Environmental Behavior of the Chiral Organophosphorus Insecticide

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Environmental Behavior of the Chiral Organophosphorus Insecticide Acephate and Its Chiral Metabolite Methamidophos: Enantioselective Transformation and Degradation in Soils Xiangyun Wang,† Zhen Li,† Hu Zhang, Junfeng Xu, Peipei Qi, Hao Xu, Qiang Wang, and Xinquan Wang* State Key Laboratory Breeding Base for Zhejiang Sustainable Pest and Disease Control, Agricultural Ministry Key Laboratory for Pesticide Residue Detection, Zhejiang Province Key Laboratory for Food Safety, Institute of Quality and Standard on Agricultural Products, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, People’s Republic of China S Supporting Information *

ABSTRACT: Acephate is a widely used organophosphorus insecticide globally, although there are some concerns about its usage with regard to acute consumer exposure and side-effects on nontarget organisms. These concerns are always attributed to the acephate metabolite methamidophos. In the many reports about the environmental behavior of acephate and its metabolite, none pay any attention to the chirality of them. In this study, the enantiomeric transformation and degradation of acephate was investigated in three soils under laboratory conditions using enantioselective GC-MS/MS. Racemic and enantiopure compounds were incubated in separate experiments. The degradation of racemates was shown to be enantioselective in unsterilized soils but not in the sterilized soils, thus confirming the enantioselectivity was microbially based. The priority of enantiomer degradation and transformation varied among soils and racemates. R-(+)-methamidophos was enriched in the Zhengzhou soil, but degraded faster in the Changchun and Nanchang soils than its antipode. For acephate, the Nanchang soil enriched R(+)-acephate, and S-(−)-acephate accumulated in the other two soils. Acephate and methamidophos were both configurationally stable in soil, showing no interconversion of R-(+)- to S-(−)-enantiomers, or vice versa. The conversion of acephate to methamidophos proceeded with retention of configuration. Generally, the degradation followed approximate first-order kinetics, but showed significant lag phases.

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INTRODUCTION Acephate (AC), (RS)-(O,S-dimethyl acetylphosphoramidothioate), with a phosphorus atom as a chiral center (Figure 1), was one of the 10 most important organophosphorus

ME is the major metabolite of AC not only in insects but also in various components of the environment such as nontarget animals, plants, waters, and soils.4 Then, with regard to the ecotoxicity of ME derived from AC application, there is dispute about the use of AC which has been forbidden by the EU since 2005,5 but is still legal in the U.S. with a series of new and more stringent measures imposed to mitigate the residual, occupational and ecological risks of AC. There will be a final decision of its registration review in 2015.2 Obviously, such a dispute signifies a requirement for a better understanding of the fate of AC in the environment, which will play an important role in the risk evaluation. The environmental behavior of AC and its metabolite ME in soils has been investigated in many reports. It is well-known that both AC and ME are readily biodegraded and nonpersistent in native soils, with half-lives ranging from 0.5 to 4.0 days in most of the cases, depending on the varied conditions of soils,4,6−9 whereas ME has been found to be degraded faster than AC under the same conditions.10 Also, AC and ME are

Figure 1. Chemical structures of acephate (AC) and methamidophos (ME) (the asterisks indicate chiral centers).

insecticide in the 1990s,1 and is still being widely used today.2 It was registered to control a wide range of chewing and sucking insects on various agricultural crops including fruits, vegetables, and cereals. The insecticidal potency of acephate is due to the inhibition of acetyl-cholinesterase (AChE) activity, and further research has revealed that the toxicity might be the result of its chiral metabolite methamidophos (ME),3 (RS)-(O,S-dimethyl phosphoramidothioate), which also has a stereogenic phosphorus atom and two enantiomers (Figure 1). Furthermore, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9233

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obtained from local distributors. Deionized water was produced by a Milli-Q system (Bedford, MA). Three typical soil samples, a black soil, red soil and loam, were collected for use in the experiment. The black soil was collected from a soybean field in Changchun (CC soil), the red soil was from a park in Nanchang (NC soil), and the loam was an agricultural soil collected from Zhengzhou (ZZ soil). After collection and air drying, the soils were sieved to 72% with relative standard deviations (RSDs) < 15%, the data of residues was present without recovery corrected.

