Two-electron Oxidation Reaction Research Articles

Ascorbic acid oxidation in SDS micellar systems

In this contribution the results of atmospheric and electrochemical oxidation of AA in the SDS micellar solutions and in the microemulsions pentanol/water stabilized by SDS are presented. The hydrophilic vitamin C readily dissolves in water and O/W microemulsions as well as undergoes the irreversible two-electron oxidation reaction. It was found that the atmospheric oxidation of AA is accelerated by the SDS up to the CMC and inhibited in the concentrated SDS solutions. We also found that the rate of atmospheric oxidation of ascorbic acid is higher in the water-in-oil (W/O) than in the oil-in-water (O/W) microemulsions and increases with the increasing oil content. The influence of the SDS on the electrochemical behavior of vitamin C was also studied. The general conclusion emerging from this investigation is that the increasing surfactant concentration shifts the ascorbic acid oxidation potential to higher values whereas the corresponding peak current values diminish. In the microemulsions the AA oxidation is slowed down by increasing the pentanol amount in the system.

Kinetics of Two-Electron Oxidations by the Compound I Derivative of Chloroperoxidase, a Model for Cytochrome P450 Oxidants

[structure: see text] Rate constants for two-electron oxidation reactions of Compound I from chloroperoxidase (CPO) with a variety of substrates were measured by stopped-flow kinetic techniques. The thiolate ligand of CPO Compound I activates the iron-oxo species with the result that oxidation reactions are 2 to 3 orders of magnitude faster than oxidations by model iron(IV)-oxo porphyrin radical cations containing weaker binding counterions.

A Coupled Dinuclear Iron Cluster that Is Perturbed by Substrate Binding in myo-Inositol Oxygenase

myo-Inositol oxygenase (MIOX) uses iron as its cofactor and dioxygen as its cosubstrate to effect the unique, ring-cleaving, four-electron oxidation of its cyclohexan-(1,2,3,4,5,6-hexa)-ol substrate to d-glucuronate. The nature of the iron cofactor and its interaction with the substrate, myo-inositol (MI), have been probed by electron paramagnetic resonance (EPR) and Mössbauer spectroscopies. The data demonstrate the formation of an antiferromagnetically coupled, high-spin diiron(III/III) cluster upon treatment of solutions of Fe(II) and MIOX with excess O(2) or H(2)O(2) and the formation of an antiferromagnetically coupled, valence-localized, high-spin diiron(II/III) cluster upon treatment with either limiting O(2) or excess O(2) in the presence of a mild reductant (e.g., ascorbate). Marked changes to the spectra of both redox forms upon addition of MI and analogy to changes induced by binding of phosphate to the diiron(II/III) cluster of the protein phosphatase, uteroferrin, suggest that MI coordinates directly to the diiron cluster, most likely in a bridging mode. The addition of MIOX to the growing family of non-heme diiron oxygenases expands the catalytic range of the family beyond the two-electron oxidation (hydroxylation and dehydrogenation) reactions catalyzed by its more extensively studied members such as methane monooxygenase and stearoyl acyl carrier protein Delta(9)-desaturase.

Formation of PPh2C6F5 through Phosphido Platinum and/or Palladium(III) Intermediates,

The two-electron oxidation reactions of the neutral [(C6F5)2M(μ-PR2)2M‘(NCCH3)2] (M = M‘ = Pt or Pd, M = Pt, M‘ = Pd) complexes using I2 as oxidant have been investigated by experimental (R = Ph) a...

