Biodegradation of the cyclic nitramine explosives RDX, HMX, and CL-20

Abstract

Cyclic nitramine explosives are synthesized globally mainly as military munitions, and their use has resulted in environmental contamination. Several biodegradation pathways have been proposed, and these are based mainly on end-product characterization because many of the metabolic intermediates are hypothetical and unstable in water. Biodegradation mechanisms for cyclic nitramines include (a) formation of a nitramine free radical and loss of nitro functional groups, (b) reduction of nitro functional groups, (c) direct enzymatic cleavage, (d) alpha-hydroxylation, or (e) hydride ion transfer. Pathway intermediates spontaneously decompose in water producing nitrite, nitrous oxide, formaldehyde, or formic acid as common end-products. In vitro enzyme and functional gene expression studies have implicated a limited number of enzymes/genes involved in cyclic nitramine catabolism. Advances in molecular biology methods such as high-throughput DNA sequencing, microarray analysis, and nucleic acid sample preparation are providing access to biochemical and genetic information on cultivable and uncultivable microorganisms. This information can provide the knowledge base for rational engineering of bioremediation strategies, biosensor development, environmental monitoring, and green biosynthesis of explosives. This paper reviews recent developments on the biodegradation of cyclic nitramines and the potential of genomics to identify novel functional genes of explosive metabolism.

Summary Review of the Aquatic Toxicology of Munitions Constituents

“…Anaerobically, sequential reduction of the nitro groups to produce mono-, di-and trinitroso derivatives (MNX, DNX and TNX, respectively) is most frequently observed. The nitroso derivatives may be further transformed to produce the unstable hydroxylamine derivatives, leading to ring cleavage (Crocker et al 2006). Degradation of HMX does not appear to occur in the field under aerobic conditions, with half-lives in the environment spanning decades (Clausen and Korte 2011).…”

Regulation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) metabolism in Shewanella halifaxensis HAW-EB4 by terminal electron acceptor and involvement of c-type cytochrome

“…Both aerobic and anaerobic bacteria have been found to be capable of biotransforming RDX (Zhao et al, 2004a;Lewis et al, 2004;Crocker et al, 2006). Previously, strains of Shewanella were isolated from an unexploded ordnance (UXO)-contaminated marine sediment site near Halifax Harbor for their potential to mineralize RDX under oxygen-limiting conditions via the intermediary formation of nitroso derivatives and ring cleavage products ( Fig.…”

Relating Carbon and Nitrogen Isotope Effects to Reaction Mechanisms during Aerobic or Anaerobic Degradation of RDX (Hexahydro-1,3,5-Trinitro-1,3,5-Triazine) by Pure Bacterial Cultures

“…The specific mechanism of ring destabilization and the steps that lead to ring cleavage may be variable and remain subject to debate (6,7,10). A denitration reaction that converts hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) (in pathway A) through multiple intermediates to methylenedinitramine (MEDINA) (in pathway B) under anaerobic conditions has also been proposed (7,11,12). Aerobic degradation proceeds via two single-electron transfer steps resulting in sequential denitration (2 NO 2 Ϫ removed) followed by ring cleavage and production of 4-nitro-2,4-diazabutanal (NDAB) (pathway C) (7).…”