pH dependent hydrolysis of 2-hydroxypropyl methanethiolsulfonate

Article abstract of DOI:10.1021/jf950722n

The hydrolysis of 2-hydroxypropyl methanethiolsulfonate (HPMTS), an industrial water treatment microbicide, was investigated at 25°C and at three pH values (pH 5, 7, and 9). Radiolabeled (¹⁴C) HPMTS at a concentration of 100 mg/L was used to determine rate constants and to identify hydrolytic degradation products. The hydrolysis at all three pHs followed pseudo-first-order kinetics with rate constants of 5.8 × 10^-4, 7.6 × 10^-2, and 7.3. h^-1 at pH 5, 7, and 9, respectively. Two major radiolabeled hydrolytic degradation products were observed at all three pH values and were identified to be bis(2-hydroxyisopropyl) disulfide and bis(2-hydroxypropyl) disulfide by particle beam LC/MS. On the basis of the observed degradation products, a hydrolytic degradation pathway for HPMTS using propylene epoxide as a reactive intermediate was postulated.

Full text of DOI:10.1021/jf950722n

1896
J. Agric. Food Chem. 1996, 44, 1896−1899
p H Dep en d en t Hyd r olysis of 2-Hyd r oxyp r op yl
Meth a n eth iolsu lfon a te
J ohn Mao,*,† Carl F. Watson,‡ and Marjorie E. Dix†
Springborn Laboratories, Inc., Wareham, Massachusetts 02571, and Buckman Laboratories
International, Inc., Memphis, Tennessee 38108
The hydrolysis of 2-hydroxypropyl methanethiolsulfonate (HPMTS), an industrial water treatment
microbicide, was investigated at 25 °C and at three pH values (pH 5, 7, and 9). Radiolabeled (14C)
HPMTS at a concentration of 100 mg/L was used to determine rate constants and to identify
hydrolytic degradation products. The hydrolysis at all three pHs followed pseudo-first-order kinetics
with rate constants of 5.8 × 10-4, 7.6 × 10-2, and 7.3 h-1 at pH 5, 7, and 9, respectively. Two major
radiolabeled hydrolytic degradation products were observed at all three pH values and were identified
to be bis(2-hydroxyisopropyl) disulfide and bis(2-hydroxypropyl) disulfide by particle beam LC/MS.
On the basis of the observed degradation products, a hydrolytic degradation pathway for HPMTS
using propylene epoxide as a reactive intermediate was postulated.
Keyw or d s: 2-Hydroxypropyl methanethiolsulfonate; HPMTS; hydrolysis; kinetics
INTRODUCTION
Hydrolysis is one of the most important naturally
occurring reactions in the environment and conse-
quently represents one of the most important degrada-
tion pathways (Mabey and Mill, 1978; U.S. EPA, 1982).
The knowledge of hydrolytic degradation pathways and
kinetics over the normal pH ranges of the aquatic
environment is critical for predicting the fate and
transport mechanisms of a chemical. Because of its
compatibility with aqueous samples, HPLC has been
used extensively to monitor hydrolytic degradations of
pesticides and industrial chemicals (Szeto, 1993; Smith
and Aubin, 1993; Szeto et al., 1989). The objective of
this study was to determine the kinetics (rate constants
and half-lives) of hydrolysis of HPMTS in water at three
different pH values, so that the importance of hydrolysis
as a transformation route could be better assessed with
regard to the overall potential for persistence or elimi-
nation of HPMTS in the environment. Reversed phase
HPLC and ion exclusion HPLC with radiometric detec-
tion were used to monitor the disappearance of [14C]-
HPMTS and the formation of radiolabeled hydrolytic
products. Particle beam LC/MS was used for positive
structural identification of hydrolytic products.
MATERIALS AND METHODS
Ch em ica ls. Nonradiolabeled HPMTS, an off-white solid
(CAS Registry No. 41206-16-0, molecular weight 170, chemical
purity 96.5%), was obtained from Buckman Laboratories, Inc.,
F igu r e 1. Representative HPLC radiochromatograms (A,
Memphis, TN. [14C]HPMTS, specific activity of 30.0 mCi/mmol
reversed phase; B, ion exclusion) of a pH 9 test sample showing
and radiochemical purity of 91% (by HPLC), was obtained from
