The nitro-aromatic compound 2,4,6-trinitrotoluene (TNT) is of greatest concern as an environmental contaminant (Figure 1a). It is intrinsically difficult for microorganisms to mineralize as the three symmetrically placed nitro- groups on the aromatic ring reduce its electron density and limits attack by electrophilic di-oxygenase enzymes (Nishino et al. 2000). Moreover, TNT is poorly soluble in water (140mg/l at 25°C), relatively stable and persistent in the environment (Gorontzy et al. 1994). Its partial breakdown products are more hazardous, recalcitrant, carcinogenic, and mutagenic than the parent compound (Bennett 1994; Whong and Edwards 1984; Won et al. 1976).
Natural decomposition is often the result of co-metabolic bio-transformation but not mineralization and breakdown products include mutagenic reduced amines (Yinon 1990). Nitroaromatic compounds such as TNT are very rarely produced in nature. A few antibiotics containing nitro-aromatic groups are produced by Pseudomonas sp., for example nitropyrrolluteorin (Ohmori et al. 1978) and oxypyrrolenitrin (Hattari et al. 1970). Streptomyces venezuelae has also been reported to produce chloramphe-nicol, and bryozoans have been reported to produce the antibiotic phidolophin (Tischler et al. 1986). Nitrophenolic compounds such as aristolochic acid can be produced by plants (Williams and Barnaby 1977) and this compound has been identified as a potent carcinogen, intercalating or forming adducts with DNA leading to oncogensis (Arlt et al. 2000).
Figure 1 Chemical structures of the major classes of explosives. (a) TNT. (b) PETN. (c) Nitrocellulose. (d) RDX. (e) HMX.
RDX production with a higher melting point than RDX. This explosive has a very low aqueous solubility of 5 mg/l at 25°C. These compounds are truly xenobiotic, and do not occur naturally. Nitramines are acutely toxic and are class C carcinogens (Hawari et al. 2000a).
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