Enantioselective Degradation of Dufulin in Four Types of Soil - Journal

Feb 11, 2014 - In this study, enantioselective degradation of dufulin in four types of soil (Guiyang silty loam, Nanning silty clay, Hefei silty clay,...
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Enantioselective Degradation of Dufulin in Four Types of Soil Kan-Kan Zhang, De-Yu Hu, Hui-Jun Zhu, Jin-Chuan Yang, and Bao-An Song* State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University, Huaxi District, Guiyang 550025, People’s Republic of China S Supporting Information *

ABSTRACT: 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(3,5dimethylphenylcarbamate) (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. KEYWORDS: dufulin, degradation, soil, enantioselective, enantiomer

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INTRODUCTION α-Aminophosphonic acids, which are bioisosteres of natural amino acids, have been found to exhibit a wide range of biological activities. Research over the past 20 years has revealed important information on their synthesis and biological activities. Compounds containing heterocyclic moieties such as 1,3,4-thiadiazole, furan, thiophene, and benzothiazole have been reported as representative compounds of this class because of their excellent antiviral bioactivity.1−3 As a result of these findings, our research team designed a series of new heterocyclic α-aminophosphonic acid esters.4−8 On this basis, we carried out intensive research to discover novel antiviral structures for plants, optimizing α-aminophosphonic acids as the primary compounds. A new commercially registered plant antiviral product, dufulin (((2-fluorophenyl)-(4-methylbenzothiazol-2-ylamino)methyl)phosphonic acid diethyl ester), was developed. Dufulin, belonging to the α-aminophosphonate family, contains an asymmetric center on a carbon atom (Figure 1).9 It is highly effective against plant viruses as it is capable of activating systemic acquired resistance (SAR) in plants. In recent years, it has been highly effective in preventing infection caused by rice viruses, tobacco mosaic virus, and tomato mosaic virus and as a result has obtained a national

invention patent in the People’s Republic of China. It was registered by the Ministry of Agriculture of China (LS 20071280, 20071282, and 20130359) and was subsequently industrialized for large-scale field application. It has been widely used to prevent and control rice, vegetable, and tobacco viral diseases in China.10 The agricultural application was conducted using a 30% dufulin wettable powder as a foliar spray in a racemic mixture with a dosage of 202.5−337.5 g of active ingredient per hectare. An increasing number of investigations have shown that the toxicity of chiral amino acid pesticides is enantiomer-specific. This suggests that the environmental behavior of these pesticides should be studied using individual enantiomers.11−16 However, most chiral pesticides are released into the environment in their racemic forms, which are equimolar mixtures of enantiomers. Given that microorganisms and associated enzymes in soil constitute a chiral environment, degradation of chiral pesticides is often enantioselective.17−19 For instance, the degradation of S-(−)-pyraclofos prevailed over the degradation of R-(+)-pyraclofos in Nanchang soil, and the result was opposite in Hangzhou soil.20 Also, active R(+)-malathion degraded more slowly than inactive S(−)-malathion in chosen soil and water samples, resulting in a relative enrichment of the R-form.21 In addition, growing concern regarding the side effects of chiral agrochemicals on nontarget organisms and natural resources has promoted the use of enantiomerically pure or stereochemically enriched compounds.22,23 For example, biotransformations of racglufosinate, L-glufosinate, and D-glufosinate have been reported Received: Revised: Accepted: Published:

Figure 1. Structure of dufulin. © 2014 American Chemical Society

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Table 1. Sampling Sites and Characterization of the Soils Studied particle size

a

soil sitea

sand (%)

silt (%)

clay (%)

pHb

Corg (%)

CECc (%)

TNd(g/kg)

TPe(g/kg)

