Enantioselectivity in environmental safety of current chiral insecticides

Abstract

Chiral pesticides currently constitute about 25% of all pesticides used, and this ratio is increasing as more complex structures are introduced. Chirality occurs widely in synthetic pyrethroids and organophosphates, which are the mainstay of modern insecticides. Despite the great public concerns associated with the use of insecticides, the environmental significance of chirality in currently used insecticides is poorly understood. In this study, we resolved enantiomers of a number of synthetic pyrethroid and organophosphate insecticides on chiral selective columns and evaluated the occurrence of enantioselectivity in aquatic toxicity and biodegradation. Dramatic differences between enantiomers were observed in their acute toxicity to the freshwater invertebrates Ceriodaphnia dubia and Daphnia magna, suggesting that the aquatic toxicity is primarily attributable to a specific enantiomer in the racemate. In field sediments, the (؊) enantiomer of cis-bifenthrin or cispermethrin was preferentially degraded, resulting in relative enrichment of the (؉) enantiomer. Enantioselective degradation was also observed during incubation of sediments under laboratory conditions. Enantioselectivity in these processes is expected to result in ecotoxicological effects that cannot be predicted from our existing knowledge and must be considered in future risk assessment and regulatory decisions. chiral contaminants ͉ chirality ͉ enantiomers ͉ chiral pesticides ͉ chiral selectivity T he significance of molecular chirality is widely recognized in life sciences (1, 2). A lesser-known fact is that many modern pesticides also contain chiral structures and thus consist of enantiomers (3, 4). About 25% of currently used pesticides are chiral, and this ratio is increasing as compounds with more complex structures are introduced into use (3). Enantiomers of the same compound have identical physical-chemical properties and thus appear as a single compound in standard analysis. For economic reasons, chiral pesticides are primarily used as mixtures of enantiomers, or racemates. However, enantiomers are known to selectively interact with biological systems that are usually enantioselective and may behave as drastically different compounds. The role of enantioselectivity in environmental safety is poorly understood for pesticides, and the knowledge gap is reflected in that the great majority of chiral pesticides are used and regulated as if they were achiral, that is, single compounds.Studies on chiral pesticides started to appear in the early 1990s (4,(5)(6)(7)(8)(9)(10)(11)(12). Studies so far show that microbial degradation of chiral pesticides is commonly enantioselective. As one enantiomer is preferentially degraded, the enantiomer ratio (ER), defined as the ratio of (ϩ) enantiomers to (Ϫ) enantiomers, increasingly deviates from the original value (typically 1.0) (8, 9). Enantioselectivity was found to result in changes of ER in ␣-HCH along the polar bear food chain, causing ER to increase from Ϸ1.0 in cod to 2.3 in liver samples of polar be...

Synthetic Pyrethroid Use Patterns, Properties, and Environmental Effects

“…One consequence is that application rates expressed on an AI basis are lower for the more active enriched products due to their enhanced activity. However, several recent articles have reported differences in persistence also, likely due to differences in biodegradability among different isomers (16)(17)(18). Data on stereoselective biodegradation of pyrethroids are relatively sparse, so the practical signficance of stereoselective biodegradation is not well understood.…”

“…permethrin, Table III). In general the biological activity of different stereoisomers varies substantially (15)(16)(17)(18), so that enrichment of the most active isomer(s) yields a product with greatly enhanced insecticidal activity.. In recent years several isomerically enriched pyrethroid active ingredients have been introduced into commercial use.…”

Effects of the pyrethroid insecticide gamma-cyhalothrin on aquatic invertebrates in laboratory and outdoor microcosm tests

“…Therefore, the microcosm data illustrate that the two enantiomers making up LCH have similar fate profiles, with the single enantiomer GCH demonstrating up to twice the level of toxicity to aquatic invertebrates as the racemate LCH. Therefore, the concerns associated with potential enantioselectivity when assessing the aquatic risk of pyrethroids, as raised by Ali et al (2003) and Liu et al (2005), are not manifested in the case of LCH when compared to the fate and effects of its active enantiomer. Besides the information for the cyhalothrins, a considerable amount of data is available from model ecosystem studies with other synthetic pyrethroids performed under various experimental conditions (see review Van Wijngaarden et al 2005).…”

Enantioselective Oxidative Damage of Chiral Pesticide Dichlorprop to Maize

“…Chiral enantiomers have identical physical and chemical properties in achiral environments (e.g., airÀwater exchange, sorption, and abiotic transformation), but they show different activities in biological systems because individual enantiomers can interact enantioselectively with enzymes and biological receptors in organisms. 1,2 Enantioselectivity of chiral pesticides in the environment, such as enantioselective degradation and toxicity, is receiving increasing attention. 3,4 Despite their application to soil and direct exposure to plants, enantioselective toxicity of chiral pesticides to plants has not received enough attention.…”