Reductive anaerobic transformation

Important reductions are involved in the anaerobic transformations of steroid and flavanoids ... 

Dos Santos AB, Cervantes FJ, Van Lier JB (2004) Azo dye reduction by thermophilic anaerobic granular sludge, and the impact of the redoxmediator anthraquinone-2,6-disulfonate (AQDS) on the reductive biochemical transformation. Appl Microbiol Biotechnol 64 62-69... 

Chen et al. (1999) studied the anaerobic transformation of 1,1,1-trichloroethane, 1,1-dichloroethane, and choroethane in a lab-scale, municipal wastewater sludge digester. 1,1,1-Trichloroethane degraded via reductive dechlorination to give 1,1-dichloroethane, chloroethane, and then ethane. When cell-free extracts were used, 1,1,1-trichloroethane degraded to acetic acid (90% yield) and 1,1-dichloroethylene, the latter degrading to ethylene. 

Since the reduction of 2,4-diamino-6-nitrotoluene to triaminotoluene is the rate-limiting step in reductive TNT transformation and can only be achieved under anaerobic conditions, this reaction is of special interest and will be described in more detail in the next section. Figure 7 shows a tentative scheme of the reduction of 2,4-diamino-6-nitrotoluene to triaminotoluene, assuming that the reduction proceeds in two-electron steps. 

Reduction of Nitro Substituents. These reactions are very common in anaerobic environments and result in amine-substituted pesticides anaerobic bacteria capable of reducing nitrate to ammonia appear to be primarily responsible. All nitro-substituted pesticides appear to be susceptible to this transformation, eg, methyl parathion (7) (eq. 9), triduralin, and pendimethalin. 

Degradation of trichloroethylene by anaerobes via reductive dehalogenation can be problematic because a common product is vinyl chloride, a known carcinogen (Ensley 1991). In an anaerobic colunm operated under methanogenic conditions, 100% transformation of injected tetrachloroethylene and trichloroethylene to... 

A cardinal issne is the species of the metal or metalloid that is examined. Metals snch as mercury or tin are methylated from cationic Hg + or Sn", whereas the metalloids are transformed from the oxyanions of As, Sb, Se, or Te. The classical Challenger mechanism that involves seqnential reductions and methylations is well established, at least for fungal methylation of the oxyanions of As (Bentley and Chasteen 2002), and Se—and is assumed to be—for Te (Chasteen and Bentley 2003). Methylation may take place under aerobic conditions for fungi or anaerobic conditions for bacteria. 

The anaerobic degradation of halogenated alkanoic acids has, however, been much less exhaustively examined. Geobacter (Trichlorobacter) thiogenes was able to transform trichloroacetate to dichloroacetate by coupling the oxidation of acetate to CO2 with the reduction of sulfur to sulfide that carries out the dechlorination (De Wever et al. 2000). 

In anaerobic microcosms, l,l,2-trichloro-l,2,2-trifiuoroethane (CFC-113) was transformed by successive reductive dechlorination to l,2-dichloro-l,2,2-trifiuoroethane (HCFC-123a), and under methanogenic conditions to l-chloro-l,2,2-trifiuoroethane (HCFC-133) and l-chloro-l,l,2-triflnoroethane (HCFC-133b) without evidence for the reductive replacement of fluorine (Fignre 7.70b) (Lesage et al. 1992). 

Scheme 5b, solutions (pH=7.5) of these DNA strands were subsequently irradiated under anaerobic conditions, after reduction of the flavin with sodium dithionite [51]. HPLC analysis of samples removed from the assay solution during such an irradiation experiment showed clean transformation of the Tf=T-containing DNA strands into the repaired TfT-containing DNA strands (Fig. 2). These experiments showed for the first time that a reduced flavin, if incorporated into DNA, can inject an extra electron into a DNA du-...

Halogenated aliphatics can be partially or completely degraded under anaerobic conditions through a transformation reaction called reductive de-halogenation. Often a co-metabolic degradation step, reductive dehalogenation... 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

The redox potential in subsurface water varies with alterations from aerobic to anaerobic conditions. In and around anaerobic environments, conditions for reduction exist and contaminants are transformed accordingly. Under aerobic conditions, is the predominant oxidation agent (mainly through biological processes), because the transformation of contaminants is mainly through oxidative pathways. Aerobic and anaerobic states may occur both in surface waters and in deeper subsurface water. 

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