Transformation kinetics and mechanism of the sulfonylurea herbicides pyrazosulfuron ethyl and halosulfuron methyl in aqueous solutions

Article abstract of DOI:10.1021/jf800899e

Pyrazosulfuron ethyl (PE) and halosulfuron methyl (HM) are two new highly active sulfonylurea herbicides that have been widely used for weed control in a variety of vegetables and other crops. These two herbicides have similar molecular structures, differing only in the substitutions on the pyrazole ring. Chemical hydrolysis is a primary process affecting the environmental fate of sulfonylurea pesticides. The hydrolytic transformation kinetics of PE and HM were investigated as a function of pH and temperature. For both herbicides, the hydrolysis rate was pH-dependent and increased with increasing temperature. The hydrolysis of both sulfonylureas was much faster in acidic or basic media than under neutral conditions. Identification of hydrolytic products by liquid chromatography-mass spectrometry (LC-MS) suggested that both PE and HM were subject to cleavage and contraction of the sulfonylurea bridge. The hydrolysis rate of HM was significantly higher than that of PE in alkaline solutions, despite their structural similarity. A chlorine substitution on HM's pyrazole ring makes HM more susceptible to bridge contraction than PE under basic conditions. The hydrolysis of HM and PE was relatively unaffected by the presence of cyclic oligosaccharides (cyclodextrins), indicating that natural OH-containing organic compounds occurring in aquatic environments may have little impact on the transformation of these sulfonylurea herbicides.

Full text of DOI:10.1021/jf800899e

J. Agric. Food Chem. 2008, 56, 7367–7372 7367
Transformation Kinetics and Mechanism of the
Sulfonylurea Herbicides Pyrazosulfuron Ethyl and
Halosulfuron Methyl in Aqueous Solutions
§
WEI ZHENG,†,‡ SCOTT R. YATES,*,‡ AND SHARON K. PAPIERNIK
Illinois Waste Management and Research Center, 1 East Hazelwood Driver, Champaign,
Illinois 61820, Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture,
450 West Big Springs Road, Riverside, California 92507, and North Central Soil Conservation
Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture,
803 Iowa Avenue, Morris, Minnesota 56267
Pyrazosulfuron ethyl (PE) and halosulfuron methyl (HM) are two new highly active sulfonylurea
herbicides that have been widely used for weed control in a variety of vegetables and other crops.
These two herbicides have similar molecular structures, differing only in the substitutions on the
pyrazole ring. Chemical hydrolysis is a primary process affecting the environmental fate of sulfonylurea
pesticides. The hydrolytic transformation kinetics of PE and HM were investigated as a function of
pH and temperature. For both herbicides, the hydrolysis rate was pH-dependent and increased with
increasing temperature. The hydrolysis of both sulfonylureas was much faster in acidic or basic media
than under neutral conditions. Identification of hydrolytic products by liquid chromatography-mass
spectrometry (LC-MS) suggested that both PE and HM were subject to cleavage and contraction of
the sulfonylurea bridge. The hydrolysis rate of HM was significantly higher than that of PE in alkaline
solutions, despite their structural similarity. A chlorine substitution on HM’s pyrazole ring makes HM
more susceptible to bridge contraction than PE under basic conditions. The hydrolysis of HM and
PE was relatively unaffected by the presence of cyclic oligosaccharides (cyclodextrins), indicating
that natural OH-containing organic compounds occurring in aquatic environments may have little
impact on the transformation of these sulfonylurea herbicides.
KEYWORDS: Sulfonylurea; pyrazosulfuron ethyl; halosulfuron methyl; cyclodextrin; hydrolysis
INTRODUCTION
runoff and leaching after their application (3, 4). Because of
their high herbicidal activity, some crops (e.g., legumes and
pastures) are highly sensitivite to trace-level residues of sulfo-
nylurea herbicides in soils (2). An increased understanding of
the environmental fate and behavior of sulfonylureas is impera-
tive to reduce their potential negative effects on agronomic
systems.
Sulfonylureas, a modern class of herbicides, are extensively
used to control a wide range of weeds in many crops. These
herbicides exhibit a simple but effective biological mode of
action through inhibiting acetolactate synthase, a key enzyme
that participates in the protein synthesis of plants. Because of
the high herbicidal activity of sulfonylureas, they are effective
at application rates as low as g ha-1 (1), which are about
10-1000 times less than those of conventional herbicides such
as triazines and chloroacetanilides. Also, sulfonylureas exhibit
extremely low acute and chronic mammalian toxicities in
comparison with most other herbicides (2). Therefore, the use
of sulfonylurea herbicides is increasing steadily worldwide.
