Showing papers on "Redox published in 1993"

A redox-based mechanism for the neuroprotective and neurodestructive effects of nitric oxide and related nitroso-compounds

Abstract: Congeners of nitrogen monoxide (NO) are neuroprotective and neurodestructive. To address this apparent paradox, we considered the effects on neurons of compounds characterized by alternative redox states of NO: nitric oxide (NO.) and nitrosonium ion (NO+). Nitric oxide, generated from NO. donors or synthesized endogenously after NMDA (N-methyl-D-aspartate) receptor activation, can lead to neurotoxicity. Here, we report that NO.- mediated neurotoxicity is engendered, at least in part, by reaction with superoxide anion (O2.-), apparently leading to formation of peroxynitrite (ONOO-), and not by NO. alone. In contrast, the neuroprotective effects of NO result from downregulation of NMDA-receptor activity by reaction with thiol group(s) of the receptor's redox modulatory site. This reaction is not mediated by NO. itself, but occurs under conditions supporting S-nitrosylation of NMDA receptor thiol (reaction or transfer of NO+). Moreover, the redox versatility of NO allows for its interconversion from neuroprotective to neurotoxic species by a change in the ambient redox milieu. The details of this complex redox chemistry of NO may provide a mechanism for harnessing neuroprotective effects and avoiding neurotoxicity in the central nervous system.

Ferrous iron oxidation by anoxygenic phototrophic bacteria

Abstract: NATURAL oxidation of ferrous to ferric iron by bacteria such as Thiobacillus ferrooxidans or Gallionella ferruginea1, or by chemical oxidation2,3 has previously been thought always to involve molecular oxygen as the electron acceptor. Anoxic photochemical reactions4–6 or a photobiological process involving two photosystems7–9 have also been discussed as mechanisms of ferrous iron oxidation. The knowledge of such processes has implications that bear on our understanding of the origin of Precambrian banded iron formations10–14. The reducing power of ferrous iron increases dramatically at pH values higher than 2–3 owing to the formation of ferric hydroxy and oxyhydroxy compounds1,2,15 (Fig. 1). The standard redox potential of Fe3+/Fe2+ (E0 = +0.77 V) is relevant only under acidic conditions. At pH 7.0, the couples Fe(OH)3/Fe2+ (E′0 = -0.236V) or Fe(OH)3 + HCO−3FeCO3 (E′0 = +0.200 V) prevail, matching redox potentials measured in natural sediments9,16,17. It should thus be possible for Fe(n) around pH 7.0 to function as an electron donor for anoxygenic photosynthesis. The midpoint potential of the reaction centre in purple bacteria is around +0.45 V (ref. 18). Here we describe purple, non-sulphur bacteria that can indeed oxidize colourless Fe(u) to brown Fe(in) and reduce CO2 to cell material, implying that oxygen-independent biological iron oxidation was possible before the evolution of oxygenic photosynthesis.

An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals

Abstract: Laboratory experiments were performed to investigate TiO 2 photocatalysis for treating water contaminated with dissolved metals (Ag, Au, Cd, Cr, Cu, Hg, Ni, and Pt) and a variety of organics (e.g., methanol, formic acid, salicylic acid, EDTA, phenol, and nitrobenzene). It was found that only those metals with half-reaction standard reduction potentials more positive than 0.3 V (vs normal hydrogen electrode) can be treated using TiO 2 as the photocatalyst. Kinetic data illustrating the synergism between oxidation and reduction are presented. Experiments using singly substituted benzenes as electron donors show that the rate of reduction of Cr(VI) is correlated with Hammett a constants

