Enantioselective degradation of metalaxyl in soils: chiral preference changes with soil pH

Chiral pesticides are often degraded enantio-/stereoselectively in soils. Degradation is typically studied with one or a small number of soils so that it is not possible to extrapolate the findings on chiral preference to other soils. For this study, the fungicide metalaxyl was chosen as a "chiral probe" to investigate its enantioselective degradation in 20 different soils, selected primarily to cover a wide range of soil properties (e.g., acidic/alkaline, aerobic/ anaerobic) rather than to consider soils of agricultural importance. Racemic metalaxyl was incubated in these soils under laboratory conditions, and the degradation of the enantiomers as well as the enantioselective formation/ degradation of the primary major metabolite, metalaxyl acid, was followed over time, using enantioselective GC-MS after ethylation with diazoethane. In aerobic soils with pH > 5, the fungicidally active R-enantiomer was degraded faster than the S-enantiomer (k(R) > k(S)), leading to residues with a composition [S] > [R]. However, in aerobic soils with pH 4-5, both enantiomers were degraded at similar rates (k(R) approximately k(S)), and in aerobic soils with pH < 4 and in most anaerobic soils, the enantioselectivity was reversed (k(R) < k(S)). These considerable soil-to-soil variations were observed with soils from locations close to each other, in one case even within a single soil profile. Liming and acidification of a "nonenantioselective" soil prior to incubation resulted in enantioselective degradation with k(R)> k(S) and k(R) < k(S), respectively. While the enantioselectivity (expressed as ES = (k(R) - k(S))/(k(R) + k(S))) of metalaxyl degradation in aerobic soils apparently correlated with soil pH, no such correlation was found for metalaxyl acid. Reevaluation of published kinetic data for the herbicides dichlorprop and mecoprop indicated similar correlations between soil pH and ES as for metalaxyl.     

Emission of chiral organochlorine pesticides from agricultural soils in the cornbelt region of the U.S

Several organochlorine pesticides are chiral molecules manufactured as racemic mixtures. Past research has shown that selective degradation of pesticide enantiomers by microorganisms occurs resulting in nonracemic signatures in soils. In this work, volatilization of chiral pesticides from soil was investigated to determine if enantioselective breakdown in soils could be used as a source signature to track releases of chiral pesticides to the atmosphere. Air samples were taken directly above agricultural soils at several sites, and enantiomeric signatures were found to be nonracemic following patterns found in the soil. A follow up study at one site showed that for most compounds concentration decreased with increasing height above the soil, while enantiomer fractions for chiral pesticides were similar to that found in the soil, signifying the soil as a source to the air. The enantiomer fractions of ambient air samples from rural nonagricultural areas in the region were also found to be nonracemic.     

Enantioselective degradation, abiotic racemization, and chiral transformation of triadimefon in soils

Triadimefon is a widely used triazole fungicide with one chiral carbon center. In soils, plants, and animals, triadimefon could be metabolized to triadimenol by reduction of the carbonyl group to an alcohol, resulting in the occurrence of a second chiral carbon in triadimenol. The enantioselective degradation of triadimefon and its chiral transformation to triadimenol in two soils, a Baoding alkaline yellow soil and a Wuhan acidic red soil, were investigated. The results showed the occurrence of enantioselectivity with R-(-)-triadimefon preferentially degraded in both soils. Abiotic racemization was observed by incubation of enantiopure triadimefon enantiomers. The racemization was clearly pH dependent and took place much more rapidly in Baoding alkaline soil than in Wuhan acidic soil. Further enantioselective analysis of converted triadimenol showed that triadimenol stereoisomer concentration invariably followed the order 1R,2R>1S,2S>1S,2R>1R,2S in Baoding soil, regardless of racemic triadimefon or single enantiomers initially treated. However, in the case of Wuhan soil, different triadimenol stereoisomer patterns could be produced depending on initial triadimefon composition at the time of application. The abiotic racemization was documented to have a great influence on the chiral profiles of triadimefon and its metabolite triadimenol. The mechanism and structural consideration of the racemization were further discussed, underscoring the importance of considering configurational stability in proper evaluation of environmental fate and risk of chiral pesticides.     

