Showing papers on "Redox published in 1987"

Direct electrical communication between chemically modified enzymes and metal electrodes. I. Electron transfer from glucose oxidase to metal electrodes via electron relays, bound covalently to the enzyme

Abstract: Glucose-reduced glucose oxidase does not directly transfer electrons to conventional electrodes because the distance between its redox centers and the electrode surface exceeds, even on closest approach, the distance across which electrons are transferred at sufficient rates. Therefore, electrical communication between the redox centers of this enzyme and electrodes required either the presence, and diffusion to and from the enzyme's redox center, of O/sub 2/ and H/sub 2/O/sub 2/, or the presence of members of a redox couple, or the use of special electrodes like TTF/TCNQ. They show here that direct electrical communication between the redox center of a large enzyme molecule and a simple metal electrode can be established through chemical modification of the enzyme. When a sufficient number of electron-relaying centers are attached through covalent bonding to the protein of glucose oxidase, electrons are transferred from the enzyme's redox centers to relays that are closer to the periphery of the enzyme. Because some of the relays are located sufficiently close to the enzyme's surface, electrons are transferred at practical rates to the electrode. As a result, a glucose-concentration-dependent current flows in an electrochemical cell made with conventional electrodes when the electrolytic solution contains the relay-modified enzyme. Such amore » current does not flow when the solution contains the natural enzyme. Specifically, electrical communication is established between the FAD/FADH/sub 2/ centers of glucose oxidase and gold, platinum, or carbon electrodes through the covalent bonding of an average of 12 molecules of ferrocenecarboxylic acid per glucose oxidase molecule.« less

Determination of ion populations and solvent content as functions of redox state and pH in polyaniline

Abstract: The piezoelectric quartz crystal microbalance (QCM) is used to investigate the ion populations and solvent content of thin films of polyaniline (PA) on electrode surfaces as functions of redox state and pH. The data are shown to be consistent with a model for PA in which the initial oxidation at a potential of ca. 0.2 V vs. SCE to give the conductive form of the polymer creates charged sites by oxidation of amine moieties along the polymer chains. Higher potentials cause further (pH dependent) oxidation with cumulative removal of exactly one electron per ring to produce a polymer containing imine groups which can hydrolyze to form quinone and quinone/imine types of structures. The initial oxidation process is accompanied by proton expulsion at low pH values, indicating partial protonation of the PA amine nitrogens in the reduced form with loss of these protons on oxidation. In addition to the proton expulsion, anion insertion also takes place during oxidation under all conditions that were studied. Data from such measurements as a function of pH allow estimation of the pK/sub a/ values of the reduced (insulating) and oxidized (conducting) forms of PA, which are given as ca. -0.3 and 3. respectively. Experiments inmore » strong (10 M) acid solutions reveal changes in solvent content during oxidation which are observed to influence the electrochemical response of PA. The ability to measure both electrochemical and gravimetric data simultaneously is shown to greatly constrain possible models for systems such as PA, leading to less ambiguity in their description.« less

Reaction kinetics of hydrogen peroxide with copper and iron in seawater.

Abstract: The oxidation of Fe(II) and Cu(I) and the reduction of Fe(III) and Cu(II) by hydrogen peroxide in sea water have been studied to understand their mechanisms and probable significance in the upper marine water column. At 10/sup -7/ M H/sub 2/O/sub 2/, a level commonly found in surface sea water, reaction with H/sub 2/O/sub 2/ is the dominant oxidation pathway for Fe(II). Reduction of Fe(III) by peroxide was not observed in the pH range 7-8. Reduction of Cu(II) and oxidation of Cu(I) by H/sub 2/O/sub 2/ contribute to a dynamic redox cycling of that element in the upper water column. Calculations based on these data indicate that CU(I) oxidation and FE(II) oxidation by H/sub 2/O/sub 2/ are at least as important as nitrite photolysis as a source of OH radicals in the ocean. 47 references, 6 figures, 2 tables.

