Probing the Enantioselectivity of Chiral Pesticides

U.S. EPA National Exposure Research Laboratory, Athens, GA, USA.
Environmental Science and Technology (Impact Factor: 5.33). 02/2006; 40(1):16-23. DOI: 10.1021/es063022f
Source: PubMed


As much as 25% of the pesticides in use are chiral, but the compounds are typically supplied as racemic mixtures. While one enantiomer may have the desired effect on a target species, the other enantiomer or enantiomers may not. Moreover, the various chiral forms could have different toxicities, degradation rates, and environmental effects. Therefore, a comprehensive risk assessment requires determining the enantiomer selectivity of these pesticides.

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    • "Stereoselective transformation and residues have been reported for chiral herbicides(Garrison, 2006; Liu et al., 2009), insecticides(Faller et al., 1991; Müller and Kohler, 2004), and other organic compounds (Wiberg et al., 2000). While most chiral pesticides are usually used as a racemic mix, only a few studies have investigated the effect of the individual enantiomers. "
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    ABSTRACT: Cycloxaprid (CYC) is a novel broad-spectrum neonicotinoid insecticide that has been developed for agricultural pest control. The fate of the (14)C-labeled racemic and enantio-pure CYC isomers in flooded and anaerobic soil was investigated using radioisotope tracing techniques. After 100d of incubation, only a minor portion (<1%) of the applied CYC isomers is mineralized by each of the four tested soil types. The fraction of initially applied radioactive CYC dissipated into the bound or non-extractable residues (BR) increases with increase in the length of the incubation period, reaching up to 53.0-81.6%. The dissipation of the CYC through mineralization or formation of BR is strongly influenced by soil properties, such as humic content, pH value, and retained microbial activity. Amongst the soils studied, the fluvio-marine yellow loamy soil displayed the highest tendency to mineralize CYC while the coastal saline soil exhibited the strongest tendency to form BR. The observation that the water phase retained the large portion(>60%) of the radioactivity attributed to the total extractable residue suggested that under the experimental condition, the initially applied (14)C-labeled CYC residues were readily available for leaching or offsite transport. Additionally, no enantiomer-specific behaviors are observed. The results from this study provide a framework for assessing the environmental impact resulting from the use of this pesticide.
    Science of The Total Environment 02/2016; 543(Pt A):116-122. DOI:10.1016/j.scitotenv.2015.11.018 · 4.10 Impact Factor
    • "Increasing knowledge of the factors controlling the environmental fate of chiral pesticide enantiomers is a recent matter of concern (Garrison, 2006; Petrie et al., 2015; Sekhon, 2009). Over 25% of pesticides currently in use are chiral, and in China up to 40% of the pesticides in use are classified as chiral compounds (Garrison, 2006; Ye et al., 2009). The presence of chirality in pesticides has received very little attention . "
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    ABSTRACT: A large number of pesticides are chiral and reach the environment as mixtures of optical isomers or enantiomers. Agricultural practices can affect differently the environmental fate of the individual enantiomers. We investigated how amending an agricultural soil with composted olive-mill waste (OMWc) or its biochar (BC) at 2% (w:w) affected the sorption, degradation, and leaching of each of the two enantiomers of the chiral fungicide metalaxyl. Sorption of metalaxyl enantiomers was higher on BC (Kd≈145Lkg(-1)) than on OMWc (Kd≈22Lkg(-1)) and was not enantioselective in either case, and followed the order BC-amended>OMWc-amended>unamended soil. Both enantiomers showed greater resistance to desorption from BC-amended soil compared to unamended and OMWc-amended soil. Dissipation studies revealed that the degradation of metalaxyl was more enantioselective (R>S) in unamended and OMWc-amended soil than in BC-amended soil. The leaching of both S- and R-metalaxyl from soil columns was almost completely suppressed after amending the soil with BC and metalaxyl residues remaining in the soil columns were more racemic than those in soil column leachates. Our findings show that addition of BC affected the final enantioselective behavior of metalaxyl in soil indirectly by reducing its bioavailability through sorption, and to a greater extent than OMWc. BC showed high sorption capacity to remove metalaxyl enantiomers from water, immobilize metalaxyl enantiomers in soil, and mitigate the groundwater contamination problems particularly associated with the high leaching potential of the more persistent enantiomer.
    Science of The Total Environment 10/2015; 541:776-783. DOI:10.1016/j.scitotenv.2015.09.097 · 4.10 Impact Factor
    • "In this regard, although several authors have reported results of batch sorption experiments where different chiral compounds seemed to exhibit strong enantioselectivity in their sorption on natural soil and sediment materials [23] [24] [25], others have failed to identify sorption enantioselectivity [21,26–28]. Since sorption is a key process determining the amount of pesticide available for other transport and degradation processes, enantioselective sorption to chiral soil, sediment, or aquatic components would greatly affect the fate of chiral pesticides in the environment by making one of the enantiomers more available than the other [2] [9] [17]. Metalaxyl [methyl-N-(2-methoxyacetyl)-N-(2,6-xylyl)-DL- alaninate] is a chiral phenylamide fungicide widely used in the control of plant diseases caused by pathogens of the Oomycota division in several crops. "
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    ABSTRACT: Mechanisms governing the enantioselectivity of the processes that determine the behavior of chiral pollutants in the environment need to be better understood. Understanding these mechanisms should help improve predictions of the hazards and risks chiral compounds can pose to people and the environment. We report the results of batch sorption experiments indicating that the sorption of the chiral fungicide metalaxyl on soil from non-racemic initial solutions was enantioselective. While from a racemic initial solution the two enantiomers of metalaxyl were sorbed on the soil to the same extent, increasing the fraction of R-enantiomer in the initial solution led to enhanced sorption of this enantiomer and to reduced sorption of the S-enantiomer. Considering the shape of the sorption isotherms (S-type) and the sorption behavior of model sorbents, we attributed this effect to molecular interactions between metalaxyl enantiomer species at the sorbed state, where R-R metalaxyl interactions appeared to be more favorable than R-S metalaxyl interactions. We discuss important environmental implications of the proposed mechanism, such as those related to the fact that the biological degradation of metalaxyl is known to be an enantioselective process that can yield non-racemic residues in soils shortly after application of the fungicide as a racemic mixture. Copyright © 2015 Elsevier B.V. All rights reserved.
    Journal of hazardous materials 07/2015; 300:581-589. DOI:10.1016/j.jhazmat.2015.07.059 · 4.53 Impact Factor
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