Hydrogen peroxide—reductant system

The influence of the structure of the ad-atom layers on the reduction of oxygen was examined by using in situ optical and scaiming probe microscopies. With these techniques reliable information on the structure of the ad-atom layers can be obtained during the course of oxygen or hydrogen peroxide reduction. In most systems studied, the UPD adlattice structures are not affected by the presence of molecular oxygen in solution. On the basis of... 

Glassy carbon electrodes have been modified with a hydrophilic, permeable film of horseradish peroxidase (HRP) covalently bound to a polyvinylpyridine polymer complexed with osmium to enhance the detection of hydrogen peroxide. Vreeke et al demonstrated that such a system could be used to quantify hydrogen peroxide (reduction at 0.0 V, SCE) produced from complex coupled reactions in their assay of NADH ... 

Chemical kinetics at tubular electrodes (systems of circular cross-section) have been considered recently in [77, 78]. Both for catalytic ErevCcat and for E vCirr (follow-up) mechanisms the theory of linear-sweep voltammetry and cyclic voltammetry was elaborated. The effects of the reaction rate constant, of the flow velocity, and of the potential scan rate on the shape of current curves are presented graphically. The deductions derived from both theories were tested on the reduction of Fe(III) in the presence of hydrogen peroxide (catalytic system) and on the oxidation of 1,4-phenylenediamine in alkaline medium (E vCin. mechanism). 

Staircase current response A staircase current is observed when the particles collide with the electrode surface and stick (or adsorb). A steady-state current response is obtained with the following electrocatalytic systems proton reduction, hydrazine oxidation, and hydrogen peroxide reduction. In many cases, the staircase current is distorted by a current decay after collision events. This decay is caused by a deactivation process, such as the adsorption of impurities, and the rate of decay depends on the experimental conditions. 

The aim of this complex system is to obtain optically pure amino acid from the corresponding racemate without the necessity to separate the a-keto acid. D-Methionine is oxidized to 2-oxo-4-methylthiobutyric acid by D-amino acid oxidase (D-AAO). Catalase is used to decompose formed hydrogen peroxide. Reduction of 2-oxo-4-methylthiobutyric acid to L-methionine is accompHshed with L-phenylalanine dehydrogenase (L-PheDH), requiring a coenzyme NADH. The latter needs regeneration, which explains a necessity to include formate dehydrogenase in this system. 

They showed that molecular oxygen was a requirement and, in the peanut system, that a hydrogen peroxide generating system was involved. Enzymes which catalysed the reduction of peroxides, such as glutathione peroxidase, reduced a-oxidation and increased the production of D-hydroxypalmitate. This pointed to the existence of a peroxide, 2-hydroperoxypalmitate,... 

As in the case of the steroids, introduction of additional nuclear substituents yields morphine analogs of increased potency. The more important of these are derived from one of the minor alkaloids that occur in opium. Thebaine (14), present in crude opium in about one-tenth the amount of morphine, exhibits a reactive internal diene system that is well known to undergo various addition reactions in a 1,4 manner (e.g., bromination). Thus, reaction with hydrogen peroxide in acid may be visualized to afford first the 14-hydroxy-6-hemiketal (15). Hydrolysis yields the isolated unsaturated ketone (16). Catalytic reduction... 

DEPENDENCE OF INDUCED REDUCTION OF HYDROGEN PEROXIDE ON EXPERIMENTAL CONDITIONS IN THE H2O2-H2SO5-SCN SYSTEM... 

The heart has a relatively low catalase activity, which, together with the superoxide dismutase (SOD) system, acts to remove hydrogen peroxide and superoxide radicals. In addition, in man, dietary vitamin C plays an important role in the reduction of vitamin E, an intrinsic antioxidant component of biological membranes (Chen and Thacker, 1986 Niki, 1987). Both vitamins C and E can also react directly with hydroxyl and superoxide radicals (HalliwcU and Gutteridge, 1989 Meister, 1992). 

In the M. capsulatus (Bath) system, all three components are necessary to obtain turnover with NADH as the reductant (57). With the M. trichosporium OB3b system, protein B is apparently not required (27). Instead, in this latter system, protein B increases the initial rates of the catalytic hydroxylation reaction (27). Catalysis can be achieved by means of a shunt pathway with hydrogen peroxide and Hox alone from both organisms (58-60). The efficiency of the shunt pathway, however, varies significantly. With M. trichosporium OB3b, alcohol yields greater than those obtained with the completely reconstituted system have been observed (58). Furthermore, upon addition of protein... 

The reduction of Fe(III) by carotenoids may have deleterious consequences. The reduced iron Fe(II) can react with hydrogen peroxide leading to the formation of hydroxyl radical, the most reactive free radical encountered in biological systems (Equation 15.10) ... 

Sonoelectrochemistry has also been used for the efficient employment of porous electrodes, such as carbon nanofiber-ceramic composites electrodes in the reduction of colloidal hydrous iron oxide [59], In this kind of systems, the electrode reactions proceed with slow rate or require several collisions between reactant and electrode surface. Mass transport to and into the porous electrode is enhanced and extremely fast at only modest ultrasound intensity. This same approach was checked in the hydrogen peroxide sonoelectrosynthesis using RVC three-dimensional electrodes [58]. 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

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