Peroxynitrous acid generation

Peroxynitrous acid is a powerful oxidizing agent with estimated one- and two-electron reduction potentials of ° (ONOOH, H+/"N02, HjO) = 1.6-1.7 V and ° (ONOOH, H /N02 , H2O) = 1.3-1.4 V, respectively . In addition, it was reported that, upon protonation, ONOO can undergo decomposition via homolytic 0—0 cleavage to generate nitrogen dioxide radical ("NO2) and hydroxyl radical ( OH) in approximately 30% yields... 

Finally, the decomposition of peroxynitric acid, H02N02, which is strongly temperature dependent, generates H02 directly ... 

Figure 7.9, for example, shows the decay of H02N02 and the formation of HONO and HN03 in their chamber. The peroxynitric acid was generated by reaction (27), where the H02 was formed by the bromine atom initiated oxidation of formaldehyde in air. Zhu et al. 

Peroxynitrite is capable of initiating many of the reactions commonly attributed to hydroxyl radical, particularly under acidic conditions. Halfpenny and Robinson (1952a,b) showed that nitrous acid plus hydrogen peroxide in aqueous solutions at pH 2, which generates peroxynitrous acid, initiated the polymerization of methylmethacrylate (the precurser to Plexiglas) as well as the hydrox-ylation, nitration, and polymerization of benzene. 

Also, a novel nonphotolytic method for generating HOz and HO radicals has been successfully used for FTIR-based kinetic and mechanistic studies [32,33]. This method involves the use of peroxynitric acid (H00N02, PNA) as a thermal source of H02 radicals which in the presence of NO react rapidly to give HO radicals ... 

The rate constants for oxidation of alkanes and alkenes with peroxynitrous acid in aqueous-gas phase are bell-shaped functions of the volume ratio between the liquid and gas phases. The kinetics of the generation of OH radicals and its importance in understanding the mechanism of lipid membrane oxidation has been stressed.236... 

Peroxynitrous acid, ONOOH, forms in another photochemical channel at shorter wavelengths but is absent at k > 300 nm. The O-atoms generated in reaction 99 may react with 02([02 Water 0.3 mM) via reaction 100 or, preferably, with nitrate via reaction 101. Nitrite (smax = 22.5 M 1 cm1 at 360 nm) will undergo secondary photolysis, reaction 102, and oxidation by OH radicals, reaction 103 ... 

In the presence of organic compounds, peroxynitrous acid induces oxidation and nitration processes [72,74,76,77]. Oxidation reactions are due to the generation of hydroxyl (reaction 28). In the case of phenol, nitration is most likely to be electrophilic as evidenced by the very steep pH trend of nitrophenol initial formation rates ( Rate oc [H+] [57]). 

Generation of Fe2+ also takes place upon irradiation of dissolved Fe(III) in acidic solution (photo-Fenton reaction [96]). It is, however, difficult to study phenol nitration in the Fe(III)/H202/HN02/UV system due to the very fast thermal reaction between H2O2 and HNO2 to yield peroxynitrous acid, HOONO, that also nitrates phenol [57]. 

Peroxynitrite is converted to its protonated form, peroxynitrous acid (ONOOH) with a pRa of 6.8. This acid form undergoes isomerization to form nitrate anion with a rate constant of 1.3 s [Eq. (12)]. Peroxynitrite is a powerful oxidant and during oxidation reactions with substrates, it undergoes homolysis, generating two powerful oxidants OH and NO2 radicals (-28%) [Eq. (13)], which can mediate oxidation, nitration and nitrosation reactions leading to impaired function of biomolecules. 

Peroxynitrous acid, however, is not stable and decomposes to yield HO and NO2 [224]. When NO2 reacts with NO, N2O3 is generated. A comprehensive discussion of these RNS is outside the scope of this paper. However, it should be noted that all these RNS are highly reactive, short-lived species. Therefore, their quantitative assay is a challenging task particularly under in vivo conditions Nitrite (NO2 ) that results e.g. from the decomposition of N2O3, is often used as a marker of the NO production under in vivo conditions and it is known that under inflammatory conditions the concentration of nitrite is strongly elevated. For instance it was shown that nitrite concentrations of up to 4 mmol/1 can be detected in synovial fluids of the patients suffering from RA [226]. 

Uric acid is present in human plasma at much higher levels than those encountered in other primates because the enzyme urate oxidase is absent from human tissues (Cutler 1984). it has therefor e been proposed that uric acid is an important antioxidant for humans (Ames et al. 1981). Urate reacts with peroxynitrite with an apparent second order rate constant of 4.8 x 10 M s in a complex process, which is accompanied by oxygen consumption and formation of allantoin, alloxan, and urate derived radicals (Santos et al. 1999). The main radical was identified as the aminocarbonyl radical by the electrospray mass spectra of its 5,5-dimethyl-/-pyrroline-N-oxide adduct. Mechanistic studies suggested that urate reacts with peroxynitrous acid and with the radicals generated from its decomposition to form products that can further react with peroxynitrite anion. 

The oxidation of thiophene (Th) by peroxynitrous acid (HOONO), generated in the H2O2-HNO2 system, proceeds simultaneously in both the gaseous and the liquid phases of the reactor. The conclusion that the active oxidant in both cases is the HO radical, formed by HO-ONO bond homolysis, is based on the equality of the rate constants of the reaction in the two phases. ... 

Peroxynitrite. - In terms of its ability to induce biomolecular injury, by far the most important reaction of NO is its combination with 02 to form the oxoperoxonitrate(— 1) ion (ONOO , peroxynitrite ) [k = (6.7-19) x 10 M s ]. Whereas ONOO is relatively stable, peroxynitrous acid (ONOOH, pKa 6.5-6.S) undergoes rapid decay to the harmful OH and NO2 radicals at a yield of ca. 30% (see Tsai et al. and references therein ). Although there are several cellular sources of 02 for combination with NO, spin trapping studies have shown that NOS itself may generate the radical, but not without attracting controversy. 

In ambient air, the primary removal mechanism for acrolein is predicted to be reaction with photochemically generated hydroxyl radicals (half-life 15-20 hours). Products of this reaction include carbon monoxide, formaldehyde, and glycolaldehyde. In the presence of nitrogen oxides, peroxynitrate and nitric acid are also formed. Small amounts of acrolein may also be removed from the atmosphere in precipitation. Insufficient data are available to predict the fate of acrolein in indoor air. In water, small amounts of acrolein may be removed by volatilization (half-life 23 hours from a model river 1 m deep), aerobic biodegradation, or reversible hydration to 0-hydroxypropionaldehyde, which subsequently biodegrades. Half-lives less than 1-3 days for small amounts of acrolein in surface water have been observed. When highly concentrated amounts of acrolein are released or spilled into water, this compound may polymerize by oxidation or hydration processes. In soil, acrolein is expected to be subject to the same removal processes as in water. 

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