Guizhou Guiyang Guangxi Nanning Anhui Hefei Heilongjian Harbin

21.0 8.5 6.8 3.7

62.0 38.8 37.9 31.6

17.0 52.7 55.3 64.7

4.8 5.2 7.8 8.5

1.797 2.192 3.809 4.348

18.6 11.4 18.2 24.3

1.4 1.6 2.1 3.9

0.4 0.5 0.6 0.8

Sites in China. bSuspension of soil in water, 1:2.5 (w/w). cCation exchange capacity. dTotal nitrogen. eTotal phosphorus. adding 1.2 mL of purified water (about 60% of the field holding capacity (w/w)) and incubated at 25 °C in the dark. To compensate for water loss throughout the experiments, the samples were weighed regularly and purified water was added. Triplicate samples were removed from each treatment at different time intervals (0, 1, 3, 5, 7, 10, 14, 21, 28, 49, 63, and 91 days) and immediately transferred to a freezer (−40 °C). Incubation of Dufulin in Soils under Sterile Conditions. To determine if enantioselective degradation was a result of microbemediated transformations, portions of 5.0 g of soil set in 50 mL polypropylene centrifuge tubes were subjected to sterilization treatment, which was achieved by autoclaving the samples twice at 121 °C for 40 min 24 h apart. On a clean bench, the sterilized samples were treated with 30 μg of S-(+)- or R-(−)-dufulin or 60 μg of racdufulin (S:R = 1:1), and sterile water was added. The samples were then covered with glass paper to maintain sterile conditions. Extraction of Soil Samples. Samples were thawed at room temperature, and 10 mL of water was added to each polypropylene centrifuge tube containing 5.0 g of soil. To extract the dufulin residues, 25 mL of acetonitrile, 1.5 g of NaCl, and 6 g of anhydrous MgSO4 were added to the samples. Then the tube was stirred on a vortex shaker for 3 min, ultrasonically extracted for 10 min, and centrifuged at 6000 rpm for 5 min. A portion of 20 mL of supernatant was transferred and concentrated using a rotary evaporator and a nitrogen blow-dry instrument. The residue was dissolved in 1.0 mL of methanol for analysis. Chromatographic Measurements. Analytical HPLC was performed using an Agilent 1200 series apparatus consisting of a quaternary pump, an autosampler, a diode array detector, a vacuum degasser, a column oven, and Agilent Chemstation software. After filtration, 5 μL of the sample was injected into a Daicel Chiralpak IA column. The temperature of the column was adjusted to 25 °C. The flow rate of the mobile phase, n-hexane/ethanol (90:10 by volume), was 1 mL/min. Method Validation. For this study, a series of dufulin standard working solutions (12−1200 μg/mL) were prepared for determining the linearity of the two enantiomers by HPLC analysis. Calibration curves were generated by plotting the peak area versus concentration for each enantiomer. Linear regression analysis was performed using Microsoft Excel 2010. The LODs and the limits of quantitation (LOQs) were defined as signal-to-noise (S/N) ratios of 3:1 and 10:1, respectively. Precision and accuracy of the methods were evaluated by recovery studies using spiked samples at three concentration levels (0.6, 1.2, and 6.0 μg/g for each dufulin enantiomer based on five replicates). For method recovery studies, samples without residue (5.0 g) were spiked prior to extraction by the addition of appropriate volumes of the pesticide standard solution in acetone. The treated samples were analyzed following the described procedure, and the recoveries were calculated. The method precision was assessed by determining repeatability and reproducibility, and both were expressed as relative standard deviation (RSD). Repeatability (RSDr) was measured by comparing the SD of the recovery percentage of spiked samples run on the same day. Reproducibility (RSDR) was determined by analyzing spiked samples from three different days. Kinetic Analysis and Calculation. For all treatment conditions, data were assumed to follow the first-order kinetics model. The corresponding rate constant k for (+)-enantiomer and (−)-enantiomer was calculated on the basis of eq 1 by regression analysis. The starting

to be different depending on the plant species. rac-Glufosinate and L-glufosinate were metabolized in nontransgenic and transgenic plant cell cultures, whereas D-glufosinate was not metabolized. Furthermore, the absorption rate of D-glufosinate was substantially lower than that of rac- or L-glufosinate in sugar beet.24,25 Thus, considering the effect of chiral pesticides on the environment, enantioselectivity is an important factor that should not be ignored.26,27 Some methods for the achiral analysis and residue determination of dufulin in various matrices have been reported. However, information related to the degradation of dufulin enantiomers is limited. To evaluate the enantioselective degradation of dufulin enantiomers in the environment, it is necessary to understand dufulin enantiomer degradation in different soils under agricultural cultivation. Therefore, we investigated the enantioselective degradation of rac-dufulin and its two enantiomers in four types of soil.

