Reduction of hydrogen peroxide

These and similar results can be explained if the simultaneous reduction of hydrogen peroxide is due to an induced reaction. To show the characteristic features of this reaction some results are presented in Table 19 and Table 20. The procedure for these measurements was as follows. The solution of peroxy compounds given in columns 1 and 2 was made up to 20 ml and the pH was adjusted to the given value. Then potassium thiocyanate solution was added and, after the reaction time noted, the process was quenched by adding potassium iodide solution (0.3 g KI). After 5 sec the solution was acidified with 1 ml 2 iV sulphuric acid then using, molybdate catalyst solution, the iodine liberated was titrated with standard thiosulphate. 

The data in Tables 19 and 20 show that both peroxysulphuric and peroxyacetic acid give the induced reduction of hydrogen peroxide by thiocyanate. The induced reduction depends strongly on the pH of the solution and on the concentration of thiocyanate. At pH > 3 both peroxy acids reacts more slowly... 

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

On the basis of these results it can be stated that the fast oxidation of thiocyanate by peroxy acids gives rise to the induced reduction of hydrogen peroxide. In order to elucidate the mechanism of this interesting reaction let us have a look at reactions of thiocyanate with peroxy compounds of different types. 

Reactions noninvertible on principle or, more probably, reactions for which no conditions have been found so far under which they will proceed in the opposite direction. An example of such a reaction is the cathodic reduction of hydrogen peroxide ... 

Polarization is much higher for the electrochemical reduction of hydrogen peroxide. The slope has the unusually high value of about 0.3 V. At a given current density the electrode potential in this reaction is again independent of solution pH. These and certain other data indicate that addition of the first electron to the peroxide molecule and simultaneous peroxide decomposition is the rate-determining step ... 

Armstrong FA, Bond AM, Buchi FN, Hanmett A, Hill HAO, Lannon AM, Lettington OC, Zoski CG. 1993. Electrocatalytic reduction of hydrogen-peroxide at a stationary pyrol3ftic-graphite electrode surface in the presence of cytochrome-c peroxidase— A description based on a microelectrode array model for adsorbed enzyme molecules. Analyst 118 973-978. 

The reduction of hydrogen peroxide at a mercury electrode is also a stepwise electrode reaction, where the second step once again is fast, so that the observed electrode reaction corresponds to the transfer of two electrons ... 

Peroxidases have been implicated in carcinogenesis by PAH, aromatic amines, and estrogens inter alia (6-9). These enzymes catalyze the reduction of hydrogen peroxide and organic... 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

However, at high rates of nitric oxide flux, the formation of nitrated and oxidized products became insensitive to the presence of catalase or MPO inhibitors but increasingly inhibited by SOD, suggesting the participation of peroxynitrite. (It is interesting that Reaction (30) might be a one-electron reduction of hydrogen peroxide by nitrite ion. If such a process really takes... 

It is readily seen that tha e are also two types of oxidizing molecular species. The reduction of oxygen according to (2) was included in the basic considerations. Reduction of hydrogen peroxide is of the same type... 

Coulter et al. (1999,2000b) showed that, in vitro, D. vulgaris Rbr has the ability to function as the terminal component of an NADH peroxidase, catalyzing the reduction of hydrogen peroxide to water (Eq. 10.4). 

Another pure-chemical route to the generation of the carbon dioxide anion-radical is the reduction of hydrogen peroxide with Ti + in the presence of formate ions (Morkovnik and Okhlobystin 1979) Ti + -E H2O2 TU+ -E QH -E OH and OH -E HCOO H2O + COj . ... 

The application of CPO, HRP and CiP is limited to sterically unencumbered substrates and all these peroxidases produce the same absolute configuration of the chiral hydroperoxide. To overcome this limitation, the semisynthetic enzyme selenosubtilisin, a mimic for glutathione peroxidase, with the peptide framework of the serine protease subtilisin was developed by Bell and Hilvert. This semisynthetic peroxidase catalyzes the reduction of hydrogen peroxide and hydroperoxides in the presence of 5-mercapto-2-nitrobenzoic acid. It was utilized by Adam and coworkers and Schreier and coworkers for the kinetic resolution of racemic hydroperoxides (equation 17) . The results obtained were very promising. 

SCHEME 5. Mechanism of electrocatalytic reduction of hydrogen peroxide with a biosensor. Top Direct reduction with enzyme-modified CPE. Bottom Accelerated reduction with enzyme-modified CPE, incorporating a mediator M (e.g. 150). The potential is relative to SCSE... 

In aqueous solution, the catalytic reduction of hydrogen peroxide was observed on the wave corresponding to the reduction of Fe(III) to Fe(II) within a-[Fe(0H2)SiWii039] and a-[Fe(OH2)P2Wi706i] [156]. The rate constants for the process triggered by the... 

The reactions of dioxygen have been amply documented The reduction can occur by a one-, two- or four-electron transfer reaction, the first of which is energetically unfavorable. The one-electron reduction of hydrogen peroxide yields the extremely reactive hydroxyl radical (Scheme 1). 

GSH see Fig. 22-27) donates electrons for the reduction of hydrogen peroxide (H202) and of oxidized Cys residues (—S—S—) in proteins, and GSH is regenerated from the oxidized form (GSSG) by reduction with NADPH. 

A. Structure of glutathione (G-SH). [Note Glutamate is linked to cysteine through a y-carboxyl, rather than an a-carboxyl.] B. Glutathione-mediated reduction of hydrogen peroxide by NADPH. 

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