The occurrence of sulfonylurea herbicides in aquatic environ-
ments is receiving public attention (3). Residues of sulfonylureas
have been detected in surface water and groundwater due to
Microbial degradation and chemical hydrolysis are two
primary transformation mechanisms of sulfonylureas in the
environment. Photolysis is a minor decomposition process for
sulfonylureas and only occurs under UV light (5). The chemical
hydrolysis kinetics and pathways of many sulfonylurea herbi-
cides in soils and water have been previously reviewed by
Sarmah and Sabadie (6). It has been well-demonstrated that the
hydrolysis of sulfonylureas is pH- and temperature-depen-
dent (2, 6–10). The dissipation of sulfonylureas is usually more
rapid in acidic than in neutral or weakly basic conditions (9–11).
The primary pathway of hydrolysis under mildly acidic condi-
tions is the cleavage of the sulfonylurea bridge, producing CO2
and the corresponding sulfonamide and heterocyclic amine (6–8).
In addition to the cleavage of the sulfonylurea bridge, some
sulfonylureas may undergo a base-catalyzed hydrolysis through
* To whom correspondence should be addressed. Tel: 951-369-4803.
Fax: 951-342-4964. E-mail: scott.yates@ars.usda.gov.
† Illinois Waste Management and Research Center.
‡ Salinity Laboratory, U.S. Department of Agriculture.
§ North Central Soil Conservation Research Laboratory, U.S. Depart-
ment of Agriculture.
10.1021/jf800899e CCC: $40.75  2008 American Chemical Society
Published on Web 07/24/2008

7368 J. Agric. Food Chem., Vol. 56, No. 16, 2008
Zheng et al.
Figure 1. Molecular structures of PE and HM and their hydrolysis pathways.
MATERIALS AND METHODS
bridge contraction and rearrangement, which results in high
hydrolysis rates in alkaline conditions (6, 12). Under strong
alkaline conditions, some sulfonylureas with ester groups are
also subject to ester hydrolysis to form corresponding acids (6, 9).
A variety of minerals and organic materials usually exist in
natural soil and aquatic systems. Minerals have been well-
documented to affect the hydrolysis of sulfonylureas (6, 13).
Organic matters in aquatic environments include polysaccharides
and humic substances that contain a large amount of phenolic
hydroxyl groups. The presence of these polyhydroxy compounds
may induce an alcoholysis reaction (14) and thereby alter the
environmental fates of pesticides. There are few reports on the
effects of these natural hydroxyl compounds on sulfonylurea
hydrolysis (15). Cyclodextrins are cyclic oligosaccharides
consisting of six or more cyclically linked glucopyranose units
with numerous primary and secondary hydroxyl groups. Previ-
ous studies have shown that cyclodextrins may catalyze the
hydrolysis of the pesticides malathion (16) or prevent rimsul-
furon hydrolysis and photolysis (17).
Pyrazosulfuron ethyl (PE) and halosulfuron methyl (HM) are
two relatively new postemergence sulfonylurea herbicides. They
share the same basic structure (Figure 1): a pyrazole ring linked
to an identical pyrimidine through a sulfonylurea bridge. The
only structural distinction of these two sulfonylurea herbicides
lies in the substitutions on the pyrazole ring. This class of
sulfonylureas does not include a triazinic and pyridinic ring (6).
Therefore, some common degradation pathways occurring on
sulfonylureas, such as O- and N-dealkylation of the group on
the triazine ring or triazine ring opening to form a triuret (18, 19),
would not be expected for PE and HM. Information pertain-
ing to the metabolism or transformation of these two novel
herbicides in the environment is poorly documented (20). To
date, very few studies are available on their hydrolysis kinetics
and mechanisms, especially for PE. Moreover, their hydrolysis
products have not been fully investigated.
Chemicals. Standards of PE {ethyl 5-[(4,6-dimethoxypyrimidin-2-
ylcarbamoyl)sulfamoyl]-1-methylpyrazole-4-carboxylate, purity 98%}
and HM [methyl 3-chloro-5-(4,6-dimethoxypyrimidin-2-ylcarbamoyl-
sulfamoyl)-1-methyl-pyrazole-4-carboxylate, purity 99%] were pur-
chased from Chem Service (West Chester, PA). Stock solutions of these
two sulfonylurea herbicides were prepared in acetonitrile. R-Cyclo-
dextrin and ꢀ-cyclodextrin were obtained from Sigma-Aldrich (St.
