Enantiomer-specific formulations could decrease
pesticide use and protect the environment from
Arthur W. GArrison
U.S. EPA NAtioNAl ExPoSUrE
curring chiral molecules (1). This difference may
lead to variations in microbial degradation rates
and would mean that one enantiomer is more per-
sistent in the environment than the other. This has
led to increased research on the enantioselectivity
of compounds such as the phenoxypropionic acid
herbicides, especially dichlorprop and mecoprop
(Figures 1a and 1b).
In many cases, exposure of racemic herbicides
to natural soils or water, in either field applications
or laboratory microcosms, results in selective mi-
crobial degradation of one of the enantiomers. In
addition, enantiomers often exhibit different ef-
fects or toxicity: The “active” enantiomer of a chi-
ral pesticide would have the desired effect on a
target species, whereas the other enantiomer may
not. Moreover, one or both enantiomers may have
adverse effects on some nontarget species. There-
fore, as this article will discuss, assessing enan-
tiomer selectivity for both exposure and effects
is required for comprehensive risk assessments.
s many as 25% of all pesticide active
ingredients are chiral, existing as two
mirror images called enantiomers. En-
antiomers usually differ in their bio-
logical properties as a result of their
interaction with enzymes or other naturally oc-
For more than two decades, environmental scientists
have been investigating the phenomenon of enan-
tiomer selectivity of chiral pollutants and its impact
on pollutant exposure and fate. Most of this research
has been related to the chirality of PCBs (19 of the 209
congeners are chiral) and the older chlorinated pesti-
cides such as o,p-DDT, o,p-DDD, -HCH (hexachlo-
rocyclohexane), and cis- and trans-chlordane.
But many chiral environmental pollutants ex-
ist besides pesticides and PCBs. For example, some
chiral pharmaceutical, health-care, and cosmetic
ingredients have been identified in the environ-
ment (2). Chiral bromochloroacetic acid, a drink-
ing-water disinfection byproduct, was observed in
our lab to degrade enantioselectively when incu-
bated in several surface waters (Figure 2). In addi-
tion, liver samples from whiting and bib fish caught
in the Western Scheldt Estuary of The Netherlands
were found to be enriched in the (+)-enantiomer of
-hexabromocyclododecane, a chiral high-produc-
tion-volume flame retardant (3).
Several chiral PCB congeners are found in non-
racemic concentrations in lake and river sediments.
Because abiotic reactions are not enantioselective,
the discovery indicates that a biotransformation had
occurred (4). Some of these same congeners also exist
© 2006 American Chemical SocietyjAnuAry 1, 2006 / EnvironmEntAl SCiEnCE & tEChnology n 17
jAnuAry 1, 2006 / EnvironmEntAl SCiEnCE & tEChnology n 17
in nonracemic mixtures in associated aquatic and ri-
parian biota (5). In another study, o,p-DDD, a chiral
metabolite, was found in fish tissue after exposure to
DDT; it occurs in these tissues primarily as the (S)-
(–)-enantiomer. The enantiomer fraction [EF, equal
to the area of the (+)-enantiomer divided by the area
of both enantiomers] for o,p-DDD in two-thirds of
these fish samples was between 0.29 and 0.44 (6).
Other investigators have reported enantioselec-
tivity in microbial degradation of -HCH, the chlor-
danes, and the DDT analogs (1, 7−9). In general, the
enantiomers of these persistent pollutants exist in
nonracemic proportions in various environmental
compartments, such as biota (3, 10−12) and human
tissues. These occurrences are indicative of enantio-
selective metabolism, microbial transformation, or
other biological processes. Enantiomer ratios have
been used in several pollutant transport studies.
For example, ratios of chlordane enantiomers in air
above chlordane-polluted soil were found to differ
from those in the soil; this indicates that the atmo-
sphere is contaminated by the pesticide carried to
the site by wind currents (13).
Single-enantiomer drugs are now routinely syn-
thesized or separated from their racemic mixtures.
These formulations compose a large fraction of the
total market (14). On the other hand, the great ma-
jority of chiral pesticides are produced and marketed
as racemates. For example, of the 67 organophos-
phorus (OP) insecticides described in one current
F I G U R E 1
Asterisks indicate chiral centers; some structures have two centers. (c) Metolachlor has a chiral center between the nitrogen
and the benzene ring because bulky groups restrict rotation about this bond.
