Study On The Stereoseiective Environmental Behaviors Of Two Chirai Pesticides In Aquatic Organisms

Along with the wide application of pesticides, the usage of chiral pesticides is also on the rise. The enantiomers of chiral pesticides are different in some biological processes and thus pose enantioselective behavior and ecotoxicity to non-target biology. Therefore, for their use and environmental safety, research on enantioselective behaviors has become the new focus of environmental toxicology. Diclofop-methyl (DM) is one kind of AOPP herbicides which is still of massive use. At present most of the AOPP herbicides are produced and used in racemic form. Although there are some reports about degradation and environmental behavior of DM, the reports about aquatic toxicity at the level of enantiomers was rare. Hexachlorocyclohexane (HCII) had been extensively used as a classical organochlorine pesticide and was forbidden in the1960s, but it can be detected for decades in various kinds of environmental samples due to its persistence and higher bioaccumulation, posing a serious risk to ecosystem. Cyclodextrins (CDs) are a kind of very important remediation agents for environmental contamination. CDs also have chiral recognition ability, so they might have an asymmetric impact on the enantioselectivity of chiral contaminants. Our work focused on the environmental behaviors and eco-effects of two chiral pesticides a-HCH and diclofop-methyl, studied their enantioselective bioaccumulation and elimination in several fish and evaluate the effects of β-cyclodextrin on the toxicology of DM.Three kinds of freshwater fish as well as daphnia magna were employed to evaluate the aquatic toxicity of DM (diclofop-methyl) enantiomers and diclofop. The acute toxicity of DM shows a certain degree of enantioselectivity:for3species of fish, the acute toxicity of R-DM is higher than that of S-enantiomer; on the contrary, the toxicity of R-DM to daphnia magna was lower than its antipode. The inclusion complex of DM/β-cyclodextrin was prepared in solution and solid phase. The inclusion complexation enhanced the DM water solubility and reduced its acute toxicity to several aquatic organisms.In order to investigate the β-cyclodextrin effect on DM enantioselective behaviors in loach, two types of contamination were adopted. One group was spiked with racemic diclofop-methyl and the other was treated with cyclodextrin/DM inclusion complexation. The first group revealed DM in loaches immediately transformed into diclofop and no DM was detected. The concentration of diclofop enantiomers differed significantly. Loach metabolized DC enantiomers quickly, with half-lives of about0.3d and R-DC faster than S-DC; In the second group, a small quantity of DM could be detected in the initial stage, and loaches up-took the DM enantiomers with no stereoselectivity. DC enantiomers had relatively long half-lives of1.2-1.5days with R-diclofop degraded more slowly than S-diclofop.An efficient and stable method for the simultaneous chiral analysis of DM and DC enantiomers on Chiralpak IC column was developed. The degradation of racemic DM and single enantiomer as well as diclofop in loach liver microsomes in vitro was conducted. The results suggested that the biotransformation process was dominated by microsomal esterases. The rac-DM degradation in loach liver microsomes was enantioselective with the conversion rate of (S)-DM markedly greater than that of the R-enantiomer. No enantiomerization was observed, suggesting the biotransformation was configurationally stable. In contrast, DC was not metabolized in liver microsomes even with NADPH present. Further research on the mechanism of DC metabolism and enzyme involved was needed.The impacts of β-cyclodextrin on interaction between DM and Scendesmus quadricauda were investigated through toxicity, enzymatic activity and degradation process. The results showed there were obvious differences in toxicity between DC enantiomers to S. quadricauda. The acute toxicity of S-DM was much higher than that of the R-DM. Compared to the control EC50values, the EC50values corresponding to every stage increased in the presence of cyclodextrin. DM exerted enantioselective induction effects on the activity of SOD and CAT of s. quadricauda. The degradation of DC by S. quadricauda was enantioselective but a bit slow. In single enantiomer cultivation, it was found that only S-DC could transform into R-DC. Cyclodextrin could accelerate the R-DC degradation and promote the R-DC conversion.The enantioselective bioaccumulation and degradation behavior of a-HCH in pseudorasbora parva was studied. A valid chiral residue analysis method for the a-HCH enantiomers in the water and fish samples was established using capillary column BGB-172combined with GC-ECD. The bioaccumulation of a-HCH in loach was quickly and the highest concentration point was reached within2days, with a maximum BCF1d of830. After that the body burden began to decrease and a companying re-uptake process was observed. The degradation kinetics of a-HCH fitted typical first order kinetic, and the half-life was about1.2days. Both the bioaccumulation and degradation processes showed obviously enantioselectivity.(+)-a-HCH was preferential accumulated and degraded than (-)-a-HCH with the EFs in fish head higher than0.5during bioaccumulation and lower than0.5during elimination.Crucian carp, a kind of edible freshwater fish was chosen to study the a-HCH bioaccumulation and metabolism behavior in various tissues through a single dose exposure. The fish were exposed to racemic a-HCH orally and the concentrations of its two enantiomers were measured in the liver, kidney, brain, muscle, skin and ovary at different time intervals. The absorption and elimination of a-HCH in liver were fast which can be characterized by a two-compartmental kinetic model. The accumulation rates were different among tissues depending on blood flow rate and fat content. Liver, kidney and brain tissue had the highest enrichment ability. The enantiomers distribution in most tissues suggested an apparent enantioenrichment of (+)-a-HCH. On the contrary, a-HCH in brain showed a significant enantioselectivity. The EFs for brain decreased dramatically after the exposure and reached to0.45, suggesting a preferential clearance of (+)-a-HCH.