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MATERIALS AND METHODS

Chemicals and Reagents. rac-Dufulin (purity > 99%) was synthesized in pure form. The products were unequivocally characterized by nuclear magnetic resonance spectral and elemental analyses. Enantiopure enantiomers of dufulin were prepared on an Agilent HPLC system with a semipreparative chiral column. The purity of both enantiomers was >99%. Water was purified using a Milli-Q system. All other chemicals and solvents were analytical reagents or of HPLC grade and were obtained from common commercial sources. The chiral analytical column amylose tris(3,5dimethylphenylcarbamate) (Chiralpak IA, 4.6 mm × 250 mm i.d., 5 μm) was purchased from Daicel (Tokyo, Japan). Electronic Circular Dichroism (ECD) Spectroscopy. ECD spectroscopy was carried out using a Jasco-J810 spectropolarimeter at room temperature. The spectra were obtained from 200 to 330 nm at a scan speed of 50 nm/min. ECD Calculations. The optimized geometry of the two dufulin enantiomers was obtained by Gaussian 09 with density functional theory (DFT) at the B3LYP/6-31G* level. ECD calculations of the two enantiomers of dufulin were obtained by Gaussian 09 with timedependent density functional theory (TDDFT) methods at the B3LYP/6-311++G (2d, 2p) level following reported protocols.28,29 Soil Samples. Four types of soil were collected using a 0−20 cm plow layer from geographically distinct agricultural regions of China. The level of dufulin in these soil samples was lower than the limit of detection (LOD). All samples were air-dried at room temperature, homogenized, passed through a 2 mm sieve, and stored in the dark for several days until use. More details on soil sites and specific physicochemical characteristics are presented in Table 1. Incubation of Dufulin in Soils under Nonsterile Conditions. Separate incubation experiments with pure (better biological activity)30 S-(+)-dufulin and R-(−)-dufulin and with racemic compounds in soils were conducted in 50 mL polypropylene centrifuge tubes covered with aluminum foil. Approximately 5.0 g of soil (dry weight equivalent) was fortified with 30 μg (50 μL of 600 μg/ mL stock solution in acetone) of S-(+)- or R-(−)-dufulin or 60 μg of rac-dufulin (S:R = 1:1), and they were set to air-dry for 5 min before thorough homogenization. The soil samples were then rehydrated by 1772

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point was the maximum value of the concentration, which declined in subsequent days. The half-life (t1/2, days) was determined on the basis of eq 2:

C t = C 0 × e −k t

(1)

t1/2 = ln 2/k

(2)

Ct and C0 refer to the concentration of the S-(+)- or R-(−)-enantiomer at time t and time 0, respectively. The enantiomer fraction (EF) was used to express enantioselectivity as defined by the equation EF = peak area of the (+)-enantiomer/ [(+)-enantiomer + (−)-enantiomer]. EF values ranged from 0 to 1, with an EF value of 0.5 indicating racemic mixture.31,32

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RESULTS AND DISCUSSION Optical Isomerism Characteristics of Dufulin. The individual enantiomers of dufulin were stereochemically analyzed by ECD spectroscopy, and almost mirror-image ECD curves were obtained. The overall curves of ECD obtained by TDDFT calculations (Figure 2a) and experimental

Figure 3. Chromatograms of the S-(+)- and R-(−)-dufulin enantiomers in the standard solution (a) and degradation experiments with the S-(+)-isomer for (b) Guiyang, (c) Nanning, (d) Hefei, and (e) Harbin soils and with R-(−)-isomer for (f) Guiyang, (g) Nanning, (h) Hefei, and (i) Harbin soils after 21 days of incubation under nonsterile condition.