Myeloperoxidase Metabolizes Thiocyanate in a Reaction Driven by Nitric Oxide

We examined the potential physiological relevance of myeloperoxidase (MPO)-nitric oxide (NO) interactions as they may relate to the cosubstrate, pseudo halide thiocyanate (SCN(-)), and substrate switching. Direct spectroscopic and rapid kinetics studies revealed that SCN(-) interaction with MPO facilitates formation of the MPO catalytic intermediate Compound II, limiting overall activity. However, a physiological NO concentration (2 microM or less) dramatically influences the build-up, duration, and decay of the steady-state level of MPO Compound II during the metabolism of SCN(-), allowing the enzyme to function at full capacity. At higher NO concentrations, we observed significant increases in the rate of MPO Compound II formation, along with proportional increases in its duration as determined by the time elapsed during catalysis. Surprisingly, the decay rate of MPO Compound II remained unaltered as NO concentrations were increased. Computer simulations were carried out to model the kinetics of MPO Compound II formation, duration, and decay during the metabolism of SCN(-) as a function of NO concentration. These simulation traces closely approximate what was observed experimentally and support the involvement of a conformational intermediate of MPO Compound II complex decay, altering the overall capacity of MPO to promote two electrons versus one-electron oxidation reactions during steady-state catalysis. Collectively, the present studies reveal that (patho)physiologically relevant levels of NO have significant effects on MPO Compound II accumulation. Thus, NO affects the overall rate of peroxidation of substrates and the overall ability of the peroxidase to execute one- versus two-electron oxidation reactions.

Reaction of ferrous lactoperoxidase with hydrogen peroxide and dioxygen: an anaerobic stopped-flow study

Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions including milk, tears, and saliva and has physiological significance in antimicrobial defense which involves (pseudo-)halide oxidation. LPO compound III (a ferrous–dioxygen complex) is known to be formed rapidly by an excess of hydrogen peroxide and could participate in the observed catalase-like activity of LPO. The present anaerobic stopped-flow kinetic analysis was performed in order to elucidate the catalytic mechanism of LPO and the kinetics of compound III formation by probing the reactivity of ferrous LPO with hydrogen peroxide and molecular oxygen. It is shown that ferrous LPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl LPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (7.2 ± 0.3) × 10 4 M −1 s −1 at pH 7.0 and 25 °C. The H 2O 2-mediated conversion of compound II to compound III follows also second-order kinetics (220 M −1 s −1 at pH 7.0 and 25 °C). Alternatively, compound III is also formed by dioxygen binding to ferrous LPO at an apparent bimolecular rate constant of (1.8 ± 0.2) × 10 5 M −1 s −1. Dioxygen binding is reversible and at pH 7.0 the dissociation constant ( K D) of the oxyferrous form is 6 μM. The rate constant of dioxygen dissociation from compound III is higher than conversion of compound III to ferric LPO, which is not affected by the oxygen concentration and follows a biphasic kinetics. A reaction cycle including the redox intermediates compound II, compound III, and ferrous LPO is proposed, which explains the observed (pseudo-)catalase activity of LPO in the absence of one-electron donors. The relevance of these findings in LPO catalysis is discussed.

Experimental and Quantum Chemical Study of the Mechanism of an Unexpected Intramolecular Reductive Coupling of a Bridging Phosphido Ligand and a C6F5 Group and the Reversible Oxidative Addition of PPh2C6F5,

The two-electron oxidation reactions of the [NBu4]2[(C6F5)2M(μ-PPh2)2M‘(C6F5)2] (M = M‘ = Pt, 1a; M = M‘ = Pd, 1b; M = Pt, M‘ = Pd, 1c) complexes using I2 as oxidant have been investigated by exper...

Cyclobutadiene Dianion

Abstract In this account, we present the synthesis and chemistry of the dilithium salts of the cyclobutadiene dianion (CBD2−), which contains a doubly charged four-membered ring system. The transmetalation reaction of tetrasilyl-substituted cyclobutadiene (CBD) cobalt complexes (5,6) with lithium metal led to the formation of the corresponding dilithium salts of CBD2− (10,11). The aromaticity of the CBD2− species has been studied on the basis of structural characteristics and magnetic properties. The dilithium salts of CBD2− stabilized by both phenyl and silyl groups (12–14) were also prepared by a similar procedure, starting from the corresponding CBD cobalt complexes (7–9). The CBD2− species have been fully characterized by NMR studies and X-ray crystallography. The two-electron oxidation reaction of the tetrasilyl-CBD2− (10,11) with 1,2-dibromoethane produced the neutral CBD derivatives (17,22). The structures, properties, and reactivities of the tetrasilyl-substituted CBD are also reviewed here. Tetrakis(trimethylsilyl)tetrahedrane (26) was synthesized by irradiation of tetrakis(trimethylsilyl)cyclobutadiene (17). The structure and properties of tetrahedrane (26) are presented.