Louis, MO). All chemicals were used as received.
Experimental Systems. All aqueous solutions were prepared using
high-purity deionized water (E-pure, Barnstead, Dubuque, IA). To avoid
microbial degradation, all glassware and solutions were autoclaved prior
to use.
The effect of pH on the kinetics of PE and HM hydrolysis was
determined in aqueous buffer solutions of different pH values at 25 (
0.5 °C. Aqueous buffer solutions (pH 3-10) were prepared according
to ref 21, using KCl and HCl for pH 3, acetate buffer for pH 5,
phosphate buffer for pH 7, and borate buffer for pH 9 and 10. Stock
solutions containing 1.0 mM PE or HM were spiked to buffer solutions
in serum bottles, yielding initial herbicide concentrations of 50 µM.
Bottles were capped with Teflon-faced butyl rubber septa and incubated
at 25 ( 0.5 °C in the dark. At regular time intervals, aliquots were
removed from each bottle, adjusted pH to 7, and stored at -20 °C
until analysis via high-performance liquid chromatography (HPLC).
The effect of temperature on the rate of hydrolysis of PE and HM
was determined in sterile aqueous buffer solutions at pH 3 and 9. The
kinetic experiments were initiated by spiking stock solution of PE or
HM to buffer solutions to generate initial herbicide concentrations of
50 µM and were then incubated at 15, 25, 35, and 45 °C in the dark.
Aliquots were periodically sampled using the procedure described
above. The activation energies of hydrolysis for PE and HM were
calculated using the data obtained from this experiment.
To investigate the effect of cyclodextrins on sulfonylurea hydrolysis,
PE and HM stock solutions were added into deionized water and pH
7 buffer solutions containing 1.0 or 5.0 mM cyclodextrins, respectively.
The initial concentrations of herbicides were 50 µM. After 7 days of
incubation in the dark at 25 ( 0.5 °C, the solutions were sampled as
described above and analyzed by HPLC. Control experiments were
concurrently performed in deionized water and buffer solution contain-
ing 50 µM PE or HM without cyclodextrin.
HPLC and LC/MS Analysis. The disappearance of PE and HM
and appearance of their hydrolysis products were analyzed using an
Agilent 1100 series HPLC/diode array detector (DAD) and LC/mass
selective detector (MSD) equipped with an electrospray ionization (ESI)
source (Agilent Technologies, Palo Alto, CA). Separation for HPLC/
DAD analysis was performed using an Eclipse C18 column (250 mm
The objectives of this study are (i) to comprehensively
investigate the stability of the herbicides PE and HM in aqueous
solutions under different hydrolytic conditions including dif-
ferent pH values and temperatures, (ii) to characterize their
hydrolysis mechanisms and pathways, (iii) to compare the
distinct chemical hydrolysis processes of these two sulfony-
lureas, and (iv) to assess the potential effect of cyclodextrins
on the transformation of PE and HM in aquatic environments.

Transformation of Two Sulfonylureas in Aqueous Solutions
J. Agric. Food Chem., Vol. 56, No. 16, 2008 7369
Figure 3. Hydrolysis rate constants of PE and HM in aqueous buffer
solutions as a function of pH. Error bars represent standard deviations of
means (triplicate samples).
Generally, the hydrolysis of pesticides is treated as a pseudo-
first-order reaction (22). In the experiment, a pseudo-first-order
kinetic model was applied for the sulfonylurea hydrolysis:
d[SU]/dt ) -kobs[SU]
(1)
Upon rearrangement and integration, eq 1 becomes
ln([SU]) ) -kobst + ln([SU]0)
(2)
where kobs is the observed pseudo-first-order rate constant at a
fixed pH, [SU] is the concentration of PE or HM, and [SU]0 is
the initial concentration of PE or HM. Values of kobs were
calculated from the slope of semilogarithmic plots of sulfony-
lurea concentration vs time using data collected over at least
two half-lives (for acidic and basic solutions) and over one half-
life (for neutral solutions). Actually, kobs represents the sum of
three separate reactions: the acid-catalyzed, neutral, and base-
catalyzed hydrolysis (23). At a constant pH, kobs can be
expressed as:
Figure 2. Example time courses for hydrolysis of sulfonylureas at 25 °C
and pH 3, 7, and 9 buffer solutions: (A) PE and (B) HM. C0 (50 µmol/L)
is the initial concentration of herbicide in solution. The formations of
metabolites 1-3 vs time in the aqueous solution at pH 9 are presented
as relative values (chromatographic peak area for metabolite/peak area
of herbicide at the initial concentration, C0; the detector wavelength was
242 nm). Error bars represent standard deviations of triplicate samples.
