Showing papers on "Redox published in 2002"

The catalytic pathway of horseradish peroxidase at high resolution

Abstract: A molecular description of oxygen and peroxide activation in biological systems is difficult, because electrons liberated during X-ray data collection reduce the active centres of redox enzymes catalysing these reactions. Here we describe an effective strategy to obtain crystal structures for high-valency redox intermediates and present a three-dimensional movie of the X-ray-driven catalytic reduction of a bound dioxygen species in horseradish peroxidase (HRP). We also describe separate experiments in which high-resolution structures could be obtained for all five oxidation states of HRP, showing such structures with preserved redox states for the first time.

Properties of O2.- and OH. formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions

Abstract: We have investigated the effect of H2O2 on the behavior of O2•- and OH• produced in photocatalysis of aqueous TiO2 suspensions by means of luminol chemiluminescence probing and terephthalic acid fluorescence probing, respectively. The reduction of O2 by photoinduced conduction band electrons (e-) was increased by the addition of H2O2, since the consumption of photoinduced valence band holes (h+) in the oxidation of H2O2 caused the repression of e-−h+ recombination. After the end of the light irradiation, the amount of O2•- decreased based on the fractal-like kinetics at the heterogeneous surface of the TiO2 particle. The decay process might be caused by trapped h+, which cannot react with water and then remains on the TiO2 particle after the irradiation. The energy level of the trapped h+ was estimated to be above the redox potential of SCN-, since it could react with the adsorbed H2O2 and I- ions but not with other ions such as SCN-, Br-, and Cl-. The formation rate of OH• was increased by the addition o...

Direct electron transfer of glucose oxidase on carbon nanotubes

Abstract: In this report, exploitation of the unique properties of single-walled carbon nanotubes (SWNT) leads to the achievement of direct electron transfer with the redox active centres of adsorbed oxidoreductase enzymes. Flavin adenine dinucleotide (FAD), the redox active prosthetic group of flavoenzymes that catalyses important biological redox reactions and the flavoenzyme glucose oxidase (GOx), were both found to spontaneously adsorb onto carbon nanotube bundles. Both FAD and GOx were found to spontaneously adsorb to unannealed carbon nanotubes that were cast onto glassy carbon electrodes and to display quasi-reversible one-electron transfer. Similarly, GOx was found to spontaneously adsorb to annealed, single-walled carbon nanotube paper and to display quasi-reversible one-electron transfer. In particular, GOx immobilized in this way was shown, in the presence of glucose, to maintain its substrate-specific enzyme activity. It is believed that the tubular fibrils become positioned within tunnelling distance of the cofactors with little consequence to denaturation. The combination of SWNT with redox active enzymes would appear to offer an excellent and convenient platform for a fundamental understanding of biological redox reactions as well as the development of reagentless biosensors and nanobiosensors.

Electrochemical oxidation of phenol at boron-doped diamond electrode. Application to electro-organic synthesis and wastewater treatment.

Abstract: The electrochemical behaviour of a synthetic boron-doped diamond thin film electrode (BDD) has been studied in acid media containing phenol using cyclic voltammetry and bulk electrolysis. The results have shown that in the potential region of water stability direct electron transfers can occur on BDD surface resulting in electrode fouling due to the formation of a polymeric film on its surface. During electrolysis in the potential region of oxygen evolution, complex oxidation reactions can take place due to electrogenerated hydroxyl radicals. Electrode fouling is inhibited under these conditions. Depending on the experimental conditions, the electrogenerated hydroxyl radicals can lead to the combustion of phenol or to the selective oxidation of phenol to benzoquinone. The experimental results have also been compared to a theoretical model that permits the prediction of the evolution with time of phenol concentration, during its combustion, or during its selective oxidation to benzoquinone.

Nitric Oxide in Biological Denitrification: Fe/Cu Metalloenzyme and Metal Complex NOx Redox Chemistry

Abstract: Department of Chemistry, The Johns Hopkins University, Charles and 34th Streets, Baltimore, Maryland 21218, Kluyver Laboratory for Biotechnology, Delft University of Technology, 2628 BC Delft, The Netherlands, Department of Biochemistry and Molecular Biology, OGI School of Science and Engineering at OHSU, Beaverton, Oregon 97006, and Department of Microbiology and Molecular Genetics, University of California, Los Angeles, California 90095

Effect of Transition Metal Ions on the TiO2-Assisted Photodegradation of Dyes under Visible Irradiation: A Probe for the Interfacial Electron Transfer Process and Reaction Mechanism

