Oxidation of nitrite

Nitrification consists of two energy yielding steps the oxidation of ammonium to nitrite, and the oxidation of nitrite to nitrate. These equations are generally represented as follows ... 

The catalytic surface being saturated to a large extent by N02 adsorbed species. NO and N02 are detected in the exhaust stream, The higher the reaction temperature, the more efficient the progressive adsorption of N02 and the oxidation of nitrites into nitrates. 

It has been suggested that N02 might be formed by the oxidation of nitrite by numerous biological oxidants. Thus, Shibata et al. [90] reported that horse radish peroxidase (HRP) + hydrogen peroxide oxidized nitrite by the following mechanism ... 

It has been proposed [91] that nitric dioxide radical formation during the oxidation of nitrite by HRP or lactoperoxidase (LPO) can contribute to tyrosine nitration and be involved in cell and tissue injuries. This proposal was supported in the later work [92] where it has been shown that N02 formed in peroxide-catalyzed reactions is able to enter cells and induce tyrosyl nitration. Reszka et al. [93] demonstrated that N02 mediated the oxidation of biological electron donors and antioxidants (NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C) catalyzed by lactoperoxidase in the presence of nitrite. 

The ability of MPO to catalyze the nitration of tyrosine and tyrosyl residues in proteins has been shown in several studies [241-243]. However, nitrite is a relatively poor nitrating agent, as evident from kinetic studies. Burner et al. [244] measured the rate constants for Reactions (24) and (25) (Table 22.2) and found out that although the oxidation of nitrite by Compound I (Reaction (24)) is a relatively rapid process at physiological pH, the oxidation by Compound II is too slow. Nitrite is a poor substrate for MPO, at the same time, is an efficient inhibitor of its chlorination activity by reducing MPO to inactive Complex II [245]. However, the efficiency of MPO-catalyzing nitration sharply increases in the presence of free tyrosine. It has been suggested [245] that in this case the relatively slow Reaction (26) (k26 = 3.2 x 105 1 mol-1 s 1 [246]) is replaced by rapid reactions of Compounds I and II with tyrosine, which accompanied by the rapid recombination of tyrosyl and N02 radicals with a k2i equal to 3 x 1091 mol-1 s-1 [246]. 

Energy is given as AG° for each reaction. The microbial oxidation of ammonia to nitrite in soil is found to be slower than the oxidation of nitrite... 

Oxidized species of nitrogen, chiefly nitrite and nitrate, occur in all soils and in the soil solution. Nitrite in the environment is of concern because of its toxicity. Its occurrence is usually limited because the oxidation of nitrite to nitrate is more rapid than the oxidation of ammonia to nitrite. Both nitrite and nitrate move readily in soil and nitrate is available to plants as a source of nitrogen and can move to plant roots with water. 

Oxidation by Nitrosomas (NFl4 NO2-) followed by oxidation by Nitrobac-ter (N02fiN03). Becanse the oxidation of nitrite to nitrate is generally rapid, most of the N-isotope fractionations is cansed by the slow oxidation of ammoninm by Nitrosomas. In N-limited systems fractionations are minimal. 

NO is effectively stored through a stepwise oxidation at a Pt site followed by adsorption at a neighboring Ba site to form Ba nitrites these species are then oxidized to nitrates. This pathway is referred to as the nitrite route . The oxidation of nitrites to nitrates is catalyzed by Pt and probably involves NO2 formed by NO oxidation on Pt... 

Free radical oxidation of nitrite by OH, for example, can also occur. This reaction (which will occur during daylight hours when photolytic sources of OH are present) and the chemistry of associated nitrogen oxides in solution have been studied by Lpgager and Sehested (1993) ... 

The fates of the G(-H) radicals in DNA are mostly determined by reactions with other substrates. Here, we consider the reactions of the G(-H) radicals with types of free radicals that are generated in vivo under conditions of oxidative stress. One of these radicals is the nitrogen dioxide radical, NO2. This radical can be generated in vivo by the oxidation of nitrite, N02, a process that can be mediated by myeloperoxidase [111, 112] as well as by other cellular oxidants [113, 114]. An alternative pathway of the generation of NO2 is the homolysis of peroxynitrite [102, 115] or nitrosoperoxycarbonate formed by the reaction of peroxynitrite with carbon dioxide [99-101]. The redox potential, E°( NO2/NO2")=1.04 V vs NHE [116] is less than that of guanine, E7[G(-H)7G] = 1.29 V vs NHE [8]. Pulse radiolysis [117] and laser flash photolysis [109] experiments have shown that, in agreement with these redox potentials, N02 radicals do not react with intact DNA. However, N02 radicals can oxidize 8-oxo-dG that has a lower redox potential ( 7=0.74 vs NHE [56]) than any of the normal nucleobases [109]. 