Environmental behavior of the chiral triazole fungicide fenbuconazole and its chiral metabolites: enantioselective transformation and degradation in soils

Fenbuconazole is a widely used systemic agricultural fungicide of the triazole class with one chiral center. In the present study, the enantioselective degradation of fenbuconazole and its chiral metabolites, RH-9129 and RH-9130, in two soils under aerobic and anaerobic conditions were investigated using a chiral OD-RH column on a reversed-phase liquid chromatography-tandem mass spectrometry system. Under aerobic or anaerobic conditions, the results showed the occurrence of enantioselectivity with (-)-fenbuconazole preferentially degraded in both soils. Further enantioselective analysis of converted products showed that the concentrations of four RH-9129 and RH-9130 stereoisomers were different from each other under both aerobic and anaerobic conditions. The four stereoisomer concentrations followed the order (-)-RH-9129 > (+)-RH-9129 > (-)-RH-9130 > (+)-RH-9130 in Langfang alkaline soil. However, in the case of Changsha acidic soil, different RH-9129 and RH-9130 stereoisomer patterns were produced in the order (-)-RH-9129 > (+)-RH-9129 > (+)-RH-9130 > (-)-RH-9130. The (-)-RH-9129 stereoisomer had the highest concentration formed by transformation of fenbuconazole in both soils. The degradation of RH-9129 and RH-9130 in the two soils is also stereoselective under both aerobic and anaerobic conditions, the results indicating that the (+)-RH-9130 enantiomer degraded faster than the (-)-RH-9130 enantiomer and the (+)-RH-9129 enantiomer degraded faster than the (-)-RH-9129 enantiomer. In addition, the (-)-RH-9129 isomer exhibited the slowest degradation rate in both soils. This study provides the first experimental evidence of stereoselective degradation and transformation of fenbuconazole as well as its chiral metabolites in the environment.     

Degradation of racemic and enantiopure metalaxyl in tropical and temperate soils

The degradation of the racemic mixture and the enantiomers of metalaxyl in typical soils from Germany and Cameroon has been studied. Formulated and unformulated R-metalaxyl were studied as well as racemic (rac) metalaxyl in controlled incubation experiments. The kinetics of the degradation or transformation were determined by means of reversed phase HPLC, while the enantiomeric ratios were measured by HPLC with a chiral Whelk O1 column. The degradation followed first-order kinetics (R2 > or = 0.96). Higher metalaxyl acid metabolite concentrations were found in German soil than in Cameroonian soil. The enantiomers of the fungicide each had different degradation rates in both soils, with half-lives ranging from 17 to 38 days. All forms of metalaxyl had lower degradation rates in the Cameroonian soil than in the German soil. The degradation. of the R-enantiomer was much faster than the S-enantiomer in the German soil and slower than the S-enantiomer in the Cameroonian soil suggesting that different microbial populations, which may be using different enzymes, have different degradation preferences. The results for the major differences in the degradation of the enantiomers may have some implication for the frequency of use as well as the environmental assessment for chiral pesticides.     

Organochlorine pesticide residues in archived UK soil

Archived background soils ("Broadbalk', 1944-1986) and sludge-amended soils ("Luddington", 1968-1990), collected from long-term agricultural experiments in the UK, were analyzed for a range of organochlorine (OC) pesticides to establish trends over time. Concentrations typically ranged from 0.1 to 10 ng/g of soil (dry weight), with gamma-hexachlorocyclohexane (gamma-HCH), dieldrin, and p,p'-DDE consistently having the highest concentrations. The trends in the Broadbalk background soils are largely consistent with usage patterns, with peak concentrations occurring in the 1960s for DDTs and between the 1960s and the 1980s for the other OCs. In the Luddington control and sludge-amended soils, several of the OCs show a significant decline in concentrations from the late 1960s to 1990, with half-lives ranging from approximately 7 years (alpha-HCH) to approximately 25 years (dieldrin). The sludge-amended plot received 125 tonnes of sludge per ha in 1968, which was mixed in to a depth of 15 cm. It appears that the sludge treatment had little effect on concentrations in the soil, with no significant difference between control soil and sludge-amended soil for most compounds, except for HCB, p,p'-DDE, and dieldrin. Enantiomeric fractions (EFs) of some chiral pesticides (alpha-HCH, cis- and trans-chlordane, and o,p'-DDT) were determined in the Luddington soils. Results reveal that enantioselective degradation of OC pesticides is occurring in these soils for trans-chlordane (TC) and cis-chlordane (CC). However, the depletion over time is not statistically significant, and there is no statistically significant difference between EFs in the control soil and sludge-amended soil. This indicates that enantioselective microbial degradation was not consistent over time and that the addition of sludge to soil did not significantly alter the enantiomeric preference of the microbial community.     

