Surface modifiers for the promotion of direct electrochemistry of cytochrome c

TiO2 phytate films as hosts and conduits for cytochrome c electrochemistry

Cytochrome c is accumulated into a film of TiO(2) nanoparticles and phytate by adsorption from an aqueous solution into the mesoporous structure. Stable voltammetric responses and high concentrations of redox protein within the TiO(2) phytate layer can be achieved. Two types of electrode systems are reported with (i) the modified TiO(2) phytate film between electrode and aqueous solution phase and (ii) the modified TiO(2) phytate film buried under a porous gold electrode ('porotrode'). The electrical conductivity of TiO(2) phytate films is measured and compared in the dry and in the wet state. Although in the dry state essentially insulating, the TiO(2) phytate film turns into an electrical conductor (with approximately 4 Omega cm specific resistivity assuming ohmic behaviour) when immersed in aqueous 0.1 M phosphate buffer solution at pH 7. The redox protein cytochrome c is therefore directly connected to the electrode via diffusion and migration of electrons in the three dimensional mesoporous TiO(2) phytate host structure. Electron transfer from cytochrome c to TiO(2) is proposed to be the rate-determining step for this conduction mechanism.

KCTCCA, a peptide-based facilitator for bioelectrochemistry

Electrochemical and scanning tunnelling microscopy (STM) studies have been carried out to investigate the suitability of the hexapeptide KCTCCA as a facilitator for bioelectrochemistry. The stable, quasi-reversible electrochemical response of cytochrome b562 on a KCTCCA modified gold electrode and the high degree of surface coverage of KCTCCA on gold (111), as observed by STM, indicate applicability of the molecule as an electrochemical facilitator

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.

Direct electron transfer of heme- and molybdopterin cofactor-containing chicken liver sulfite oxidase on alkanethiol-modified gold electrodes

Direct heterogeneous electron transfer (ET) of sulfite oxidase (SOx), a heme- and molybdopterin cofactor-containing intermembrane enzyme, was studied on alkanethiol-modified Au electrodes both with SOx entrapped between the modified Au electrode and a permselective membrane and with SOx adsorbed at the electrode surface, in the absence of any membrane. SOx in direct electronic communication with the electrode surface gave a quasi-reversible electrochemical signal with a midpoint potential of--120 mV vs Ag/AgCl corresponding to the redox transformations of the heme domain of SOx and with a heterogeneous ET constant in the order of 15 s(-1). The efficiency of the bioelectrocatalytic 2e- oxidation of sulfite catalyzed by SOx in direct ET exchange with the electrode was shown to depend essentially on the nature of the alkanethiol layer. Adsorption and orientation of SOx on an 11-mercapto-1-undecanol (MuD-OH) self-assembled monolayer, i.e., terminally functionalized with OH groups, provided efficient catalytic oxidation of sulfite, contrary to nonfunctionalized alkanethiols, e.g., 1-decanethiol, or alkanethiol layers terminally functionalized with NH2 groups. Comparative studies with short-chain alkanethiols, e.g., cysteamine and 2-mercaptoethanol, revealed an evidently different mode of adsorption of SOx on these layers, onto which SOx was not catalytically active. Coadsorption of MuD-OH and 11-mercapto-1-undecanamine improved the surface properties of the SAM, resulting in a higher surface coverage with bioelectrocatalytically active SOx but not in an increased apparent catalytic rate constant, kcat, ranging in the order of 18-24 s(-1) at pH 7.4. The achieved efficiency of SOx bioelectrocatalysis in direct ET reaction between the modified electrode and the enzyme approached the rates characteristic for the catalysis mediated by cytochrome c, the natural redox partner of SOx, thus implying the retention of the biological function of SOx under the heterogeneous electrode reaction conditions. Results obtained enable the development of a third-generation biosensor for sulfite monitoring.

The [Ru(CN)5(pyS)](4-) complex, an efficient self-assembled monolayer for the cytochrome c heterogeneous electron transfer studies

The organothiol 4-mercaptopyridine (pyS) has been used extensively as facilitator for the assessment of heterogeneous electron transfer reaction of cytochrome c (cyt c). Its efficiency, however, is strongly affected by the instability of the adlayer due to the C-S bond cleavage. The K(4)[Ru(CN)(5)(pyS)].3H(2)O complex was synthesized and characterized aiming its utilization as an inorganic self-assembled monolayer (SAM) that would enhance the gold adlayer stability. The SAM formed by this complex onto gold (RupySAu) was characterized by spectroscopic (FTIRRAS and SERS) and electrochemical (LSV) techniques. The ex situ vibrational SERS and FTIRRAS spectra data of this SAM formed onto gold suggest a sigma interaction between the gold and sulfur atoms of the complex, inducing a perpendicular arrangement in relation to the surface normal. Additionally, SERS and FTIRRAS spectra performed for freshly prepared RupySAu adlayer and for large immersion times in the precursor solution have not shown any significant change that would reflect the degradation of the adlayer. The LSV desorption curves of this SAM indicate an enhancement in the C-S bond strength of the pyS ligand when coordinated to the [Ru(CN)(5)](3-) moiety. Comparatively to the data obtained for the desorption process of the pyS monolayer, the reductive desorption potential, E(rd), of the RupySAu presents a shift of -17 mV. This bond strength intensification leads to an increase in the stability of the monolayer. The voltammetric curves of cyt c carried out with the RupySAu electrode showed electrochemical parameters consistent with those reported for the native protein, as well as the maintenance of the electrochemical kinetic data after repetitive cycles. The results all together suggest that the pi back-bonding effect from the [Ru(CN)(5)](3-) metal center plays an important role in the stability of the RupySAu adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.

