Kankan Zhang

Enantioselective Degradation of Dufulin in Four Types of Soil

In this study, enantioselective degradation of dufulin in four types of soil (Guiyang silty loam, Nanning silty clay, Hefei silty clay, and Harbin silty clay) was investigated under sterile and nonsterile conditions. Pesticide residues in soil samples were extracted with acetonitrile. S-(+)-Dufulin and R-(-)-dufulin were separated and determined on an amylose tris(3,5-dimethylphenylcarbamate) (Chiralpak IA) chiral column by normal phase high-performance liquid chromatography (HPLC). The absolute configurations of dufulin enantiomers were determined by obtaining experimental and computed circular dichroism spectra. Dufulin enantiomers were found to be configurationally stable in the selected soils, and no interconversion was observed during the incubation of enantiopure S-(+)- or R-(-)-dufulin under nonsterile conditions. Compared to the half-life (t1/2) of dufulin in sterile soils, the degradation rate was higher in nonsterile soils, which suggests that dufulin degradation can be attributed primarily to microbial activity in soils used for agricultural cultivation. Furthermore, enantiopure S-(+)-dufulin degraded more rapidly than its antipode. This suggests that use of enantiopure S-(+)-dufulin could exert less disturbance to soil bioactivity and contribute less to environmental pollution.

One-Pot Synthesis of Novel Chiral β-Amino Acid Derivatives by Enantioselective Mannich Reactions Catalyzed by Squaramide Cinchona Alkaloids

An efficient one-pot synthesis of novel β-amino acid derivatives containing a thiadiazole moiety was developed using a chiral squaramide cinchona alkaloid as organocatalyst. The reactions afforded chiral β-amino acid derivatives in moderate yields and with moderate to excellent enantioselectivities. The present study demonstrated for the first time the use of a Mannich reaction catalyzed by a chiral bifunctional organocatalyst for the one-pot synthesis of novel β-amino acid derivatives bearing a 1,3,4-thiadiazole moiety on nitrogen.

Determination of RH-5849 and indoxacarb in rice straw, rice husk, brown rice and soil using liquid chromatography-tandem triple quadrupole mass spectrometry following extraction with QuEChERS method.

A fast and simple analytical method was developed for the simultaneous determination of RH-5849 and indoxacarb in soil, rice straw, rice husk and brown rice. QuEChERS (quick, easy, cheap, effective, rugged and safe) method was used for extraction, and liquid chromatography with tandem triple quadrupole mass spectrometry was used for quantification. The matrix-matched calibration plots were linear in the range between 25 and 5000 μg/L for soil, rice straw, rice husk and brown rice samples. All determination coefficients (R(2) ) were ≥0.9962. The limits of detection and quantification were 1.5 and 5 μg/kg, respectively. Recoveries at three fortification levels ranged between 79.5 and 97.9% with relative standard deviations <11%. The developed method was validated and applied for the analysis of dissipation study samples. For field experiments, the half-lives of RH-5849 and indoxacarb in rice straw were 11.93 and 5.83 days, respectively. The method was demonstrated to be reliable for the routine monitoring of RH-5849 and indoxacarb in rice samples. Copyright

Determination of Dufulin Residue in Vegetables, Rice, and Tobacco Using Liquid Chromatography with Tandem Mass Spectrometry.

A rapid and accurate LC/MS/MS method using positive electrospray ionization was established for the determination of residues of the novel plant antiviral agent dufulin in samples of tobacco leaf (dry), tomato, cucumber, and rice. Samples were extracted with acetonitrile; cleaned up by dispersive SPE using primary secondary amine, C18, and graphitized carbon black sorbents; separated on a C18 column; and confirmed by multiple reaction monitoring mode MS with a matrix effect of -21.5-19.6%. The method showed satisfactory linearity (R2≥0.9912) for the target compound. The LOD and the LOQ were 0.05 and 0.15 μg/kg, respectively. The mean recoveries from four matrixes varied from 71.9 to 93.6% with intraday RSD in the range of 2.9 to 9.0% and interday RSD 6.9 to 15.2%. The method was successfully applied for analysis of dufulin in actual trial samples.

Dissipation rates of dufulin residues in paddy, soil, and water determined by ultra-performance liquid chromatography coupled with photo-diode array detection

A sensitive and selective analytical method for dufulin detection was developed using ultra-performance liquid chromatography coupled with photo-diode array detection (UPLC-PDA). Dufulin analysis was carried out with 1 μl sample injection on BEH C18 column (50.0 mm × 2.1 mm i. d., 1.7 μm film thickness) thermostatted at 40°C with a mobile phase of methanol/water (58/42 by volume) at flow rate of 0.3 ml min−1. Dissipation rates of dufulin in paddy plant, soil, water, and brown rice in three districts in China were studied using the method developed. Dufulin (30% WP) was applied at two dosages, 337.50 g a. i. ha−1 (recommended dosage) and 506.25 g a. i. ha−1 (1.5 times the recommended dosage) in experimental fields in Guizhou, Anhui, and Tianjin provinces in China. The limit of detection (LOD) and the limit of quantification (LOQ) of dufulin were 2.5 and 7.5 μg kg−1, respectively. The average recoveries were 72.76–95.43, 97.84–101.7, 96.99–100.8, and 80.91–100.5% in paddy plant at four spiking levels (0.04, 0.20, 2.0, 50.4 mg kg−1), in water at three spiked levels (0.0025, 0.025, 0.25 mg l−1), in soil at three spiking levels (0.04, 0.13, 1.3 mg kg−1), and in brown rice at four spiking levels (0.05, 0.20, 2.0, 10.0 mg kg−1), respectively. Our results showed that half-lives of dufulin in paddy plant and water in the three districts were 7.7–11.3 and 2.1–4.8 days, respectively. At harvest, dufulin residues in brown rice samples collected 35 days after the last dufulin application at the recommended dosage were below 0.5 mg kg−1. Our results provided data to help establish appropriate application frequency and harvest intervals in the use of dufulin for rice pest control in China.