Energetic comparison between photoinduced electron-transfer reactions from NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from NADH model compounds to p-benzoquinone derivatives

Abstract: Kinetic studies on photoinduced electron-transfer reactions from dihydropyridine compounds (PyH/sub 2/) as being NADH model compounds to organic and inorganic oxidants and hydride-transfer reactions from PyH/sub 2/ to p-benzoquinone derivatives (Q) in the absence and presence of Mg/sup 2 +/ ion are reported by determining over 150 rate constants. These results, combined with the values of Gibbs energy change of the photoinduced electron-transfer reactions as well as those of each step of the hydride-transfer reactions as being the e/sup -/-H/sup +/-e/sup -/ sequence, which are determined independently, revealed that the rate constants of the photoinduced electron-transfer reactions obey the Rehm-Weller-Gibbs energy relationship and that the activation barrier of the hydride-transfer reactions from PyH/sub 2/ to Q is dependent solely on the Gibbs energy changes of the initial electron transfer from PyH/sub 2/ to Q and the following proton transfer from PyH/sub 2//sup .+/ to Q/sup .-/ and thus independent of the Gibbs energy change of the final electron transfer from PyH/sup ./ to QH/sup ./. The retarding effect of Mg/sup 2 +/ ion observed on the photoinduced electron transfer and hydride-transfer reactions of PyH/sub 2/ is ascribed to the positive shifts of the redox potentials of the ground and excitedmore » states of PyH/sub 2/ due to the complex formation with Mg/sup 2 +/ ion.« less

Efficient Vanadium Redox Flow Cell

Abstract: Excellent performance characteristics have been obtained with the all‐vanadium redox flow cell employing V(II)/V(III) and V(IV)/V(V) redox couples for the negative and positive half‐cells, respectively. Using 1.5M vanadium solutions in , carbon felt electrodes and a polystyrene sulfonic acid cation selective membrane, the cell was charged at 40 mA/cm2 and discharged across various loads. The coulombic efficiency was 90%, while the voltage efficiency calculated over the range 10–90% state‐of‐charge was 81%. An overall energy efficiency for the cell of 73% which, together with the simplicity of the system, makes the vanadium redox cell one of the promising energy storage systems currently under development.

Chemical Reactions on Clays

Abstract: Layer aluminosilicates catalyze reactions in numerous ways. They stabilize high-energy intermediates. They can store energy in their lattice structures and can release it in the form of chemical energy. They can catalyze redox reactions and can serve as photocatalytic devices. They often exhibit high surface acidity. Organic reactions that are catalyzed by the agency of days are reviewed. The role of clays in prebiotic chemistry is also examined.

Fate of superoxide in coastal sea water

Abstract: The superoxide anion (O2.− ) is a key intermediate in oxygen redox chemistry, and may mediate many chemical transformations in the ocean. The photochemical formation of hydrogen peroxide in sea water has been postulated1 to result from the disproportionation of superoxide, and here we report the results of a study which used the formation of H2O2 to characterize the dynamics of superoxide in coastal sea water. Midday rates of superoxide generation averaged ∼5 x 10-7 mol 1-1h-1. In addition to rapid disproportionation to hydrogen peroxide, a substantial fraction of the superoxide flux was shunted off through unknown pathways; 24–41% of the superoxide flux did not lead directly to H2O2 formation. Our calculations predicted accelerated decomposition of super-oxide in natural sea water relative to pure water; nevertheless, superoxide should be a relatively long-lived transient in sea water. We calculate the 50% decay time of a 10-8mol 1-1 steady-state level to be 20 min.

Photoassisted C–C coupling via electron transfer to benzylic halides by a bis(di-imine) copper(I) complex

Abstract: Owing to its special topography Cu(dap)2+[dap = 2,9 bis(p-anisyl)-1,10-phenanthroline] displays photophysical properties which allow its use as an efficient redox photocatalyst in reductive coupling of p-NO2C6H4CH2Br.

Iron uptake by the yeast Saccharomyces cerevisiae: involvement of a reduction step.

