Jacques Moiroux

Immobilization of glucose oxidase on a carbon surface derivatized by electrochemical reduction of diazonium salts

Glassy carbon disk electrodes grafted with superficial 4-phenylacetic groups through electrochemical reduction of the corresponding diazonium salt were used for glucose oxidase covalent immobilization. Protein attachment was confirmed by means of X-ray photoelectron spectroscopy and the activity of the enzyme modified electrode was ascertained by performing enzymatic electrocatalysis. The present method does not cause any roughening of the electrode surface and concomitant increase of a background current thus allowing precise measurements using transient electrochemical techniques such as cyclic voltammetry. As diazonium salts (which upon reaction provide the anchoring site) can be synthesized with various substituents, it is conceivable that customized attachment of enzymes and biochemical reagents could be designed.

Electrochemical Measurement of Lateral Diffusion Coefficients of Ubiquinones and Plastoquinones of Various Isoprenoid Chain Lengths Incorporated in Model Bilayers

The long-range diffusion coefficients of isoprenoid quinones in a model of lipid bilayer were determined by a method avoiding fluorescent probe labeling of the molecules. The quinone electron carriers were incorporated in supported dimyristoylphosphatidylcholine layers at physiological molar fractions (<3 mol%). The elaborate bilayer template contained a built-in gold electrode at which the redox molecules solubilized in the bilayer were reduced or oxidized. The lateral diffusion coefficient of a natural quinone like UQ10 or PQ9 was 2.0 +/- 0.4 x 10(-8) cm2 s(-1) at 30 degrees C, two to three times smaller than the diffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be determined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ2 or PQ2 to UQ10, the diffusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior predicted by a continuum fluid model in which the isoprenoid chains are taken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The present study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer.

Electrochemical reduction of NAD+ and pyridinium cations adsorbed at the mercury water interface. Electrochemical behavior of adsorbed pyridinyl radicals

Abstract Neutralization of the positive charge of the pyridinium cation (Pyr + ) ads adsorbed at the mercury/water interface following electrochemical reduction is likely to provoke a very fast flat-to-perpendicular reorientation of the adsorbed species, thus rendering the perpendicularly adsorbed radical (Pyr ) ads electrochemically inactive. Therefore, it cannot be ascertained by means of cyclic voltammetry whether dimerization of the (Pyr ) ads radicals, which occurs in solution, also occurs at the mercury/water interface.

Oxydation électrochimique des aldéhydes en solution aqueuse: I. Oxydation de solutions aqueuses basiques d'aldéhydes à des électrodes d'or et de platine

Abstract Voltemmetric experiments have been performed upon 9 aldehydes in aqueous basic solutions from pH 9 to pH 14, at rotating gold and platinum electrodes. The potential of the first oxidation process, which is either a wave or a peak, depends linearly on pH. It is identical for all the studied aldehydes but differs at both electrodes. From the relation between the maximal intensity and pH, it can be concluded that the electroactive species is in all cases the geminal diol anion RCHOHO − . In order to explain all the experimental results we propose the following assumption: aldehyde oxidation proceeds from a chemical reaction between adsorbed monoionized aldehyde hydrate and superficial metal oxides.

Construction of multicomponent catalytic films based on avidin-biotin technology for the electroenzymatic oxidation of molecular hydrogen

Two methods based on the avidin-biotin technology were developed for the multimonolayer immobilization of Desulfovibrio gigas hydrogenase on glassy carbon or gold electrodes. In both methods the molecular structure of the modified interface was the result of a step-by-step process. The first method alternates monolayers of avidin and biotinylated hydrogenase, the mediator (methyl viologen) being free to diffuse in the structure. In the second method, the avidin monolayers were used to immobilize both the biotinylated enzyme and a long-chain biotinylated viologen derivative. The viologen head of this hydrophilic arm shuttles the electrons between the electrode and the enzyme. The modified electrodes were evaluated for the electroenzymatic oxidation of molecular hydrogen, which has interest for the development of enzymatic fuel cells. The parameters that affect the current density of mediated oxidation of H(2) at the modified electrodes was studied. The second structure, which has given typical catalytic currents of 25 microA per cm(2) for 10 monolayers, was found clearly less efficient than the first structure (500 microA per cm(2) for 10 monolayers). In both methods the catalytic currents increased linearly with the number of monolayers of hydrogenase immobilized, which indicates that the multilayer structures are spatially ordered.

