Probing the Enantioselectivity of CHIRAL Pesticides Enantiomer-specific formulations could decrease
pesticide use and protect the environment from unintended effects. A rthur W. Ga rrison U.S. EPA National Ex posure Resea rch La bor atory
A
s many as 25% of all pesticide active ingredients are chiral, existing as two mirror images called enantiomers. Enantiomers usually differ in their biological properties as a result of their interaction with enzymes or other naturally occurring chiral molecules (1). This difference may lead to variations in microbial degradation rates and would mean that one enantiomer is more persistent 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 microbial degradation of one of the enantiomers. In addition, enantiomers often exhibit different effects or toxicity: The “active” enantiomer of a chiral 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. Therefore, as this article will discuss, assessing enantiomer selectivity for both exposure and effects is required for comprehensive risk assessments.
© 2006 American Chemical Society
Historical perspective For more than two decades, environmental scientists have been investigating the phenomenon of enantiomer 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 pesticides such as o,p-DDT, o,p-DDD, -HCH (hexachlorocyclohexane), and cis- and trans‑chlordane. But many chiral environmental pollutants exist besides pesticides and PCBs. For example, some chiral pharmaceutical, health-care, and cosmetic ingredients have been identified in the environment (2). Chiral bromochloroacetic acid, a drinking-water disinfection byproduct, was observed in our lab to degrade enantioselectively when incubated in several surface waters (Figure 2). In addition, 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-production-volume flame retardant (3). Several chiral PCB congeners are found in nonracemic 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
january 1, 2006 / Environmental Science & Technology n 17
FIGURE 1
Pesticide structures 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. (a)
(b) CH3 Cl
H3C
C
O
C
H
COOH
HOOC
O
CH3
Cl
H
Cl
CH3 *C
O
C
OH
H Cl
Mecoprop
Cl
(R)-(+)-Dichlorprop
O
(S)-(–)-Dichlorprop (d)
(c) CH3 H
CH2Cl
C
CH3
O
CF3
CH3
* N CH2 O
CH2OCH3
*
*
Cl
O
*
Bifenthrin
H3C Metolachlor (e)
(f) CH3 P* S CH3
CH2CH3
CH3
COOCH3
* CH3
Metalaxyl
Fonofos (h)
(g) S CH3O
O N
O CH3O
O
H
P
S
Cl *
O CH3
COOC2H5
in nonracemic mixtures in associated aquatic and riparian 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 enantioselectivity in microbial degradation of -HCH, the chlordanes, 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 18 n Environmental Science & Technology / january 1, 2006
..
N
CF3
*
CH2COOC2H5
Malathion
CN
N
Cl
NH2
S
CF3
O
Fipronil
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 atmosphere is contaminated by the pesticide carried to the site by wind currents (13). Single-enantiomer drugs are now routinely synthesized or separated from their racemic mixtures. These formulations compose a large fraction of the total market (14). On the other hand, the great majority of chiral pesticides are produced and marketed as racemates. For example, of the 67 organophosphorus (OP) insecticides described in one current
pesticide handbook (15), 20 are chiral, but none are formulated as single- or enriched-enantiomer products. All fungicides in the relatively new conazole class are chiral, but none are sold as single enantiomers. Only a few of the many chiral pyrethroid insecticides are formulated as single- or enrichedenantiomer 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 physical 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 governments 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 configuration, or the orientation in space of the groups around the chiral center of the enantiomer. This orientation is the important factor in determining the fit with enzymes and other biological molecules. Either 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 pesticides marketed as single- or enriched-enantiomer products, metolachlor (Figure 1c), an important acetanilide herbicide, has been enriched by its manufacturer (Syngenta) to contain 86% of the active S enantiomers. This enrichment allows a 40% reduction 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 Senriched formulation was used for two years, the pesticide in runoff contamination of a nearby lake changed from one dominated by the racemic mixture to one containing the S-enriched metolachlor (18). Note that the total concentration of metolachlor in the lake was not affected by the switch from the racemate to (S)-metolachlor. Possible explanations for this finding include more precipitation 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 racemate use had continued, the total concentration of metolachlor would have increased over the twoyear period. Requests for registration of single- or enrichedenantiomer pesticides are likely to increase as the agrochemical industry develops more complex pesticides with more chiral centers, such as the conazoles; develops more synthetic routes for single- or enriched-enantiomer compounds; and becomes increasingly conscious of green chemistry. Production 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 pollution prevention. More specifically, green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances”(19). EPA assesses the potential for enantioselectivity in its risk assessments of chiral pesticides. In 2000, the agency implemented an interim policy that focuses on optical stereoisomers and evaluates the relative risks of enantiomers (20). The strategy in this policy is to bridge the biological-effects and environmental-fate data for racemates and single- or enriched-enantiomer forms of the pesticide, focusing on biological transformation in soils and water. If substantial differences in ecotoxicity and transformation exist, then additional toxicity and fate data are requested for these single- or enriched-enantiomer forms.