× 4.6 mm i.d.; particle size, 5 µm). The mobile phase consisted of
acetonitrile/water with 0.1% trifluoroacetic acid (60:40, v/v), the flow
rate was 1.0 mL min-1, and the detector wavelength was 242 nm. Under
these conditions, the retention times for PE and HM were 6.3 and 7.4
min, respectively.
kobs ) ka[H+] + kn + kb[OH-] ) ka[H+] + kn + kb(Kw ⁄ [H+])
(3)
Analyses by LC/MSD were performed using the same chromato-
graphic conditions described above, except that the flow rate was 0.8
mL min-1. LC/MS total ion current (TIC) chromatograms were
recorded between m/z 50 and 500 at a rate of 2 scans per second.
Positive polarity ionization mode was operated to obtain mass spectra
for the identification of hydrolysis products of PE and HM. The
electrospray source parameters were optimized by infusion of analyte
standard solutions. The operating conditions for ESI were as follows:
capillary voltage, 4000 V for positive mode; drying gas (nitrogen) flow
rate, 10 L min-1 at 350 °C; and nebulizer gas pressure, 60 psi.
where ka, kn, and kb represent the rate constants for acid-
catalyzed, neutral, and base-catalyzed hydrolysis reactions and
Kw is the equilibrium constant for the dissociation of pure water.
To better demonstrate pH effects on hydrolysis, the logarithm
of hydrolysis rate constants (kobs) of PE and HM was plotted
against the pH values (Figure 3). The hydrolytic dissipation of
these two herbicides shows a similar tendency in the studied
pH range (pH 3-10). The hydrolysis rates of both PE and HM
decreased significantly from pH 3 to 7, then substantially
increased at pH > 7. The hydrolysis half-lives of PE and HM
at different pH values are available in the Supporting Informa-
tion. The reaction rate constants for the acid-catalyzed, neutral,
and base-catalyzed hydrolyses (ka, kn, and kb) of PE and HM
were estimated using nonlinear regression according to eq 3
(Table 1). The observed hydrolysis rate constants (kobs) from
different pH buffer solutions were well-described by eq 3 (r2
> 0.99). These model-calculated kinetic parameters varied
widely, which indicates that acid-catalyzed, neutral, and
base-catalyzed hydrolyses of PE and HM are three totally
different reactions resulting from different degradation mech-
anisms (23).
RESULTS AND DISCUSSION
Acid- and Base-Catalyzed Hydrolysis Kinetics. Initial ex-
periments focused on the hydrolysis kinetics of PE and HM in
aqueous solutions of different pH values. Example time courses
for hydrolysis of these two herbicides at pH 3, 7, and 9 are
depicted in Figure 2, which represent their hydrolytic trans-
formations in acidic, neutral, and basic media. As shown in
Figure 2, the dissipation of PE and HM was slower at neutral
pH than at acidic and basic pH. This result suggests that these
two sulfonylureas are more susceptible to chemical hydrolysis
in acidic and basic solutions than in neutral media.

7370 J. Agric. Food Chem., Vol. 56, No. 16, 2008
Zheng et al.
Table 1. Rate Constants for Acid-Catalyzed, Neutral-, and Based-Catalyzed Hydrolyses of the Sulfonylurea Herbicides PE and HM in Buffer Solutions (pH
3-10) at 25 °C
hydrolysis rate constants
herbicides
acid-catalyzed (ka) (M-1 h-1
)
neutral (kn) (h-1
)
based-catalyzed (kb) (M-1 h-1
)
PE
HM
1.75 × 101 ( 0.03 × 101
9.01 × 10-4 ( 1.37 × 10-4
1.06 × 102 ( 0.03 × 102
6.90 ( 3.35
2.48 × 10-3 ( 1.72 × 10-3
3.17 × 103 ( 0.31 × 103
at typical environmental temperatures (23). In these experiments,
each 10 °C increase in temperature increased the average rate
of PE hydrolysis by 2.4 times at pH 3 and 4.5 times at pH 9.