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RESULTS AND DISCUSSION The Elution Order of Enantiomers of rac-AC and racME on the Cyclosil-B Column. The optical rotation of the enantiomers of AC and ME was measured by a CHIR9235

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degradation rate constant of ME. The fit was good with R2 values ranging from 0.962 to 0.995. The half-lives of sum of ME were less than 1.08 days and shorter than that of sum of AC in the same soil which indicated that ME degraded more rapidly than AC under the same conditions (details are shown in Table 2) in accordance with the fore investigation.10

ALYSER−MP optical rotation detector produced by IBZ MESSTECHNIK Company (Hannover, Germany) (SI Figure S2). As described by Miyazaki et al.,11 the (+)-ME has been determined to be of (R)P configuration by 1H NMR with Eu(hfc)3 and the (+)-AC was synthesized by (+)-ME with the same configuration. Finally, after the enantiomers were individually eluted on Cyclosil-B column, the following elution order was confirmed: R-(+)-ME, S-(−)-ME, R- (+)-AC, and S(−)-AC as shown in SI Figure S1. The Chiral Stability of AC and ME in the Experiment. The abiotic enantiomerization/racemization of some chiral pesticides, especially some synthetic pyrethroid insecticides, during the process of storage, analysis and degradation in soils has been observed and investigated.17−22 Here, by the fortification-recovery of enantiopure enantiomers which were spiked into the soils separately, the chiral stability of AC and ME during the analysis process was confirmed which prevented any significant analytical biases caused by the instability of enantiomers happening. As described in SI Table S1, every enantiomer was spiked into each soil and incubated separately. Because of that the trends were found to be quite similar among our three soils. Given this similarity, the results for the NC soil are used as the example to describe the stability of enantiomers (Figure 2). Figure 2 shows the degradation of enantiopure enantiomers in the sterilized (a−d, 7 day) NC soil and the unsterilized (e−h, 3 day) NC soil under aerobic conditions. Panels a, b, e, and f show the results for AC, and panels c, d, g, and h represent the results for ME. As described above, the initial enantio-purity of the spiked enantiomers was more than 98% (chromatograms are not shown), and there was no distinct antipode detected in the soils (Figure 2a−h), suggesting that there was no conversion from R-(+)- to S-(−)-enantiomers, and vice versa, during the incubation. Although there appears to be some S(−)-ME observed in the panel g, the amount was below the limit of quantification, and is therefore considered to be negligible. Furthermore, the degradation of R-(+)- and S(−)-AC produced their respective ME concurrently. That is; R(+)-ME was only derived from R-(+)-AC and S-(−)-AC was the only source of S-(−)-ME. From these data we conclude that the enantiomerization and configuration conversion is insignificant compared to degradation/dissipation for AC and its chiral metabolite ME. Also, under the same conditions, the degradation of an enantiopure enantiomer should be theoretically the same as the identical enantiomer in its racemate with the potential influence of one enantiomer on the degradation of the other excluded. However, in the study, the half-lives of enantiopure enantiomers were not equal to that in racemates which should be attributed to the not exactly same conditions during the incubation rather than the potential influence between a pair of enantiomers.23 Accordingly, the results of enantiopure enantiomer degradation are listed briefly in SI Table S2 (Unsterilized soils) and SI Table S3 (Sterilized soils) without further discussion. Degradation of rac-ME in Unsterilized Soils under Aerobic Conditions. The rac-ME was dissipated rapidly in unsterilized soils under aerobic conditions, with none of the initially spiked ME detectable after 14 days of incubation. Assuming that the degradation of ME was a first-order reaction, the degradation kinetics are calculated as C = C0 × e−kt by regression analysis (Excel 2003, Microsoft), and the half-life (t1/2, day) was estimated from t1/2 = ln2/k, where C0 and C are the concentrations at time 0 and t, respectively, and k is the