Enantiomer-specific fate Past studies have focused on the role of chirality of persistent pesticides and PCBs in the environment, 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 (+)-enantiomer is known to be the active herbicide, whereas FIGURE 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 transformation. (a) Initially, EF = 0.50 and (b) at 8 days, EF = 0.43. The initial racemate concentration was 30 mg/L. O H C* Br Cl (a)
(–)
C
OH
(b) (+) (–) (+)
january 1, 2006 / Environmental Science & Technology n 19
the (–)-enantiomer is simply “ballast”. Several European 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 insecticides relative to their enantioselectivity (22). All pyrethroids are chiral; they usually have more than one chiral center and, therefore, exist as several enantiomers. Approximately 30% of OP insecticides are chiral, usually because of asymmetry about the phosphorus atom. The new study found that enantioselectivity 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 concentration for 50% of the population; LC50) to both Ceriodaphnia 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 (–)-enantiomer 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. FIGURE 3
Enantioselective transformation of metalaxyl in Ohio soil 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 ).
Metalaxyl (mg/L)
25 20 15 10 5 0 0
100
200 Time (days)
300
400
with a wide variety of soils showed that not only the enantiomer degradation rate but even the enantiomer preference changed with soil pH (26). Other research has shown that microbial transformation 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 enantioselectivity and kinetic rates of biodegradation for three chiral pesticides—ruelene (also called crufomate), dichlorprop, and methyl dichlorprop—were measured. 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 values 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 disturbance (deforestation) and treatment (nutrient amendments and warming at 5 °C above ambient temperature to simulate global warming) changed degradation rates. In some cases, enantiomer specificity 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 comparison, the microbes in pasture samples almost exclusively preferred the (–)-enantiomer. Seven caveats regarding research on the environmental occurrence, fate, and exposure of chiral compounds that should be considered in risk assessments are highlighted in Table 1.
Enantiomer-specific effects One complicating factor of including enantioselectivity in risk assessments is the possible shift of selectivity in transformation with changes in environmental conditions; this leads to a corresponding 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 unnecessary enantiomer. However, subsequent research 20 n Environmental Science & Technology / january 1, 2006
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-active or it is more target-active than the other enantiomer, 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 using various endpoints, sometimes as prerequisites to developing single-enantiomer products. Examples for pyrethroids, acylanilides, OPs, and other pesticides are in the literature (30). Despite few effects data in the literature, enough exist to establish that
FIGURE 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 enantiomers in soil was followed by gas chromatography with a Chirasil-Dex column used for enantiomer separation. (c) Initial separation, (d) 24 h later, and (e) after 48 h (28 ). O H3CO
P*
H3CNH (a)
(+)
C(CH3)3 O CI
Additional research is needed on two fronts: to investigate the potential for target-inactive enantiomers to produce unintended effects on nontarget species, and to determine how environmental conditions affect the relative persistence of enantiomers. For the second case, microbial degradation studies should be conducted with important modern chiral pesticides by using a wider variety of soil/sediment and natural water matrices to establish correlations between environmental properties and enantioselectivity. Effects and toxicity studies are more demanding because they require separated
Cl CH3 O
*C H
(d)
C
TA B L E 1
Caveats for research on the environmental occurrence, fate, and exposure of chiral compounds that should be considered in risk assessments
(+)
(c)
Future research opportunities
(b)
(–)
(–)
Cl
enantiomer (Figure 1g; 32). (+)-Fipronil, a phenylpyrazole 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 progesterone activity than the (+) form. Finally, (–)-o,p-DDT has much greater estrogen receptor affinity than the (+)-enantiomer (34).