Abstract: The effect of metal ions (Cu2+, Fe3+, Zn2+, Al3+, and Cd2+) on the photodegradation of several dyes: sulfo-rhodamine B (SRB), alizarin red (AR), and malachite green (MG) has been investigated in aqueous TiO2 dispersions under visible irradiation (λ > 420 nm). Trace quantities of transition metal ions such as Cu2+ and Fe3+ having suitable redox potentials alter the electron-transfer pathway involving the dye, O2 and TiO2 particles, and markedly depress the photodegradation of all three dyes under visible irradiation. Other metal ions, such as Zn2+, Cd2+, and Al3+, have only a slight influence on the photoreaction by altering the adsorption of dyes. Photogeneration of H2O2 and reactive radicals, and the changes in fluorescence emission of SRB in TiO2 aqueous dispersions were examined to elucidate the role of the metal ions. Addition of Cu2+ or Fe3+ decreases the reduction of O2 by the conduction electrons, subsequently blocks the formation of reactive oxygen species (O-•, •OH), and depresses the degradatio...

Comparing the catalytic efficiency of some mediators of laccase

Abstract: The mechanism of oxidation of non-phenolic substrates by laccase/mediators systems has been investigated. Oxidation of 4-methoxybenzyl alcohol (1), taken as a benchmark reaction, enabled us to compare and to rank the relative ability of twelve mediators: TEMPO proved most effective, and a ionic mechanism is suggested for its action. Data on intermolecular selectivity of substrate oxidation are in favour of an electron transfer (ET) mechanism in the case of ABTS-mediated oxidations, and of a radical mechanism in HBT- and HPI-mediated reactions. Investigation by cyclic voltammetry (CV) of some of the mediators revealed that an important role in determining the mechanism of substrate oxidation may be played by the stability of the oxidised form of the mediator, as well as by its redox potential.

Computing Redox Potentials in Solution: Density Functional Theory as A Tool for Rational Design of Redox Agents

Abstract: High-level density functional theory in combination with a continuum solvation model was employed to compute standard redox potentials in solution phase for three different classes of electrochemically active molecules: small organic molecules, metallocenes, and M(bpy)3x (M = Fe, Ru, Os; x = +3, +2, +1, 0, −1). Excellent agreement with experimentally determined redox potentials is found with an average deviation of approximately 150 mV when four different solvents commonly in use for electrochemical measurements were included. To obtain quantitative agreement between theory and experiment, the use of a large basis set is crucial especially when the redox couple includes anionic species. Whereas the addition of diffuse functions improved the results notably, vibrational zero-point-energy corrections and addition of entropy effects are less important. The computational protocol for computing redox potentials in solution, which has been benchmarked, is a powerful and novel tool that will allow a molecular-l...

Electrodeposition of redox polymers and co-electrodeposition of enzymes by coordinative crosslinking

Abstract: Thin films of transition metal complex-based redox polymers are electrodeposited on electrodes via application of fixed or cycled potential. When hydrated, an electrodeposited film conducts electrons by electron exchange between backbone-bound, but mobile, functional segments of its redox polymer constituents. These functional segments, or redox complexes, have labile ligands, such as chloride anions, for example, in their inner coordination spheres. The backbones of the redox polymers have strongly coordinating ligands, such as pyridine- or imidazole-containing functions, for example, which are generally not coordinated prior to deposition. Electrodeposition results from coordinative crosslinking by exchange of labile ligands and strongly coordinating ligands between polymer chains, provided sufficient functional segments of the redox polymers are present at the electrode surface. When a biological macromolecule or protein, such as a redox enzyme, is added to the solution from which the redox polymer is electrodeposited, it is co-electrodeposited on the electrode surface. When the co-deposited film contains redox enzymes, for example, the modified electrode may be used to catalyze the electrooxidation or electroreduction of substrates of the enzymes. Electrodes modified according to the invention also have application in chemical or biological assays.

Mechanistic Study of Alcohol Oxidation by the Pd(OAc)2/O2/DMSO Catalyst System and Implications for the Development of Improved Aerobic Oxidation Catalysts

Abstract: The Pd(OAc)2/O2/DMSO catalyst system displays impressive versatility in the aerobic oxidation of organic substrates, ranging from alcohols to olefins This report details mechanistic insights into these reactions Dimethyl sulfoxide (DMSO) plays no redox role in the chemistry, and kinetic experiments identify the turnover-limiting step as DMSO-promoted oxidation of palladium(0) by molecular oxygen The “chemical oxidase” pathway characterized for this catalyst system holds great promise for the design of new aerobic oxidation reactions

