassy carbon) electrodes. Two different strategies were used: one with the molecules confined to the electrode surface and a second with DgAOR in solution. In all of the cases studied, electron transfer took place, although different redox reactions were responsible for the voltammetric signal. From a thorough analysis of the voltammetric responses and the structural properties of the molecular surface of DgAOR, the redox reaction at the carbon electrodes could be assigned to the reduction of the more exposed iron cluster, [2Fe-2S] II, whereas reduction of the molybdopterin cofactor occurs at the gold electrode. Voltammetric results in the presence of aldehydes are also reported and discussed.

Covalent attachment of osmium complexes to glucose oxidase and the application of the resulting modified enzyme in an enzyme switch responsive to glucose

Pyridine-based osmium complexes bearing either a carboxylate or aldehyde group were covalently attached to glucose oxidase and were shown to work as mediators for the reoxidation of the enzyme. For the complex containing the carboxylate group, the binding was made through carbodiimide coupling to the amine residues in the protein. For the complex containing the aldehyde group, the reductive coupling was carried out by condensation with the amino groups on the protein in the presence of sodium cyanoborohydride. Electrochemical studies show evidence for both intramolecular and intermolecular redox mediation for the electrochemical reoxidation of the modified glucose oxidases in the presence of glucose. The modified enzymes adsorbed on glassy carbon and platinum show different electrochemical responses for the two electrode materials, suggesting that orientation of the adsorbed enzyme is induced due to the interaction of the osmium complex with the different surfaces. Construction of enzyme switches based on these modified enzymes was carried out, and their responses were compared with those obtained using native glucose oxidase and a soluble redox mediator.

Recent progress in the electrochemistry of c-type cytochromes

C-type cytochromes are classified into two main groups: i) cytochromes which give fast electrochemical responses at the conventional electrodes in the absence of any promoter (eg multi-heme cytochromes c3); ii) cytochromes which need the presence of promoters or the use of modified electrodes to exhibit fast electrochemical responses (eg one-heme mitochondrial cytochrome c). In the latter case, careful design of electrode surface and composition of the solution are required for the attainment of rapid and reversible electron-exchange reactions. Some general considerations are given on the 'electrochemical model'. In particular, binding interactions between the electrode and the protein can take place in a similar manner to that occurring between physiological partner proteins. Electrochemistry when coupled to other physical techniques can give more complete insights in the relationship between the redox properties, structure and function of c-type cytochromes. In particular, in the case of polyheme cytochromes, promising results are expected from the study of site-directed mutagenesis-modified cytochrome c3.

Voltammetric studies of the catalytic electron-transfer process between the Desulfovibrio gigas hydrogenase and small proteins isolated from the same genus

The kinetics of electron transfer between the Desulfovibrio gigas hydrogenase and several electron-transfer proteins from Desulfovibrio species were investigated by cyclic voltammetry, square-wave voltammetry and chronoamperometry. The cytochrome c3 from Desulfovibrio vulgaris (Hildenborough), Desulfovibrio desulfuricans (Norway 4), Desulfovibrio desulfuricans (American Type Culture Collection 27774) and D. gigas (NCIB 9332) were used as redox carriers. They differ in their redox potentials and isoelectric point. Depending on the pH, all the reduced forms of these cytochromes were effective in electron exchange with hydrogenase. Other small electron-transfer proteins such as ferredoxin I, ferredoxin II and rubredoxin from D. gigas were tentatively used as redox carriers. Only ferredoxin II was effective in mediating electron exchange between hydrogenase and the working electrode. The second-order rate constants k for the reaction between reduced proteins and hydrogenase were calculated based on the theory of the simplest electrocatalytic mechanism [Moreno, C., Costa, C., Moura, I., Le Gall, J., Liu, M. Y., Payne, W. J., van Dijk, C. & Moura, J. J. G. (1993) Eur. J. Biochem. 212, 79-86] and the results obtained by cyclic voltammetry were compared with those obtained by chronoamperometry. Values for k of 10(5)-10(6) M-1 s-1 (cytochrome c3 as electron carrier) and 10(4) M-1 s-1 (ferredoxin II as the electron carrier) were determined. The rate-constant values are discussed in terms of the existence of an electrostatic interaction between the electrode surface and the redox carrier and between the redox carrier and a positively charged part of the enzyme.

Electrochemical studies of the hexaheme nitrite reductase from Desulfovibrio desulfuricans ATCC 27774

The electron-transfer kinetics between three different mediators and the hexahemic enzyme nitrite reductase isolated from Desulfovibrio desulfuricans (ATCC 27774) were investigated by cyclic voltammetry and by chronoamperometry. The mediators, methyl viologen, Desulfovibrio vulgaris (Hildenborough) cytochrome c3 and D. desulfuricans (ATCC 27774) cytochrome c3 differ in structure, redox potential and charge. The reduced form of each mediator exchanged electrons with nitrite reductase. Second-order rate constants, k, were calculated on the basis of the theory for a simple catalytic mechanism and the results, obtained by cyclic voltammetry, were compared with those obtained by chronoamperometry. Values for k are in the range 10(6)-10(8) M-1 s-1 and increase in the direction D. desulfuricans cytochrome c3-->D. vulgaris cytochrome c3-->methyl viologen. An explanation is advanced on the basis of electrostatic interactions and relative orientation between the partners involved. Chronoamperometry (computer controlled) offers advantages over cyclic voltammetry in the determination of homogeneous rate constants (faster, more accurate and better reproducibility). Direct, unmediated electrochemical responses of the hexaheme nitrite reductase were also reported.

