Riverside, California
January 14, 2005
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.
The University of California, Riverside is a major
research institution and a national center for the
humanities. Key areas of research include nanotechnology,
genomics, environmental studies, digital arts and
sustainable growth and development. With a current
undergraduate and graduate enrollment of nearly 17,000, the
campus is projected to grow to 21,000 students by 2010.
Located in the heart of inland Southern California, the
nearly 1,200-acre, park-like campus is at the center of the
region's economic development.
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