Nature of the active site for CO oxidation on highly active Au/γ-Al2O3

Abstract: The deactivation and regeneration phenomenon during room temperature CO oxidation was studied over a Au/γ-Al2O3 catalyst, which was as active as the most active supported Au catalysts reported in the literature. The initial rapid loss of activity could be prevented if either hydrogen or water vapor was present in the reaction mixture. Otherwise, it could be recovered by exposure of the deactivated catalyst to either hydrogen or water vapor at room temperature. Thermal treatment above 100 °C in a dry atmosphere also deactivated the catalyst. These results suggested that hydroxyl group, most likely associated with a Au(I) cation, is associated with the active site and support the proposal that the active site is an ensemble of Au+OH− together with Au(0) atoms. The CO oxidation reaction was proposed to proceed via the insertion of CO into the Au+OH− bond to form a hydroxycarbonyl, which is oxidized to a bicarbonate. Decarboxylation of the bicarbonate completes the reaction cycle.

The organic chemistry of enzyme-catalyzed reactions

Abstract: Preface About the Author Enzymes as Catalysts What Are Enzymes, and How Do They Work? Historical Specificity of Enzyme-Catalyzed Reactions Rate Acceleration Mechanisms of Enzyme Catalysis Approximation Covalent Catalysis General Acid/Base Catalysis Electrostatic Catalysis Desolvation Strain or Distortion Enzyme Catalysis in Organic Media Enzyme Nomenclature Epilogue References Group Transfer Reactions: Hydrolysis, Amination, Phosphorylation Hydrolysis Reactions Amide Hydrolysis: Peptidases Ester Hydrolysis: Esterases and Lipases Aminations Glutamine-Dependent Enzymes Aspartic Acid as a Source of Ammonia Phosphorylations: Transfers of Phosphate and Phosphate Esters to Water or Other Acceptors Phosphatases Phosphodiesterases Kinases References Reduction and Oxidation General Redox without a Coenzyme Redox Reactions That Require Coenzymes Nicotinamide Coenzymes (Pyridine Nucleotides) Flavin Coenzymes Quinone-Containing Coenzymes Other Redox Enzymes References Monooxygenation General Flavin-Dependent Hydroxylases No Reducing Agent Required Hydroxylases Requiring an External Reducing Agent Pterin-Dependent Hydroxylases General Mechanism Heme-Dependent Monooxygenases General Molecular Oxygen Activation Mechanistic Considerations Nonheme Iron Oxygenation Methane Monooxygenase Copper-Dependent Oxygenation Dopamine a-Monooxygenase References Dioxygenation General Intramolecular Dioxygenases Catechol Dioxygenases Prostaglandin H Synthase (Cyclooxygenase) Intermolecular Dioxygenases a-Keto Acid-Dependent Dioxygenases

A reversible solid-state crystalline transformation in a metal phosphide induced by redox chemistry.

Abstract: We demonstrate low-potential intercalation of lithium in a solid-state metal phosphide. A topotactic first-order transition between different but related crystal structures at room temperature takes place by an electrochemical redox process: MnP4 ↔ Li7MnP4. The P-P bonds in the MnP4 structure are cleaved at the time of Li insertion (reduction) to produce crystalline Li7MnP4 and are reformed after reoxidation to MnP4, thereby acting as an electron storage reservoir. This is an unusual example of facile covalent bond breaking within the crystalline solid state that can be reversed by the input of electrochemical energy.

Copper bis(thiosemicarbazone) complexes as hypoxia imaging agents: structure-activity relationships.

Abstract: Copper(II) bis(thiosemicarbazone) complexes labelled with Cu-60/62/64 are useful radiopharmaceuticals for imaging blood flow and hypoxic tissues in vivo. The aim of this study was to identify structure-activity relationships within a series of analogues with different alkyl substitution patterns in the ligand, in order to design improved hypoxia imaging agents and elucidate hypoxia selectivity mechanisms. Thirteen such complexes were synthesised and characterised spectroscopically and electrochemically. The uptake of each (labelled with Cu-64) in EMT6 tumour cells in vitro under normoxic and hypoxic conditions was studied. All complexes were taken up efficiently into cells, and some showed strong hypoxia selectivity, which was highly correlated with the Cu(II/I) redox potential. Redox potentials at the low end of the range were found to be essential for hypoxia selectivity. In turn, the redox potential was strongly correlated with alkyl substitution pattern, and the most important determinant of the redox potential was the number of alkyl groups on the diimine backbone of the ligand. Several complexes in the series warrant further evaluation as hypoxia imaging agents. The radioactivity uptake/release behaviour in the cells provides insight into possible mechanisms, and a model for hypoxia-selective intracellular trapping is discussed.