Microbial transformation of triadimefon to triadimenol in soils: selective production rates of triadimenol stereoisomers affect exposure and risk

The microbial transformation of triadimefon, an agricultural fungicide of the 1,2,4-triazole class, was followed at a nominal concentration of 50 μg/mL over 4 months under aerobic conditions in three different soil types. Rates and products of transformation were measured, as well as enantiomer fractions of parent and products. The transformation was biotic and enantioselective, and in each soil the S-(+)-enantiomer reacted faster than the R-(-) one. Rates of the first-order reactions were 0.047, 0.057, and 0.107 d(-1) for the three soils. The transformation involves reduction of the prochiral ketone moiety of triadimefon to an alcohol, resulting in triadimenol, which has two chiral centers and four stereoisomers. The abundances of the four product stereoisomers were different from each other, but abundance ratios were similar for all three soil types. Triadimenol is also a fungicide; the commercial product is composed of two diastereomers of unequal amounts (ratio of about 4.3:1), each having two enantiomers of equal amounts. However, the triadimenol formed by soil transformation of triadimefon exhibited no such stereoisomer profile. Instead, different production rates were observed for each of the four triadimenol stereoisomers, resulting in all stereoisomer concentrations being different from each other and very different from concentration/abundance patterns of the commercial standard. This result is important in risk assessment if the toxicity of the environmental transformation product were to be compared to that of the commercial triadimenol. Because triadimenol stereoisomers differ in their toxicities, at least to fungi and rats, the biological activity of the triadimenol formed by microbes or other biota in soils depends on the relative abundances of its four stereoisomers. This is an exposure and risk assessment issue that, in principle, applies to any chiral pesticide and its metabolites.     

Enantioselective degradation of organochlorine pesticides in background soils: variability in field and laboratory studies

Variability in the enantioselective degradation of chiral organochlorine pesticides (alpha-HCH, cis- and trans-chlordane (CC and TC), and o,p'-DDT) in the field and laboratory was investigated. Background soils presumably receive the same EF signature of a compound via atmospheric deposition and then degrade that compound in a way that can vary over small spatial areas. Background soils from woodland and grassland areas were sampled to compare chiral signatures and determine the spatial variability within a few square meters. The enantiomer fractions, EF = areas of the (+)/[(+)+(-)]-enantiomers, showed variability between and within ecosystems. For example, the EF of CC varied between 0.272 -and 0.558 in nine samples taken over a few square meters, and a range of 0.431-0.506 was found within depths of a few centimeters. Woodland and grassland soils were spiked with alpha-HCH, TC, CC, and o,p'-DDT, and one portion was placed in the field to monitor changes in EF under in situ conditions and the other taken to the laboratory. In general, the enantiomer degradation preferences in the laboratory paralleled those in the field, with some differences. Soil organic matter content and pH exerted a minor influence on this variability. The results of this study have implications for the use of chiral compounds to make inferences about air-soil exchange and for the mechanisms of biodegradation/ biotransformation of anthropogenic compounds in soils.     

粉唑醇在草莓生长和加工过程中对映体选择性降解行为规律

为研究手性农药粉唑醇在草莓生长及草莓酒加工过程中的选择性降解行为,对草莓进行田间施药,并将草莓模拟家庭酿酒方式加工。样品采用超高效合相色谱-串联质谱进行手性拆分并检测。结果表明,在草莓生长中两个对映体没有明显的选择性降解行为,（－）-R-粉唑醇在草莓上的半衰期为13.80?d,（＋）-S-粉唑醇在草莓上的半衰期为13.60?d。而在草莓酒发酵过程中,统计学分析显示存在明显的选择性降解行为（P＜0.05）,（＋）-S-粉唑醇优先降解。清洗处理组（－）-R-粉唑醇在草莓酒中的半衰期为38.06?d,（＋）-S-粉唑醇的半衰期为29.90?d。未清洗处理组（－）-R-粉唑醇在草莓酒发酵中的半衰期为29.31?d,（＋）-S-粉唑醇半衰期为22.14?d。     

Analysis of Tebuconazole and Tetraconazole Enantiomers by Chiral HPLC-MS/MS and Application to Measure Enantioselective Degradation in Strawberries