Enantioselective hydrolyzation and photolyzation of dufulin in water

BackgroundDufulin is a novel, highly effective antiviral agent that activatives systemic acquired resistance of plants. This compound is widely used in China to prevent and control viral diseases in tobacco, vegetable and rice. Dufulin can treat plants infected by the tobacco mosaic virus and the cucumber mosaic virus. However, the achiral analysis and residue determination of dufulin remain underdeveloped because of its high enantioselectivity rates and high control costs. The enantioselectivity of an antiviral compound is an important factor that should be considered when studying the effect of chiral pesticides on the environment. The enantioselective degradation of dufulin in water remains an important objective in pesticide science.ResultsThe configuration of dufulin enantiomers was determined in this study based on its circular dichroism spectra. The S-(+)-dufulin and R-(−)-dufulin enantiomers were separated and identified using an amylose tris-(3,5-dimethylphenylcarbamate) chiral column by normal phase high-performance liquid chromatography. The degradation of the rac-dufulin racemate and its separate enantiomers complied with first-order reaction kinetics and demonstrated acceptable linearity. The enantioselective photolysis of rac-dufulin allowed for the faster degradation of R-(−)-dufulin, as compared with S-(+)-dufulin. However, S-(+)-dufulin was hydrolyzed faster than its antipode.ConclusionThe photolysation and hydrolyzation of dufulin in water samples normally complied with the first-order kinetics and demonstrated acceptable linearity (R2>0.66). A preferential photolysation of the R-(−)-enantiomer was observed in water samples. Moreover, the S-(+)-enantiomer was hydrolyzed faster than its antipode.

Multiresidue determination of pyrethroid pesticide residues in pepper through a modified QuEChERS method and gas chromatography with electron capture detection

This study developed and used a modified quick, easy, cheap, efficient, rugged and safe (QuEChERS) method coupled with gas chromatography with electron capture detection to determine eight pyrethroid pesticide residues in green, red and dehydrated red peppers. Pyrethroids were extracted with acetonitrile, partitioned with sodium chloride and purified with primary secondary amino and graphitized carbon black in hexane. The QuEChERS extraction conditions were optimized, and the matrix effects that might influence recoveries were evaluated and minimized using matrix-matched calibration curves. Under the optimized conditions, the calibration curves for pyrethroid pesticides showed good linearities in the concentration range of 0.05-20 µg/mL with determination coefficients (R(2) ) >0.997. The limits of quantification of eight pyrethroids were 0.004-0.04 mg/kg for green and red pepper and 0.04-0.5 mg/kg for dehydrated red pepper. These values are below the suggested regulatory maximum residue limits. The mean recoveries ranged between 79.0 and 104%, and the relative standard deviations were <11%. The developed method was successfully applied to commercial samples. Some samples were found to contain pyrethroid pesticides with levels below the legal limits.

Enantioselective Degradation of Indoxacarb From Different Commercial Formulations Applied to Tea

The stereoselective degradation of indoxacarb enriched with (+)-S-indoxacarb (S/R:70/30) was investigated in three typical green teas. A convenient and precise chiral method was developed and validated for measuring indoxacarb enantiomers in green tea. The developed method was based on high-performance liquid chromatography coupled with tandem mass spectrometry using a Chiralpak IC column. The stereoselective degradation of indoxacarb enantiomers showed that the (+)-S-enantiomer dissipated faster than the (-)-R-enantiomer in all three typical tea farms. However, no enantiomerization was observed after applying pure (+)-S-indoxacarb. Residues on tea plant of the active ingredient (+)-S-indoxacarb from suspension concentrate (SC) was more persistent than that from emulsifiable concentrate (EC).

Enantiomeric separation of indoxacarb on an amylose‐based chiral stationary phase and its application in study of indoxacarb degradation in water

Direct semipreparative enantioseparation of indoxacarb was performed on a semipreparative Chiralpak IA column using normal-phase high-performance liquid chromatography (HPLC) with n-hexane-isopropanol-ethyl acetate (70:20:10) mixture as mobile phase. Degradation of indoxacarb (2.33S + 1R) and its two enantiopure isoforms in three aqueous buffer solutions and four water samples collected from natural water sources was then elucidated by HPLC analysis on Chiralpak IA column. Degradation of all three indoxacarbs complied with first-order kinetics and demonstrated linearity with regression coefficients R(2) > n0.88. Indoxacarb (2.33S + 1R) underwent enantioselective degradation in river water, rain water, and buffer solution of pH 7.0. Enantiopure S-(+)-indoxacarb and R-(-)-indoxacarb were both found to be configurationally stable in water.

Comparative Study of the Selective Degradations of Two Enantiomers in the Racemate and an Enriched Concentration of Indoxacarb in Soils

In this study, selective degradations of the two enantiomers of indoxacarb in the concentrate (2.33S/1R) and racemate (1S/1R) are examined. The absolute configurations of indoxacarb enantiomers were determined using X-ray diffraction. The results showed that in two alkaline soils, the S-(+)-indoxacarb was preferentially degraded in both the concentrate and racemate. In one acid soil, the two enantiomers degraded no-selectivity. In another acid soil and one neutral soil, the R-(-)-indoxacarb was preferentially degraded in both the concentrate and racemate. Indoxacarb enantiomers were configurationally stable in the five soils, and no interconversion was observed during the incubation. Because no significant difference in degradation was observed after samples were sterilized, the observed enantioselectivity may be attributed primarily to microbial activity in soils. The results indicate that the selective degradation behavior was the same for both formulations that were tested.