Peroxynitrous acid oxidant

In continental clouds, hydrogen peroxide is the most important oxidant of suhur dioxide dissolved in the aqueous phase, contributing about 80% to the total oxidation rate. Ozone and peroxynitric acid oxidize up to 10% each, and the gas-phase reaction of SO2 with OH radicals adds about 3%. Clouds are estimated to occupy about 15% of the airspace in the lower troposphere. In-cloud reactions thus oxidize 70-80% of SO2 in the troposphere, the remaining 20-30% of SO2 is oxidized in the gas-phase by reaction with OH radicals in cloud-free air. 

Kikugawa, K., Hiramoto, K., Tomiyama, S., and Asano, Y. 1997. P-Carotene effectively scavenges toxic nitrogen oxides Nitrogen dioxide and peroxynitrous acid. FEBS Lett. 404 175-178. 

At physiological pH, ONOO- protonates to peroxynitrous acid (ONOOH) which disappears within a few seconds, the end product being largely nitrate. The chemistry of peroxynitrite/peroxynitrous acid is extremely complex, although addition of ONOO to cells and tissues leads to oxidation and nitration of proteins, DNA and lipids with a reactivity that is comparable to that of hydroxyl radicals. 

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. 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

The second proposal is a bit more imaginative and arises from the above arguments that 0—0 bond homolysis is much too slow to be involved in oxidations by peroxynitrate. Pryor and coworkers invoked the intermediacy of a metastable form of peroxynitrous acid (HO—ONO ) in equilibrium with its ground state. This so-called excited state of peroxynitrous acid has, to date, eluded detection or characterization by the experimental community. However, recent high-level theoretical calculations by Bach and his collaborators have presented plausible evidence for the intermediacy of such a shortlived species with a highly elongated 0—0 bond and have confirmed its involvement in the oxidation of hydrocarbons (see below). The discovery of this novel series of biologically important oxidants has fostered a new area of research in both the experimental and theoretical communities. In this chapter we will describe many of the more pertinent theoretical studies on both the physical properties and chemical reactivity of peroxynitrous acid. 

The similarity of the structure of peroxynitrous acid to the simplest peroxy acid, per-oxyformic acid, immediately raised the question as to its relative reactivity as an oxygen atom donor. This became particularly relevant when it was recognized that the 0—0 bond dissociation energy (AG° = 21 kcalmoR ) of HO—ONO was much lower than that of more typical peroxides. Consequently, peroxynitrous acid (HO-ONO) can be both a one- and two-electron oxidant. Since the 0-0 bond in HO-ONO is so labile, its chemistry is also consistent in many cases with that of the free hydroxyl radical. 

The relatively short half-life of peroxynitrous acid hampers experimental smdies of its reactions as an oxidizing agent. Therefore, theoretical methods can play a particularly useful role in describing the chemistry of peroxynitrous acid. For comparison of the HO-ONO fragment geometries found in transition structures, the geometric parameters of HO-ONO at various levels of theory are presented in Figure 3. 

For practical reasons the two-electron oxidations of such prototypical substrates as alkenes, sulfides, amines and phosphines with peroxynitrous acid have been smdied since this type of oxygen atom transfer reaction is similar to the corresponding oxidations with peroxyformic acid, where both experimental and computational data are available The oxidative reactions (equations 3-6) of these key substrates can... 

FIGURE 4. Transition structures for the oxidation of tiimethylamine (a), trimethylphosphine (b) and dimethyl sulfide (c,d) with peroxynitrous acid optimized at the B3LYP/6-311G, MP2(full)/6-31G (in parentheses) and QCISD/6-31G (in brackets) levels... 

Peroxynitric acid (O2NO—OH) is another important reactive two-electron oxidative species in this series of nitrogen-containing peroxides. The activation barriers reported by Houk and coworkers for two-electron oxidation of NH3, H2S and H2C=CH2 are similar to those found for HO—ONO . It differs from peroxynitrous acid mainly in its... 

One-electron oxidation. The oxidation of methane with metastable peroxynitrous acid... 

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

Recent studies by Bach and coworkers at the B3LYP/6-311- -G(d,p) level also suggest a classical activation barrier for methane oxidation on the restricted ( S > = 0.0), two-electron surface, of AE = 31.1 kcalmol (TS-6r, Figure 11). However, the more important question is whether hydrocarbon oxidation proceeds by a one-electron process involving the metastable form of peroxynitrous acid. 

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... 

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. 

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