Separation, bioactivity, and dissipation of enantiomers of the organophosphorus insecticide fenamiphos.

Most chiral pesticides are used as racemates despite the fact that the pesticidal activity of the given pesticide is usually the result of the preferential reactivity of only one enantiomer while the other enantiomers may have toxic effects against other nontarget organisms. Accordingly, the enantiomer of fenamiphos, an organophosphorus pesticide, was separated by high-performance liquid chromatography with reverse phase and normal phase on a Pirkle model chiral stationary phase column. It was found that n-hexane/isopropanol (95/5) was the best solvent system for enantiomer resolution of fenamiphos. Chromatographic data including capacity factor (k'), separation factor (alpha), and resolution (Rs) are presented. Inhibitory activity to enzyme butyrylcholinesterase and toxicity to Daphnia of enantiomers and racemic insecticide fenamiphos were also studied. In the toxicity tests of Daphnia, the lethal concentration (LC50) of (+)-fenamiphos, (-)-fenamiphos, and racemate were 0.0016, 0.0061, and 0.0019 microg/mL, respectively. No significant difference of LC50 values between (+)-fenamiphos and racemate were found, but (-)-fenamiphos showed significantly lower toxicity to Daphnia. The inhibitory concentration (IC50) to the cholinesterase were 0.008, 0.15, and 0.46 microg/mL for (+)-fenamiphos, (-)-fenamiphos, and racemate, respectively. Both enantiomers and the racemate showed significant difference in inhibiting the cholinesterase. However, (+)-fenamiphos proved to be about 20 times more toxic to Daphnia and only about four times more inhibitory activity to butyrylcholinesterase than (-)-fenamiphos. The dissipation of (+)-fenamiphos, (-)-fenamiphos, and racemate in selected soils and natural water samples were also studied. The half-life (t(1/2)) of (+)-, (-)-, and racemate in soils showed no related to the soil texture, pH, or organic carbon content. By comparing the residues of (+)-, (-)-, and racemate in the water, it was found that (+)-fenamiphos was degraded faster than the others after 21 days. Briefly, (+)-fenamiphos is more toxic than (-)-fenamiphos to a nontarget organism (Daphnia), but the environmental persistence of the two compounds showed no significant difference.