&EPA
U.S. Environmental Protection Agency
Chiral Chemistry: the ultimate in pollutant speciation
(RM+l-dichloiprap (S M -)-dichlorprap
KiiantiomiTS of the thiral horbkick1 dk'hlorprop
What's in a molecule? We ordinarily think that the
diagram of a molecule's structure shows everything about
the way the atoms connect to make a specific chemical with
defined properties. However, with some molecules, a
careful look at their three dimensional structure offers a
surprise - there are two ways to connect the atoms. These
are called chiral molecules, or chiral chemicals, from the
Greek cheir (hand), because, like hands, the two forms of
the molecule are non-superimposible mirror images of each
other. These two species are called enantiomers.
Objective: Enantiomers have identical physical and chemical properties except when they interact with
enzymes or with other chiral molecules; then they usually react differently, or selectively. This
enantioselectivity results in different rates of microbial transformation and differences in activity and
toxicity of the two enantiomers. Up to 25% of pesticides are chiral molecules, as are some PCBs and many
other pollutants. However, almost all chiral pesticides are manufactured and applied as mixtures of equal
amounts of the two enantiomers. On the other hand, the agrochemical industry and government regulators
are beginning to take enantioselectivity into account. For example, the (R)-(+)-enantiomer of the herbicide
dichlorprop (as well as the (R)-(+)-enantiomers of all the phenoxypropionic acid herbicides) is the active
enantiomer, killing the weeds, while the (S)-(-)-enantiomer is inactive (see the dichlorprop structures
above); 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. To make more accurate risk assessments of chiral pesticides, it is necessary to understand the
relative persistence and effects of their enantiomers. The objective of our research is to determine the
environmental occurrences, fate and effects of the enantiomers of selected chiral pesticides and other
chiral organic pollutants.
Our research approach
Separation: develop analytical techniques (GC, HPLC or CE) to separate enantiomers (I).
Occurrence: analyze water, soil, sediment, biota and food samples expected to contain chiral
pollutants to determine occurrences and ratios of the enantiomers
Transformation: conduct degradation experiments in selected environmental matrices to measure
enantioselectivity and rates of enantiomer degradation
Effects: for especially important pollutants, separate and collect enough of each enantiomer for
individual toxicity testing.
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Chiral Chemistry: the ultimate in pollutant speciation I
Some results
The enantioselectivity occurring during microbial transformation of chiral pesticides in soils may be
substantially altered by environmental changes imposed on the soils, thus changing the relative persistence of
the enantiomers (2).
Bromochloroacetic acid, formed by chlorination of drinking waters containing naturally occurring bromide,
degraded enantioselectively in all six natural waters and a municipal wastewater effluent into which it was
spiked.
o,p'-DDD, a chiral residue that remains in fish tissue after exposure to DDT, occurs primarily as the (S)-
enantiomer.
The (R)-enantiomer of o,p'-DDT was shown by endocrine disrupter screening tests to have much more
estrogenic activity than the (S)-enantiomer.
Several chiral PCB congeners occur enantioselectively in lake and river sediments (3), indicating the
biotransformation has occurred, as well as in associated biota.
Useful publications
1. Garrison, A.W. "Analysis of Chiral Pesticides and Polychlorinated Biphenyl Congeners in Environmental
Samples" in Encyclopedia of Analytical Chemistry, R.A. Meyers, ed., pp. 6147-6158, John Wiley & Sons,
2000. (A general reference to chiral chemistry in the environment.)
2. Lewis, D.L., Garrison, A.W., Wommack, K.E., Whittemore, A., Steudler, P., Melillo, J. "Influence of
environmental changes on degradation of chiral pollutants in soils" Nature, 1999, 401, 898-901.
3. Wong, C.S., Garrison, A.W., Foreman, W.T "Enantiomeric composition of chiral polychlorinated biphenyl
atropisomers in aquatic bed sediment" Environmental Science and Technology, 2001, 35, 33-39.
Selected milestones
June 2001 - APM 127: Characterize the enantiomeric ratios of selected organophosphorus (OP) and other widely
used and less persistent pesticides in environmental and food samples
2001 - Measure the endocrine disrupting activity of the enantiomers of selected persistent organochlorine (OC)
pesticides
2002 - Measure the degradation rates and endocrine disrupting activity of the enantiomers of selected OP and
other less persistent pesticides
2003 — Measure the relative microbial degradation rates of the enantiomers of selected OC pesticides and PCBs in
soil and sediment systems
Expected Benefits
• Risk assessment - increased accuracy of environment and human risk assessment will result from
consideration of enantioselectivity in exposure and effects of chiral pollutants
Pollution prevention - use of only the target-active enantiomer of pesticides will reduce the pollutant load and
avoid any adverse effects of the other enantiomer.
Participants/affiliations
ORD/NERL/ERD, Athens: Wayne Garrison*, Jackson Ellington, David Lewis, Jack Jones, Charles Wong
ORD/NERL/HEASD, RTF: Renee Falconer
National Exposure Research Laboratory
For More Information Contact:
Wayne Garrison
garrison.wayne@epa.gov
National Exposure
Research Laboratory
Ecosystems Research Division
Website
http://www.epa.gov/athens
For more information about this and other NERL science projects, visit our Website
http://www.epa.gov/nerl/
WEB SITE ANNOUNCEMENT April 2001
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