Chemical Kinetics and Process Dynamics in Aquatic Systems

Abstract: Overview: Introduction. Natural Waters as Nonequalibrium Systems. Scope of This Book. Rate Expressions for Chemical Reactions: Introduction. Rate Equations for Elementary Reactions. Rate Equations for More Complicated Reactions. Determination of Rate Equations and Rate Parameters from Experimental Data. Relationships between Rate Equations and Reaction Mechanisms. Experimental Aspects of Kinetic Studies. Theoretical Aspects of Kinetics and Effects of Physical Conditions on Reaction Rates: Introduction. The Arrhenquation and Related Temperature Relationships. Properties of Water and Reactants in Aqueous Solution. Encounter Theory for Reactions in Solution. Transition State/Activated Complex theory. Effects of Pressure on Rates of Reactions in Solution. Effects of Ionic Strength on Reaction Rates. Ranges of Values for Kinetic Parameters: Summary and perspective . Kinetics of Chemical Reactions in Aquatic Systems: From Homogeneous Catalysis to Reactions at Interfaces: Homogeneous Catalysis. Introduction. Types and Mechanisms of Homogeneous Catalysis. Rate Equations for Catalyzed Reactions. Kinetics of Chemical Reactions in Aqueous Solution: Kinetics of Dissociation and Hydrolysis Reactions in Natural Waters. Kinetics of Chlorination Reactions. Inorganic Redox Reactions. Processes at Interfaces. Gas Transfer Kinetics. Mineral Dissolution and Formation. Reactors, Mass Transport, and Process Models: Reactor Theory. Fundamentals of Chemical Reactors. Principles of Mass Balance Analysis. Comparative Behavior of CFSTRs and PFRs. Applications of Reactor Models to Natural Aquatic Systems. Equilibrium versus Kinetic Models for Open Systems. Nutrient Models. Models of Organic Contaminant Transport and Fate. CFSTR Models for Sulfate and Alkalinity on Lakes. Kinetics of Multi-Compartment Systems. Nature of Multi-Compartment Systems. Analysis of Compartment Models by Matrix Methods. The Inverse Problem. Kinetics of Biochemical Reactions and Microbial Processes in Natural Waters. Biochemical Kinetics: Enzyme Catalyzed Reactions. Reactions Involving a Single Substrate. Multisubstrate Reactions. Enzyme Inhibition. Theoretical Aspects of Enzyme Catalysis. Kinetics of Microbial Processes. Kinetics of Nutrient-Limited Microbial Growth. Continuous Culture of Microorganisms. Effects of Temperature on Microbial Processes. Kinetic Models of Heterotrophic Growth in Aquatic Systems. Kinetic Models for Autotrophic Growth and Nutrient Uptake. Diffusion-Limited Transport and Growth. Microbial Dieoff and Disinfection Kinetics. Prediction Methods for Reaction Rates and Compound Reactivity. Linear Free energy Relationships. Introduction. The Bronsted Relationship. Sigma Relationships for Organic Substituent Effects. LFERs for Redox Reactions. Applications of LFERs to Reactions in Natural Waters. Property-Activity and Structure-Activity Relationships in Environmental Biology and Chemistry. Nature and Importance of PARs and SARs. Physical-Chemical Measures of Compound Availability and Transportability. Linear Solvation Energy Relationships. A Simple Property-Activity Model for Biological Systems. Property-Activity Relationships for Aquatic Organisms. Quantitative Structure-Activity Relationships. Use of Statistical Techniques to Develop and Analyze PARs and SARs. Summary. Photochemical Reactions in Natural Waters: Introduction. Kinetics of Photochemical Reactions. Inorganic Photochemistry of Natural Waters. Nature and Photochemistry of Natural DOM. Photochemistry of Organic Contaminants in Natural Waters. Photochemistry on Suspended Oxide Particles. References. Index.

The Asp-His-Fe triad of cytochrome c peroxidase controls the reduction potential, electronic structure, and coupling of the tryptophan free radical to the heme.