For HM, the average rate of hydrolysis increased by 2.9 (pH 3)
and 3.2 times (pH 9) with each 10 °C increase in tem-
perature.
Hydrolysis Mechanism and Pathway. To elucidate the
pathways of PE and HM hydrolysis, aliquots of hydrolysis
solutions at different pH values were periodically withdrawn
and monitored by HPLC/DAD and LC/MS. Representative
HPLC/DAD chromatograms after the hydrolysis of PE and HM
in acidic and basic solutions are shown in the Supporting
Information (Figures 1S and 3S). Peak identification was
performed by LC/MS. Prior to the identification of the hydrolysis
products, PE and HM standards were run to verify chromato-
graphic separation and mass spectra with desired fragmentations.
In these hydrolysis experiments, three products of each sulfo-
nylurea were characterized based on their mass spectra and
retention times. The mass spectra (Figure 2S and 4S) and a
description of the interpretation of fragmentation used to identify
PE, HM, and their hydrolysis products are available in the
Supporting Information.
For PE, two primary products were detected and identified
in acidic and neutral solutions: 1-methyl-4-ethylcarboxylate
pyrazolesulfonamide (metabolite 1) and 2-amino-4,6-dimethoxy-
pyrimidine (metabolite 2) (Figure 1). The latter was also
detected as a hydrolysis product for other sulfonylureas with a
pyrimidine ring, for example, azimsulfuron (7) and rimsulfuron
(24). The concentrations of these two products increased
gradually with the dissipation of the corresponding parent
compound. Like all sulfonylureas, the cleavage of the sulfony-
lurea bridge was a predominant hydrolysis pathway for PE,
especially in acidic water and soils. In this case, the hydrolysis
process occurs through an acid catalysis (AAC2) mechanism
involving a protonation of the carbonyl oxygen initially (22),
producing carbon dioxide, the pyrazolesulfonamide, and pyri-
midine amine (Figure 1). The mechanism differs from the
normal behavior observed for carbonate derivatives, which
exhibit base catalysis (BAC2) (22). Generally, sulfonylureas are
weak acids and have pKa values ranging from 3 to 5. The
sulfonyl group may significantly enhance the acidity of the
proton on the adjacent nitrogen atom. Deprotonation of the sul-
fonylurea bridge results in a distribution of the negative charge
throughout the sulfonylurea moiety (Figure 1), which largely
deactivates the direct nucleophilic attack of H2O or OH to the
carbonyl group. The transformation process pertaining to the
cleavage of the sulfonylurea bridge is relatively slow under
neutral and basic conditions as compared to acidic media, which
is similar to previous observations of the hydrolysis of azim-
sulfuron (7).
Figure 4. Example time courses for hydrolysis of sulfonylureas in pH 3
buffer solutions at different temperatures: (A) PE and (B) HM. Error bars
represent standard deviations of triplicate samples.
Temperature Effects on Hydrolysis. Temperature generally
plays an important role in hydrolysis of pesticides. Example
time courses for hydrolysis (pH 3) of PE and HM at different
temperatures are shown in Figure 4. Hydrolysis of these
herbicides consistently increased with increasing temperature
at a fixed pH. Hydrolysis of PE and HM followed pseudo-first-
order kinetics for each temperature with correlation coefficients
(r) of more than 0.90 (Table 2).
The Arrhenius equation offers a simple and direct method to
characterize the effects of temperature into a mathematical
relationship:
ln kobs ) ln A - Ea ⁄ RT
(4)
where A is called the preexponential factor, Ea is the activation
energy, R is the universal gas constant, and T is the absolute
temperature. The activation energies of PE and HM were
calculated by fitting the hydrolysis rates (kobs) to the Arrhenius
equation (Table 2). The magnitude of activation energy corre-
sponds to the effect of temperature on hydrolysis rate constants.