O OCH3 (e)
enantiomers may produce different effects. The most recent examples of the differing fates for enantiomers were previously described for pyrethroids and OP insecticide enantiomers (22). However, 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 (–)-
• Differences or changes in microbial populations can change, even reverse, the enantiomer fraction (EF) of a given pollutant. • Some microbial degradation processes are not enantioselective with certain pollutants, and their disappearance causes no change in EF. • F or pollutants with short half-lives for microbial transformation, enantioselectivity may not be important. For example, there may be little or no consequence if one enantiomer has a half-life of one day and another two days; some of the more modern and low-persistence pesticides may fall into this category (Figures 4c–4e). • Some pollutants could have relatively long halflives for microbial activity but degrade significantly faster by competing abiotic reactions, which makes enantioselectivity a secondary effect. • Regardless of half-life, the transformation products of chiral pollutants may be chiral themselves (metalaxyl acid, Figure 3), and these product enantiomers may differ in toxicological significance. On the other hand, achiral pollutants can also be transformed into chiral products, and those enantiomers may differ in biological properties. • Microbial activity may convert one enantiomer to the other—its mirror image (16 ). • Some enantioselective environmental processes are not yet proven or fully understood. For example, enantioselective sorption to chiral clays or chiral soil, sediment, or aquatic organic matter could affect the fate of chiral compounds in the environment (29 ).
january 1, 2006 / Environmental Science & Technology n 21
enantiomers. Such separations require extra time and equipment; for example, preparative HPLC using chiral columns may be needed (35). (A few enantiomers of important pesticides are available from chemical standard suppliers.) Most enantiomer effects endpoints, such as lethality (LC50), death (LD50), and relative fungicidal activity, are for acute toxicity. Other endpoints include chlorinesterase inhibition, rates of metabolism by enzymatic hydrolysis and oxidation (30), and relative enantiomer receptor binding (36). Such measurements supply valuable information on mechanisms of toxicity and the selective effects of enantiomers. However, the modern and sophisticated “—omics” tools—microarray analysis, proteomics, and metabolomics—measure gene expression, protein production, and changes in endogenous 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 rainbow trout are exposed separately to each of the two enantiomers of triadimefon, significantly different 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 differentiate between enantiomer activities at the molecular level and provide additional insight into enantiomer toxicity pathways. The ultimate goal of such enantiomer-specific research 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, including humans. For example, in 2001, U.S. farmers used ~10,000 t of racemic metolachlor. Using (S)metolachlor instead would have decreased the environmental 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 enantiomers 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 compound with its own set of biological properties and would provide a sound scientific base for regulatory decisions. Arthur W. Garrison is a research chemist in the Ecosystems Research Division of EPA’s National Exposure Research Laboratory in Athens, Ga. 22 n Environmental Science & Technology / january 1, 2006
Acknowledgments The author expresses his appreciation to several others who have contributed to the work on chiral chemistry in the environment conducted at the National Exposure Research Laboratory in Athens, Ga.: Jimmy Avants, Jack Jones, Mac Long, Charles Wong, David Lewis, Brad Konwick, Lorrie Howell, Jessica Jarman, and Tracey Cash. Reviews by the journal referees and EPA staff, especially staff of the Office of Pesticide Programs, have resulted in increased accuracy and other improvements in this manuscript. This article has been reviewed in accordance with EPA’s peer and administrative review policies 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 constitute endorsement or recommendation for use.