Abstract: The buried charge of Asp-235 in cytochrome c peroxidase (CCP) forms an important hydrogen bond to the histidine ligand of the heme iron. The Asp-His-metal interaction, which is similar to the catalytic triad of serine proteases, is found at the active site of many metalloenzymes and is believed to modulate the character of histidine as a metal ligand. We have examined the influence of this interaction in CCP on the function, redox properties, and iron zero-field splitting in the native ferric state and its effect on the Trp-191 free radical site in the oxidized ES complex. Unlike D235A and D235N, the mutation D235E introduces very little perturbation in the X-ray crystal structure of the enzyme active site, with only minor changes in the geometry of the carboxylate-histidine interaction and no observable change at the Trp-191 free radical site. More significant effects are observed in the position of the helix containing residue Glu-235. However, the small change in hydrogen bond geometry is all that is necessary to (1) increase the reduction potential by 70 mV, (2) alter the anisotropy of the Trp-191 free radical EPR, (3) affect the activity and spin-state equilibrium, and (4) reduce the strength of the iron ligand field as measured by the zero-field splitting. The changes in the redox potential with substitution are correlated with the observed zero-field splitting, suggesting that redox control is exerted through the heme ligand by a combination of electrostatic and ligand field effects. The replacement of Asp-235 with Glu appears to result in a significantly weaker hydrogen bond in which the proton resides essentially with His-175. This hydrogen bond is nevertheless strong enough to prevent the reorientation of Trp-191 and the conversion to one of two low-spin states observed for D235A and D235N. The Asp-His-Fe interaction is therefore as important in defining the redox properties and imidazolate character of His-175 as has been proposed, yet its most important role in peroxidase function may be to correctly orient Trp-191 for efficient coupling of the free radical to the heme and to maintain a high-spin 5-coordinate heme center.

Partial Oxidation of Methane Using the Redox of Cerium Oxide

Abstract: Direct conversion of methane into synthesis gas of H2/CO ratio of 2 has been demonstrated using CeO2 as an oxidant at 873–1073 K. The reaction was accelerated in the presence of Pt black (1 wt%). The reduced cerium oxide after the oxidation of methane can be used to convert carbon dioxide into carbon monoxide.

Redox chemistry of iron in fog and stratus clouds

Abstract: The redox chemistry of Fe in fog and cloudwater has been investigated at coastal and inland locations in the Los Angeles basin, in Bakersfield California, and in Delaware Bay. Samples were collected and analyzed for Fe (Fe(II)), Fe(III), total(Fe), sulfur (S(IV), S(VI)), organic ligands (formate, acetate, oxalate), total organic carbon (TOC), pH, major cations (sodium, calcium, magnesium, potassium, ammonium), chloride, sulfate, nitrate, peroxides, and aldehydes (HCHO); the amount of sunlight was also measured. The ratio Fe(II)/Fe(total) varied between 0.02 and 0.55. The concentration of Fe(II) varied between 0.1 and 5 micromole, and the concentration of total Fe varied between 2 and 27 micromole. The atmospheric redox cycle of Fe involves both dissolved and aerosol surface species and appears to be related to the presence of organic compounds which act as electron donors for the reduction of Fe(III). Fe(III) reduction is enhanced by light but significant Fe(II) levels were observed in the dark. We suggest that reduction of Fe(III) species by organic electron donors may be an important pathway that affects the speciation of Fe in both urban and rural atmospheres. It is possible that reactions involving Fe and organic compounds might be an important source of carboxylic acids in the troposphere.

Reversible Electrochemistry of Fumarate Reductase Immobilized on an Electrode Surface. Direct Voltammetric Observations of Redox Centers and Their Participation in Rapid Catalytic Electron Transport

