Separation and toxicity of salithion enantiomers

Enantioseletive toxicities of chiral pesticides have become an environmental concern recently. In this study, we evaluated the enantiomeric separation of salithion on a suite of commercial chiral columns and assessed the toxicity of enantiomers toward butyrylcholinesterase and Daphnia magna. Satisfactory separations of salithion enantiomers could be achieved on all tested columns, that is, Chiralcel OD, Chiralcel OJ, and Chiralpak AD column. However, the Chiralpak AD column offered the best separation and was chosen to prepare micro-scale of pure salithion enantiomers for subsequent bioassays. The first and second enantiomers eluted on the Chiralpak AD column were further confirmed to be (-)-S-salithion and (+)-R-salithion, respectively. The half inhibition concentrations to butyrylcholinesterase of racemate, (+)-R-salithion, and (-)-S-salithion were 33.09, 2.92, and 15.60 mg/l, respectively, showing (+)-R-enantiomer being about 5.0 times more potent than its (-)-S-form. However, the median lethal concentrations (96 h) of racemate, (+)-R-salithion, and (-)-S-salithion toward D. magna were 3.54, 1.10, and 0.36 microg/l, respectively, suggesting that (-)-S-salithion was about 3.0 times more toxic than (+)-R-form. Racemic salithion was less toxic than either of the enantiomers in both bioassays, suggesting that antagonistic interactions might occur between the enantiomers during the toxication action. This work reveals that the toxicity of salithion toward butyrylcholinesterase and D. magna is enantioselective, and this factor should be taken into consideration in the environmental risk assessment of salithion.