Nitrogen oxides oxidation pathways

Water-soluble sdanols such as (1) were found to undergo successive oxidative demethylations with tropospheric ultraviolet irradiation in the presence of suitable chromophores, such as nitrogen oxides (516). The water-soluble methylated sdicones did not promote diatom (Nap cu/apelliculosd) growth but the demethylated photo products did. The sequence of sod-induced degradation of sdicones to water-soluble species such as (1), followed by light-induced conversion to sdicate, suggests a pathway, conceptually at least, for the mineralization of sdicones. 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Denitrification, a dissimilatory pathway of nitrate reduction (see Section 3.3 also) into nitrogen oxides, N2O, and dinitrogen, N2, is performed by a wide variety of microorganisms in the forest ecosystems. Measurable rates of N20 production have been observed in many forest soils. The values from 2.1 to 4.0 kg/ha/yr are typical for forest soils in various places of Boreal and Sub-Boreal Forest ecosystems. All in situ studies (field monitoring) of denitrification in forest soils have shown large spatial and temporal variability in response to varying soils characteristics such as acidity, temperature, moisture, oxygen, ambient nitrate and available carbon. 

Amides that undergo N-hydroxylation are often amides of arylamines (Fig. 4.87) some of which are carcinogens such as 2-acetylaminofluorene (Fig. 4.87). Initial N-hydroxylation of a hydrazine is similar to that of an amine however, further oxidation can lead to the formation of nitrogen gas and reactive species. The two-electron oxidation pathway is shown in Figure 4.87. Hydrazines also undergo one-electron oxidations but the intermediates are short-lived and these pathways are less well defined (154). 

The a-oxidation pathway ofTV-nitrosodiethanolamine metabolism (Figure 2) leads to the formation of an a-hydroxynitrosamine that rapidly decomposes, producing glycol aldehyde, acetaldehyde, ethylene glycol and molecular nitrogen. The latter is assumed to arise from a reactive (2-hydroxyethyl)diazonium ion, which probably is responsible for the formation of 2-hydroxyethylated adducts in DNA (Scherer et al., 1991 Loeppky etal., 1998 Loeppky, 1999). 

The redox potentials of various oxidants derived from nitric oxide and peroxynitrite are summarized in Table 4. Clearly, as the adducts of molecular oxygen and nitric oxide become more reduced, they form substantially stronger oxidizing agents. In effect, addition of one electron makes these nitrogen oxides more ready to accept the next. The precise pathway of decomposition followed is influenced by what types of target molecules come in contact with peroxynitrite and is... 

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. 

With regards to thermodynamic aspects, Chapter 4 discussed practically all known oxidation pathways of molecular nitrogen fixation, including reaction with hydrogen peroxide. 

R. F. Nelson, "Anodic Oxidation Pathways of Aliphatic and Aromatic Nitrogen Functions," in Technique of Electroorganic Synthesis, N. Weinburg, ed., Wiley, New York, 1974, pp. 535-792. 

Commeyras and co-workers proposed for the first time a prebiotically relevant synthetic pathway [143,144]. They established that the nitrosation of N-carbamoylamino acids 25 (otherwise believed as unreactive in prebiotic environments) by nitrogen oxides (typically obtained by mixing O2 and NO) quantitatively releases NCA. Although first tested in organic solvent [ 143], the reaction proved to work in the solid state [144], and also in aqueous solution [145], where the NCA can be observed for ca. 1 h prior to its conversion into either free amino acids or peptides. 

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