Abstract: Fumarate reductase (Escherichia coli) can be immobilized in an extremely electroactive state at an electrode, with retention of native catalytic properties. The membrane-extrinsic FrdAB component adsorbs to monolayer coverage at edge-oriented pyrolytic graphite and catalyzes reduction of fumarate or oxidation of succinate, depending upon the electrode potential. In the absence of substrates, reversible redox transformations of centers in the enzyme are observed by cyclic voltammetry. The major component of the voltammograms is a pair of narrow reduction and oxidation signals corresponding to a pH-sensitive couple with formal reduction potential E degree' = -48 mV vs SHE at pH 7.0 (25 degrees C). This is assigned to two-electron reduction and oxidation of the active-site FAD. A redox couple with E degree' = -311 mV at pH 7 is assigned to center 2 ([4Fe-4S]2+/1+). Voltammograms for fumarate reduction at 25 degrees C, measured with a rotating-disk electrode, show high catalytic activity without the low-potential switch-off that is observed for the related enzyme succinate dehydrogenase. The catalytic electrochemistry is interpreted in terms of a basic model incorporating mass transport of substrate, interfacial electron transfer, and intrinsic kinetic properties of the enzyme, each of these becoming a rate-determining factor under certain conditions. Electrochemical reversibility is approached under conditions of low turnover rate, for example, as the supply of substrate to the active site is limited. In this situation, electrocatalytic half-wave potentials, E1/2, are similar for oxidation of bulk succinate and reduction of bulk fumarate and coincide closely with the E degree' value assigned to the FAD. At 25 degrees C and pH 7, the apparent KM for fumarate reduction is 0.16 mM, and kcat is 840 s-1. Accordingly the second-order rate constant, kcat/KM, is 5.3 x 10(6) M-1 s-1. Under the same conditions, oxidation of succinate is much slower. As the supply of fumarate to the enzyme is raised to increase turnover, the electrochemical reaction eventually becomes limited by the rate of electron transfer from the electrode. Under these conditions a second catalytic wave becomes evident, the E1/2 value of which corresponds to the reduction potential of the redox couple suggested to be center 2. This small boost to the catalytic current indicates that the low-potential [4Fe-4S] cluster can function as a second center for relaying electrons to the FAD.

Redox chemistry of soils

Abstract: Soil chemical reactions involve some combination of proton and electron transfer. Oxidation occurs if there is a loss of electrons in the transfer process while reduction occurs if there is a gain of electrons. The oxidized component or oxidant is the electron acceptor and the reduced component or reductant is the electron donor. Table 8.1 lists oxidants and reductants found in natural environments. The electrons are not free in the soil solution, thus the oxidant must be in close contact with the reductant. Both oxidation and reduction must be co idered to completely describe oxidation-reduction (redox) reactions (Barden n James, 1993; Patrick et at., 1995). To determine if a particular reaction will occur (i.e., the Gibbs free energy for the rea 'Oll, j,Gr < 0), one can write reduction and oxidation halfreactions (a a '-reaction or half-cell reaction can be referred to as a redox couple) and al ulate equilibrium constants for the half-reactions. Redox reactions 0 oil oxidants can be defined conventionally by the following general hali-re ucrion reaction (Patrick et at., 1995);

Effect of redox potential on methanogenesis by Methanosarcina barkeri

Abstract: Concentrations of 0.5% O2 immediately inhibited CH4 production from methanol by Methanosarcina barkeri. Simultaneously, the redox potential of the medium increased to about +100 mV. However, the rates of CH4 production were not significantly affected, when the redox potential of an anoxic medium was adjusted to values between -420 mV and +100 mV by addition of titanium (III) citrate, sodium dithionite, flavin adenine dinucleotide, or sodium ascorbate. When the redox potential was adjusted to values between -80 mV and +550 mV by means of mixtures of ferrocyanide and ferricyanide, CH4 production was not inhibited until a redox potential of about +420 mV was reached. M. barkeri was able to reduce 0.5 mM ferricyanide solution at +430 mV within 0.005%.

Resolution of the reaction sequence during the reduction of O2 by cytochrome oxidase

Abstract: Time-resolved resonance Raman spectroscopy has been used to study the reduction of dioxygen by the mitochondrial enzyme, cytochrome oxidase. In agreement with earlier reports, Fe(2+)-O2 and Fe(3+)-OH- are detected in the initial and final stages of the reaction, respectively. Two additional intermediates, a peroxy [Fe(3+)-O(-)-O-(H)] and a ferryl (Fe4+ = O), occur transiently. The peroxy species shows an oxygen-isotope-sensitive mode at 358 cm-1 that is assigned as the nu(Fe(3+)-O-) stretching vibration. Our kinetic analysis indicates that the peroxy species we detect occurs upon proton uptake from bulk solution; whether this species bridges to Cu(B) remains uncertain. For the ferryl, nu(Fe(4+) = O) is at 790 cm-1. In our time-resolved spectra, the 358 cm-1 mode appears prior to the 790 cm-1 vibration. By using kinetic parameters deduced from the time-resolved Raman work and from a variety of time-resolved optical studies from other laboratories, we have assigned rate constants to several steps in the linear reaction sequence proposed by G. T. Babcock and M. Wikstrom [(1992) Nature (London) 356, 301-309]. Simulations of this kinetic scheme provide insight into the temporal behavior of key intermediates in the O2 reduction process. A striking aspect of the reaction time course is that rapid O2-binding and trapping chemistry is followed by a progressive slowing down of succeeding steps in the process, which allows the various transient species to build up to concentrations sufficient for their detection by our time-resolved techniques. Our analysis indicates that this behavior reflects a mechanism in which conditions that allow efficient dioxygen bond cleavage are not inherent to the active site but are only established as the reaction proceeds. This catalytic strategy provides an effective means by which to couple the free energy available in late intermediates in the reduction reaction to the proton-pumping function of the enzyme.

