Surface modifiers for the promotion of direct electrochemistry of cytochrome c

Interface analysis in biosensor design

In a survey, the analytical tools to characterise and optimise properties and stabilities of interfaces in thin film biosensors are discussed. After an introduction to microscopic and spectroscopic techniques and different transducers, case studies are presented. They concern bioaffinity sensors with particular emphasis on biomimetic recognition structures, catalytic sensors, transmembrane sensors, cell sensors, and the ambitious goal of addressing individual biomolecular function units.

Direct electron transfer bioelectronic interfaces: application to clinical analysis

Bioelectronic interfaces based on direct electron transfer to proteins and enzymes immobilised at functional electrode surfaces are currently under development and the potential of two such systems for application to clinical measurement will be outlined. The first is the detection of free radical production via direct electrochemistry of cytochrome c immobilised covalently at modified gold electrodes. The redox protein cytochrome c has been immobilised covalently to gold electrodes surface-modified with N-acetyl cysteine via carbodiimide condensation. The electrodes thus produced were used to measure directly the enzymatic and cellular production of the superoxide anion radical (O2(-). The superoxide radical reduced the immobilised cytochrome c which was immediately re-oxidised by the surface-modified gold electrode poised at a potential of +25 mV (vs Ag/AgCl). The electron transfer rate constant (ket) of this process was 3.4 +/- 1.2 s(-1). The rate of current generation was directly proportional to the rate of O2(-) production. The essentially reagentless system produced was designed to be applied ultimately to continuous monitoring of free radical activity in vivo since there is evidence that oxygen-derived free radical species act as mediators which cause and perpetuate inflammation in disease states, including rheumatoid arthritis and neurodegenerative disorders. The second systems are pseudo-homogeneous immunoassays based on direct electron transfer to horseradish peroxidase. Horseradish peroxidase enzyme electrodes based on activated carbon (HRP-ACE) have been constructed by simple passive adsorption.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Disproportionation involving an organomercurial and a sulfhydryl-containing protein. Reaction between chloromercuryferrocene and bakers'-yeast cytochrome c

Reaction between iso-1 cytochrome c from bakers' yeast and chloromercuryferrocene, FcHgCl, does not result in simple replacement of the sulfhydryl hydrogen atom in Cys 102 with the ferrocenylmercury group, FcHg. Instead, this reaction yields the protein monomer modified at Cys 102 with an HgCl+ group and the protein dimer in which the thiolate groups of Cys 102 are bridged by a mercury(II) atom. These proteins and other organometallic products are identified by chromatographic, spectroscopic, and electrochemical methods. Organomercurials of the type RHgX and biological thiols can undergo not only substitution reactions, but disproportionation reactions as well.

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.

Direct electrochemistry of proteins. Investigations of yeast cytochrome c mutants and their complexes with cytochrome b5

Direct electrochemistry of site-specific mutants of yeast iso-1-cytochrome c (cyt c) and their complexes with bovine cytochrome b5 (cyt b5) has been investigated at edge-plane pyrolytic graphite (EPG) and bis(4-pyridyl)-disulphide-modified gold electrodes. Structure/function relationships have been investigated with the particular aim of clarifying the factors controlling the interactions of proteins at electrode/electrolyte interfaces and the determinants for direct electrochemistry in ternary protein/protein/electrode adducts, e.g. cyt c/cyt b5/EPG. Investigations of the cyt c mutants alone revealed a variety of electrochemical responses: all the mutants show similar voltammetric reversibility at modified gold electrodes, whereas at EPG electrodes the reversibility follows the order: Asn52Ile-Cys102Thr greater than Cys102Thr greater than Asn52Ala-Cys102Thr. Mid-point potentials follow the order: Arg13Ile (+60 +/- 5 mV vs. standard calomel electrode) greater than Cys102Thr (+40 +/- 5 mV) greater than Lys27Gln (+30 +/- 5 mV) approximately Lys72Asp (+30 +/- 5 mV) greater than Asn52Ala-Cys102Thr (+15 +/- 5 mV) greater than Asn52Ile-Cys102Thr (-10 +/- 5 mV). The structural basis for these differences is briefly discussed. When these mutants are bound to cyt b5, the differences in electrochemical response are greatly enhanced in the ternary cyt c/cyt b5/EPG adducts. A minimal analysis of these differences supports a model of multiple overlapping binding and recognition domains on cyt c which may be finely tuned to allow ternary complex formation so that a single-site variation could modify or abolish direct electrochemistry in the ternary adduct.

