Behavior of metalaxyl and its pure R-Enantiomer in sunflower plants (Helianthus annus)

Article abstract of DOI:10.1021/jf020310w

A possible stereospecific and/or stereoselective mechanism of biodegradation for metalaxyl and metalaxyl-M was studied to elucidate their behavior in sunflower plants and to compare their biodegradation. Greenhouse experiments were carried out to confirm

Full text of DOI:10.1021/jf020310w

J. Agric. Food Chem. 2002, 50, 5373−5377 5373
Behavior of Metalaxyl and Its Pure R-Enantiomer in Sunflower
Plants (Helianthus annus)
‡
C. ZADRA,† C. MARUCCHINI,*,† AND A. ZAZZERINI
Dipartimento di Scienze Agroambientali e della Produzione Vegetale (Di.SA.Pro.V.) Chimica Agraria,
and Dipartimento di Arboricoltura e Protezione delle Piante Patologia Vegetale, Universita` degli Studi
di Perugia, Borgo XX Giugno 72, 06121 Perugia, Italy
A possible stereospecific and/or stereoselective mechanism of biodegradation for metalaxyl and
metalaxyl-M was studied to elucidate their behavior in sunflower plants and to compare their
biodegradation. Greenhouse experiments were carried out to confirm the same efficacy of the two
fungicides against infections by Plasmopara helianthi in sunflower plants. The two fungicides appear
to have the same behavior regarding both the protection against plant infections and the mode of
translocation and the rate and pathway of biotransformation, but we have evidence that this
biotransformation process is enantioselective. Furthermore, we propose procedures for a chromato-
graphic separation of enantiomers and acid metabolites of the fungicides and for the determination
of the R:S ratio by HPLC chiral analyses. This study emphasizes the importance of examining the
fate of both stereoisomers of a chiral agrochemical in an environmental system for the correct use of
enantiomerically pure agrochemical compounds.
KEYWORDS: Metalaxyl; chirality; Oomycetes; enantiomers; biodegradation
INTRODUCTION
Many agrochemicals, about 25% of the total (1), are chiral
molecules and may exist in different stereoisomeric forms which
may have different properties in asymmetrical environments
such as living systems. At the present time, a significant number
of these chiral agrochemicals are released into the environment
as racemates or a mixture of stereoisomers, but in some cases
the biological activity of a pesticide may be attributed to one
stereoisomer, while the other stereoisomers have little or no
activity (2). Indeed, the stereoisomers may have complementary
biological activity, or one enantiomer may produce a completely
different biological response than its antipode, and therefore
the stereoisomers have different biological and physiological
properties in terms of qualitative and/or quantitative effects. It
is important to point out that stereochemistry is a factor
influencing not only biological activity but also processes such
as uptake, distribution, and metabolic behavior in organisms
and in the environment. After their field application, chiral
pesticides generally undergo a series of biologically mediated
reactions and, if a racemic mixture of stereoisomers is applied,
there may well be a predominance of an isomer in the residue,
depending on the stereospecificity in the biological processes
that may be responsible for the preferential biodegradation of
one of the stereoisomers (3). Therefore, the stereochemical
Figure 1. Chemical structure of metalaxyl. *Asymmetric carbon atom.
control in biodegradation process has important implications
in the manufacture and use of chiral agrochemicals, and the
separation, identification, and use of the biologically active
isomer is essential to minimize contaminants in the environment.
Indeed, regulatory actions have already limited the use of
racemates in The Netherlands and in Switzerland for chiral
herbicides such as dichlorprop and mecoprop, for which only
one form is herbicidally active (4).
Metalaxyl [(R,S)-methyl-N-(2-methoxyacetyl)-N-(2,6-xylyl)-
DL-alaninate] is an important acylanilide fungicide introduced
in 1977, widely used in the control of plant diseases caused by
pathogens of the Oomycota division. It is a systemic, apoplas-
tically transported fungicide, highly active against fungi of the
order Peronosporales by selectively interfering with the syn-
thesis of ribosomal RNA (5, 6). Metalaxyl is chiral due to the
presence of the stereogenic center in the alkyl moiety (Figure
1) and consists of a pair of enantiomers, S(+) and R(-) (7). A
comparative study of the biochemical effects of the stereoiso-
mers of metalaxyl indicated that the R- and S-enantiomers have
the same mode of action, but show considerable differences in
their effectiveness in reaching or binding to the receptor (8).