Reversible photoreductive deposition and oxidative dissolution of copper ions in titanium dioxide aqueous suspensions

Abstract: Particulate titanium dioxide was used to remove and concentrate Cu(II) ions in aqueous solutions through a cyclic process of photodeposition, separation, and oxidation. Illuminated, nitrogen-purged solutions containing copper sulfate, excess sodium formate (pH 3.6), and titanium dioxide formed a purple Cu-TiO[sub 2] species. Cu(II) concentrations in the supernatant were driven from 51 to [le] 0.018 [mu]g/mL. Upon purging with oxygen, this purple Cu-TiO[sub 2] system reverted back to white along with a corresponding increase in the Cu(II) supernatant concentration. The photodeposition step and the air oxidation step were utilized to demonstrate a volume reduction process. Eighty-six percent of the Cu(II) in a synthetic waste stream was concentrated to an organic-free solution having 7% of the initial volume. The remaining waste solution contained only 1% of the initial Cu(II), and in a subsequent step, the remaining formate ion was destroyed using conventional TiO[sub 2] photocatalytic oxidation. The overall process demonstrated the ability to separate copper ions from organics using only light and air. The reversible photoreduction deposition of Cu(II) from solution was observed in the pH range 1.84-6.60. The reversible photoreduction deposition of copper (II) was dependent on the organic used to scavenge holes and independent of the copper salt used.more » 33 refs., 4 figs., 4 tabs.« less

Electron Diffusion Coefficients in Hydrogels Formed of Cross-Linked Redox Polymers

Abstract: : The electron diffusion coefficient (De) for the redox polymer POs-EA, an ethylamine quaternized poly(vinylpyridine) complex of Os(bipyridine)2Cl2, has been directly measured by steady-state voltammetry at interdigitated array (IDA) electrodes. In cross-linked POs-EA De decreased upon increasing the ionic strength and upon changing the hydrophilic chloride counterion to the hydrophobic perchlorate anion. In 5.0 wt% cross-linked POs-EA De is pH dependent increasing from 4.5 x 10(-9) to 1.6 x 10(-8) cm2 sec(-1) as the pyridine rings are protonated. In highly (25 wt%) cross-linked POs-EA, the motion of the chains is restricted, De is independent of pH. The results show that De increases steeply upon hydration of POs-EA. It is proposed that the extent of electron diffusion in redox hydrogel is determined by segmental motion of the polymer backbone and is therefore enhanced, rather than diminished upon swelling of the redox polymer network, that increases the static distances between redox centers.... Hydrogels, Cross-linked Redox Polymers, Poly(vinylpyridine), Os(bipyridine)2Cl2.

Models for the redox active site in galactose oxidase

Abstract: Modeling approaches have been used to develop insight into the nature of the redox active site of the free-radical-containing copper metalloenzyme galactose oxidase. The optical spectrum of the free radical generated by low-temperature UV irradiation of (methylthio)cresol is nearly identical to that observed for the free-radical site in metal-free apo galactose oxidase, supporting the assignment of the protein radical to a novel tyrosine-cysteine covalent cross-link structure recently reported from X-ray crystallographic studies. Basic characterization of the chemistry for this new type of biological redox group includes measurements of the substituent effects on phenolic proton acidic and observation of the oxygenation of (methylthio)cresol with peroxides