Enantiomeric enrichment of chiral pesticides in the environment.

Enantiomer fractions (EFs) of chiral compounds have been used to explain the mechanisms of enantiomer enrichment in air, soil, water, and biota. The EFs were calculated from enantiomeric ratios (ERs) of chiral compounds measured by researchers during the past 10 years. Six compounds were selected from different abiotic and biotic compartments: alpha-hexachlorocyclohexane (alpha-HCH), mecoprop, cis-chlordane (CC), trans-chlordane (TC), heptachlor exo-epoxide (HEPX), and oxychlordane (OXY). The EF was used as a general descriptor for enantiomer enrichment. In environmental compartments the EFs of chiral pesticides deviated from those of the racemic composition (EF = 0.5). The deviations from EF = 0.5 in the different compartments show similar patterns for several compounds, i.e., air < water < soil < biota. In biota the order was lower trophic level < higher trophic level and liver or kidney tissue < brain tissue. Explanations for stereoselective behavior were found in pharmacology and brain research. The enantiomeric enrichments in environmental compartments were visualized in a general scheme applicable to other persistent chiral compounds. The mechanisms of enantiomer enrichment were conceptualized by a hypothetical model of a chiral machine (enzymatic degradation) and a chiral guard (stereospecific efflux). Environmental regulation authorities should treat chiral pesticides as a composition of enantiomers because biotic processes handle enantiomers as separate chemical entities.