Enantioselective degradation of dufulin in four types of soil

Article abstract of DOI:10.1021/jf404130d

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 (t_1/2) 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.

Full text of DOI:10.1021/jf404130d

Article
pubs.acs.org/JAFC
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(3,5-
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.
KEYWORDS: dufulin, degradation, soil, enantioselective, enantiomer
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 rac-
glufosinate, L-glufosinate, and D-glufosinate have been reported
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 bio-
logical 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-methylbenzo-
thiazol-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
Received: September 16, 2013
Revised: February 10, 2014
Accepted: February 10, 2014
Published: February 11, 2014
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
b
c
d
e
soil site
sand (%)
silt (%)
clay (%)
pH
Corg (%)
CEC (%)
TN (g/kg)
TP (g/kg)
Guizhou Guiyang
Guangxi Nanning
Anhui Hefei
21.0
8.5
62.0
38.8
37.9
31.6
17.0
52.7
55.3
4.8
5.2
7.8
8.5
1.797
2.192
3.809
18.6
11.4
18.2
1.4
1.6
2.1
3.9
0.4
0.5
0.6
0.8
6.8
Heilongjian Harbin
3.7
64.7
4.348
24.3
a
b
c
d
e
Sites in China. Suspension of soil in water, 1:2.5 (w/w). Cation exchange capacity. Total nitrogen. Total 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).
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.
Incubation of Dufulin in Soils under Sterile Conditions. To
determine if enantioselective degradation was a result of microbe-
mediated 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 rac-
dufulin (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.
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(3,5-
dimethylphenylcarbamate) (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 time-
dependent 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
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
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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:
Ct = C0 × e−kt
(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
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 rac-
dufulin. 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.
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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 enantiose-
lectivity 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). Degrada-
tion 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
incubated compound k (day−1
)
t1/2 (days)
R2
Guizhou Guiyang Soil
nonsterile
sterile
S-(+)-dufulin
0.0245
0.0120
0.0088
0.0083
28.29
57.76
82.52
83.51
0.9966
0.9841
0.9974
0.9976
R-(−)-dufulin
S-(+)-dufulin
R-(−)-dufulin
Guangxi Nanning Soil
nonsterile
sterile
S-(+)-dufulin
0.0263
0.0136
0.0091
0.0087
26.36
50.97
78.68
79.67
0.9901
0.9718
0.9972
0.9967
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
R-(−)-dufulin
S-(+)-dufulin
R-(−)-dufulin
incubated
dufulin
k
t1/2
Anhui Hefei Soil
condition
compound
enantiomer
(day−1
)
(days)
R2
nonsterile
sterile
S-(+)-dufulin
0.0289
0.0172
0.0115
0.0117
23.98
40.30
60.27
59.24
0.9987
0.9938
0.9969
0.9964
Guizhou Guiyang Soil
R-(−)-dufulin
S-(+)-dufulin
R-(−)-dufulin
nonsterile
sterile
rac-dufulin
rac-dufulin
S
R
S
0.0181
0.0182
38.30
38.09
79.67
76.17
0.9987
0.9982
0.9858
0.9892
0.0087
0.0091
Heilongjiang Harbin Soil
R
nonsterile
sterile
S-(+)-dufulin
0.0339
0.0201
0.0164
0.0134
20.45
34.48
50.97
51.73
0.9935
0.9954
0.9994
0.9987
Guangxi Nanning Soil
R-(−)-dufulin
S-(+)-dufulin
R-(−)-dufulin
nonsterile
sterile
rac-dufulin
rac-dufulin
S
0.0201
34.48
33.98
64.18
64.18
0.9965
0.9961
0.9930
0.9933
R
0.0204
0.0108
0.0108
S
R
Degradation of Racemic Dufulin, Enantiopure S-
(+)-Dufulin, and R-(−)-Dufulin in Four Soil Types under
Nonsterile Conditions. The plots of concentration of rac-
dufulin 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-(−)-enan-
tiomer 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 rac-
malathion 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
Anhui Hefei Soil
nonsterile
sterile
rac-dufulin
rac-dufulin
S
R
S
0.0213
0.0200
0.0107
0.0112
32.54
34.66
64.78
61.89
0.9910
0.9884
0.9920
0.9955
R
Heilongjiang Harbin Soil
nonsterile
sterile
rac-dufulin
S
R
S
0.0259
26.76
25.86
54.58
57.76
0.9746
0.9724
0.9950
0.9965
0.0268
0.0127
0.0120
rac-dufulin
R
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.
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be enantioselective in one or more soils, but the actual
degradations of rac-dufulin in all tested soils were non-
enantioselective. This result assumed that the microorganisms
could not conduct the enantioselective degradation of rac-
dufulin 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.
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.
AUTHOR INFORMATION
Corresponding Author
*(B.-A.S.) Phone: +86(851)362-0521. Fax: +86(851)362-2211.
E-mail: songbaoan22@yahoo.com.
■
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 enantio-
pure S-malathion in Nanchang soil.20 However, no enantiome-
rization 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.
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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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures of calibration curves of two enantiomers of rac-dufulin
(Figure S1), figures of concentration−time curves rac-dufulin,
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Journal of Agricultural and Food Chemistry
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