Direct conversion of ferrous myeloperoxidase to compound II by hydrogen peroxide: an anaerobic stopped-flow study

Myeloperoxidase (MPO) is one of the essential components of the antimicrobial systems of polymorphonuclear neutrophils. It is unique in having a globin-like standard reduction potential of the ferric/ferrous couple. Here, it is shown that ferrous MPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl MPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (6.8 ± 0.6) × 10 4 M −1 s −1 at pH 7.0. After depletion of (micromolar) H 2O 2 compound II slowly decays to ferric MPO, whereas upon addition of millimolar H 2O 2 to ferrous MPO, compound III (oxyperoxidase) is formed in a sequence of two reactions involving compound II formation and its direct reaction with H 2O 2, which also follows second-order kinetics [(78 ± 2) M −1 s −1 at pH 7.0]. It is discussed how these reactions contribute to the interconversion of compound II and compound III and could explain the catalase activity of MPO.

One- and two-electron oxidation reactions of trolox by peroxynitrite.

One and two electron oxidation reactions of peroxynitrite with trolox, an analogue of vitamin E were studied. Kinetics of the reaction was studied using a single mixing stopped-flow spectrometer in the pH range from 6 to 11 and the products were analyzed by HPLC/HPIC. One-electron oxidation reaction produced the phenoxyl radical of trolox having a characteristic absorption spectrum in the 300-500 nm region with absorption maximum at 430 nm. The rate constant for the formation of the trolox radical at 430 nm was determined to be 1.8 x 10(5) M(-1) s(-1). At low concentration of peroxynitrite the radical did not show any noticeable decay in the time scale of 20-50 s. However, when the concentration of peroxynitrite was increased, there was an appreciable increase in the decay of the trolox radical due to its reaction with excess peroxynitrite yielding a bimolecular rate constant of 6.0 x 10(3) M(-1) s(-1) for the reaction of trolox radical with peroxynitrite. The released nitrite and nitrate anions, and the two-electron oxidation product quinone produced as a result of these reactions, were estimated using ion chromatography and HPLC analysis. By following the changes in the yields of nitrite and quinone as a function of peroxynitrite concentration and from the above determined rate parameters, the relative importance of the one and two electron processes has been discussed.

Characterization of Polyethylene Glycolated Horseradish Peroxidase in Organic Solvents: Generation and Stabilization of Transient Catalytic Intermediates at Low Temperature

Polyethylene glycolated horseradish peroxidase (PEG-HRP) can catalyze one- and two-electron oxidation reactions in organic solvents as well as in aqueous buffer. Even though the oxidation of guaiacol in benzene and chlorobenzene is 5 orders of magnitude slower than in phosphate buffer, compounds I and II are involved in the catalytic cycle in organic media. Factor analysis and global fittings of rapid scan data set reveal that the formation of compound I of PEG-HRP in organic media consists of two steps (the first fast and the second slow) and suggest the involvement of a H2O2−HRP complex in the catalytic cycle. The labile precursor of compound I is stabilized when PEG-HRP reacts with hydrogen peroxide in chlorobenzene at −20 °C. The absorption spectrum of the precursor does not exhibit the features of hyperporphyrin spectrum but has a normal Soret as previously observed in R38L HRP. More importantly, compound I of PEG-HRP can be maintained for more than an hour at −20 °C in chlorobenzene.