A rapid and sensitive enantioselective method for the determination of tebuconazole and tetraconazole enantiomers in strawberry has been developed. The enantiomers were resolved by high-performance liquid chromatography on a cellulose tris (3-chloro-4-methylphenylcarbamate) (Lux Cellulose-2) column using methanol?C0.1% formic acid solution (70:30, v/v) as mobile phase. The chiral liquid chromatographic separation was combined with an extract/cleanup procedure known as the quick, easy, cheap, effective, rugged, and safe (QuEChERS) method. High-performance liquid chromatography-tandem mass spectrometry with electrospray ionization was then used for qualitative and quantitative determination of the enantiomers of tebuconazole and tetraconazole. Hexaconazole was used as an internal standard. The method provided high selectivity and sensitivity, and the limits of quantification for tebuconazole and tetraconazole enantiomers in strawberry were both 2.5???g/kg. The method was successfully utilized to investigate the probable enantioselective degradation of tebuconazole and tetraconazole in strawberry. The results showed that the degradation of the tebuconazole and tetraconazole enantiomers in strawberry followed first-order kinetics (R ²?>?0.96). The results from this study revealed the degradation of tetraconazole in strawberry was not enantioselective, while the degradation of tebuconazole was enantioselective, and the (+)-tebuconazole showed a faster degradation than (?)-tebuconazole in strawberry. The results could provide a reference to fully evaluate risks of these two fungicides.     

Fate of brominated flame retardants and organochlorine pesticides in urban soil: volatility and degradation

As the uses of polybrominated diphenyl ethers (BDEs) are being phased out in many countries, soils could become a secondary emission source to the atmosphere. It is also anticipated that the demand for alternative brominated flame retardants (BFRs) will grow, but little is known about their environmental fate in soils. In this study, the volatility and degradation of BFRs and organochlorine pesticides (OCPs) in soil was investigated. A low organic carbon (5.6%) urban soil was spiked with a suite of BFRs and OCPs, followed by incubation under laboratory condition for 360 days. These included BDE- 17, -28, -47, -99; α- and β-1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH), β-1,2,5,6-tetrabromocyclooctane (TBCO), and 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE), OCPs: α-hexachlorocyclohexane (α-HCH) and (13)C(6)-α-HCH, trans-chlordane (TC), and (13)C(10)-TC. The volatility of spiked chemicals was investigated using a fugacity meter to measure the soil-air partition coefficient (K(SA)). K(SA) of some spiked BFRs and OCPs increased from Day 10 to 60 or 90 and leveled off afterward. This suggests that the volatility of BFRs and OCPs decreases over time as the chemicals become more strongly bound to the soil. Degradation of alternative BFRs (α- and β-TBECH, β-TBCO, DPTE), BDE-17, and α-HCH ((13)C-labeled and nonlabeled) was evident in soils over 360 days, but no degradation was observed for the BDE-28, -47, -99, and TC ((13)C-labeled and nonlabeled). A method to separate the enantiomers of α-TBECH and β-TBCO was developed and their degradation, along with α-HCH ((13)C-labeled and nonlabeled) was enantioselective. This is the first study which reports the enantioselective degradation of chiral BFRs in soils. Discrepancies between the enantiomer fraction (EF) of chemicals extracted from the soil by dichloromethane (DCM) and air were found. It is suggested that DCM removes both the sequestered and loosely bound fractions of chemicals in soil, whereas air accesses only the loosely bound fraction, and these two pools are subject to different degrees of enantioselective degradation. This calls for caution when interpreting EFs obtained from DCM extraction of soil with EFs in ambient air.     

Environmental behavior of the chiral acetamide pesticide metalaxyl: enantioselective degradation and chiral stability in soil

Racemic metalaxyl is currently being replaced in many countries by metalaxyl-M, the fungicide enantiomerically enriched with the biologically active R-enantiomer. This "chiral switch" is expected to reduce the amount of pesticide released into the environment as well as potential side-effects on nontarget organisms. Detailed knowledge of the environmental behavior of such chiral compounds should include information on the chiral stability (interconversion of enantiomers). In the present study, the degradation/ dissipation of metalaxyl and its primary carboxylic acid metabolite (MX-acid) in soil was investigated under laboratory conditions using enantioselective gas chromatography mass spectrometry (GC-MS). Racemic and the enantiopure R- and S-compounds were incubated in separate experiments. The degradation of metalaxyl was shown to be enantioselective with the fungicidally active R-enantiomer being faster degraded than the inactive S-enantiomer, resulting in residues enriched with S-metalaxyl when the racemic compound was incubated. The relatively high enantioselectivity suggests that degradation/dissipation was largely biological. The data indicated a conversion of 40-50% of metalaxyl to MX-acid, and the remaining metalaxyl being degraded via other pathways. The degradation of MX-acid was also enantioselective. Metalaxyl and MX-acid were both configurationally stable in soil, showing no interconversion of R- to S-enantiomers, and vice-versa. Furthermore, the conversion of metalaxyl to MX-acid proceeded with retention of configuration. Degradation followed approximate first-order kinetics but showed significant lag phases.     

