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OBSERVATIONS OF ENANTIOSELECTIVITY IN THE FATE, PERSISTENCE AND EFFECTS OF MODERN PESTICIDES
Citation:
Garrison, A W., W J. Jones, T. E. Wiese, J W. Washington, J. L. Jarman, AND J Avants. OBSERVATIONS OF ENANTIOSELECTIVITY IN THE FATE, PERSISTENCE AND EFFECTS OF MODERN PESTICIDES. Presented at 23rd International Symposium on Halogenated Organic Pollutants and Persistent Organic Pollutants, Boston, MA, August 24-29, 2003.
Impact/Purpose:
Extend existing model technologies to accommodate the full range of transport, fate and food chain contamination pathways, and their biogeographical variants, present in agricultural landscapes and watersheds. Assemble the range of datasets needed to execute risk assessments with appropriate geographic specificity in support of pesticide safety evaluations. Develop software integration technologies, user interfaces, and reporting capabilities for direct application to the EPA risk assessment paradigm in a statistical and probabilistic decision framework.
Description:
Chiral pollutants exist as 2 (or more) species, -- enantiomers -- that are non-superimposable mirror images of each other. Enantiomers have identical physical and chemical properties except when they interact with enzymes or other chiral molecules; then they usually react selectively. This enantioselectivity often results in different rates of microbial/biological transformation, differences in toxicity of the two enantiomers, and differences in activities toward target species. Up to 25% of pesticides are chiral molecules, and almost all are manufactured and applied as their racemates, mixtures of equal amounts of the enantiomers. Recently, however, the agrochemical industry and government regulators are beginning to take enantioselectivity into account. For example, the (R)-(+)-enantiomer of dichlorprop (as well as the (R)-enantiomers of all the phenoxypropionic acid herbicides) is the herbicidally active species, while the (S)-(-)-enantiomer is inactive; so, to reduce the amount of herbicide used and avoid the possibility of the unnecessary enantiomer causing some adverse impact, several European countries have decreed that only the (R)-enantiomers will be used. In addition, the two (S)-enantiomers of metolachlor, one of the most widely used herbicides in the USA, are nine times more herbicidally active than the (R)-enantiomers, so its manufacturer successfully petitioned the EPA for registration of a formulation enriched to contain 88% of the (S)-enantiomers . This allows a 35% reduction in the amount of the herbicide applied, with the same effect. It seems obvious that the enantiomers of chiral pesticides should be treated as separate compounds and that accurate environmental and human risk assessments require an understanding of the relative persistence and effects of each enantiomer. This paper emphasizes results of recent research on the fate and effects of enantiomers of specific chiral pesticides in use today; the so-called modern pesticides.