Simulation of the electrochemical behavior of multi-redox systems. Current potential studies on multiheme cytochromes

The direct unmediated electrochemical response of the tetrahemic cytochrome c3 isolated from sulfate reducers Desulfovibrio baculatus (DSM 1743) and D. vulgaris (strain Hildenborough), was evaluated using different electrode systems [graphite (edge cut), gold, semiconductor (InO2) and mercury)] and different electrochemical methods (cyclic voltammetry and differential pulse voltammetry). A computer program was developed for the theoretical simulation of a complete cyclic voltammetry curve, based on the method proposed by Nicholson and Shain [Nicholson, R.S. & Shain, I. (1964) Anal. Chem. 36, 706-723], using the Gauss-Legendre method for calculation of the integral equations. The experimental data obtained for this multi-redox center protein was deconvoluted in to the four redox components using theoretically generated cyclic voltammetry curves and the four mid-point reduction potentials determined. The pH dependence of the four reduction potentials was evaluated using the deconvolution method described.

Biosensors for process control

Biosensors have been extensively studied during the last 20 years, and a myriad of laboratory biosensors have been developed. Improvements are required in biosensor design and performance before they become widely accepted in industrial process monitoring. However, as the biotechnology industry expands, biosensors may become more acceptable because, despite their limitations, they are the only devices capable of delivering the information required.

Direct electron transfer of redox proteins at the bare glassy carbon electrode

A simple system is presented for the microscale, direct voltammetry of redox proteins, typically 25 micrograms, in the absence of mediators and/or modifiers. The sample consists of a droplet of anaerobic solution laid onto an oversized disc of nitric-acid-pretreated glassy carbon as the working electrode. Very reproducible, Nernstian responses are obtained with horse heart cytochrome c. The midpoint potential Em (pH 7.0) is dependent on the ionic strength, ranging from $293 mV in 1 mM potassium phosphate to $266 mV in 0.1 M phosphate. At fixed buffer and cytochrome concentrations the magnitude of the voltammetric response is found to be independent of pH over six pH units around neutrality. It is suggested that the response of the present system is not complicated by pH-dependent properties of the electrode surface around physiological pH and, therefore, that the use of this system is practical in biochemically oriented studies. Direct, quasi-reversible responses have also been obtained at pH 7.0 (5 mM phosphate) from Desulfovibrio vulgaris. Hildenborough strain, tetraheme cytochrome c3 (pI = 10.0 at 4 C; 3 X Em = -0.32 mV, Em = -0.26 V), and cytochrome c553 (pI = 9.3; Em = +60 mV), and from Megasphaera elsdenii rubredoxin (pI congruent to 3; Em = -353 mV). The latter protein absorbs onto the glassy carbon surface, thus forming a system with possible applications in the electrochemical study of ferredoxin-linked enzymes.

The electrode reaction of Euglena gracilis cytochrome c-552 at edge-oriented pyrolytic graphite

The direct electron transfer reaction of Euglena gracilis cytochrome c-552 at edge-oriented pyrolytic graphite electrodes was determined by cyclic voltammetry to be quasi-reversible, stable and reproducible. The presence of a persistent layer of irreversibly adsorbed cytochrome c-552 on the electrode surface was detected in these experiments. Heterogeneous electron transfer rate constants for both the diffusing and adsorbed forms of the protein are reported, and mechanistic aspects are addressed. The applicability of cytochrome c-552 as a complementarily charged analog of eucaryotic cytochrome c in interfacial bioelectrochemical studies is discussed.

Electron transfer between the hydrogenase from Desulfovibrio vulgaris (Hildenborough) and viologens. 1. Investigations by cyclic voltammetry

The electron transfer kinetics between the hydrogenase from Desulvovibrio vulgaris (strain Hildenborough) and three different viologen mediators has been investigated by cyclic voltammetry. The mediators methyl viologen, di(n-aminopropyl) viologen and propyl viologen sulfonate differ in redox potential and in net charge. Dependent on the pH both the one- and two-electron-reduced forms or only the two-electron-reduced form of the viologens are effective in electron exchange with hydrogenase. Calculations of the second-order rate constant k for the reaction between reduced viologen and hydrogenase are based on the theory of the simplest electrocatalytic mechanism. Values for k are in the range of 10(6)-10(7) M-1 s-1 and increase in the direction propyl viologen sulfonate----methyl viologen----di(n-aminopropyl) viologen. An explanation is based on electrostatic interactions. It is proposed that the electron transfer reaction is the rate-determining step in the catalytic mechanism.