Enantioselective Damage of Diclofop Acid Mediated by Oxidative Stress and Acetyl-CoA Carboxylase in Nontarget Plant Arabidopsis thaliana

Diclofop-methyl (DM) is a widely used chiral herbicide, which rapidly hydrolyzes to its major metabolite diclofop acid (DC) after application. With a carbon chiral center, DC not only is an important ingredient of herbicidal activity, but also has a long half-life in soil. Studies so far have only considered the activity of racemic DM in target organisms, and the enantioselective toxicity in nontarget plants of DM and DC has yet to be explored. In this study, the enantioselective phytotoxicity of DC mediated by oxidative stress and the key enzyme ACCase in the fatty acid synthesis system on the model plant Arabidopsis thaliana was investigated. Significant differences between the two enantiomers were observed in phytotoxicity including growth inhibition, oxidative damage and alteration of key genes expression of ACCase, with R-DC showing greater toxicity to Arabidopsis thaliana than S-DC. The results of molecular docking showed that there was a stronger affinity between R-DC and the target enzyme carboxyltransferase domain of ACCase, likely leading to the enantioselective phytotoxicity of DC. This study suggested that chirality of both parent compounds and metabolites should be considered to improve our understanding of the environmental fate and risks of chiral pesticides.     

Chiral organochlorine pesticide signatures in global background soils

Chiral pesticides frequently undergo enantioselective degradation in soils. Prior studies to characterize chiral signatures have focused on treated agricultural soils, rather than background (untreated) soils, and tracking signatures in the atmosphere for source apportionment purposes. In this study, we investigated the chiral signatures in 65 background soils collected from different locations across the world. The soils were taken from different ecosystems (e.g., grasslands, forests), and the enantiomeric fractions (EFs) of chiral chlordanes, alpha-hexachlorocyclohexane (alpha-HCH), and o,p'-DDT were determined. Chlordanes in most of the soils showed the usual pattern of enantioselective degradation seen in agricultural soils, depletion of (+)-trans-chlordane (TC) and (-)-cis-chlordane (CC). However, some samples showed opposite enantiomer degradation patterns for TC, CC, and chlordane compound MC5. Correlations were tested between the deviation of EFs from racemic (DEVrac = absolute value of 0.500 - EF), the percent soil organic matter (% SOM), annual mean temperature, and the ratio of TC to the more stable compound trans-nonachlor (TN). Significant positive correlations were found between DEVrac and % SOM for TC and CC (p = 0.0022 and 0.0031), but not for the other OCPs. No significant correlations were found between DEVrac and annual mean temperature for any of the OCPs. DEVrac for TC was negatively correlated with the TC/TN ratio, but the regression was driven by two points with high ratios of TC/CC. Removing these two points resulted in a nonsignificant regression. The range of EFs for TC, CC, and alpha-HCH in soils was greater than in ambient air, providing evidence of in situ degradation after atmospheric deposition in some cases. Variable EFs in soil suggest that caution is needed when considering the enantiomer signatures in air as a marker of volatilization of weathered soil-derived organochlorines.     

Stereoselective degradation kinetics of theta-cypermethrin in rats

The enantioselective degradation and chiral conversion of theta-cypermethrin (TCYM) in rats have been studied via intravenous (i.v.) injection. The degradation kinetics and the enantiomer fraction (EF) were determined by means of normal-phase high-performance liquid chromatography (HPLC) with diode array detection (DAD) and a cellulosetris-(3,5-dimethylphenylcarbamate)-based chiral stationary phase (CDMPC-CSP). The degradation followed pseudo-first-order kinetics. The degradation of the (+)-TCYM was much faster than that of the (-)-TCYM in plasma, heart, liver, kidney, and fat after administration of racemic TCYM (rac-TCYM). The EFs were over 0.500 in these tissues and muscle. The results showed the conversion of (+)-enantiomer to (-)-enantiomer in plasma after injection of (-)- and (+)-TCYM separately. The results for the major differences in the degradation of the enantiomers may have some implication for the environmental and ecological risks assessment for chiral pesticides.     

