Some compounds show dramatic differences in toxicity and rate of break down between isomers
Researchers at the University of California, Riverside have demonstrated that isomers – or the mirror-image structures – of some pesticides, although chemically identical, have very different biological and environmental impacts between the two sides. This may have significant implications for risk assessment and research and development directions of new products.
The environmental risks of pesticides have been traditionally evaluated on the basis of their specific chemical structure, according to Jay Gan, a UCR professor of environmental chemistry. He found, however, that this group, known as chiral pesticides, including many widely used organophosphates and synthetic pyrethroids, pose previously uncalculated toxic risks due to the differing biological reactions of the isomers in the environment.
A characteristic of chiral compounds is that they occur as isomers with two (or more) identical but mirror-image structures that, as Gan's research indicates, while chemically identical, may behave biologically differently. These mirror-image molecules are known as enantiomers. Currently about 25 percent of pesticides fall into this classification and this ratio is expected to increase as new products are being introduced into the market.
Gan's findings add weight to the argument that regulators should consider whether a product is a chiral compound when assessing its risk, and that the chemical industry should pursue the value of producing single isomer products instead of mixed isomer products.
By using pesticides with just the active isomer, farmers will likely achieve the same degree of pest control at a much-reduced rate of chemical use. This will have environmental benefits as much less chemical is introduced into the environment.
The findings were published in a paper titled Enantioselectivity in Environmental Safety of Current Chiral Insecticides in last week's online edition of the Proceedings of the National Academy of Sciences. Gan published the paper in cooperation with a team of UCR colleagues including Daniel Schlenk, professor of aquatic ecotoxicology; Soil Physics Professor, William A. Jury; and visiting professor Weiping Liu.
Gan and his colleagues at UC Riverside decided to look at chiral insecticides that are widely used today. They examined five common insecticides, including the organophosphates, such as profenofos, and synthetic pyrethroids, such as permethrin. For all these compounds, one of the optical isomers, or enantiomers, was consistently over 10 times more toxic than the other to Ceriodaphia, a small crustacean often used to assess water toxicity.
The researchers also found that a specific enantiomer lingered longer in the environment than the other enantiomers, making one enantiomer of permethrin almost twice as prevalent in sediment or runoff water. This means that the environmental impact of these pesticides may depend on the behavior of a particular enantiomer instead of the whole compound, the team concluded.
Regulators currently examine the safety of the pesticide straight from the factory, in which both enantiomers are normally present in an equal ratio. On the other hand, knowing about such selectivity would be valuable for the chemical industry. For instance, if only one enantiomer is known to contribute to the pest control efficacy, it would be environmentally advantageous to manufactured products containing just the active component. The rate of use may be cut in half, and the chemical load into the environment will also be halved.
"The difference in terms of pesticide regulation and future R&D directions could be pretty drastic for chiral pesticides," said Gan.
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