Figure 2. Calculation (a) and experimental measurement (b) of the ECD spectra of dufulin enantiomers in methanol (12 μg/mL).

ECD (Figure 2b) are similar. By comparison of the absolute configurations from computed ECD, the configurations of dufulin enantiomers eluted from the columns could be correctly assigned. As shown in Figure 2a, enantiomers with peaks 1 and 2 in Figure 2b are assigned to S-(+)-dufulin and R-(−)-dufulin, respectively. Method Validation. Calibration curves were obtained over a concentration range of 12−1200 μg/mL (n = 7) for the S(+)-enantiomer (y = 12.1432x + 12.8549, R2 = 1.0000) and R(−)-enantiomer (y = 12.0948x + 16.1483, R2 = 0.9999) of racdufulin. The chromatogram of rac-dufulin in the standard solution is shown in Figure 3a. The precision data for S-(+)-

and R-(−)-enantiomers of rac-dufulin are summarized in Table S1 in the Supporting Information. For both enantiomers, the recoveries obtained were in the acceptable range of 77.21− 109.95%, whereas repeatability (RSDr) ranged from 2.13 to 13.09% and reproducibility (RSDR) ranged from 2.19 to 10.57% (Supporting Information Table S2). The LODs were 0.004 μg/g and the LOQs were 0.013 μg/g for both dufulin enantiomers. 1773

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Degradation of Racemic Dufulin, Enantiopure S(+)-Dufulin, and R-(−)-Dufulin in Four Soil Types under Sterile Conditions. No significant difference in enantioselectivity was found for rac-dufulin degradation in four soils under sterile conditions. The concentrations of the two enantiomers of dufulin in sterile soils were determined. The EF values of dufulin in four sterile soils were approximately 0.5 after treatment (Supporting Information Figure S2f). Degradation of each enantiomer in the four soil types generally fit a first-order kinetics decay model (Table 2). Data from

Table 3. Degradation Rate Constant (k), Half-Life (t1/2), and Correlation Coefficient (R2) Values for the Degradation of S-(+)- and R-(−)-Dufulin in Four Types of Soil condition nonsterile sterile

nonsterile

Table 2. Degradation Rate Constant (k), Half-Life (t1/2), and Correlation Coefficient (R2) Values for the Degradation of rac-Dufulin in Four Types of Soil condition nonsterile sterile

nonsterile sterile

nonsterile sterile

nonsterile sterile

incubated compound

dufulin enantiomer

k (day−1)

Guizhou Guiyang Soil rac-dufulin S 0.0181 R 0.0182 rac-dufulin S 0.0087 R 0.0091 Guangxi Nanning Soil rac-dufulin S 0.0201 R 0.0204 rac-dufulin S 0.0108 R 0.0108 Anhui Hefei Soil rac-dufulin S 0.0213 R 0.0200 rac-dufulin S 0.0107 R 0.0112 Heilongjiang Harbin Soil rac-dufulin S 0.0259 R 0.0268 rac-dufulin S 0.0127 R 0.0120

sterile

t1/2 (days)

R2

38.30 38.09 79.67 76.17

0.9987 0.9982 0.9858 0.9892

sterile

34.48 33.98 64.18 64.18

0.9965 0.9961 0.9930 0.9933

sterile

32.54 34.66 64.78 61.89

0.9910 0.9884 0.9920 0.9955

26.76 25.86 54.58 57.76

0.9746 0.9724 0.9950 0.9965

nonsterile

nonsterile

incubated compound

k (day−1)