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 electrochemistry of the enzyme, methylamine dehydrogenase, from bacterium W3A1

The electrochemical response of methylamine dehydrogenase from bacterium W3A1 at edge-plane-oriented pyrolytic graphite (epg) and modified gold electrodes has been investigated. Quasi-reversible electron transfer has been observed. Variations in concentration of different cations and anions gave rise to both promotion and inhibition of the direct response. A catalytic response of the enzyme in the presence of methylamine has been observed at both an epg electrode and a 2,2'-dithiodiglycolic-acid-modified gold electrode surface, and the effects of various cations and anions on the catalytic peak current have been investigated. The spectroelectrochemical results obtained at an optically transparent thin-layer electrode, modified with 2,2'-dithiodiglycolic acid, are also reported. In the presence of 1,1'-dimethylferrocene-3-(1-ethanol-2-amine) (14.8 microM), the results reveal a midpoint potential of -148 mV for methylamine dehydrogenase from bacterium W3A1. This is in very close agreement to the value obtained in the cyclic voltammetric investigations of -140 mV.

Electrochemical, kinetic, and circular dichroic consequences of mutations at position 82 of yeast iso-1-cytochrome c

Replacement of Phe-82 in yeast iso-1-cytochrome c with Tyr, Leu, Ile, Ser, Ala, and Gly produces a gradation of effects on (1) the reduction potential of the protein, (2) the rate of reaction with Fe(EDTA)2-, and (3) the CD spectra of the ferricytochromes in the Soret region under conditions where contributions from the alkaline forms of these proteins are absent. The reduction potential of cytochrome c is lowered by as little as 10 mV (Tyr-82) or by as much as 43 mV (Gly-82; pH 6.0) as the result of these substitutions. The second-order rate constants for reduction of these cytochromes range from a low of 6.20 (2) x 10(4) for the Tyr-82 variant to a high of 14.8 x 10(4) M-1 s-1 for the Ser-82 variant [pH 6.0, 25 degrees C, mu = 0.1 M (sodium phosphate)]. Analysis of these rates by use of relative Marcus theory produces values of k11corr that range from 10.9 M-1 s-1 for the wild-type protein to 190 M-1 s-1 for the Gly-82 mutant [25 degrees C, mu = 0.1 M, pH 6.0 (sodium phosphate)]. Reinvestigation of the effect of substituting Phe-82 by a Tyr residue on the CD spectrum of the protein now reveals little alteration of the intense, negative Cotton effect in the Soret CD spectrum of ferricytochrome c. On the other hand, substitution of nonaromatic residues of various sizes at this position results in loss of this spectroscopic feature, consistent with previous findings.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for fast and discriminatory electron transfer of proteins at modified gold electrodes

The electrochemistry of the redox proteins, cytochrome c, cytochrome b5, plastocyanin and ferredoxin at modified gold electrodes has been examined on the basis that electron transfer takes place at electroactive sites which are microscopic in size. Using this model, it is now proposed that electrochemistry of these proteins occurs at suitably modified sites with fast rates at potentials near the standard redox potential. The microscopic model implies that redox proteins and enzymes take part in fast electron transfer at specific sites on the electrode, other sites being completely ineffective. This form of molecular recognition, i.e. the ability to discriminate between the different sites on an electrode surface, mimics homogeneous redox reactions wherein redox active proteins 'recognize' their biological partners in a very specific sense. Previously, protein electrochemistry has been interpreted via use of a macroscopic model in which the proteins are transported to the electrode surface by linear diffusion followed by quasi-reversible or irreversible electron transfer to the electrode surface. The microscopic model, which assumes that the movement of the protein occurs predominantly by radial diffusion to very small sites, would appear to explain the data more satisfactorily and be consistent with biologically important, homogeneous redox reactions which are known to be fast.

Electron transfer reactions of metalloproteins at peptide-modified gold electrodes

The electron transfer reactions of four small redox proteins, cytochrome c. ferredoxin, plastocyanin and azurin, have been investigated at novel peptide-modified gold electrodes. These proved to be effective and selective in facilitating electron transfer. Good, quasi-reversible electron transfer was achieved selectively at different peptide-protein configurations by changing the pH or the ionic strength of the solution. The use of peptides as promoters for protein electrochemistry opens up the possibility of designing very specific electrode surfaces for larger molecules like enzymes.

Modified electrode surface in amperometric biosensors

The electron transfer reactions of biological molecules are frequently very slow at ordinary electrodes. To overcome this problem, and thus to facilitate the direct coupling of biological redox reactions to electrodes for biosensor or bioelectronic applications, various types of modified electrode have been used. These include electrodes modified by the covalent attachment of species to the surface, by the reversible adsorption of promotors, or by the deposition of polymeric species, and the use of conducting polymers or conducting organic salts as electrode materials. Some of these different approaches are reviewed and their applications to biosensors and bioelectrochemistry are discussed.