Nitrogen dioxide, free-radical nitration

NO can interact with other free radicals most notably, NO reacts with the superoxide anion (O 2) to produce the potent oxidant peroxynitrite (ONOO") (Radi et al., 1991). Although it is a simple molecule, ONOO is chemically complex. It can display hydroxyl radical-like and nitrogen dioxide (NO2) radical-like activity, can oxidize lipids, and can directly nitrate proteins. ONOO" has been shown to nitrate a tyrosine residue on superoxide dismutase (SOD) (Beckman et al., 1992). 

The sulfate and nitrate content of atmospheric particles comes primarily from the conversion of sulfur dioxide and nitrogen dioxide. Photochemi-cally initiated atmospheric reactions and transient free radicals are... 

Photolytic. Photolysis of acetone in air yields carbon monoxide and free radicals, but in isopropanol, pinacol is formed (Calvert and Pitts, 1966). Photolysis of acetone vapor with nitrogen dioxide via a mercury lamp gave peroxyacetyl nitrate as the major product with smaller quantities of methyl nitrate (Warneck and Zerbach, 1992). 

For example, tyrosine residues on proteins are readily nitrated by a free radical mechanism (Prutz et al., 1985), where one nitrogen dioxide oxidizes the tyrosine to a phenyl radical that reacts with a second nitrogen dioxide to give nitroty-... 

In recent years, nitric oxide (NO) has emerged as one of the most interesting mediators of normal and patho physiological processes. NO is a highly reactive free radical, a lipophilic gas with a very short half-life in the range of 5 - 30 s under bioassay conditions (Palmer et al., 1987). NO is rapidly converted to nitrogen dioxide (N02), which again rapidly forms the more stable metabolites nitrite (N02 ) and nitrate (N03). 

Much experimental evidence established that the reaction occurs by a free-radical mechanism164 173 similar to that suggested above [Eqs. (10.26)—(10.28)] for liquid-phase nitration. The nitrous acid produced during the transformation is unstable under the reaction conditions and decomposes to yield nitric oxide, which also participates in nitration, although less effectively. It was found that nitric acid and nitrogen dioxide yield identical products but that the former gives better yields and higher rates.172... 

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

As discussed in Chapters 1 and 5, nitrogen dioxide (NO ) is a relatively low-active free radical. Therefore, it virtnally does not react at ambient temperatures with polymers that do not contain specific fnnctional groups. However, NO interacts very actively with the double C-C bonds of macromolecules as well as with macroradicals. These reactions result in the formation of various products of nitration and nitrosation. Subsequent processes can involve the degradation of macromolecules, and the formation of stable nitrogen-containing radicals. 

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