Biological in vitro tests against Phythophthora infestans and
Pythium ultimum showed that the R-enantiomer was ap-
proximately 1000 times more active than the S-enantiomer, but
in vivo the activity of the R-enantiomer was still 3-10 times
* Corresponding author: Cesare Marucchini, Dipartimento di Scienze
Agroambientali e della Produzione Vegetale, Chimica Agraria, Universita`
degli Studi di Perugia, Borgo XX Giugno, 72, 06121 Perugia, Italy. Tel
++39-0755856234. Fax ++39-0755856239. E-mail: cmaru@unipg.it.
† Dipartimento di Scienze Agroambientali e della Produzione Vegetale,
Chimica Agraria.
‡ Dipartimento di Arboricoltura e Protezione delle Piante Patologia
Vegetale.
10.1021/jf020310w CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/10/2002

5374 J. Agric. Food Chem., Vol. 50, No. 19, 2002
Zadra et al.
covering the infected plants with plastic bags for 48 h to provide a
saturated atmosphere (18). Zoosporangia (and zoosporangiophores) were
gently brushed from the cotyledons and leaves into distilled water; their
concentration was determined with a haemocytometer and adjusted as
necessary with distilled water. Seven days after sowing, the infection
of the experimental plants was obtained by distributing the zoosporangia
suspension (700 zoosporangia/g of soil) over the soil surface (20 mL/
box). The controls consisted of plants treated with the same chemicals
without the inoculation of P. helianthi, plants treated with water, and
untreated plants inoculated with P. helianthi.
In the second experiment, the seeds were treated with the fungicides
in the same way as experiment 1, but without pathogen inoculum.
Plants of all experiments were grown in a greenhouse at an
alternating day-night temperature of 20/18 ( 2 °C, 60-70% day/
night relative humidity with 12 h of daylight (180 µE/m-2 s-1) for 80
days. Plants were fertilized one per week with 20-5-10 (N-P-K).
These experiments were repeated 4 times, and 7 days after planting
the plant material was sampled periodically.
Evaluation of Systemic Infection by P. helianthi. Plants from the
first experiment were also placed for 48 h in a saturated atmosphere to
assess fungal sporulation on hypocotyls, cotyledons, and true leaves.
Plants were removed from the soil, and sporulation of P. helianthi was
assessed visually with or without a dissecting microscope. All hypo-
cotyls that seemed to be healthy were cut into sections 2-3 cm long
and were then incubated on moist filter paper in Petri dishes at 20 °C
for 24-48 h in the dark to induce sporulation. Percentages of infected
plants were therefore determined by counting plants with systemic
infection (showing chlorosis, stunting, and sporulation on cotyledons
and true leaves) and plants with sporulation on hypocotyls.
Plant Samples Preparation. Metalaxyl, metalaxyl-M, and their acid
metabolites were extracted from plant leaves (of the second experiment)
following this procedure. Five grams of vegetable matter was homog-
enized in a high-speed blender with methanol (2 × 20 mL) for 15 min.
The supernatant was filtered through filter paper and the filtrate
evaporated to 5 mL by using a rotary evaporator at 45 °C; 25 mL of
distilled water was added to the residue, and this solution was extracted
with ethyl-acetate (3 × 30 mL). Organic phases were collected, dried
with 2 g of anhydrous sodium sulfate, filtered, and evaporated under
vacuum at 40 °C to dryness. The residue, dissolved in 2 mL of hexane,
was transferred into an SPE column (1 g of silica) for the cleanup step;
impurities were washed out by rinsing with 20 mL of hexane, hexane-
ethyl acetate 5% v/v, hexane-ethyl acetate 10% v/v, and hexane-
ethyl acetate 15% v/v. The desorption of compounds was achieved using
30 mL of hexane-ethyl acetate 20% v/v. The eluate was evaporated
to dryness.
Figure 2. Hydrolysis reaction of metalaxyl to its acid metabolite.
more than the S-enantiomer. The S-enantiomer of metalaxyl is
inactive in the herbicide test, and therefore metalaxyl, as the
technical racemate, is not phytotoxic. Nevertheless, the implica-
tions of stereoisomerism for the biological activity of fungicides
are difficult to characterize, since the receptors and the metabolic
pathways must be considered in both the pathogens and the host
plant (9).