Reactions of Pyridine Coenzyme Dimers and Monomers with Viologens

DCMV++(1,1′-dimethyl-2,2′-dicyano-4,4′-bipyridinium,bis-methylsulfate) promotes the aerobic oxidation of the NAD(P) dimers (NADP)2and (NAD)2with the formation of 2 mol of NADP+or NAD+per mole of dimers. The reaction appears to follow a pseudo-first-order kinetics with respect to the dimer concentration. One mole of oxygen was consumed in the reaction per mole of NAD(P) dimer oxidized and hydrogen peroxide was produced. The monomers NADPH and NADH under the same reaction conditions were not oxidized by DCMV++. In anaerobiosis NAD(P) dimers but not NAD(P)H rapidly reduced DCMV++to its radical cation DCMV•+, which was rapidly back-oxidized by air to its parent dication. Paraquat (MV++) was also able to catalyze the aerobic oxidation of NAD(P) dimers and, at a much lower extent, NADPH and NADH, but only under light irradiation. In anaerobiosis and upon light irradiation all the above nucleotides were able to convert paraquat to its radical cation MV•+, reoxidized to MV++by air admission. This study shows the different ability of NAD(P) dimers and NAD(P)H to undergo one-electron and two-electron oxidation reactions, with different viologens.

The electrochemical reactivity of [Pt(AuPPh3)8]2+ and [HPt(AuPPh3)8]+ under dihydrogen

Abstract [(H) 2 Pt(AuPPh 3 ) 8 ] 2+ is electrochemically reduced in a dihydrogen atmosphere at about −1.62 V in various solvents following a CEE reaction path. A chemical step (C), i.e. the fast dissociation into dihydrogen and [Pt(AuPPh 3 ) 8 ] 2+ , precedes the two closely spaced one-electron reduction steps (EE) shown by [Pt(AuPPh 3 ) 8 ] 2+ . Long-term experiments (controlled potential electrolysis) revealed that in a chemical follow-up reaction [HPt(AuPPh 3 ) 8 ] + is formed. The reduced cluster, [Pt(AuPPh 3 ) 8 ] O reacts as a base with the starting dihydride cluster as an acid. In a basic solvent like pyridine [HPt(AuPPh 3 ) 8 ] + undergoes a two-electron oxidation reaction. In chemical follow-up reactions the formed 3+ cluster reacts immediately with the solvent and dihydrogen and the starting material [HPt(AuPPh 3 ) 8 ] + is regenerated and C 5 H 5 NH + is formed. Thus an EC′ reaction mechanism is operative (C′ = catalytic reaction) in which dihydrogen is oxidised and whereby [HPt(AuPPh 3 ) 8 ] + acts as the catalyst.

4] Glutathione : Dehydroascorbate oxidoreductases

This chapter focuses on the dehydroascorbate oxidoreductases of glutathione. Dehydro- L -ascorbic acid (DHA), the first chemically stable oxidation product of L -ascorbic acid (AAH 2 ), is generated in plant or animal cells from L -ascorbic acid by one- or two-electron oxidation reactions in which semidehydroascorbic acid (AAH . ) may or may not be a required intermediate. Superoxide is produced by respiration and by a number of enzymes, for example, xanthine oxidase, aldehyde oxidase, and others. Oxidation of many compounds including hydroquinone, flavins, thiols, catecholamines, dialuric acid, herbicides, paraquat, and CCl 4 produces O 2 - . Two mammalian DHA reductases have been identified. The cytoplasmic reductase, also known as thioltransferase or glutaredoxin, has been purified from numerous sources, including rat liver, pig liver, calf thymus, rabbit bone marrow, human placenta, and human erythrocytes. The DHA reductase properties of thioltransferase and protein disulfideisomerase have been reported. By using site-directed mutagenesis techniques, the essential amino acids at the catalytic center of porcine thioltransferase (dehydroascorbate reductase) are determined.