Dissociation, peroxynitrous acid

Dissociation, peroxynitrous acid, 13-14 Dissolved organic carbon (DOC), in water, 606, 607-8... 

Density functional theory has also been recently applied to several dissociative pathways of HO—ONO and its anion (ONO—0 ) in aqueous solution. For example, the Gibbs free energy for the homolysis of peroxynitrous acid (HO—ONO HO -h ONO) is calculated to be AG (aq.) = 11.1 kcalmol" , a value in good agreement with experiment (13.6 kcalmoD ). For peroxynitrite homolysis (ONO—O NO2 -b O ) the calculated AG (aq.) = 13.0 kcalmoD was obtained for ion-molecule complexes with watef ... 

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. 

At physiological pH the protonated form of ONOO-, the peroxynitrous acid (ONOOH), is unstable and decomposes to nitrate (N03). ONOOH can also react directly with reductants or can decompose by homolytic dissociation to form nitrogen dioxide (N02) and hydroxyl radical (OH"), or can dissociate by a heterolytic mechanism to yield nitryl cation (NOj), which reacts with thiol, methionyl, tyro-syl, and tryptophanyl residues in proteins. 

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