For the most part, nitrification involves the activity of two very different classes of bacteria (Wallace and Nicholas, 1969 Kuenen and Robertson, 1987). The first step, the 6-electron oxidation of NH, to nitrite (NO2 ), is carried out by a small numbet of genera of autotrophic bacteria, exemplified by Nitrosomonas the second step, the 2-electron oxidation of nitrite to nitrate (NO3 ) by a sim-... 

Other authors (Kabin et al., 2006 Nova et al., 2004, 2006b) proposed alternative pathways in which, in the presence of oxygen, NO is directly adsorbed to form Ba nitrites which are progressively oxidized to nitrates, without a previous formation of gaseous N02. The summary reaction for this NO adsorption route (including the oxidation of nitrites to nitrates) is... 

As is indicated in Fig. 24-1, the interconversions of nitrate and nitrite with ammonia and with organic nitrogen compounds are active biological processes. Two genera of nitrifying soil bacteria, which are discussed in Chapter 18, oxidize ammonium ions to nitrate. Nitrosomas carries out the six-electron oxidation to nitrite (Eq. 18-17) and Nitrobacter the two-electron oxidation of nitrite to nitrate (Eq. 18-18).79... 

The oxidation of ammonia to nitrite, in the process of nitrification, is brought about mainly by autotrophic bacteria such as Nitrosomonas species. The oxidation of nitrite to nitrate is due to the action of Nitrobacter species. 

A tetraruthenated porphyrin was electropolymerised onto glassy carbon and used to catalyse the oxidation of nitrite to nitrate, with the resultant current giving a selective measure of the concentration of nitrite ion [81]. As an alternative method, soluble poly(3-octyl thiophene) [82] was cast along with tridodecylmethylammonium chloride onto glassy carbon, to give electrodes with superior selectivity over PVC-based membranes to lipophilic ions such as bromide or nitrate. 

Thus the overpotential was lowered from 0.92 V at the bare electrode to 0.65 and 0.63 V at the polymerized MnPc (NH2)4 and OTiPc (NH2)4 modified electrodes, respectively. Hence the central Mn and Ti metals did not differ much in terms of lowering the nitrite oxidation potential. Also there was no significant influence on the point of substitution (OTiPc (NH2)4 vs. OTiPc (NH2)4) on the oxidation potential of nitrite, Table 3. Oxidation of nitrite occurred at potentials ranging from 0.55 to 0.75 V on TiPc and MnPc derivatives, Table 3 [34,45,46,95],... 

Tau P, Nyokong T (2007) Electrocatalytic oxidation of nitrite by tetra substituted oxotita-nium(IV) phthalocyanines adsorbed or polymerized on glassy carbon electrode. J Electroanal Chem 611 (1—2) 10—18... 

Agboola BO, Ozoemena KI, Nyokong T (2006) Electrochemical properties of benzylmer-capto and dodecylmercapto tetra substituted nickel phthalocyanine complexes electrocatalytic oxidation of nitrite. Electrochim Acta 51(28) 6470-6478... 

Tau P, Nyokong T (2007) Electrocatalytic activity of aryl thio tetra substituted oxotitanium(IV) phthalocyanines towards the oxidation of nitrite. Electrochim Acta 52(13) 4547-4553... 

Fig. 5.11 Modification of the vinyl groups of the prosthetic heme of HRP upon catalytic oxidation of nitrite to N02 or Br to HOBr. For each reaction, two different modifications of the vinyl groups are shown. These modifications can occur in different combinations. For example in the Br reaction, both of the vinyls could he present as bromohydrin [-CHBrCH2OH] adducts...

Around neutral pH, by far the most common Fe(III) species are the oxides and hydroxides. Of these, a-Fc203 and /I-FeOOH have been most studied from a photochemical point of view [88-92], They are semiconductor oxides and irradiation in the visible promotes electrons from the valence band to the conduction one, leaving holes (h+) in the valence band [90,91]. Electrons and holes can either thermally recombine or migrate to the oxide surface, where they can be trapped by surface species and react with dissolved molecules [93]. In aerated solution, trapped electrons are likely to reduce oxygen to superoxide, while trapped holes can oxidise various molecules. Quite interestingly, when irradiated in the visible, g -Fc203 and /J-FcOOH are not able to transform phenol. However, in the presence of phenol and nitrite, quantitative yield in nitrophenols is observed as a consequence of the oxidation of nitrite to nitrogen dioxide [85]. 

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