Biodegradation of metalaxyl in different soils and plants has
been extensively studied (10-14), and many studies have
demonstrated that the degradation of this chemical in soil is a
microbiologically mediated process (15). The biological pro-
cesses generally tend to stereospecifically degrade chiral organic
compounds. Indeed, a enantiospecific microbial degradation of
racemic metalaxyl in two media (soil and sewage sludge) was
reported (16), but these studies provided no information on the
fate of the individual enantiomers of either the parent compound
or their chiral hydrolysis products.
In our previous study (17), we demonstrated that the
biodegradation of metalaxyl and metalaxyl-M in soil is ste-
reospecific; likewise, it can be expected that the behavior and
the reactions of these compounds with chiral structures (protein
such as metabolizing enzymes) of biological systems (plants)
are stereospecific.
In light of the recent recommendations by Sukul and Spiteller
(15) that precautions be taken in the continued application of
metalaxyl to crops, the purpose of this work is to investigate
the behavior of (R,S)-metalaxyl and of the pure active R-
enantiomer (metalaxyl-M) in infected sunflower plants as well
as to verify and monitor the possible chiral switch of the racemic
mixture and/or of the single enantiomer. Moreover, because both
in soil and in plants metalaxyl undergoes a hydrolysis reaction,
we investigated the rate of the formation of the acid metabolite
(Figure 2) and the stereochemistry of this reaction. Indeed, a
stereospecific biodegradation may lead either to enantiomeric
ratios of parent compounds and metabolites which differ from
the 1:1 ratio (when the pesticide is applied as a racemic mixture)
or to a racemization of the pesticide applied as a pure
enantiomer.
Preparation of the Comparison Compound Metalaxyl Acid
Metabolite [N-(2,6-Dimethylphenyl)-N-(methoxyacetyl)-alanine]. One
gram of metalaxyl or metalaxyl-M was added to 100 mL of 10%
sulfuric acid solution at 100 °C by refluxing for 10 h. Fifty milliliters
of water was added to the cool solution, the pH of which was adjusted
to 9-10 with 1 N NaOH, and the solution was then extracted with
ethyl-acetate (3 × 100 mL). The aqueous phase was adjusted to pH 2
with 2 N HCl and further extracted with ethyl-acetate (3 × 100 mL).
Organic phases were collected, dried with 2 g of anhydrous sodium
sulfate, and the solution was then filtered and evaporated in vacuo to
dryness. The resulting white crystals were collected and washed with
water. The identity of the acid was confirmed by NMR data. The NMR
MATERIALS AND METHODS
Chemicals. All reagents, Analar or Hipersolv grade, were obtained
from BDH. The certified standard of metalaxyl [(R,S)-metalaxyl)]
(chemical purity greater than 99.3%) was purchased from Dr. Ehren-
storfer G.m.b.H. (Germany). The certified standard of metalaxyl-M
R(-)enantiomer (chemical purity greater than 94.7%), the com-
mercial products Apron SD35 (35% of metalaxyl), and CGA329351
ES350 (35% of metalaxyl-M) were provided by Syngenta (formerly
Novartis).
Biological Materials. All experiments were carried out with
susceptible sunflower plants (Helianthus annuus L. cv. Ala). Isolate
of Plasmopora helianthi (race 1 or “European race”) was used for the
induction of infection.
Fungicides Application, Plant Care, and Pathogen Inoculum. In
the first experiment, the fungicides Apron SD35 and CGA 329351 ES
350 were applied as a seed dressing to 5 g of sunflower seeds at the
rate of 6 and 3 g/kg of seed, respectively. The sunflower seeds were
sown (15 seeds/box) in boxes (20 × 40 × 10 cm) each containing 1.5
kg of a sterilized sandy-peat mixture (1:1, v:v). The isolate of P.
helianthi was maintained on susceptible sunflower plants by inoculating
pregerminated seeds and growing them in pots containing perlite in a
separate growth chamber. After 2 weeks, sporulation was induced by
1
spectra were obtained with a Bruker DRX 400 (400 MHz, H; 400
MHz 13C) spectrometer equipped with a 5-mm probe.
1H NMR (CDCl3). δ 1.14 (d, 3H, J ) 7.4 Hz); 2.2 (s, 3H); 2.41 (s,
3H); 3.37 (s, 3H); 3.56 (qAB, 2H, J ) 15.6 Hz); 4.59 (q, 1H, J ) 7.4
Hz); 7.2 (m, 3 ArH); 10.64 (bs, OH).