The calculated values of the activation energy (55-110 kJ
mol-1) for PE and HM at pH 3 and 9 indicate that an increase
of 10 °C accelerates the rate of hydrolysis by a factor of 2.2-4.7
In addition to the cleavage of the sulfonylurea bridge, another
transformation mechanism of PE is the contraction of the
sulfonylurea bridge in alkaline solutions, yielding metabolite 3
(Figure 1). This contraction mechanism is suggested to proceed
through an intramolecular SNAr reaction (25). Unlike the
cleavage of sulfonylurea bridge, the contraction only occurs in

Transformation of Two Sulfonylureas in Aqueous Solutions
J. Agric. Food Chem., Vol. 56, No. 16, 2008 7371
Table 2. Hydrolysis Pseudo-First-Order Rate Constants (h-1) and Activation Energies (kJ mol-1) of Sulfonylurea Herbicides PE and HM in Buffer Solutions
(pH 3 and 9) at Different Temperatures
PE
HM
temperature
pH 3
pH 9
pH 3
pH 9
15 °C
1.76 × 10-2 ( 0.19 × 10-2
1.99 × 10-2 ( 0.20 × 10-2
4.27 × 10-2 ( 0.32 × 10-2
1.69 × 10-1 ( 0.11 × 10-1
56.8 ( 16.2
3.57 × 10-4 ( 0.19 × 10-4
1.95 × 10-3 ( 0.01 × 10-3
1.04 × 10-2 ( 0.01 × 10-2
2.91 × 10-2 ( 0.04 × 10-2
113.7 ( 6.7
4.24 × 10-3 ( 0.75 × 10-3
9.40 × 10-3 ( 1.78 × 10-3
3.50 × 10-2 ( 0.18 × 10-2
9.39 × 10-2 ( 0.70 × 10-2
80.7 ( 6.1
8.22 × 10-3 ( 0.16 × 10-3
3.76 × 10-2 ( 0.06 × 10-2
1.26 × 10-1 ( 0.06 × 10-1
2.16 × 10-1 ( 0.24 × 10-1
84.4 ( 10.0
25 °C
35 °C
45 °C
activation energy
some sulfonylureas, indicating that this transformation is
structurally dependent. Example time courses for the formation
of three hydrolysis products of PE in the pH 9 buffer solution
are represented in Figure 2A. The decrease in concentration of
PE was accompanied by a simultaneous increase of all three
products at pH 9, suggesting that the cleavage and contraction
of PE’s sulfonylurea bridge occur simultaneously in alkaline
solutions.
Similarly, a cleavage of HM’s sulfonylurea bridge was
observed in acid and neutral aqueous buffer solutions (pH e
7), producing carbon dioxide and the corresponding pyrazole-
sulfonamide and pyrimidine amine (metabolites 1 and 2 in
Figure 1). Unlike PE, however, only the contraction product
(metabolite 3) was detected shortly after HM was spiked to basic
solutions (Figure 2B and Figure S3 of the Supporting Informa-
tion). This result suggests that the cleavage of HM’s sulfonylurea
bridge is insignificant and that the bridge contraction is a
predominant hydrolysis mechanism for HM in basic solution.
The maximum concentration of the contraction product was
observed when HM had completely disappeared (Figure 2B).
The concentration of the contraction product then decreased
gradually, with a concurrent gradual increase in the pyrimidine
amine (metabolite 2). This result indicates that the contraction
product of HM is not stable and it may decompose to its
corresponding products, for example, pyrimidine amine, via
further hydrolysis (Figure 1).
environmental effects of these daughter products should be
considered to provide a better understanding of the environ-
mental effects of these herbicides.
Effect of Cyclodextrins on Hydrolysis. A large number of
polyhydroxy molecules such as polysaccharides, polyphenols,
and humic substances exist in aqueous and soil environments.
These components may react with sulfonylureas to form
transformation products not observed in pure water (14) and
thereby impact the herbicide hydrolysis. The effect of cyclo-
dextrin on the hydrolysis of PE and HM in deionized water
and buffer solutions is summarized in Figure 5. Results show
that the presence of either R- or ꢀ-cyclodextrin did not promote
or decelerate the dissipation of either PE or HM in buffer
solutions. In addition, no extra products were detected in the
experimental system, suggesting that no nonhydrolysis reactions
for PE and HM occurred in pH 7 buffer solution.
