Caveats in Using Chirality to Probe Biologically Mediated Environmental Processes

7 Caveats in Using Chirality to Probe Biologically Mediated Environmental Processes 

While chirality is a powerful tool for detecting and understanding biochemical weathering processes in the environment, some caveats and precautions should be kept in mind. First, a compound must be asymmetric in order for chirality to be used. Thus, while extrapolations to structurally similar compounds may be plausible [194], such interpretations must be made cautiously in light of the variability in enantiomer behavior of even structurally similar chemicals. An example of such variability is the observation of enantioselective degradation of PCB 136 by rainbow trout, while structurally similar PCB 95 was not degraded [227,232]. 

In addition, although most abiotic processes are nonenantioselective, not aU are indeed the case. Nucleophilic 5 jv2-substitution reactions at a chiral center will result in chiral inversion to the antipodal enantiomer. While such processes are often biologically mediated, as for the nonsteroidal anti-inflammatory drugs [328], they can also be abiotic. Appropriate sterile controls should be used for experiments with such compounds, as was done in the demonstration of microbial chiral inversion of ibuprofen in Swiss lake water [329]. Photolysis of a-HCH [114], /3-PCCH [114], and chlordane compounds [116] was demonstrated not to be enantioselective, as expected for an abiotic process. However, this may not be the case for some pyrethroids, known to isomerize photolytically. 

Caution must be exercised when analyzing some pyrethroids, given possible isomerization of pyrethroids with cyano substituents at the asymmetric a-carbon atom, such as cypermethrin and cyfluthrin. Isomerization may occur in the presence of heat, polar solvents, or light. About 9% interconversion of these two compounds was observed at 

260 °C, but was negligible at 180 °C or when on-column GC injection was used [96]. Cypermethrin [96] and cyfluthrin [96,330] also isomerized slowly (half-life ca. 160 days) at the asymmetric a-carbon atom in sterile water, as does deltametrin, which also has a cyano substituent at the asymmetric a-carbon atom in polar solvents [331, 332]. Cypermethrin isomerized rapidly in isopropanol (half-life of 3-7 days) and methanol (half-life of 2-3 days), as well as in organic solvent-water mixtures depending on water content and temperature [333]. Photolytic epimerization was observed for deltamethrin [331, 334] and for cyhalothrin, another cyano-bearing pyrethroid [335]. No isomerization by any means was observed for bifenthrin [96] and permethrin [96, 333], both of which lack cyano substituents. Thus, caution should be applied to cyano-bearing pyrethroids to avoid exposure to light and use of incompatible solvents (e.g. HPLC mobile phases), and in interpretation of enantiomer composition from environmental data to account for abiotic isomerization. 

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