Azaarene

The aerobic biodegradation of monocyclic azaarenes frequently involves reduction (Chapter 10, Part 1), but purely chemical reduction may take place under highly anaerobic conditions. This has been encountered with the substituted l,2,4-triazolo[l,5a]pyrimidine (Flumetsulam) (Wolt et al. 1992) (Figure 1.27). 

A wide range of azaarenes including acridines and benzacridines, 4-azafluorene, and 10-azabenzo[fl]pyrene (Figure 1.32) has been identified in particulate samples of urban air, and some of them have been recovered from contaminated sediments (Yamauchi and Handa 1987). 

FIGURE 1.32 Azaarenes identified in particulate samples of urban air. 

Beitz T, W Bechmann, R Mitzner (1998) Investigations of reactions of selected azaarenes with radicals in water. 1. Hydroxyl and sulfate radicals. J Phys Chem A 102 6760-6765. 

Herod AA (1998) Azaarenes and thiaarenes. Handbook Environ Chem 31 271-323. 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Quinaldine 4-oxidase from Arthrobacter sp. Rii61a that mediates the hydroxylation of a number of bicyclic azaarenes has been termed an oxidase since oxygen functions effectively as an electron acceptor (Stephan et al. 1996). 

The aerobic degradation of several azaarenes involves reduction of the rings at some stage, and are discussed in Chapter 10, Part 1. Illustrative examples include the degradation of pyridines (3-alkyl-pyridine, pyridoxal) and pyrimidines (catalyzed by dihydropyrimidine dehydrogenases). Reductions are involved in both the aerobic and the anaerobic degradation of uracil and orotic acid. 

These oxidoreductases are widely used for the introduction of the oxygen atom from HjO into heteroarenes, especially azaarenes including pyridine, quinoline, pyrimidine, and purine, and they generally contain molybdenum. 

The metabolism of 2-methylquinoline in Arthrobacter sp. strain Rii 61a is comparable, with introduction of oxygen at C4 (Hund et al. 1990). The enzymes (oxidoreductases) that introduce oxygen into the azaarene rings in Rhodococcus sp. strain Bl, Arthrobacter sp. strain Rii61a, and Pseudomonas putida strain 86 are virtually identical and, like those already noted have molecular masses of 300-320 kDa and contain Mo, Fe, FAD, and acid-labile sulfur (De Beyer and Lingens... 

Azaarenes are structural elements of coal and petroleum, and are found in products derived from them by pyrolysis and distillation. They are components of creosote, which has been widely used as a timber preservative. 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

As for the aerobic degradation of pyridines, hydroxylation of the heterocyclic ring is a key reaction in the anaerobic degradation of azaarenes by Clostridia. Whereas in Clostridium barkeri, the end products are carboxylic acids, CO2, and ammonium, the anaerobic sulfate-reducing Desulfococcus niacinii degraded nicotinate completely to CO2 (Imhoff-Stuckle and Pfennig 1983), although the details of the pathway remain incompletely resolved. 

The degradation or transformation of only a few halogenated azaarenes has been examined under aerobic conditions ... 

It is very seldom that only a single substrate is present. It is therefore important to examine how the regulation of degradative pathways may be affected and, in particular, whether the simultaneous presence of other contaminants has an adverse effect. In addition, some of the components of a contaminant may directly inhibit degradation by toxification of the relevant organism. The example of azaarenes in groundwater at a wood preservation site that inhibit PAH degradation (Lantz et al. 1997) is noted in Chapter 14. 

Comprehensive chemical analyses of samples of water, sediment, and biota were carried out both before and after the spill. This cannot of course be carried out in most cases, and illustrates a serious limitation in field studies, in which lack of background data or difficulty in finding an uncontaminated control locality is frequently encountered. Sum parameters were sparingly employed in Baffin Island Oil Spill (BIOS), and emphasis was placed on the analysis of specific compounds attention was directed not only to PAHs, but also to azaarenes, dibenzothiophenes, and hopanes. Thereby, a clear distinction could be made between the input from the oil deliberately discharged, and that arising from natural biological reactions or mediated by atmospheric transport. 

The presence of azaarenes— primarily quinolines, acridine, and carbazole—in creosote may inhibit or be incompatible with the degradation of PAHs. 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

Fukuhara, K., A. Hakura, N. Sera, H. Tokiwa, and N. Miyata, 1- and 3-Nitro-6-azabenzo[uIpyrenes and Their At-Oxides Highly Mutagenic Nitrated Azaarenes, Chem. Res. Toxicol., 5, 149-153 (1992). 

Nielsen, T., P. Clausen, and F. P. Jensen, Determination of Basic Azaarenes and Polynuclear Aromatic Hydrocarbons in Airborne Particulate Matter by Gas Chromatography, Anal. Chim. Acta, 187, 223-231 (1986). 

As early as 1964, polycyclic aromatic hydrocarbons (PAHs) known to be carcinogenic, such as benzo[a]pyrcne, were detected in broiled meat (164). A number of papers have been published about PAHs found in smoked and thermally treated foods as a result of pyrolysis or incomplete combustion of organic matter (165 -169). Less information is available on the nitrogen analogs, the basic azaarenes polycyclic aromatic nitrogen-containing hydrocarbons (PANHs), but they have been shown to be present in association with PAHs in various samples that contain nitrogen, such as processed food (170-172). 

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