13C NMR (CDCl3). δ 14.74, 18.62, 18.98, 56.28, 59.28, 129.33,
129.68, 129.89, 135.72, 139.14, 137.56, 171.7, 176.4.
Methylation with Diazomethane. The synthesized acid and the plant
extracts containing the acid metabolite were converted into their
corresponding methyl ester to confirm their identity. The dry residue
was dissolved in 1 mL of CHCl3, and 1 mL of CH2N2 (in CHCl3) was
added. Diazomethane was produced by the reaction of potassium
hydroxide dissolved in diethylene glycol and a solution of N-nitro-N-
methyl-p-toluensulfonamide in diethyl ether. After methylation, the

Behavior of Metalaxyl in Sunflower Plants
J. Agric. Food Chem., Vol. 50, No. 19, 2002 5375
solution was evaporated to dryness and the residue dissolved in 1 mL
of hexane for GC-MS analysis.
Table 1. Concentrations of Metalaxyl and Metalaxyl-M in Sunflower
Leaves over Time
High Performance Liquid Chromatography (HPLC). A modular
instrumentation comprising a solvent delivery system constaMetric 4100
(LDC Analytical), a variable UV wavelength detector spectroMonitor
3200 (Thermo Separation Products), a Rheodyne injection system with
a 10 µL loop, and a Alltima Alltech C18 column (5 µm 150 × 4.6 mm)
was used to monitor the disappearance of the parent metalaxyl and the
production of its acid metabolite. Peak area was integrated using a
computer with a data system Chrom-Card. The operating parameters
were as follows: solvent A (H2O) and solvent B (CH3CN) with a
gradient system (0-10 min 50% A and 50% B and 10-20 min 10%
B), and a flow rate of 0.8 mL/min. Ten microliters of the samples,
redissolved in 1 mL of the same mobile phase, were injected and
detected at a wavelength of 254 nm. Calibration was performed daily
using external standards and linear regression analysis.
days after
treatment metalaxyl metalaxyl-M
ppm
ppm
days after
treatment metalaxyl metalaxyl-M
ppm
ppm
7
10
14
17
21
25
31
38
0.50
0.93
1.28
1.50
1.68
1.70
1.73
2.66
44
50
56
60
66
75
85
1.89
1.58
1.24
1.14
1.07
0.96
0.87
1.44
1.00
0.96
0.92
0.70
0.67
0.33
0.60
0.70
0.81
1.28
1.82
1.49
Chiral High Performance Liquid Chromatography (HPLC).
Enantiomers of metalaxyl were separated using the HPLC apparatus
previously described equipped with a OJ Chiralcel column with
cellulose derivative as the stationary phase (Daicel, 250 × 4.6 mm).
The operating conditions for the determination of (R,S)-metalaxyl and
(R)-metalaxyl were as follows: mobile phase n-hexane/2-propanol (85:
15 by volume), flow rate 0.8 mL/min, and detector wavelength 254
nm. The metalaxyl peak was split into two components, the enantiomers
(R) and (S), whose elution order was established by comparison with
the retention time of the R-pure enantiomer.
The operating conditions for the determination of corresponding acid
metabolites were as follows: mobile phase n-hexane/2-propanol/formic
acid (95:5:0.5 by volume), flow rate 0.8 mL/min, and detector
wavelength 254 nm. The enantiomers were identified as described
above.
Gas Chromatography-Mass Spectroscopy (GC-MS). A Varian
Star 3400 gas chromatograph equipped with a split-splitless injector
was combined by direct coupling to a Varian Saturn II mass
spectrometer, operating in the electron impact mode (EI), equipped with
a multiple-ion detector. A CP-Sil8CB capillary column (Chiralpack 25
m × 0.25 mm, F.t. 0.25 µm) was used. The chromatographic conditions
were as follows: the temperature was programmed from 60 °C (1 min)
to 260 °C (15 min) at 20 °C min-1. In this condition, the metalaxyl
retention time was 14 min 38 s. The carrier gas was helium at the flow
rate of 1 mL min-1; the temperatures of the injector port and of the
ion source were 220 and 280 °C, respectively; the emission current
was 10 µA.