Abstract: Among several parameters affecting the rate and amount of iron uptake by Saccharomyces cerevisiae, the oxidation state of iron appeared to be determinant. Iron presented as Fe(II) was taken up faster than Fe(III) and the kinetic parameters were different. Iron was taken up by the cells from different ferric chelates, at rates that did not depend on their stability constants, and uptake was strongly inhibited by an iron(II)-trapping reagent like ferrozine. Iron was physiologically reduced by a transplasmamembrane redox system, which was induced in iron-deficient conditions. We propose that iron must be reduced to be taken up by the cells in the same way as other divalent cations.

Addition–elimination paths in electron-transfer reactions between radicals and molecules. Oxidation of organic molecules by the OH radical

Abstract: Reactions between the ˙OH radical and molecules Y that ultimately lead to electron transfer from Y to ˙OH have been studied by in-situ radiolysis or photolysis, electron spin resonance and pulse radiolysis techniques with optical and conductance detection. These radical–molecule reactions proceed in aqueous solution via the intermediate formation of covalently bound adducts HO—Y˙. These radicals are able to undergo heterolysis, which may proceed by spontaneous or by catalysed paths. The heterolysis results in a one-electron oxidation of Y, and the overall reaction thus consists in a one-electron transfer from the molecule to ˙OH.[graphic omitted]In the addition step (I), a reducing radical is formed by reaction of the oxidizing ˙OH with Y, which is usually neither an oxidant nor a reductant. In the heterolysis step (II), however, the reducing HOY˙ is converted into the oxidizing Y˙+. This phenomenon, termed redox inversion, is the consequence of the change in oxidation state of Y by two units in going from HOY˙ to Y˙+. Examples of redox processes of this kind are given from the class of substituted benzenes and of N-heterocyclics, and structure–reactivity relations governing the heterolysis of HO—Y˙ are discussed.

Trace metal reduction by phytoplankton: the role of plasmalemma redox enzymes

Abstract: The phytoplankton cell surface reduces external copper(II) and iron(III) complexes and redox dyes. This reductive activity appears to be mediated by one or more plasmalemma redox enzymes. Trace metal complexes are directly reduced by the redox enzyme, therefore the reduction rate is not regulated by the metal free ion activity in solution. This is in direct contrast to previous measurements of trace metal interactions with the phytoplankton cell membrane. Half-saturation constants for the reduction of Cu(II) complexes with carbonate, phenanthroline and bathocuproinedisulfonate are in the range 2.3-14.7 ..mu..M, which suggests that trace metal complexes are not the main electron acceptor in natural waters. In the diatom Thalassiosira weissflogii there is additional reductive activity associated with the cell wall.

Photochemically induced charge separation at the molecular level. A chromophore Quencher complex containing both an electron transfer donor and an acceptor

Abstract: Examples are known of chromophore-quencher complexes where following optical excitation of a metal-to-ligand charge transfer (MLCT) or a porphyrin ..pi.. ..-->.. ..pi..* chromophore, oxidative or reductive intramolecular electron-transfer quenching occurs. In a recent example, photolysis of a porphyrin-based system containing both an electron-transfer donor and an acceptor led to a relatively long-lived (3 ..mu..s) photoinduced charge separation onto peripheral donor and acceptor redox sites where in terms of the redox potential stored, ..delta..G/sup 0/ > 1.0 eV. One value of such systems is that they begin to reveal how oxidative and reductive equivalents can be generated photochemically and stored within the same molecule. They report here that MLCT-based excitation of the complex (Ru(Me(bpy)-3DQ/sup 2 +/)(Me(bpy)-PTZ)/sub 2/)/sup 4 +/ is followed by a sequence of intramolecular events which lead with relatively high efficiency to a charge-separated state based on the PTZ and -DQ/sup 2 +/ redox sites for which the transiently stored free energy is ..delta..G/sup 0/' approx. 1.29 eV.

