Peroxynitrous acid decomposition

Coddington JW, Hurst JK, Lymar SV (1999) Flydroxyl radical formation during peroxynitrous acid decomposition. J Am Chem Soc 1999 2438-2443... 

In the last 10 to 15 years, many experimental and theoretical studies have been dedicated to the study of peroxynitrite reactions. Free radical and non-free radical mechanisms of peroxynitrite action have been proposed, which were discussed in numerous studies (see for example, Refs. [103-110]). In accord with non-radical mechanism an activated form of peroxynitrous acid is formed in the reaction of superoxide with nitric oxide, which is able to react with biomolecules without the decomposition to HO and N02 radicals. 

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 ... 

The oxidation of primary and secondary alcohols by stable organic nitroxyl radicals has been reviewed.111 The kinetics of reactions of alkanes and arenes with peroxynitrous acid suggest the participation of the same active oxidizing species in both gas and aqueous phase HOONO or its decomposition product OONO. 112 The oxidation of the alkaloids reserpine and rescinnamine by nitric acid has been studied.113... 

Keith WG, Powell RE (1969) Kinetics of decomposition of peroxynitrous acid. J Chem Soc (A) 90 Keyer K, Imlay JA (1997) Inactivation of dehytratase [4F3-4S] clusters and disruprion of iron home-stasis upon cell exposure to peroxynitrite. J Biol Chem 272 27652-27659 Khaikin Gl, Alfassi ZB, Huie RE, Neta P (1996) Oxidation of ferrous and ferrocyanide ions by peroxyl radicals. J Phys Chem 100 7072-7077... 

The first-order reaction rate constant for the isomerization of peroxynitrous acid to nitrate is 4.5 s 1 at 37°C therefore, at pH 7.4 and at 37°C the half-life of the peroxynitrite/peroxynitrous acid couple (let both these species be referred to as peroxynitrite for the sake of brevity) is less than 1 s. The reaction mechanism of peroxynitrite decomposition was a subject of controversy. Primarily proposed was that peroxynitrous acid decomposes by homolysis, producing two strong oxidants hydroxyl radical and nitrous dioxide (B15) ... 

One report of the rates of the isomerization of peroxynitrous acid to nitric acid involved interpretation of the results of a calorimetric method of estimation of residual peroxynitrite (from the heat of decomposition). Two other reports are in substantial agreement. From experiments with alkaline solutions, the isomerisation was found to be first order in OONO" and in hydrogen ion. At 25 °C and fi = 0.25, k is 5.7 x 10 l.rnole. sec and the activation energy 12.0 kcal.mole Quite irreproducible behaviour was found with solutions more alkaline than pH 9. There may well be difficulty with buffers and also with contaminants as yet undisclosed. 

Peroxynitrite itself, apparently exists in equilibrium with its conjugate acid peroxynitrous acid (ONOOH) at pH 7.2. The decomposition of ONOO is complicated [95], as the anion is stable in alkaline conditions but decays rapidly to ONOOH at physiological pH with a pKa 6.8 [96,97]. Three pathways of ONOOH decomposition have been proposed. It was suggested by Beckman et al. [98] that ONOOH decomposes to form hydroxyl and NO radicals based on the sensitivity of peroxynitrite induced reactions to hydroxyl radical scavengers. This was independently supported by EPR data suggesting evidence of free hydroxyl radicals on decomposition of peroxynitrite [32]. However Koppenol et al. [97] concluded from molecular dynamic calculations that homolytic cleavage of ONOOH was improbable. This led to the hypothesis by Piy or et al. [99] of a... 

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]. 

Peroxynitrite has a pKa of 7.5 and will be therefore substantially protonated at physiological pH, to produce peroxynitrous acid (ONOOH). One route for the decomposition of this molecule is thought to be homolytic cleavage to produce hydroxyl radical (OH-) and nitrogen dioxide (NO2-). 

Peroxynitrite itself is not a free radical because the two unpaired electrons of superoxide and nitric oxide have combined to form a new bond. Peroxynitrite is an isomer of nitrate, but is 36 kcal x mol-1 higher in energy (Koppenol et al. 1992). The half-life of peroxynitrous acid (HOONO), which is 7 s at 0 °C and 1 s at 37 °C, should be long enough to allow the molecule to diffuse inside the cell and react with DNA. The decomposition pathway of HOONO, leading to the formation of nitric acid, remains... 

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. 

Peroxynitric acid HO2NO2 is a molecule formed in the recombination reaction between HO2 radicals and NO2. Although it is not detected in the atmosphere, it is thought to be an important atmospheric species to be included in the tropospheric chemistry models. In the lower troposphere thermal decomposition reaction is as a major loss process of HO2NO2, but in the middle and upper troposphere where the ternperamre is lower, the regeneration of HO2 radicals by the photolysis becomes important. 

Peroxynitric acid (HO2NO2, PNA) plays an important role in atmospheric chemistry as a gas-phase reservoir for NO (NO and NO2) and HO in the stratosphere as well as the troposphere. The lifetime of PNA at the middle latitudes in the upper troposphere and lower stratosphere is in the range of 10-12 h. However, at the higher tonperatures found in the lower troposphere and even in the middle of the upper stratosphere, HOjNOj degradation can be dominated by thermal decomposition (reaction -1)... 

Table VIII-M-4. Rate coefficients for the thermal decomposition of peroxynitric acid and some PAN-type compounds... 

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