Hydrogen peroxide, also

The sulfur analog of hydrogen peroxide also exists and is an example of a polysulfane, a catenated molecular compound of composition HS—S —SH, where n can take on values from 0 through 6. The polysulfide ions obtained from the polysulfanes include two ions found in lapis lazuli (Fig. 15.15). 

Ascorbate is known to act as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, reduced ascorbate can react non-enzymatically with molecular oxygen to produce dehydroascorbate and hydrogen peroxide. Also, ascorbate in the presence of light, hydrogen peroxide and riboflavin, or transition metals (e.g. Fe, Cu " ), can give rise to hydroxyl radicals (Delaye and Tardieu, 1983 Ueno et al., 1987). These phenomena may also be important in oxidative damage to the lens and subsequent cataract formation. 

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

Depletion of other cofactors such as UTP, NADH, and NADPH may also be involved in cell injury either directly or indirectly. Thus, the role of NADPH in maintaining reduced GSH levels means that excessive GSH oxidation such as caused by certain quinines, which undergo redox cycling, may in turn cause NADPH depletion (see below). Alternatively, NADPH may be oxidized if it donates electrons to the foreign compound directly. However, NADPH may be regenerated by inter conversion of NAD+ to NADP+. Some quinones such as menadione, l,2-dibromo-3-chloropropane (DBCP), and hydrogen peroxide also cause depletion of NAD, but probably by different mechanisms. Thus, with menadione, the depletion may be the result of... 

Although 2e reduction with fast, chemically catalyzed H202 disproportionation, seems to be the mechanism of highest importance, it had been shown that hydrogen peroxide also can be electrocatalytically reduced to water according to... 

Aqueous hydrogen peroxide also causes the liberation of oxygen from perdisulphate solutions, probably on account of interaction with Caro s acid first formed.4 The reaction is considerably retarded by the presence of acid. 

Hydrogen peroxide also adds well to alkenes with heteroatom substituents on the double bond, as exemplified by the reactions of vinyl ethers (equation 235)368,369 and enamines (equation 236).370... 

Much attention has focused on vitamin K 1 because of its function as an obligatory cofactor in enzymic sequences central to blood clotting. The role of molecular oxygen in the formation of vitamin K oxide 2 has been studied intensively, and the mechanism of the 1 —> 2 transformation has been the subject of much controversy. Oxidation of 1 with basic hydrogen peroxide also gives 2, and two obvious mechanisms can be postulated for this model oxidation. 180 labelling studies have been used to distinguish between these mechanisms. 

The sign in front of the reaction term is positive only for hydrogen peroxide. Also, the function 3Ipn can be made constant by operating the sensor in a medium of high buffer capacity. This is clearly a distinct advantage compared to the potentiometric sensors in which the buffer capacity represented a major interference. 

Nitrous oxide is relatively soluble in water (2 x 10 2 M at atmospheric pressure), it reacts very rapidly with eaq( 7 x 109 M 1 s 1) and is practically inert toward OH and H (for the latter, k < 104 M 1 s 1). Because of partial scavenging of e q from the spurs G(OH) in N20-saturated solutions is 6-0. Hydrogen peroxide also converts ejq into OH very efficiently [reaction (6)], but it has... 

In alkaline solution potassium ferrioyanide and hydrogen peroxide also yield a steady stream of oxygen which can be immediately cheeked by the addition of an acid.6... 

All the cases reported seem to agree with this hypothesis, except of the formation of compound 9, which can be ascribed to oxidation by oxygen derived from decomposition of hydrogen peroxide. Also penta-methylpyrrole is atypical, since it is a.a -disubstituted. The following mechanism has been proposed by Seebach8 to explain the formation of the hydroperoxy derivative (56). 

Hydrogen peroxide also has a high dielectric constant, but not so great as water (73 for HjOj at 20 C). Gross and Taylor (834) concluded that H2O2 may be less H bonded than H2O because of its two equivalent nonplanar, bent forms which can be easily interconverted by rotation about the 0-0 axis. Because of this structure, the molecules do not fit such a compact lattice. ... 

Fe3+ -t H2O2 Fe2+ -1- OOH -l- H+ Hydrogen peroxide also decomposes in the presence of ozone as... 

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