Study of Surface-Enhanced Infrared Spectroscopy

In this paper we report a versatile and effective strategy to attain strong surface-enhanced infrared absorption by employing a sandwich system consisting of metal island films and self-assembled monolayers (SAMs) of 4-pyridinethiol. The observed larger enhancement factor stems from coupling of the electric fields induced by excitation of the surface plasmon resonance of the overlayer and underlayer Au island films, and from enhanced chemical interactions of the Au island films and the pyridine molecule in the sandwiched structures, compared to the corresponding SAM-Au configuration. Copyright 2001 Academic Press.

Characterization of an alkaline transition intermediate stabilized in the Phe82Trp variant of yeast iso-1-cytochrome c

In general, mutation of the phylogenetically conserved residue Phe82 in yeast iso-1-cytochrome c destabilizes the native conformation of the protein by facilitating the ligand exchange reactions that are associated with the alkaline conformational transitions of the ferricytochrome. Of the Phe82 variants surveyed thus far, Phe82Trp is unique in that it adopts a thermodynamically stable, high-spin conformation at mildly alkaline pH. This species exhibits spectroscopic features that can only be detected transiently in other ferricytochromes c within the first 100 ms immediately after a pH-jump from neutrality to pH >10. Spectroscopic characterization of this high-spin reaction intermediate suggests that in addition to an obligatory pentacoordinate heme iron, a group within the heme pocket coordinates the heme iron but is then replaced either by Met80, to revert to the native conformation, or by Lys73 or Lys79, to yield one of the conventional alkaline conformers. Evidence is presented to suggest that this group is either a hydroxide ion or Tyr67 rather than a loosely bound Met80.

Voltammetry of a flavocytochrome c(3): the lowest potential heme modulates fumarate reduction rates

Iron-induced flavocytochrome c(3), Ifc(3), from Shewanella frigidimarina NCIMB400, derivatized with a 2-pyridyl disulfide label, self-assembles on gold electrodes as a functional array whose fumarate reductase activity as viewed by direct electrochemistry is indistinguishable from that of Ifc(3) adsorbed on gold or graphite electrodes. The enhanced stability of the labeled protein's array permits analysis at a rotating electrode and limiting catalytic currents fit well to a Michaelis-Menten description of enzyme kinetics with K(M) = 56 +/- 20 microM, pH 7.5, comparable to that obtained in solution assays. At fumarate concentrations above 145 microM cyclic voltammetry shows the catalytic response to contain two features. The position and width of the lower potential component centered on -290 mV and corresponding to a one-electron wave implicates the oxidation state of the lowest potential heme of Ifc(3) as a defining feature in the mechanism of fumarate reduction at high turnover rates. We propose the operation of dual pathways for electron transfer to the active site of Ifc(3) with the lowest potential heme acting as an electron relay on one of these pathways.

Electrochemical studies of a truncated laccase produced in Pichia pastoris

The cDNA that encodes an isoform of laccase from Trametes versicolor (LCCI), as well as a truncated version (LCCIa), was subcloned and expressed by using the yeast Pichia pastoris as the heterologous host. The amino acid sequence of LCCIa is identical to that of LCCI except that the final 11 amino acids at the C terminus of LCCI are replaced with a single cysteine residue. This modification was introduced for the purpose of improving the kinetics of electron transfer between an electrode and the copper-containing active site of laccase. The two laccases (LCCI and LCCIa) are compared in terms of their relative activity with two substrates that have different redox potentials. Results from electrochemical studies on solutions containing LCCI and LCCIa indicate that the redox potential of the active site of LCCIa is shifted to more negative values (411 mV versus normal hydrogen electrode voltage) than that found in other fungal laccases. In addition, replacing the 11 codons at the C terminus of the laccase gene with a single cysteine codon (i.e., LCCI-->LCCIa) influences the rate of heterogeneous electron transfer between an electrode and the copper-containing active site (k(het) for LCCIa = 1.3 x 10(-4) cm s(-1)). These results demonstrate for the first time that the rate of electron transfer between an oxidoreductase and an electrode can be enhanced by changes to the primary structure of a protein via site-directed mutagenesis.