18 n EnvironmEntAl SCiEnCE & tEChnology / jAnuAry 1, 2006
pesticide handbook (15), 20 are chiral, but none are
formulated as single- or enriched-enantiomer prod-
ucts. All fungicides in the relatively new conazole
class are chiral, but none are sold as single enan-
tiomers. Only a few of the many chiral pyrethroid
insecticides are formulated as single- or enriched-
enantiomer products (15).
Moreover, most conazoles and pyrethroids have
more than one chiral center, resulting in two or more
diastereomers that may be manufactured in ratios
other than 1:1. Like enantiomers, diastereomers are
expected to have different biological activities, but
unlike enantiomers, they also have different physi-
cal properties. Generally, each diastereomer can be
considered a separate chiral compound that consists
of a pair of enantiomers.
In the past 5−10 years, however, several single- or
enriched-enantiomer pesticide formulations have
been developed and promoted in North America and
in Europe. For example, several European govern-
ments have required that mecoprop and dichlorprop
be used as only their active R enantiomers (16). [In
chiral notation, R and S refer to the absolute con-
figuration, or the orientation in space of the groups
around the chiral center of the enantiomer. This ori-
entation is the important factor in determining the
fit with enzymes and other biological molecules. Ei-
ther the R or the S form may rotate plane polarized
light to the right (+) or the left (–), a property that
has no bearing on biological activity but serves as an
easily measured tag to distinguish enantiomers.]
In addition to the few popular pyrethroid pesti-
cides marketed as single- or enriched-enantiomer
products, metolachlor (Figure 1c), an important ac-
etanilide herbicide, has been enriched by its manu-
facturer (Syngenta) to contain 86% of the active S
enantiomers. This enrichment allows a 40% reduc-
tion in the amount of the herbicide that needs to
be applied for the same effect to be achieved (17).
A study in Switzerland found that after the new S-
enriched formulation was used for two years, the
pesticide in runoff contamination of a nearby lake
changed from one dominated by the racemic mix-
ture to one containing the S-enriched metolachlor
(18). Note that the total concentration of metola-
chlor in the lake was not affected by the switch
from the racemate to (S)-metolachlor. Possible ex-
planations for this finding include more precipita-
tion over the two-year period, resulting in increased
metolachlor runoff, or greater use of the pesticide
after the switch. However, in either case, if the ra-
cemate use had continued, the total concentration
of metolachlor would have increased over the two-
Requests for registration of single- or enriched-
enantiomer pesticides are likely to increase as the
agrochemical industry develops more complex pes-
ticides with more chiral centers, such as the con-
azoles; develops more synthetic routes for single- or
enriched-enantiomer compounds; and becomes in-
creasingly conscious of green chemistry. Produc-
tion of single- or enriched-enantiomer pesticides is
a green-chemistry development, because it reduces
the environmental loading of the inactive enantio-
mer, the one that has no or reduced impact on the
target species yet might be active to nontargets. The
U.S. EPA has included the following in its definition
of green chemistry: “. . . the use of chemistry for pol-
lution prevention. More specifically, green chemistry
is the design of chemical products and processes
that reduce or eliminate the use and generation of
EPA assesses the potential for enantioselectivity
in its risk assessments of chiral pesticides. In 2000,
the agency implemented an interim policy that fo-
cuses on optical stereoisomers and evaluates the
relative risks of enantiomers (20). The strategy in
this policy is to bridge the biological-effects and en-
vironmental-fate data for racemates and single- or
enriched-enantiomer forms of the pesticide, focus-
ing on biological transformation in soils and water.
If substantial differences in ecotoxicity and transfor-
mation exist, then additional toxicity and fate data
are requested for these single- or enriched-enan-
Past studies have focused on the role of chirality
of persistent pesticides and PCBs in the environ-
ment, but most of these chemicals are no longer
used. Now, research studies on the environmental
impact of enantiomers of the less-persistent chiral
pesticides currently in use are growing. For many of
these pesticides with intermediate half-lives (days to
months), selective microbial transformation occurs
before significant abiotic transformation.
For example, the herbicide dichlorprop was shown
to be enantioselectively transformed in the surface
soil after application to an experimental field (21).
