Denitrification intermediates

Tolman W.B. (1995) Synthetic Modeling of the Interactions of Nitrogen Oxides with Copper Proteins Copper Nitrosyl Complexes Relevant to Putative Denitrification Intermediates, Adv. Chem. Ser., 246, 195. 

Copper Nitrosyl Complexes Relevant to Putative Denitrification Intermediates... 

Nitric oxide and NjO are direct intermediates in the denitrification pathway, the reduction of NO3 to Nj. Reduction to Nj is often incomplete, so that both NjO and Nj are equally important end products of denitrification, the ratio of NjO/Nj production being determined by soil physical properties. For example, NjO is the main end-product in acid soils, whereas low redox potentials and high organic matter content favour the further reduction to Nitric... 

Emissions of nitric and nitrous oxides are the result of microbial nitrification and denitrification in soils, controlled principally by soil water and mineral N contents, labile organic carbon, and temperature. Nitric oxide is a direct intermediate of both nitrification... 

Under field eonditions, not all intermediate products are eonverted to N. Nitrate reductase, for example, eauses a decrease in the enzymatie aetivity. Denitrifieaton in the absence of oxygen is eaused by a large number of baeteria Table 15.4 lists the main microorganisms capable of denitrification. 

As will be discussed further in this chapter, there is now much evidence to suggest that NO is an obligatory intermediate in the denitrification pathway. Furthermore, there is evidence that NH3 nitrifiers can synthesize the denitrification apparatus in addition to the nitrification apparatus and that the former system can produce NO and N2O (also N2 in at least one case) from nitrite under low partial pressures of O2. It is possible therefore that NO may be an intermediate in the denitrification activity of nitrifiers and so arise as a secondary consequence of NH3 oxidation. NO can also be ptoduced by nondenitrifying organisms under certain conditions. For example, NO can be slowly produced by the anaerobic reduction of nitrite, but only in absence of nitrate, by a variety of enteric bacteria. Some of the NO can be further reduced to N2O. 

The first product of nitrosyl transfer to nitrite in Eq. (2), E N203, contains N-N bonded N2O3 which is itself a well-known and powerful nitrosyl donor. It is reasonable to suppose therefore that nitrosyl transfer reactions with N- and O-nucleophiles could involve both E NO (or E HONO) and E N205. In addition, the involvement of a second molecule of nitrite for denitrification would require that the substrate saturation curve should be sigmoidal to reflect a term second-order in nitrite concentration. No such effect has been reported to our knowledge. The use of bimolecular substrate kinetics in dilute solutions to generate an intermediate subject to solvolysis seems metabolically unwise hut not impossible. 

III. EVIDENCE FOR AND AGAINST NITRIC OXIDE AS AN INTERMEDIATE IN DENITRIFICATION PATHWAY... 

In this section we examine the evidence as to whethet NO is an intermediate in denitrification. The weight of evidence suggests, in our opinion, that NO is in fact an obligatory, free intermediate in the major, if not sole, pathway. 

O-exchange studies of Ye et al. (1991) support, we believe, the catalysis by nitrite reductase of redox reversibility between nitrite and NO as depicted in the first line of Eq. (3). They observed by analyzing the 0 content of product N2O that all eight strains of denitrifying bacteria studied could catalyze the exchange of 0 between water and nitrite or NO by way of an electrophilic (nitrosyl donor) species of NO. The rates and extent of these exchange reactions depended on whether the bacterium made use of a heme- or Cu-type nitrite reductase. Contrary to the conclusions of Ye et al. (1991), we do not believe that this study otherwise informs about the pathway of denitrification or whether NO is an intermediate. 

If NO were an obligatory intermediate in the denitrification pathway, then there should exist a separate and specific enzyme to reduce NO to N2O. This enzyme would need to keep pace with the flux of denitrification as set by nitrite... 

In summary, a considerable body of enzymatic, genetic, and analytical data supports the view that the major, if not sole, pathway of denitrification involves NO as an obligatory intermediate and requires the action of nitric oxide reductase. On the other hand, the ability of nitrite to modify nitrosyl transfer ratios and the N isotope fractionation factor during its reduction, are consistent with the reductive scheme of Averill and Tiedje (1982). It was suggested (Goretski... 

Firestone, M. K., Firestone, R. B., and Tiedje, ]. M. (1979). Nitric oxide as an intermediate in denitrification Evidence from nitrogen-13 isotope exchange. Biochem. Biophys. Res. Commun. 91, 10-16. 

Goretski, J., and Hollocher, T. C. (1990). Tire kinetic and isotopic competence of nitric oxide as an intermediate in denitrification. J. Biol. Chem. 265, 889-895. 

Hulse, C. L., and Averill, B. A. (1989). Evidence for a copper-nitrosyl intermediate in denitrification by the copper-containing nitrite reductase of Achromobacter cycloclastes. J. Am. Chem. Soc. Ill, 2322-2323. 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the solubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are developed for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed. 

---