Table 2. First-Order Rate Constant (k), Half-Life (t1/2), and Correlation Coefficient (R2) for the Degradation of rac-AC and rac-ME in Unsterilized Soils experiment

incubated compound

CC1

rac-AC

CC4

rac-ME

NC1

rac-AC

NC4

rac-ME

ZZ1

rac-AC

ZZ4

rac-ME

enantiomer R-(+)-AC S-(−)-AC sum of AC R-(+)-ME S-(−)-ME sum of ME R-(+)-AC S-(−)-AC sum of AC R-(+)-ME S-(−)-ME Sum of ME R-(+)-AC S-(−)-AC sum of AC R-(+)-ME S-(−)-ME sum of ME

k (day−1) 0.669 0.597 0.629 1.819 0.533 0.644 0.192 0.215 0.203 1.058 0.731 0.889 0.502 0.434 0.465 0.888 1.060 0.958

t1/2 (days) a

1.05 1.17a 1.12 0.38a 1.31a 1.08 3.65a 3.30a 3.47b 0.66a 0.95a 0.79b 1.39a 1.61a 1.51b 0.79a 0.65a 0.73b

R2 0.982 0.980 0.983 0.974 0.985 0.991 0.986 0.986 0.986 0.983 0.982 0.995 0.965 0.962 0.964 0.962 0.963 0.964

a

The degradation of a pair of enantiomers was significantly different from each other in all of three soils, P < 0.05 (paired t test). bOther than in CC soils, the half-lives of sum of ME were also significantly shorter than that of sum of AC, P < 0.05 (paired t test).

In Figure 3, the panels b, d, and f show the chromatographs of ME residues in the soils after a 3 day incubation which were significantly enantioselective (paired t test, P < 0.05) and varied among the soils. During the past years, enantiomer fraction (EF) and enantiomer ratio (ER) were used to measure the enantioselectivity. However, several studies24−26 have pointed out that the EF is preferred to ER for describing the chiral signatures in environmental analysis, and then EF was present in the following discussion. EF is defined as EF = [R]/ ([S]+[R]), where [R] refers to the concentration of the R(+)-enantiomer, and [S] refers to the concentration of the S(−)-enantiomer. Consequently, EF > 0.5 indicate a more rapid dissipation of the S-(−)-enantiomer, EF < 0.5 indicate a more rapid removal of the R-(+)-enantiomer, and at an EF value of 0.5, the degradation is nonenantioselective. In this study, R-(+)-ME was found to be degraded more rapidly than its antipode in the NC (Figure 4d) and CC (Figure 4b) soils and the EF value of ME decreased from 0.50 to 0.29 and 0.014 before the R-(+)-ME was totally dissipated. However, in the ZZ soil (Figure 4f), it was S-(−)-ME which was preferentially degraded with the EF value of ME increasing continuously to 0.70 from the initial 0.50. The rate difference Δk (Δk= kS−kR) calculated from Table 2 could also help to understand the enantiomer degradation difference. For example, they were −1.286 d−1 in CC soil, −0.327 d−1 in NC soil and 0.172 d−1 in ZZ soil, repectively, indicating a result same with that concluded from Figure 4. 9236

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Figure 3. The degradation of rac-AC and rac-ME in unsterilized soils showing obviously enantioselective degradation in all of three soils. (a) rac-AC in CC soils after 3 days (exp. CC1); (b) rac-ME in CC soils after 3 days (exp. CC4); (c) rac-AC in NC soils after 3 days (exp. NC1); (d) rac-ME in NC soils after 3 days (exp. NC4); (e) rac-AC in ZZ soils after 3 days (exp. ZZ1); (f) rac-ME in ZZ soils after 3 days (exp. ZZ4).