In unbuffered deionized water, the dissipation of PE was
relatively rapid in the presence of ꢀ-cyclodextrin (Figure 5),
which may be attributable to a slight decrease in pH after
the addition of ꢀ-cyclodextrin. Only hydrolysis products were
detected in the control solution (without cyclodextrin addi-
Although PE and HM have very similar structures, their
hydrolysis rates in alkaline solutions are markedly different
(Figure 3). The rate constant of base-catalyzed hydrolysis for
HM is approximately 30-fold greater than that for PE (Table
1). This difference may be attributable to a minor structural
distinction in the pyrazole ring. Pyrazole is an electron-poor
heterocycle (electrophilic group), which may lead to the bridge
contraction of sulfonylureas under basic conditions via a five-
member transition state (7, 25). The sulfonylurea HM contains
a chlorine on the pyrazole ring, which is a strong electron-
withdrawing group that enhances the electrophilicity of HM’s
pyrazole ring. The increase in electrophilicity will favor the
occurrence of the five-member transition state in the basic
hydrolytic process of HM and thereby facilitate its bridge
contraction. The difference in substitutions on the pyrazole ring
in PE (H rather than Cl) makes PE less electrophilic and less
susceptible to bridge contraction than HM. Therefore, the
hydrolysis rates of HM were much higher than those of PE in
alkaline solutions.
Saponification reactions often occur in some sulfonylureas
with ester groups under alkaline conditions (6, 26). Although
the saponification reactions related to the ester groups present
in PE and HM were not observed in our experimental systems
(pH e 10), the hydrolysis pathway can not be neglected,
especially in strong alkaline solutions (pH > 10) (6, 9).
Collectively, both sulfonylurea herbicides may decompose to
small molecular weight products through chemical hydrolysis
reactions (Figure 1). The herbicidal activity and potential
Figure 5. Effect of cyclodextrins on the hydrolysis of sulfonylureas after
7 days of incubation at 25 ( 0.5 °C: (A) PE and (B) HM. C0 (50 µmol/L)
is the initial concentration of two herbicides in aqueous solutions.

7372 J. Agric. Food Chem., Vol. 56, No. 16, 2008
Zheng et al.
(11) Hemmanda, S.; Calmon, M.; Calmon, J. P. Kinetics and hydrolysis
mechanism of chlorsulfuron and metsulfuron methyl. Pestic. Sci.
1994, 40, 71–76.
(12) Schneiders, G. E.; Koeppe, M. K.; Naidu, M. V.; Horne, P.;
Brown, A. M.; Mucha, C. F. Fate of rimsulfuron in the environ-
ment. J. Agric. Food Chem. 1993, 41, 2404–2410.
(13) Sabadie, J. Nicosulfuron: Alcoholysis, chemical hydrolysis, and
degradation on various minerals. J. Agric. Food Chem. 2002, 50,
526–531.
(14) Sabadie, J. Behavior of four sulfonylurea herbicides in the presence
of hydroxyl compounds. J. Agric. Food Chem. 2000, 48, 4752–
4756.
(15) Sabadie, J. Degradation of bensulfuron-methyl on various minerals
and humic acids. Weed Res. 1997, 37, 411–418.
tion) and in the sulfonylurea unbuffered solutions with R-
or ꢀ-cyclodextrin. These results suggested that cyclodextrin
would not react with the sulfonylureas in unbuffered deion-
ized water.
Cyclodextrin can reversibly host a variety of guest molecules
to form inclusion complexes by noncovalent interaction. There-
fore, there is interest in incorporating cyclodextrin into insoluble
materials, for example, zeolite (17) and silica (27), to adsorb
pesticide residues for the treatment of drinking water sources.
Whereas cyclodextrin has been reported to affect the hydrolysis
of some pesticides (16, 17), our results suggest that cyclodextrin
is not expected to react with PE and HM in aqueous solutions,
providing very useful information for the further evaluation of
using cyclodextrin-coated sorbents to remove pesticide residues
from contaminated water.
(16) Zhang, A. P.; Luo, F.; Chen, S. W.; Liu, W. P. Effects of
cyclodextrins on hydrolysis of malathion. J. EnViron. Sci.-China
2006, 18, 572–576.
Supporting Information Available: Hydrolysis half-lives of
PE and HM at different pH values, representative HPLC
chromatograms of PE and HM after hydrolysis, electrospray
LC/MS mass spectra, and a description concerning identification
of hydrolysis products. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Liguori, A.; D’Auria, M.; Emanuele, L.; Scrano, L.; Lelario, F.;
Bufo, S. A. Reactivity of rimsulfuron in newly formed inclusion
combination by using cyclodextrin and zeolite. Int. J. EnViron.
Anal. Chem. 2007, 87, 1043–1052.
(18) Braschi, H.; Calamai, L.; Cremonini, M. A.; Fusi, P.; Gessa, C.;
Pantani, O.; Pusino, A. Kinetics and hydrolysis mechanism of
triasulfuron. J. Agric. Food Chem. 1997, 45, 4495–4499.