References (1) Hegeman, W. J. M.; Laane, R. W. P. M. Enantiomeric Enrichment of Chiral Pesticides in the Environment. Rev. Environ. Contam. Toxicol. 2002, 173, 85−116. (2) Xia, K.; et al. Occurrence and Fate of Pharmaceuticals and Personal Care Products (PPCPs) in Biosolids. J. Environ. Qual. 2005, 34, 91−104. (3) Janek, K.; et al. Hexabromocyclododecane in Marine Species from the Western Scheldt Estuary: Diastereoisomer- and Enantiomer-Specific Accumulation. Environ. Sci. Technol. 2005, 39, 1987−1994. (4) Wong, C. S.; Garrison, A. W.; Foreman,W. T. Enantiomeric Composition of Chiral Polychlorinated Biphenyl Atropisomers in Aquatic Bed Sediment. Environ. Sci. Technol. 2001, 35, 33−39. (5) Wong, C. S.; et al. Composition of Chiral Polychlorinated Biphenyl Atropisomers in Aquatic and Riparian Biota. Environ. Sci. Technol. 2001, 35, 2448−2454. (6) Garrison, A. W.; et al. Determining the Environmental Enantioselectivity of o,p-DDT and o,p-DDD. Organohalogen Compd. 1997, 31, 256−261. (7) Shen, L.; et al. Hexachlorocyclohexanes in the North American Atmosphere. Environ. Sci. Technol. 2004, 38, 965−975. (8) Aigner, E.; Leone, A.; Falconer, R. Concentrations and Enantiomeric Ratios of Organochlorine Pesticides in Soils from the U.S. Corn Belt. Environ. Sci. Technol. 1998, 32, 1162−1168. (9) Vetter, W.; Schurig, V. Enantioselective Determination of Chiral Organochlorine Compounds in Biota by Gas Chromatography on Modified Cyclodextrins. J. Chromatogr., A 1997, 774, 143−175. (10) Borga, K.; Bidleman, T. F. Enantiomer Fractions of Organic Chlorinated Pesticides in Arctic Marine Ice Fauna, Zooplankton, and Benthos. Environ. Sci. Technol. 2005, 39, 3464−3473. (11) Zegers, B.; et al. Levels of Hexabromocyclododecane in Harbor Porpoises and Common Dolphins from Western European Seas, with Evidence for Stereoisomer-Specific Biotransformation by Cytochrome P450. Environ. Sci. Technol. 2005, 39, 2095−2100. (12) Lehmler, H.-J.; et al. Distribution of PCB 84 Enantiomers in C57BL/6 Mice. Fresenius Environ. Bull. 2003, 12, 254−260. (13) Renner, R. Chiral Compounds Show Promise as Environmental Tracers. Environ. Sci. Technol. 1996, 30, 16A−17A. (14) Somogyi, A.; Bochner, F.; Foster, D. Inside the Isomers: The Tale of Chiral Switches. Aust. Prescr. 2005, 27, 47−49. (15) Roberts, T., Hutson, D., Eds. Metabolic Pathways of Agrochemicals, Part Two: Insecticides and Fungicides; Royal Society of Chemistry: Cambridge, UK, 1999. (16) Buser, H.-R.; Müller, M. D. Occurrence and Transformation Reactions of Chiral and Achiral Phenoxyalkanoic Acid Herbicides in Lakes and Rivers in Switzerland. Environ. Sci. Technol. 1998, 32, 626−633. (17) Spindler, F.; Früh, T. Chiral Acylanilides and Chiral Triazole-Related Fungicides. In Kurihara, N., Miyamoto, J., Eds; Chirality in Agrochemicals; Wiley & Sons: Chichester, UK, 1998, pp 143−145. (18) Buser, H.-R.; Poiger, T.; Müller, M. D. Changed Enantiomer Composition of Metolachlor in Surface Water Following the Introduction of the Enantiomerically En-