Degradation of rac-AC in Unsterilized Soils under Aerobic Conditions. In the unsterilized soils under aerobic conditions, AC decreased readily, and more than 95% of it was eventually degraded in all three of the soils. It has been demonstrated in earlier achiral studies that the degradation of AC in soils should follow first-order kinetics.6,7,9,27 The kinetic data (degradation rate constant k and half-life t1/2) were calculated and are listed in Table 2. Overall, AC was most persistent in the NC soil with a half-life of 3.47 days, compared with 1.51 days for the ZZ soil and 1.12 days for the CC soil. This might be attributable to the significantly higher clay content in the NC soil, as found in previous investigations.7 Figure 3 (panels a, c, and e) shows the enantioselective degradations of rac-AC in the three soils (3 day), which were also evaluated with EF. In the experiment, the EF value of AC increased gradually from the initial 0.50 to 0.58 after 14 days in the NC soil (Figure 5d), whereas it decreased from 0.51 and 0.50 to 0.34 and 0.38 after 5 and 7 days in the CC soil (Figure 5b) and the ZZ soil (Figure 5f), respectively, indicating that there was significant enantioselectivity in all of the three collected soils, with the S-(−)-AC preferentially degraded in the NC soil, but enriched in the CC and ZZ soils during the incubation. Such a preferential degradation could also be confirmed by the enantiomers kinetic data of rac-AC incubated in three soils which were listed in Table 2. Briefly, the half-life of R-(+)-AC was significantly shorter (P < 0.05, paired t test) than that of S-(−)-AC in CC and ZZ soils, but half-life of R-

(+)-AC in NC soils was found to be significantly longer (P < 0.05, paired t test) than its antipode. Obviously, it could be concluded that the degradation of racAC and rac-ME was enantioselective in all of three soils with various preference. One reason for the difference in the degradation behavior of these two chemicals might be that the different soil types contain different microbial populations equipped with different enzymes, which are preferential degraders of different enantiomers.14 It could also be concluded that there might be different microbes or enzyme systems in a single soil, which degraded the enantiomers of ME and AC preferentially, respectively. The lag phases in the plots of EF versus time (Figures 4 and 5) indicated a gradually increasing enantioselectivity or activity of the microbial community during the beginning of the incubations. Such lag phases have been observed in previous studies, and the present example might be fairly precise because of the same recovery of R- and Senantiomers, which avoid the variety of analyte recovery.28 The Generation and Degradation of ME during rac-AC Incubation in Unsterilized Soils under Aerobic Conditions. Generally speaking, the soil concentrations of both enantiomers of generated methamidophos (GME) increased steadily to a maximum and then decreased gradually to the end of the incubation. However the rates of increase and decrease and the values of the maximum concentrations varied among the three soils (Figure 5a, c, e). In the CC soil (Figure 5a), the highest concentrations of R-(+)-GME and S-(−)-GME were 9237

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Figure 4. The degradation of spiked rac-ME in unsterilized soils. (a, b) CC soil (exp. CC4); (c, d) NC soil (exp. NC4); (e, f) ZZ soil (exp. ZZ4). Panels a, c, and e showing that the concentrations of ME decreased steadily during the incubation. Panels b, d, and f plot the EFs of ME versus incubation time in CC soil, NC soil, and ZZ soil, respectively.