Figure 3. Biodegradation of metalaxyl and metalaxyl-M in sunflower leaves
over time. (The starting point is the maximum value of concentration.)
to 0.87 ppm and from 1.82 to 0.33 ppm, for metalaxyl and
metalaxyl-M, respectively, 85 days after treatment (Table 1).
The biodegradation of both fungicides appeared to follow a
pseudo first-order kinetic reaction, and the degradation rate
constants were determined using regression plots of ln (c/c0)
versus time (t). The half-life time values (t1/2) of 23 and 28
days for metalaxyl and metalaxyl-M, respectively, were observed
(Figure 3).
In view of these different residual concentrations of metalaxyl
and metalaxyl-M, we decided to study and monitor the formation
of acid compound as one of the metabolic products of this
chemical. This acid metabolite is the main breakdown product,
both for metalaxyl and metalaxyl-M, arising from ester hy-
drolysis (15). The disappearance of both fungicides and the
formation of their acid metabolites were monitored by HPLC
analysis as described above. The identity of the acid metabolite
of metalaxyl was confirmed by comparing the residue retention
time with authentic standards and by following the methylation
procedure described by Businelli et al. (19). The GC-MS data
are according to those of the synthesized compound. The
retention time and the MS fragmentation of the acid metabolite
after methylation are the same as that of the parent metalaxyl
(M+ at m/z 280 and significant m/z values at 248, 220, 160)
(Figure 4). Quantitative analyses of these compounds were
carried out, and the HPLC profiles of residues of metalaxyl show
that the decrease in the unchanged parent compound was
accompanied by an increase in the concentration of the acid
metabolite. This acid metabolite did not appear until 21 days
after treatment as seed dressed, and its concentration increases
about 2-fold after 60 days, but the complete disappearance of
metalaxyl ester was not observed at least up to 90 days (Figure
5).
RESULTS AND DISCUSSION
Response of Sunflower Plants to Metalaxyl-M in Com-
parison to Metalaxyl. Plants treated with the two fungicides
did not show any symptom of disease (chlorosis, stunting, and
sporulation on cotyledons and true leaves): the percentage of
infection was 0%; therefore, Apron SD35 and CGA329351
fungicides showed the same activity toward the pathogen.
Conversely, the percentage of infection observed in control
plants untreated with fungicides, ranged from 78 to 81%
(average 79%).
Biodegradation of Metalaxyl and Metalaxyl-M in Sun-
flower Plants. The analytical method used to determine the
residue of pesticides in sunflower plants was fast and reliable.
The recoveries of pesticides from vegetables treated with
standard solutions of metalaxyl and metalaxyl-M were 80% (
0.71 (standard error of four replicates) for a concentration of 1
ppm and 75% ( 0.48 for a concentration of 0.01 ppm.
Quantitative analyses of these compounds were carried out by
comparing peak area and retention time of the residue with the
authentic standard. Concerning the behavior of metalaxyl and
metalaxyl-M in sunflower plants, both compounds were ab-
sorbed by the target plant and accumulated in leaves, but we
did not observe any difference regarding the rate of translocation.
Their maximum concentrations were achieved 38 and 31 days
after seed treatment, and these concentrations declined from 2.66
Concerning the formation of acid metabolite from metalaxyl-
M, we observed that the compound appeared later than the
previous acid compound (38 days after treatment) and also its
concentration increased about 2-fold at the end of the experiment
(Figure 6). These observations are confirmed by the very similar
values of formation rate of the acid metabolite; in fact, k is
0.0147 for the metalaxyl acid metabolite and 0.0149 for the
metalaxyl-M acid metabolite (Figure 7).

5376 J. Agric. Food Chem., Vol. 50, No. 19, 2002
Zadra et al.
Table 2. Enantiomeric Ratio (ER ) R/S) of (R,S)-Metalaxyl and
(R,S)-Metalaxyl Acid Metabolite in Sunflower Leaves
days after
ER
ER
days after
ER
ER
treatment R,S-metalaxyl R,S-metalaxyl-M treatment R,S-metalaxyl R,S-metalaxyl-M
7
10
14
17
21
25
31
38
0.75
0.67
0.75
0.70
0.49
1.04
1.19
3.33
44
50
56
60
66
75
85
3.20
3.38
3.13
3.38
3.86
3.80
4.80
0.97
0.95
0.98
0.96
0.98
0.98
0.94
0.92
0.93
0.94
0.97
Figure 4. Mass spectrum of metalaxyl ester.