Kinetics of redox conversion at a gold electrode of water-insoluble ubiquinone (UQ(10)) and plastoquinone (PQ(9)) incorporated in supported phospholipid layers

The cyclic voltammetry of UQ(10) and PQ(9) incorporated in a supported lipid bilayer and reacting at a gold electrode is quantitatively analyzed assuming that the electrochemical process consists of a nine-member square scheme and using the approach developed earlier by Laviron. The striking change affecting the shape of the cathodic peak in the cyclic voltammograms when pH is increased from less than 7.5 to higher values reflects a shift from kinetic control of the cathodic process by the equivalent first electron exchange to kinetic control by the equivalent second electron exchange. Reasonable values are found for the thermodynamic and kinetic characteristics of the electrochemical process.

Electrochemical behavior of 1,2-dithiole-3-thiones

Abstract The polarographic behavior of dithiole-thiones in DMF depends on the nature of substituent R4. When R4 is a methyl group a 2 e cathodic wave is observed. When R4 is H, two, apparently 1 e, cathodic waves are observed. The first can be ascribed to a reversible monoelectronic reduction yielding an organomercurial. The second in fact corresponds to the occurrence of the same 2 e cathodic process as observed when R4 is a methyl group. The 2 e process occurs at ca. −1050 mV vs. aqueous SCE and this potential does not markedly depend on the nature of R4 and/or R5. The 2 e reduction product is either the cyclic dianion (A2−) when R4 is H and R5 is phenyl group, or a mixture of (A2−) and A2− when R4 and/or R5 is a methyl group. The redox couple (A2−)/A exhibits reversible behavior at the mercury electrode.

Consequences of acceptor effects on the polarization of oxo and imine bonds in acyl and R-CO-COOH compounds: Part III. Acid-base equilibria antecedent to the reduction of α-ketoacids at the DME

Abstract It is usually assumed that in α-ketoacids (R-CO-COO−), when the pH increases, the antecedent protonation occurs at the carboxylate group. A comparison of the cathode half-wave potentials and the ratio ΔE1/2/ΔpH as well as the determination of Tafts polar substituents δCOO−* and δCOOH* lead us to postulate that, in neutral media, the recorded recombination current corresponds to antecedent protonation of the carbonyl function. The antecedent protonation of the carboxylate group only occurs in a more acid medium (1 The hydrate is not reducible because its protonation, followed by hydration, only occurs at the electrode at pH lower than 1.

Mechanism of the eletrochemical reduction of hydroxyiminoanthraquinones in DMF

Abstract Depending on the structure of the hydroxyiminoanthraquinone, the radical anion formed upon addition of one electron can either be stable in aprotic media, as already reported in the case of hydroxyanthraquinones, or protonated by the parent compound. Such a self-protonation induces the disproportionation of the semi-iminoquinone which becomes a shortlived species. The latter mechanism is studied in detail: it corresponds to a transition between the DISP1 and DISP2 regimes observed experimentally under mixed kinetic-diffusion control. The pertinent rate constants have been determined.

Electrochemistry of synthetic analogues of NAD dimers

Abstract As shown by direct and redox catalysis electrochemical methods, the cation A + (NAD + ) and the dimer A 2 form a chemically reversible redox couple, but with a large separation between the effective reduction potential of A + and the oxidation potential of A 2 . One cause of this large separation is that the reduction of A + into A 2 involves the irreversible radical-radical dimerization of two A . radicals whereas the oxidation of A 2 into 2A + involves the irreversible cleavage of the cation radical A 2 .+ into A + and A . . The reduction of A + is kinetically controlled by dimerization of the A . radicals. In contrast, mixed kinetic control by electron transfer and cleavage or total control by electron transfer is observed in the oxidation of A 2 .