Guizhou Guiyang Soil S-(+)-dufulin 0.0245 R-(−)-dufulin 0.0120 S-(+)-dufulin 0.0088 R-(−)-dufulin 0.0083 Guangxi Nanning Soil S-(+)-dufulin 0.0263 R-(−)-dufulin 0.0136 S-(+)-dufulin 0.0091 R-(−)-dufulin 0.0087 Anhui Hefei Soil S-(+)-dufulin 0.0289 R-(−)-dufulin 0.0172 S-(+)-dufulin 0.0115 R-(−)-dufulin 0.0117 Heilongjiang Harbin Soil S-(+)-dufulin 0.0339 R-(−)-dufulin 0.0201 S-(+)-dufulin 0.0164 R-(−)-dufulin 0.0134

t1/2 (days)

R2

28.29 57.76 82.52 83.51

0.9966 0.9841 0.9974 0.9976

26.36 50.97 78.68 79.67

0.9901 0.9718 0.9972 0.9967

23.98 40.30 60.27 59.24

0.9987 0.9938 0.9969 0.9964

20.45 34.48 50.97 51.73

0.9935 0.9954 0.9994 0.9987

Degradation of Racemic Dufulin, Enantiopure S(+)-Dufulin, and R-(−)-Dufulin in Four Soil Types under Nonsterile Conditions. The plots of concentration of racdufulin from nonsterile experiments versus incubation time (t) are shown in Figure S2a,b in the Supporting Information. The half-life (t1/2) values of S-(+)-enantiomer were 38.30, 34.48, 32.54, and 26.76 days in Guiyang, Nanning, Hefei, and Harbin soils, respectively. The half-life (t1/2) values of R-(−)-enantiomer were 38.09, 33.98, 34.66, and 25.86 days in Guiyang, Nanning, Hefei, and Harbin soils, respectively. Thus, degradation of dufulin was faster in Harbin soil, which had the highest organic carbon content and alkalinity, compared to the other three soils. Our results revealed that degradation of dufulin under sterile conditions is much slower than under nonsterile conditions. In addition, besides abiotic degradation, dufulin degradation can also be attributed to microbial community activity in agricultural soils. Microorganisms in the soil may play important roles in the enantioselective metabolism of many chiral compounds. The enantioselective degradation behavior could be conducted by comparing the half-life (t1/2) of each enantiomer. However, the half-life (t1/2) of each enantiomer of dufulin was found to be approximately the same in the four soils from nonsterile conditions in our study. Moreover, EF values for rac-dufulin degradation in the four soils from nonsterile conditions were calculated to be approximately 0.5 after treatment (Supporting Information Figure S2e). This indicated that there was no significant enantioselectivity during degradation, which was the same as the degradation of racmalathion in Nanchang soil,20 but differed from other chiral pesticides.21,32 Some studies suggested that soil properties such as texture, pH, and OM content could have major impacts on the activity of the soil microbial community and then could influence chiral signatures in soils.12,20,21,34 Therefore, the activity of the microbial community might be distinguished in the four tested soils which had differing textures, pH values, and OM contents (Table 1). The degradation of rac-dufulin should

experiments involving sterilized soils are plotted in Figure S2c,d in the Supporting Information. Half-life (t1/2) values of dufulin in the four sterile soils (Guiyang, Nanning, Hefei, and Harbin) were about 77.9, 64.2, 63.3, and 56.2 days, respectively. The degradation rates of enantiopure S-(+)-dufulin and R(−)-dufulin were also similar (Supporting Information Figure S3c,d). No interversion between the two enantiomers was observed. Rate constants (k), half-lives (t1/2), and correlation coefficients (R2) were calculated (Table 3). Half-life (t1/2) values in the four sterile soils (Guiyang, Nanning, Hefei, and Harbin) were about 82.52, 78.68, 60.27, and 50.97 days for S(+)-dufulin and 83.51, 79.67, 59.24, and 51.73 days for R(−)-dufulin, respectively. Soil pH may play an important role in the degradation of various compounds in sterile soils.33 Higher overall microbial activity in the soil with a high pH, high organic matter (OM), and high cation exchange capacity (CEC) was also the main reason for the fast degradation. Consistent with this theory we found that degradation t1/2 of rac-dufulin in Harbin soil with the highest pH (8.5), OM (4.348%), and CEC (24.3%) was 56.2 days, which was shorter compared to those in Hefei, Nanning, and Guiyang soils (Table 2). Therefore, dufulin possibly degrades more efficiently in an alkaline soil with high CEC and high OM. 1774