(19) Bray, L. D.; Heard, N. E.; Overman, M. C.; Vargo, J. D.; King,
D. L.; Lawrence, L. J.; Phelps, A. W. Hydrolysis of prosulfuron
at pH 5: Evidence for a resonance-stabilized triazine cleavage
product. Pestic. Sci. 1997, 51, 56–64.
(20) Dubelman, A. M.; Solsten, T. R.; Fujiwara, H.; Mehrsheikh, A.
Metabolism of halosulfuron-methyl by corn and wheat. J. Agric.
Food Chem. 1997, 45, 2314–2321.
(21) Dean, J. A. Ed., Analytical Chemistry Handbook; McGraw-Hill
Inc.: New York, 1995.
(22) Larson, R. A.; Weber, E. J. Reaction Mechanisms in EnViron-
mental Organic Chemistry; Lewis: Boca Raton, FL, 1994.
(23) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M.
EnVironmental Organic Chemistry; John Wiley & Sons: Hoboken,
NJ, 2003.
(24) Schneiders, G. E.; Koeppe, M. K.; Naidu, M. V.; Horne, P.;
Brown, A. M.; Mucha, C. F. Fate of rimsulfuron in the environ-
ment. J. Agric. Food Chem. 1993, 41, 2404–2410.
(25) Galeazzi, R.; Marucchini, C.; Orena, M.; Porzi, G. The cleavage
of sulfonylurea herbicide rimsulfuron under basic conditions: a
computational investigation. Heterocycles 2000, 53, 2517–2526.
(26) Cambon, J. P.; Bastide, J. Hydrolysis kinetics of thifensulfuron
methyl in aqueous buffer solutions. J. Agric. Food Chem. 1996,
44, 333–337.
LITERATURE CITED
(1) Hay, J. V. Chemistry of sulfonylurea herbicides. Pestic. Sci. 1990,
29, 247–261.
(2) Brown, H. M. Mode of action, crop selectivity, and soil relations
of the sulfonylurea herbicides. Pestic. Sci. 1990, 29, 263–281.
(3) Battaglin, W. A.; Furlong, E. T.; Burkhardt, M. R.; Peter, C. J.
Occurrence of sulfonylurea, sulfonamide, imidazolinone, and
other herbicides in rivers, reservoirs and ground water in the
Midwestern United States, 1998. Sci. Total EnViron. 2000, 248,
123–133.
(4) Okamoto, Y.; Fisher, R. L.; Armbrust, K. L.; Peter, C. J. Surface
water monitoring survey for bensulfuron methyl applied in paddy
fields. Pestic. Sci. 1998, 23, 235–240.
(5) Morrica, P.; Fidente, P.; Seccia, S. Identification of photoproducts
from imazosulfuron by HPLC. Biomed. Chromatogr. 2004, 18,
450–456.
(6) Sarmah, A. K.; Sabadie, J. Hydrolysis of sulfonylurea herbicides
in soils and aqueous solutions: A review. J. Agric. Food Chem.
2002, 50, 6253–6265.
(7) Boschin, G.; D’Agostina, A.; Antonioni, C.; Locati, D.; Arnoldi,
A. Hydrolytic degradation of azimsulfuron, a sulfonylurea her-
bicide. Chemosphere 2007, 68, 1312–1317.
(8) Saha, S.; Kulshrestha, G. Degradation of sulfosulfuron, a sulfo-
nylurea herbicide, as influenced by abiotic factors. J. Agric. Food
Chem. 2002, 50, 4572–4575.
(9) Sarmah, A. K.; Kookana, R. S.; Duffy, M. J.; Alston, A. M.;
Harch, B. D. Hydrolysis of triasulfuron, metsulfuron-methyl and
chlorsulfuron in alkaline soil and aqueous solutions. Pest Manage.
Sci. 2000, 56, 463–471.
(10) Dinelli, G.; Vicari, A.; Bonetti, A.; Catizone, P. Hydrolytic
dissipation of four sulfonylurea herbicides. J. Agric. Food Chem.
1997, 45, 1940–1945.
(27) Sawicki, R.; Mercier, L. Evaluation of mesoporous cyclodextrin-
silica nanocomposites for the removal of pesticides from aqueous
media. EnViron. Sci. Technol. 2006, 40, 1979–1983.
Received for review March 22, 2008. Revised manuscript received June
14, 2008. Accepted June 16, 2008.
JF800899E