Some aspects of diagenetic sulphate-hydrocarbon redox reactions

Abstract: Summary Sulphate-hydrocarbon redox-reactions occur at two specific diagenetic temperature/thermal maturity levels: less than about 75–85°C (0.2–0.3% R0), and more than 100–140°C (> 1.5% R0), respectively. In low-temperature/maturity environments these redox reactions take place only with the mediation of bacteria. In high-temperature/maturity environments these reactions take place thermochemically, and certain catalysts must interact in order to overcome the high activation energies and to sustain the reactions at geologically significant rates. The reaction products and by-products may be identical for both temperature/maturity levels: altered and oxidized hydrocarbons (including bitumen), hydrogen sulphide, metal sulphides (including Mississippi Valley Type deposits), elemental sulphur, carbonates (mainly calcite and dolomite), and other minerals. An important by-product of these redox reactions may be porosity resulting from the dissolution of solid sulphates and/or the carbonate host rock. The net reaction is exothermic, and the released heat may generate a geothermal hot-spot in some cases.

In situ Moessbauer study of redox processes in a composite hydroxide of iron and nickel

Abstract: In situ Moessbauer spectra revealed changes in the electronic and structural properties of Fe sites brought about by redox processes attributed formally to Ni sites in a composite iron/nickel hydroxide. Measurements on the oxidized form, polarized in 1 M KOH at 0.5 V vs. Hg/HgO, revealed a singlet with an isomer shift of 0.22 mm/s vs. ..cap alpha..-Fe. After reduction and polarization at 0.0 V, a doublet with an isomer shift of 0.32 mm/s and a quadrupole splitting of 0.44 mm/s was observed. The spectral changes, which accompanied the formal oxidation of Ni sites at 0.5 V, indicated a partial transfer of electronic density away from trivalent Fe sites and structural rearrangements toward more symmetrical coordination about these sites. This highly oxidized iron species in the composite hydroxide may be involved in the electrocatalytic mechanism for oxygen evolution in alkaline media.

Uncoupling of oxidative phosphorylation. 1. Protonophoric effects account only partially for uncoupling.

Abstract: The mechanism of uncoupling of oxidative phosphorylation by carbonyl cyanide p-trifluoromethoxy)phenylhydrazone (FCCP), a typical weak acid protonophore, oleic acid, a fatty acid, and chloroform, a general anesthetic, has been investigated by measuring in mitochondria their effect on (i) the transmembrane proton electrochemical potential gradient (delta mu H) and the rates of electron transfer and adenosine 5'-triphosphate (ATP) hydrolysis in static head, (ii) delta mu H and the rates of electron transfer and ATP synthesis in state 3, and (iii) the membrane proton conductance. Both FCCP and oleic acid increase the membrane proton conductance, and accordingly, they cause a depression of delta mu H [generated by either the redox proton pumps or the adenosinetriphosphatase (ATPase) proton pumps]. Although their effects on ATP synthesis/hydrolysis, respiration, and delta mu H are qualitatively consistent with a pure protonophoric uncoupling mechanism and an additional inhibitory action of oleic acid on both the ATPases and the electron-transfer enzymes, a quantitative comparison between the dissipative proton influx and the rate of either electron transfer or ATP hydrolysis (multiplied by either the H+/e- or the H+/ATP stoichiometry, respectively) at the same delta mu H shows that the increase in membrane conductance induced by FCCP and oleic acid accounts for the stimulation of the rate of ATP hydrolysis but not for that of the rate of electron transfer. Chloroform (at concentrations that fully inhibit ATP synthesis) only very slightly increases the proton conductance of the mitochondrial membrane and causes only a little depression of delta mu H.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron-transfer biosensors.

Abstract: The electrochemistry of redox proteins is now well established. Conditions exist which allow electron-transfer reactions of all simple proteins to proceed rapidly and reversibly at electrodes. Coupling of the electrode reaction to enzymes, for which the redox proteins act as cofactors, allows exploitation of this good electrochemistry. This is well illustrated by the enzyme-catalysed electrochemical oxidation of p -cresol to p -hydroxybenzaldehyde, which has been shown to proceed along with coupling to the electrode via the copper protein, azurin, or the organometallic compound ferroceneboronic acid. Ferrocene derivatives, in general, show a degree of versatility, coupling the electron-transfer reactions of many enzymes. Thus derivatives of the ferricinium ion act as excellent electron-transfer reagents from the enzyme glucose oxidase. The system is capable of detecting glucose in blood. Similar procedures, in conjunction with the appropriate enzyme, have yielded assays for, among others, H 2 O 2 and cholesterol.