Interpreting Nonracemic Ratios of Chiral Organochlorines Using Naturally Contaminated Fish

Abstract

Although reports of nonracemic proportions of chiral organochlorine pollutants in the environment are widespread, the interpretation of such data is not well developed. Using GC/MS and a chiral stationary phase consisting of 25% tert-butyldimethylsilylated beta-cyclodextrin in PS086 (beta-BSCD), we followed the change in the enantiomeric signature of 2-exo,3-endo,6-exo,8,9,10-hexachlorobornane (B6-923) in naturally contaminated fish maintained under toxaphene-free conditions. Whereas the enantiomeric ratio (ER) of B6-923 was near racemic at the start of the elimination experiment, it had increased severalfold by the end of 60 d. On the basis of first-order kinetics, one enantiomer of B6-923 was eliminated twice as fast as its mirror image, resulting in half-lives of 7 and 13 d, respectively. Enantioselective elimination by our test fish (Fundulus sp.) strongly suggests active biotransformation of B6-923; however, bioprocessing throughput estimates suggest a very low in situ rate of natural attenuation. These results confirm that the relatively constant ERs observed for chiral organochlorines in a given species are the result of competing processes, e.g., uptake vs elimination. Our experiments also further illustrate the utility of enantioselective analysis in characterizing the biotransformation of persistent organochlorine pollutants.

Chirality as an Environmental Forensics Tool

“…Mummichogs naturally contaminated with toxaphene from a Georgia estuary eliminated one enantiomer of the congener B6-923 twice as fast as its antipode, with halflives of 6 and 13 days, respectively [224]. This experiment confirmed previous field observations [151].…”

“…However, natural attenuation was expected to be slow, given high contamination levels and slow elimination throughput. This study suggested that relatively constant enantiomer signatures observed in wild biota may be due to a steady state between the competing processes of uptake and elimination [224]. In subsequent work, elimination of chlorobornane congeners B6-923 and B7-1001 by mummichogs at 25 C proceeded at twice the rate at 15 C [225] by lowering enzymatic activation barriers.…”

“…Enantiomer-specific rates of biotransformation, based on using EFs to determine concentrations of individual enantiomers, have been determined in a number of laboratory and field experiments, as previously discussed: bench-scale anaerobic biotransformation of a-HCH in activated sludge [132], elimination of toxaphene congeners in mummichogs taken from a contaminated estuary [224], and microbial degradation of a-HCH within the Arctic Ocean water column [127]. Alternative methods have also been proposed, particularly for assessing bioaccumulation of POPs in fish.…”

Chiral analysis for environmental applications

“…Chiral GC has been used extensively to determine whether or not biodegradation was relevant in the selected media such as vertebrates, surface water, sediment, tissue, sewage sludge, soil etc. [1,2,3,4,5,6,8,16]. These experiments work well under the assumption that only enzymes perform enantioselective reactions in the environment.…”

“…Essentially a two-step clean up, for example, consisting of size-exclusion and silica-sorption chromatography is used for clean up. Especially for ECD analysis additional normal-phase HPLC fractioning for sample preparation was K. Bester necessary in some cases [8]. Fractionation is especially important for the chiral analysis of toxaphenes, as the original toxaphene pattern is extremely complex; thus, chiral separation has to be performed in combination with a classical separation of some hundred congeners [8].…”