Use of medium effects to tune the ΔE1/2 values of bimetallic and oligometallic compounds

Abstract: The redox potentials of bis(fulvalene)dinickel, 1, and the tetrakis(ferrocenyl)nickel dithiolene complex 2 have been measured in a variety of nonaqueous electrolytes. The difference in E1/2 values of the two successive one-electron oxidations of 1 (i.e., ΔE1/2 values) increased from a low of 212 mV in anisole/[NBu4]Cl to a high of 850 mV in CH2Cl2−Na[B(C6H3(CF3)2)4], reflecting an increase of over 1010 in the comproportionation constant (Kcomp = [1+]2/[1][12+]). Six reversible one-electron processes are possible for compound 2, the four oxidations arising from the ferrocenyl substituents, and the two reductions arising from the Ni dithiolene moiety. The E1/2 spreads of the four oxidation waves and the two reduction waves are both highly sensitive to medium effects. For both 1 and 2, the largest ΔE1/2 values for cationic products are found in solvents of low polarity and donor strength containing electrolyte salts having large anions and small cations. Conversely, the smallest ΔE1/2 values for anionic prod...

On Active-Site Heterogeneity in Pyrolyzed Carbon-Supported Iron Porphyrin Catalysts for the Electrochemical Reduction of Oxygen: An In Situ Mössbauer Study

Abstract: Some FeTPP−Cl/carbon electrocatalysts, heat-treated at temperatures up to 800 °C, have been studied with cyclic voltammetry, x-ray photoelectron spectroscopy, extended X-ray absorption fine structure, and in situ Mossbauer spectroscopy. It appears that the heat treatment induces considerable site heterogeneity in electronic terms, although structurally the Fe−N4 moiety seems persistent. The data indicate that only part of these Fe−N4 sites contributes to the activity for the electrochemical reduction of O2 but that they operate according to the well-known redox mechanism.

Integration of polyaniline/poly(acrylic acid) films and redox enzymes on electrode supports: an in situ electrochemical/surface plasmon resonance study of the bioelectrocatalyzed oxidation of glucose or lactate in the integrated bioelectrocatalytic systems.

Abstract: Electropolymerization of aniline in the presence of poly(acrylic acid) on Au electrodes yields a polyaniline/poly(acrylic acid) composite film, exhibiting reversible redox functions in aqueous solutions at pH = 7.0. In situ electrochemical-SPR measurements are used to identify the dynamics of swelling and shrinking of the polymer film upon the oxidation of the polyaniline (PAn) to its oxidized state (PAn(2+)) and the reduction of the oxidized polymer (PAn(2+)) back to its reduced state (PAn), respectively. Covalent attachment of N(6)-(2-aminoethyl)-flavin adenin dinucleotide (amino-FAD, 1) to the carboxylic groups of the composite polyaniline/poly(acrylic acid) film followed by the reconstitution of apoglucose oxidase on the functional polymer yields an electrically contacted glucose oxidase of unprecedented electrical communication efficiency with the electrode: electron-transfer turnover rate approximately 1000 s(-1) at 30 degrees C. In situ electrochemical-SPR analyses are used to characterize the bioelectrocatalytic functions of the biomaterial-polymer interface. The current responses of the bioelectrocatalytic system increase as the glucose concentrations are elevated. Similarly, the SPR spectra of the system are controlled by the concentration of glucose. The glucose concentration controls the steady-state concentration ratio of PAn/PAn(2+) in the film composition. Therefore, the SPR spectrum of the film measured upon its electrochemical oxidation is shifted from the spectrum typical for the oxidized PAn(2+) at low glucose concentration to the spectrum characteristic of the reduced PAn at high glucose concentration. Similarly, the polyaniline/poly(acrylic acid) film acts as an electrocatalyst for the oxidation of NADH. Accordingly, an integrated bioelectrocatalytic assembly was constructed on the electrode by the covalent attachment of N(6)-(2-aminoethyl)-beta-nicotinamide adenine dinucleotide (amino-NAD(+), 2) to the polymer film, and the two-dimensional cross-linking of an affinity complex formed between lactate dehydrogenase and the NAD(+)-cofactor units associated with the polymer using glutaric dialdehyde as a cross-linker. In situ electrochemical-SPR measurements are used to characterize the bioelectrocatalytic functions of the system. The amperometric responses of the system increase as the concentrations of lactate are elevated, and an electron-transfer turnover rate of 350 s(-1) between the biocatalyst and the electrode is estimated. As the PAn(2+) oxidizes the NADH units generated by the biocatalyzed oxidation of lactate, the PAn/PAn(2+) steady-state ratio in the film is controlled by the concentration of lactate. Accordingly, the SPR spectrum measured upon electrochemical oxidation of the film is similar to the spectrum of PAn(2+) at low lactate concentration, whereas the SPR spectrum resembles that of PAn at high concentrations of lactate.