The (–)-enantiomer exhibited a half-life of ~4 days
and the (+)-enantiomer ~8 days. The (+)-enantio-
mer is known to be the active herbicide, whereas
F I G U R E 2
Bromochloroacetic acid spiked into
natural river water
Analysis over time by capillary electrophoresis with
a chiral selector allowed calculation of enantiomer
fraction (EF) as a measure of microbial transforma-
tion. (a) Initially, EF = 0.50 and (b) at 8 days, EF = 0.43.
The initial racemate concentration was 30 mg/L.
jAnuAry 1, 2006 / EnvironmEntAl SCiEnCE & tEChnology n 19
the (–)-enantiomer is simply “ballast”. Several Eu-
ropean countries now prescribe using only the (+)-
enantiomer (1, 16).
However, the situation can be quite complex, as
recently shown in work on the environmental safety
of chiral pyrethroid and organophosphorus insec-
ticides relative to their enantioselectivity (22). All
pyrethroids are chiral; they usually have more than
one chiral center and, therefore, exist as several en-
antiomers. Approximately 30% of OP insecticides
are chiral, usually because of asymmetry about the
phosphorus atom. The new study found that enan-
tioselectivity was observed in both the toxicity and
the microbial degradation of the same pesticide (22).
For example, the (+)-enantiomer of cis-bifenthrin
(Figure 1d) was ~20× more toxic (lethal concentra-
tion for 50% of the population; LC50) to both cerio-
daphnia dubia and c. magna than the (–) form, and
was also more persistent [i.e., the (–)-enantiomer
was preferentially degraded by microbes] at various
depths in an aged field sediment. The (–)-enantio-
mer of fonofos (Figure 1e), an OP insecticide, was
~15× more toxic to c. dubia and c. magna than the
(+)-enantiomer. The microbial degradation rates of
the fonofos enantiomers were not reported.
One complicating factor of including enantiose-
lectivity in risk assessments is the possible shift of
selectivity in transformation with changes in envi-
ronmental conditions; this leads to a correspond-
ing shift in enantiomer persistence. The herbicide
metalaxyl provides a good example (Figure 1f). Metal-
axyl is one of the few pesticides marketed both as
a racemic formulation and as a single-enantiomer
product, called metalaxyl M, which contains only
the active (R)-(+) form of the key ingredient (23). In
two recent studies, metalaxyl loss was measured in
four soil–water slurries (24, 25). The (+)-enantiomer
disappeared faster than the (–)-enantiomer in all
four soils, but much faster in one soil, with a high
degree of enantioselectivity (Figure 3; 24). So, the
initial conclusion was that microbial degradation of
metalaxyl leads to longer persistence of the unnec-
essary enantiomer. However, subsequent research
with a wide variety of soils showed that not only the
enantiomer degradation rate but even the enantio-
mer preference changed with soil pH (26).
Other research has shown that microbial trans-
formation is not necessarily enantioselective (27).
Significant nonstereoselective transformation of
-HCH was observed in parts of a Virginia estuary
where relevant bacterial activity was the highest,
whereas highly stereoselective degradation occurred
in other regions of the estuary.
In another research study, the enantioselectiv-
ity and kinetic rates of biodegradation for three
chiral pesticides—ruelene (also called crufomate),
dichlorprop, and methyl dichlorprop—were mea-
sured. The goal was to probe the differences in
activities of microbial populations in control soil
samples and samples that were purposely disturbed
or treated from field plots in Brazil, North America,
and Norway (28). Water slurries of soils from these
plots were spiked and analyzed at different times for
these pesticide residues. Although several months
of decay were required before reliable kinetic val-
ues for ruelene (Figures 4a and 4b) and dichlorprop
could be established, the methyl ester of dichlorprop
hydrolyzed enantioselectively to the corresponding
acid over a few days (Figures 4c–4e). The second
eluting enantiomer was completely gone after 48 h.
Apparently, abiotic hydrolysis was much slower than
microbial hydrolysis in these particular soils.
This research also showed that ecosystem dis-
turbance (deforestation) and treatment (nutrient
amendments and warming at 5 °C above ambient
temperature to simulate global warming) changed
degradation rates. In some cases, enantiomer speci-
ficity of the soil microbial populations shifted. For
example, soil microorganisms in most forest samples
from Brazil preferentially removed (+)-dichlorprop
acid, the active form of the herbicide. By compari-
son, the microbes in pasture samples almost exclu-
sively preferred the (–)-enantiomer.