riched Product to the Market. Environ. Sci. Technol. 2000, 34, 2690−2696. (19) Green Chemistry Program Fact Sheet. www.epa.gov/ greenchemistry/docs/general_fact_sheet.pdf. (20) EPA Environmental Fate and Effects Division Interim Policy for Stereoisomeric Pesticides, www.epa.gov/ oppefed1/ecorisk_ders/stereoisomer_policy.htm. (21) Garrison, A. W.; et al. Enantiomeric Selectivity in the Environmental Degradation of Dichlorprop as Determined by High-Performance Capillary Electrophoresis. Environ. Sci. Technol. 1996, 30, 2449−2455. (22) Liu, W.; et al. Enantioselectivity in Environmental Safety of Current Chiral Insecticides. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 701−706. (23) Spindler, F.; Früh, T. Chiral Acylanilides and Chiral Triazole-Related Fungicides. In Chirality in Agrochemicals; Kurihara, N., Miyamoto, J., Eds.; Wiley & Sons: Chichester, UK, 1998, pp 155−157. (24) Buser, H.-R.; et al. Environmental Behavior of the Chiral Acetamide Pesticide Metalaxyl: Enantioselective Degradation and Chiral Stability in Soil. Environ. Sci. Technol. 2002, 36, 221−226. (25) Jarman, J. L.; et al. Application of Capillary Electrophoresis To Study the Enantioselective Transformation of Five Chiral Pesticides in Aerobic Soil Slurries. J. Agric. Food Chem. 2005, 53, 6175–6182. (26) Buerge, I. J.; et al. Enantioselective Degradation of Metal axyl in Soils: Chiral Preference Changes with Soil pH. Environ. Sci. Technol. 2003, 37, 2668−2674. (27) Padma, T. V.; Dickhut, R. M.; Ducklow, H. Variations in aHexachlorocyclohexane Enantiomer Ratios in Relation to Microbial Activity in a Temperate Estuary. Environ. Toxicol. Chem. 2003, 22, 1421−1427.
(28) Lewis, D. L.; et al. Influence of Environmental Changes on Degradation of Chiral Pollutants in Soils. Nature 1999, 401, 898−901. (29) Wedyan, M.; Preston, M. R. Isomer-Selective Adsorption of Amino Acids by Components of Natural Sediments. Environ. Sci. Technol. 2005, 39, 2115−2119. (30) Kurihara, N., Miyamoto, J., Eds. Chirality in Agrochemicals; Wiley & Sons: Chichester, UK, 1998. (31) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; Wiley & Sons: New York, 1993; p 208. (32) Polec, I.; et al. Simple Synthesis of Malathion and Malaoxon Enantiomers and Isomalathion Diastereoisomers: Toxicity– Configuration Relationship. Pestic. Sci. 1998, 53, 165−171. (33) Konwick, B. J.; et al. Acute Enantioselective Toxicity of Fipronil and Its Desulfinyl Photoproduct to Ceriodaphnia dubia. Environ. Toxicol. Chem. 2005, 24, 2350–2355. (34) McBlain, W. A. The Levo Enantiomer of o,p-DDT Inhibits the Binding of 17 Beta-estradiol to the Estrogen Receptor. Life Sci. 1987, 40, 215−221. (35) Champion, W. L., Jr.; et al. Liquid Chromatographic Separation of the Enantiomers of trans-Chlordane, cis-Chlordane, Heptachlor, Heptachlor Epoxide and a-Hexachlorocyclohexane with Application to Small-Scale Preparative Separation. J. Chromatogr., A 2004, 1024, 55−62. (36) Wiese, T. E.; et al. Selective in vitro Hormone Activity of o,p-DDT and o,p-Methoxychlor Optical Isomers. Presented at the 218th National Meeting of the American Chemical Society, New Orleans, LA, Aug 1999. (37) Kiely, T.; Donaldson, D.; Grube, A. Pesticides Industry Sales and Usage, 2000 and 2001 Market Estimates; U.S. EPA Office of Pesticide Programs, Biological and Economic Analysis Division, May 2004, www.epa.gov/ oppbead1/pestsales/.
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