How oxygen damages microbes: oxygen tolerance and obligate anaerobiosis

Abstract: The orbital structure of molecular oxygen constrains it to accept electrons one at a time, and its unfavourable univalent reduction potential ensures that it can do so only with low-potential redox partners. In E. coli, this restriction prevents oxygen from oxidizing structural molecules. Instead, it primarily oxidizes reduced flavins, a reaction that is harmful only in that it generates superoxide and hydrogen peroxide as products. These species are stronger oxidants than is oxygen itself. They can oxidize dehydratase iron-sulphur clusters and sulphydryls, respectively, and thereby inactivate enzymes that are dependent upon these functional groups. Hydrogen peroxide also oxidizes free iron, generating hydroxyl radicals. Because hydroxyl radicals react with virtually any biomolecules they encounter, their reactivity is broadly dissipated, and only their reactions with DNA are known to have an important physiological impact. E. coli elaborates scavenging and repair systems to minimize the impact of this adventitious chemistry; mutants that lack these defences grow poorly in aerobic habitats. Some of the growth deficits of these mutants cannot be easily ascribed to sulphydryl, cluster, or DNA damage, indicating that important aspects of oxidative stress still lack a biochemical explanation. Obligate anaerobes cannot tolerate oxygen because they utilize metabolic schemes built around enzymes that react with oxidants. The reliance upon low-potential flavoproteins for anaerobic respiration probably causes substantial superoxide and hydrogen peroxide to be produced when anaerobes are exposed to air. These species then generate damage of the same type that they produce in aerotolerant bacteria. However, obligate anaerobes also utilize several classes of dioxygen-sensitive enzymes that are not needed by aerobes. These enzymes are used for processes that help maintain the redox balance during anaerobic fermentations. They catalyse reactions that are chemically difficult, and the reaction mechanisms require the solvent exposure of radicals or low-potential metal clusters that can react rapidly with oxygen. Recent work has uncovered adaptive strategies by which obligate anaerobes seek to minimize the damage done by superoxide and hydrogen peroxide. Their failure to divest themselves of enzymes that can be directly damaged by molecular oxygen suggests that evolution has not yet provided economical options to them.

Iodide Electron Transfer Kinetics in Dye-Sensitized Nanocrystalline TiO2 Films

Abstract: Electron transfer kinetics plays a key role in determining the energy conversion efficiency of dye-sensitized photoelectrochemical solar cells. Photoinduced charge separation in such cells results in oxidation of the sensitizer dye. The resulting dye cation may be rereduced by recombination with injected electrons or by electron transfer from iodide ions in the redox electrolyte, often referred to as the regeneration reaction. In this paper, we employ transient absorption spectroscopy to investigate the kinetic competition between these two pathways in Ru(dcbpy)2(NCS)2-sensitized nanocrystalline film TiO2 electrodes immersed in a propylene carbonate electrolyte. The experiments monitored both the dye cation decay kinetics and the yield of product species, assigned to I2- radicals generated by electron transfer from iodide ions to the dye cation. The kinetic competition between the recombination and the regeneration processes is found to be dependent upon both the iodide concentration and the electrical bi...

Glutathione and thioredoxin redox during differentiation in human colon epithelial (Caco-2) cells

Abstract: Cellular redox, maintained by the glutathione (GSH)- and thioredoxin (Trx)-dependent systems, has been implicated in the regulation of a variety of biological processes. The redox state of the GSH system becomes oxidized when cells are induced to differentiate by chemical agents. The aim of this study was to determine the redox state of cellular GSH/glutathione disulfide (GSH/GSSG) and Trx as a consequence of progression from proliferation to contact inhibition and spontaneous differentiation in colon carcinoma (Caco-2) cells. Results showed a significant decrease in GSH concentration, accompanied by a 40-mV oxidation of the cellular GSH/GSSG redox state and a 28-mV oxidation of the extracellular cysteine/cystine redox state in association with confluency and increase in differentiation markers. The redox state of Trx did not change. Thus the two central cellular antioxidant and redox-regulating systems (GSH and Trx) were independently controlled. According to the Nernst equation, a 30-mV oxidation is associated with a 10-fold change in the reduced/oxidized ratio of a redox-sensitive dithiol motif. Therefore, the measured 40-mV oxidation of the cellular GSH/GSSG couple or the 28-mV oxidation of the extracellular cysteine/cystine couple should be sufficient to function in signaling or regulation of differentiation in Caco-2 cells.