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S-(+)-dufulin and R-(−)-dufulin in nonsterile soils and sterile soils (Figures S2 and S3), tables of precision, accuracy, and recovery data for the determination of dufulin on three different days (Tables S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

be enantioselective in one or more soils, but the actual degradations of rac-dufulin in all tested soils were nonenantioselective. This result assumed that the microorganisms could not conduct the enantioselective degradation of racdufulin in soil. This phenomenon is possibly due to some interactions between the two enantiomers in rac-dufulin such as the mutual promotion effects, which led to the similar degradation rates of two dufulin enantiomers in soils. During the degradation of enantiopure S-(+)- or R(−)-enantiomer in nonsterile soils, no interconversion of S(+)- to R-(−)-enantiomer or vice versa was detected. This indicated that the enantiomer was configurationally stable in these four soil types. The chromatograms of S-(+)-dufulin and R-(−)-dufulin after 21 days of incubation in the four soil types we tested are shown in Figure 3. We plotted data from the degradation of enantiopure S-(+)-dufulin and R-(−)-dufulin in four nonsterile soils (Supporting Information Figure S3a,b). Unlike the degradation study in sterile soils, the difference in degradation rate between enantiopure S-(+)-dufulin and R(−)-dufulin was obvious. The half-life (t1/2) values of S(+)-dufulin and R-(−)-dufulin were 28.29 and 57.76 days, 26.36 and 50.97 days, 23.98 and 40.30 days, and 20.45 and 34.48 days in Guiyang, Nanning, Hefei, and Harbin soils, respectively (Table 3). Degradation rates for the enantiomers in the incubation with the single pure enantiomers were higher than in the incubations with the raceme; this phenomenon was consistent with the results reported by Zipper.35 The degradation rate of S-(+)-dufulin incubations with single pure enantiomers was approximately 1.68−2.04 times higher than that of its antipode in the four types of soils tested. This result was also consistent with the results reported by Sun that enantiopure R-malathion degraded more rapidly than enantiopure S-malathion in Nanchang soil.20 However, no enantiomerization was observed. These results differed from the degradation study of enantiopure malathion that converted between R-(+)-malathion and S-(−)-malathion in different soils.20 No interaction between enantiomers existed during the degradation process of enantiopure dufulin in soil. This suggests that the microbial community in agricultural soils may decompose enantiopure S-(+)-dufulin more efficiently than R-(−)-dufulin, but not lead to the enantiomerization of the two dufulin enantiomers. In conclusion, we investigated the enantioselective degradation of rac-dufulin and its two enantiopure enantiomers in four types of soil. We initially determined the absolute configuration of dufulin enantiomers on the basis of a combination of calculated and experimental ECD spectra. We found no significant enantioselectivity in the degradation of rac-dufulin in the four soil types tested under sterile and nonsterile conditions. However, enantiopure S-(+)-dufulin degraded much more quickly than its antipode R-(−)-dufulin in all nonsterile soils, which suggests that microorganisms degrade enantiopure S-(+)-dufulin more efficiently than they do R(−)-dufulin. Future studies should aim to identify the primary metabolites and metabolic processes of dufulin in soil and water to adequately assess the environmental risk associated with dufulin use.

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AUTHOR INFORMATION

Corresponding Author

*(B.-A.S.) Phone: +86(851)362-0521. Fax: +86(851)362-2211. E-mail: [email protected]. Funding

We gratefully acknowledge financial support from the National Key Program for Basic Research (no. 2010CB126105), the Key Technologies R&D Program (no. 2011BAE06B05-6), and the National Natural Science Foundation of China (no. 21132003). Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figures of calibration curves of two enantiomers of rac-dufulin (Figure S1), figures of concentration−time curves rac-dufulin, 1775

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dx.doi.org/10.1021/jf404130d | J. Agric. Food Chem. 2014, 62, 1771−1776