Seven caveats regarding research on the envi-
ronmental occurrence, fate, and exposure of chiral
compounds that should be considered in risk as-
sessments are highlighted in Table 1.
Pesticide risk assessments integrate information on
exposure and effects. Besides potentially differing
in their fate and exposure, enantiomers can also
differ in their toxic effects on target and nontarget
species. Often, only one enantiomer is target-ac-
tive or it is more target-active than the other enan-
tiomer, which is inactive or less active and simply
adds an extra chemical load to the environment.
As previously described, enantiomer exposure data
have emerged over the past several years. However,
a dearth exists for enantiomer-specific effects data.
Some manufacturers have determined the activity
of separated enantiomers of new pesticides by us-
ing various endpoints, sometimes as prerequisites to
developing single-enantiomer products. Examples
for pyrethroids, acylanilides, OPs, and other pesti-
cides are in the literature (30). Despite few effects
data in the literature, enough exist to establish that
F I G U R E 3
Enantioselective transformation of metalaxyl in
The reaction was followed by capillary electrophoresis, which
showed formation of the (R)-(+) acid product (red line). (R)-(+)-
metalaxyl is marked in blue and (S)-(–)-metalaxyl in green (25).
20 n EnvironmEntAl SCiEnCE & tEChnology / jAnuAry 1, 2006
enantiomers may produce different effects.
The most recent examples of the differing fates
for enantiomers were previously described for pyre-
throids and OP insecticide enantiomers (22). How-
ever, earlier literature examples exist. For one, the
two S enantiomers of metolachlor, a widely used
chloroacetamide herbicide, are ~10× more toxic
to target weeds than the two R enantiomers (17).
(Metolachlor has two chiral centers and thus has
two R and two S enantiomers.) All the fungicidal
activity of metalaxyl resides with the (+)-enantiomer
(23). The cholinesterase inhibition activity of chiral
OP pesticides, as well as of the very toxic OP nerve
gases, is enantioselective (31). (+)-Malathion is more
acutely toxic to anthropods and rats than the (–)-
enantiomer (Figure 1g; 32). (+)-Fipronil, a phenyl-
pyrazole broad-spectrum insecticide, is more toxic
to c. dubia than the (–)-enantiomer (Figure 1h; 33),
but in other studies the (–)-enantiomer was shown
to have significantly more androgen and progester-
one activity than the (+) form. Finally, (–)-o,p-DDT
has much greater estrogen receptor affinity than the
Future research opportunities
Additional research is needed on two fronts: to
investigate the potential for target-inactive enan-
tiomers to produce unintended effects on nontar-
get species, and to determine how environmental
conditions affect the relative persistence of enan-
tiomers. For the second case, microbial degrada-
tion studies should be conducted with important
modern chiral pesticides by using a wider variety of
soil/sediment and natural water matrices to estab-
lish correlations between environmental properties
and enantioselectivity. Effects and toxicity studies
are more demanding because they require separated
TA B L E 1
Caveats for research on the environ-
mental occurrence, fate, and expo-
sure of chiral compounds that should
be considered in risk assessments
• Differences or changes in microbial populations
can change, even reverse, the enantiomer fraction
(EF) of a given pollutant.
• Some microbial degradation processes are not en-
antioselective with certain pollutants, and their
disappearance causes no change in EF.
• For pollutants with short half-lives for microbial
transformation, enantioselectivity may not be im-
portant. For example, there may be little or no con-
sequence if one enantiomer has a half-life of one
day and another two days; some of the more mod-
ern and low-persistence pesticides may fall into
this category (Figures 4c–4e).
• Some pollutants could have relatively long half-
lives for microbial activity but degrade significant-
ly faster by competing abiotic reactions, which
makes enantioselectivity a secondary effect.
• Regardless of half-life, the transformation prod-
ucts of chiral pollutants may be chiral themselves
(metalaxyl acid, Figure 3), and these product en-
antiomers may differ in toxicological significance.
On the other hand, achiral pollutants can also be
transformed into chiral products, and those enan-
tiomers may differ in biological properties.
• Microbial activity may convert one enantiomer to
the other—its mirror image (16).