Effects of menthol stereoisomers on the growth,sporulation and fumonisin B1 production of Fusarium verticillioides

Menthol is a naturally occurring cyclic terpene alcohol of plant origin from the Lamiaceae family. It has three chiral centres, implying eight possible different stereoisomers, which in turn define four pairs of enantiomers. This is the first work that reports on the stereoselective antifungal and antitoxigenic activities of the menthol stereoisomers on Fusarium verticillioides, with the (−)-menthol and (+)-menthol enantiomers found to be the most active inhibitors of fungal growth and sporulation. The results obtained suggest the importance of the presence of these substituents in the equatorial positions of menthol stereoisomers in the antifungal activity. The stereoisomer (−)-menthol, followed by (+)-menthol, were the most active compounds in the inhibition of fumonisin B₁ (FB₁) biosynthesis. The different antitoxigenic activities of (−)-menthol and (+)-menthol revealed that the molecular requirements to affect the FB₁ production were dependent not only on the presence of the substituents in the equatorial positions, but also on their spatial arrangements.     

手性农药兽药免疫分析研究进展

手性药物对映异构体在生物体内的代谢转化和毒性作用等具有立体选择性差异,因此从对映体形态水平上加强手性农药兽药的分析研究十分必要。免疫分析法简单快捷、特异性高、灵敏度高,因而成为手性农药兽药分析研究的热点。本文介绍了近年来免疫分析法在手性农药兽药分析检测方面的应用情况,综述了抗体立体选择识别性、分子模拟技术及定量构效关系等在手性药物免疫识别机制研究中应用,并展望手性免疫分析技术的研究和应用发展趋势。     

Enantioseparation and Enantioselective Phytotoxicity of Glufosinate Ammonium on Catechin Biosynthesis in Wheat

The enantioselective phytotoxicity of chiral pesticides is a new area of research in phyto-biochemical studies. Glufosinate (GL) is a chiral herbicide, comprised of the two enantiomers. The aim of this study was to investigate the enantioselective phytotoxicity of glufosinate ammonium enantiomers on root and shoot dry weight as well as catechin biosynthesis of wheat seedlings. Separation of GL enantiomers was achieved by using high-performance liquid chromatography on Eurocel 01 column. The optical rotation of separated enantiomers confirmed the purity of each isomer. The I₅₀ parameter, which is the concentration resulting in a 50 % inhibition in root and shoot dry weight estimated using dose–response curves, showed that (+)-GL was 4.2 times more active than the (?)-GL enantiomer. It demonstrated that the effect of enantioselectivity of GL on catechin biosynthesis in wheat shoots was sensible, while the effect of enantioselectivity of GL in catechin content in wheat roots was not distinct.     

The freshwater invertebrate Mysis relicta can eliminate chiral organochlorine compounds enantioselectively

Accumulation and elimination of chiral polychlorinated biphenyls (PCBs) and organochlorine (OC) pesticides by the opossum shrimp, Mysis relicta, was investigated to determine if zooplankton can stereoselectively process chiral OC contaminants. Concentrations and enantiomer fractions were measured within mysids over a 10-day exposure followed by a 45-day depuration period. Rapid accumulation occurred within mysids exposed to sediment contaminated with racemic chiral OC compounds at microg/g levels. Enantiomer enrichment was observed within mysids for the second-eluting enantiomer and the (-)-enantiomer of PCB 95 and trans-chlordane, respectively, after 7 days of exposure to spiked sediment, and for the second-eluting enantiomers of PCBs 91 and 183 and (-)-PCB 149 over longer time periods. Enantiomer fractions decreased with time during the depuration phase of the experiment for these compounds, showing that their elimination from mysids was stereoselective. Oxychlordane was detected in nonracemic proportions after exposure, indicating that mysids can metabolize trans-chlordane enantioselectively. Minimum elimination rates calculated were higher than biotransformation rates calculated for fish in previous studies, which have been shown to metabolize OC contaminants. This study is the first to show stereoselective processing of chiral OC contaminants by aquatic invertebrates.