Figure 8. Concentration of S-enantiomer of metalaxyl and its acid
metabolite in sunflower leaves over time.
enantiomers were used for estimating the enantiomeric ratio
(ER) values during this experiment. The ER was defined as the
peak area of the first eluting R-enantiomer divided by the peak
area of the later eluting S-enantiomer (20). The initial formula-
tion (Apron) containing (R,S)-metalaxyl has an ER ) 1.02
showing that the two enantiomers are present in the same
proportion. In Table 2 are shown the ER values in plant leaves
over time; these values concurred with the enantiospecific
degradation observed from the kinetic data and are more
revealing of the enantiospecificity than the values of the half-
life. The vegetable extracts were initially rich in S-enantiomer,
whereas 25 days after treatment there was a rise in the
concentration of R-enantiomer. The ER values ranged from 0.49
(composition S > R) to 4.80 (composition R . S). The change
of ER indicated a initially predominance of S-enantiomer in
comparison to R-enantiomer, but with time the S-enantiomer
disappeared faster than R-enantiomer. This steady decrease of
S-form concentration in comparison to the R-form could indicate
a preferential biotransformation of S-isomer by the plant’s
enzymatic systems when this isomer is applied as a racemic
mixture. On the other hand, it is known that the R-enantiomeric
form is biologically more active than the S-form, so its higher
concentration and its greater fungicide activity help in the
protection against Plasmopora diseases in sunflower plants. In
turn, the decrease of S-enantiomer could be a consequence of
the chiral inversion (S f R), while it is not so surprising that
the biotransformation process of chiral fungicides could lead
to an inversion of configuration, but it was observed from the
beginning of this study that there was no chromatographic
evidence of an inversion of the R absolute configuration of
metalaxyl-M to the inactive S-form and probably the reverse is
the case.
Figure 5. Concentration of metalaxyl and its acid metabolite in sunflower
leaves over time.
Figure 6. Concentration of metalaxyl-M and its acid metabolite in sunflower
leaves over time.
Figure 7. Rate of formation of metalaxyl acid and metalaxyl-M acid.
In order to compare the fate of the two enantiomers of
metalaxyl, the ratios of R:S enantiomers in the plant extracts
containing rac-metalaxyl were investigated. The degradation rate
constants and the half-life values of the S- and R-enantiomers
were calculated. The data showed that the two enantiomers
We compared the relative concentrations of the R- and S-acid
isomers (enantiomer peak areas) of metabolite arising from
(R,S)-metalaxyl. These data showed little changes in the 1:1
ratio of the racemic mixture of acid metabolites; at any given
moment the R-isomer peak is always slightly smaller than
S-isomer with a practically constant enantiomeric ratio (ER )
0.94-0.98). The study of the formation of the two enantiomers
of metalaxyl acid has showed that the S-enantiomer appeared
degraded at slightly different rates: the R-enantiomer (t1/2
24 days) degrades more slowly than the S-enantiomer (t1/2
21 days). The data of the residual concentrations of the two
)
)

Behavior of Metalaxyl in Sunflower Plants
J. Agric. Food Chem., Vol. 50, No. 19, 2002 5377
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CONCLUSIONS
In our experiment, we have confirmed that the two fungicides,
metalaxyl and metalaxyl-M, exhibited the same efficacy against
infections by P. helianthi. Metalaxyl and metalaxyl-M were
readily biodegraded in sunflower plants by ester hydrolysis, so
that the major metabolites were the corresponding carboxylic
acids. These acid compounds arising from metalaxyl and
metalaxyl-M were formed at different times: (R,S)-metalaxyl
acid metabolite appeared before the (R)-metalaxyl acid metabo-
lite. The hydrolysis process of the two enantiomers of (R,S)-
metalaxyl occurred at different rates: the S-enantiomer was
biodegraded faster than the R-enantiomer. Furthermore, we have
observed that in sunflower plants metalaxyl was converted into
its acid metabolite with retention of configuration; in fact, the
(R,S)-metalaxyl was converted into (R,S)-metalaxyl acid me-
tabolites and metalaxyl-M was converted only into an R-acid
metabolite without inversion of configuration.
LITERATURE CITED
Received for review March 12, 2002. Revised manuscript received May
28, 2002. Accepted June 12, 2002.
(1) Williams, A. Opportunities for chiral agrochemicals. Pestic. Sci.
1996, 46, 3-9.
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