• Some enantioselective environmental process-
es are not yet proven or fully understood. For ex-
ample, enantioselective sorption to chiral clays
or chiral soil, sediment, or aquatic organic mat-
ter could affect the fate of chiral compounds in the
F I G U R E 4
Microbial transformations of ruelene
and methyl dichlorprop enantiomers
(a and b) Ruelene spiked into a soil–water slurry
at 50 mg/L of the racemate. Analysis over time
by capillary electrophoresis with a chiral selector
found that (a) the initial enantiomer fraction (EF) =
0.50, but (b) at 100 days, EF = 0.40 (25). (c–e) The
microbial transformation of methyl dichlorprop en-
antiomers in soil was followed by gas chromatogra-
phy with a Chirasil-Dex column used for enantiomer
separation. (c) Initial separation, (d) 24 h later, and
(e) after 48 h (28).
jAnuAry 1, 2006 / EnvironmEntAl SCiEnCE & tEChnology n 21
The author expresses his appreciation to several others who
have contributed to the work on chiral chemistry in the envi-
ronment conducted at the National Exposure Research Lab-
oratory in Athens, Ga.: Jimmy Avants, Jack Jones, Mac Long,
Charles Wong, David Lewis, Brad Konwick, Lorrie Howell, Jes-
sica Jarman, and Tracey Cash. Reviews by the journal refer-
ees and EPA staff, especially staff of the Office of Pesticide
Programs, have resulted in increased accuracy and other im-
provements in this manuscript. This article has been reviewed
in accordance with EPA’s peer and administrative review pol-
icies and approved for presentation and publication, but this
does not signify that the contents reflect the views of EPA.
Mention of trade names or commercial products does not con-
stitute endorsement or recommendation for use.
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tiomer Composition of Metolachlor in Surface Water
Following the Introduction of the Enantiomerically En-
enantiomers. Such separations require extra time
and equipment; for example, preparative HPLC us-
ing chiral columns may be needed (35). (A few enan-
tiomers of important pesticides are available from
chemical standard suppliers.)
Most enantiomer effects endpoints, such as le-
thality (LC50), death (LD50), and relative fungicidal
activity, are for acute toxicity. Other endpoints in-
clude chlorinesterase inhibition, rates of metabo-
lism by enzymatic hydrolysis and oxidation (30),
and relative enantiomer receptor binding (36).
Such measurements supply valuable information
on mechanisms of toxicity and the selective ef-
fects of enantiomers. However, the modern and
sophisticated “—omics” tools—microarray analy-
sis, proteomics, and metabolomics—measure gene
expression, protein production, and changes in en-
dogenous metabolites after exposure of test species
to pesticide enantiomers. For example, scientists
at our Ecosystems Research Division have recently
used NMR metabolomics to show that when rain-
bow trout are exposed separately to each of the two
enantiomers of triadimefon, significantly differ-
ent endogenous metabolite pattern responses are
observed in the fish livers. These responses also
differ from those seen in the control experiment.
Ultimately, such tools should be able to differenti-
ate between enantiomer activities at the molecular
level and provide additional insight into enantiomer
The ultimate goal of such enantiomer-specific re-
search should be the development of a predictive
capability for enantioselectivity, so that science
can guide manufacturers toward the production
of more single- or enriched-enantiomer pesticides.
Such products would relieve the environment of
thousands of tons of unnecessary chemicals that
may have adverse impacts on nontarget species, in-
cluding humans. For example, in 2001, U.S. farmers
used ~10,000 t of racemic metolachlor. Using (S)-
metolachlor instead would have decreased the en-
vironmental load by ~4000 t (37).
Of course, the target-active enantiomer of any
chiral pesticide may also have some adverse effects
on nontarget species, but the pesticide registration
process would have screened the active enantiomer
for adverse effects through the required toxicity
tests. Thus, a product with an EPA-registered active
enantiomer would be labeled for use in such a way
as to limit adverse environmental impacts (20).
Finally, regulatory authorities should be provided
with data on both the fate and effects of separate en-
antiomers so that they can make the best possible
risk assessments for single- or enriched-enantiomer
pesticides that may be submitted for registration.
These additional data would allow risk assessors
to consider each enantiomer as an individual com-
pound with its own set of biological properties and
would provide a sound scientific base for regula-
Arthur W. Garrison is a research chemist in the Eco-
systems research Division of EPA’s National Exposure
research laboratory in Athens, Ga.
22 n EnvironmEntAl SCiEnCE & tEChnology / jAnuAry 1, 2006
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