about 0.161 and 0.274 mg/kg, respectively, which occurred at days 1 and 3, respectively. In the NC soil (Figure 5c), the highest concentrations of R-(+)-GME and S-(−)-GME were about 0.029 and 0.048 mg/kg, respectively, which occurred at day 2. In the ZZ soil (Figure 5e), the concentrations of R(+)-GME and S-(−)-GME reached a maximum of 0.368 and 0.272 mg/kg, respectively, at day 3. The levels of GME residues in the NC soil were significantly lower than in the other two soils which might be due to its slowest degradation of AC which concurrently formed GME most inefficiently, while the dissipation of GME in it was similar to the other soils. It is wellknown that the GME generated from AC after application is mainly responsible for the environmental risk. Then, it might be concluded that the application of AC under conditions where the GME residue was lower such as in the NC soil, the environmental risk should consequently be considerably less.29 Other than the enantiomer concentrations of GME derived from AC, the enantiomer fraction (EF) of GME might be another factor that should be taken into account when the environment risk evaluation of AC is carried out. The EF values of GME reflect a combination of the enantioselectivity of the generation of GME from AC and the subsequent degradation of any GME. Consequently, in this study, there were three kinds of combinations for the EF values of GME generated from AC (Figure 5b, d, f). In the ZZ soil (Figure 5f), the EF

values increased consistently from the initial 0.51 to 0.58, whereas in the NC soil (Figure 5d), the EF ratios decreased to 0.21, but then increased again to 0.47. Also, in these two soils, the trend in EF values of GME was the same as that of spiked ME. This might be because the enantioselectivity of the generation of GME from AC was in the same direction as the enantioselectivity of the degradation of the GME. In the CC soil (Figure 5b), the EFs of generated ME increased from 0.49 to a maximum of 0.68 at day 0.5, and then moved downward to a minimum of 0.048 at day 5. This indicated that the GME in the soil was racemic at first and in the initial stage when the EF was increasing, the preferential degradation of R-(+)-AC which provides the enrichment of R-(+)-ME exceeded the preferential degradation of R-(+)-ME. The EFs maximum represents the turning point where a rates balance in the rates of enantioselective enrichment and degradation of R-(+)-ME is achieved. After this point, the enantioselective degradation of R-(+)-ME is stronger than its generation which leads to the enantiomer composition of ME reverting to a racemic mixture and consequent enrichment of S-(−)-ME. Degradation of rac-AC, rac-ME in Sterilized Soils. In sterilized soils, there was no enantiomer interconversion of AC and ME. Also, there was no enantioselectivity of racemate degradation, which indicates that the enantioselectivity observed in unsterilized soils under aerobic conditions is 9238

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Figure 5. The generation and degradation of GME from rac-AC incubation in unsterilized soils. (a, b) CC soil (exp. CC1); (c, d) NC soil (exp. NC1); (e, f) ZZ soil (exp. ZZ1). Panels a, c, and e showing that the concentrations of GME increased steadily to a maximum and then decreased gradually to the end of the incubation along with the steadily degradation of spiked AC. Panels b, d, and f plot the EFs of GME and AC versus incubation time in CC soil, NC soil, and ZZ soil, respectively.

Author Contributions

microbially based. Furthermore, the degradation of AC and ME was much slower than that in unsterilized soils, suggesting that the degradation of AC and ME in unsterilized soils was mainly biologically mediated. Assuming first-order kinetics, the kinetic data (degradation rate constant k and half-life t1/2) were calculated and are listed in SI Table S4. Limitations of the Present Experiment. In the present experiment, there was significant enantioselectivity loss during the later phase of the experiment which is apparent in Figure 5b and d. This phenomenon has also been found in the degradation of metalaxyl23 and triadimefon,30 but no reason for it has been provided. We suggest it could be the result of microbial change in the soils, and intend to test this in a further study.

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†

X.W. and Z. L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 20807038 and 21207118), the National High Technology Research and Development Program of China (The 863 Program, Grant No. 2011AA100806), the Special Fund for Agro-scientific Research in the Public Interest (Grant No. 201303088), and the Project of Zhejiang Province Science and Technology Innovative Team (Grant No. 2010R50028).

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ASSOCIATED CONTENT

S Supporting Information *

Two figures and four tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-57186401834; e-mail: [email protected]. 9239

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