the separation of HPMTS and its hydrolytic degradation
Sigma Chemical Co., St. Louis, MO. Analytical grade 1-mer-
products.
capto-2-propanol was obtained from Aldrich Chemical Co., Inc.,
Milwaukee, WI. Methyl iodide was obtained from Sigma.
Hydrogen peroxide solution (3%) was obtained from Mallinck-
rodt Specialty Chemicals Co., Paris, KY, and used to oxidize
thiol compounds. All other chemicals were of analytical or
reagent grade. Water and organic solvents were of HPLC
grade.
Ta ble 1. Ra te Con sta n ts a n d Ha lf-Lives of HP MTS
Hyd r olysis
rate constant
(h-1
correl coeff
half-life,
t1/2 (h)
total data
points
pH
)
(r2)
5
7
9
5.87 × 10-4
7.62 × 10-2
7.31
0.9367
0.9843
0.9897
1187.00
9.10
33
21
24
[14C]Bis(2-hydroxypropyl)disulfide was synthesized by add-
ing [14C]HPMTS in dilute NaOH solution at room temperature.
0.10
The reaction mixture was partitioned with ethyl acetate. The
extract was concentrated to dryness, and residues were
redissolved in acetonitrile. This sample was used “as is” for
† Springborn Laboratories, Inc.
‡ Buckman Laboratories International, Inc.
S0021-8561(95)00722-9 CCC: $12.00
© 1996 American Chemical Society
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pH Dependent Hydrolysis of HPMTS
J. Agric. Food Chem., Vol. 44, No. 7, 1996 1897
F igu r e 2. Formation and decline of HPMTS hydrolytic degradation products. (A) pH 5, replicate 1; (B) pH 7, replicate 1; (C) pH
9, replicate 1.
F igu r e 3. EI mass spectra of HPMTS (A) and bis(2-hydroxypropyl) disulfide (product 3) (B) in a pH 9 sample.
HPLC retention time determinations. The radio HPLC profile
of this sample showed a single major product (>90%). The
identity of this major product was confirmed by particle beam
LC/MS to be [14C]bis(2-hydroxypropyl) disulfide.
used to quantify HPMTS and its degradation products during
the study, while the ion exclusion system was used to confirm
the identification of hydrolytic degradation products. HPLC
instrumentation included a Waters 510 solvent pump, a
Hewlett-Packard 1050 or a Waters 710B autosampler, a
Beckman 171 or Radiomatic A-280 radioisotope detector with
liquid scintillation cells (800 µL cell for the Beckman instru-
ment and 1000 µL cell for the Radiomatic instrument), and a
Hewlett-Packard 1047A refractive index (RI) detector.
Reversed phase HPLC separation was achieved on a Phe-
nomenex Ultremex C18 (5 µm, 250 × 4.6 mm) column at
ambient temperature. Isocratic mobile phase was 25/75
acetonitrile/water (pH 3 with phosphoric acid) at 1 mL/min.
Ion exclusion HPLC was conducted using an Interaction ORH-
801 organic acid column (300 × 6.5 mm) at 35 °C. Isocratic
mobile phase was 10 mM H2SO4 at 0.8 mL/min. Eluted
radioactivity was monitored by the flow-through radiometric
detector, while nonradiolabeled reference standards were
monitored by the RI detector. The flow rate of the scintillation
cocktail (Beckman ReadyFlow III or Radiomatic Flo-Scint A)
was set at 3 times the flow rate of HPLC eluent. [14C]HPMTS
and its radiolabeled degradation products were identified by
comparison of their retention times with those of reference
standards and quantified on the basis of integrated peak areas
(dpms). To ensure that all injected radioactivity eluted off the
analytical column and was measured by the detector, HPLC
column recoveries were determined for each test sample. This
was accomplished by comparing the total detector integrated
Test P r oced u r es. The hydrolysis study was conducted at
three pH values (pH 5, 7, and 9). pH buffers were prepared
with sterilized HPLC grade water: pH 5, sodium acetate buffer
(100 mM); pH 7, potassium phosphate buffer (50 mM); and
pH 9, sodium borate buffer (35 mM). Triplicate test solutions
were prepared at each pH in 100- or 200-mL volumetric flasks.
The test concentration was 101 mg/L of HPMTS (containing
1.46% radiolabeled HPMTS). The specific activity of test
solutions was calculated to be 5719.8 dpm/µg. (Nonradio-
labeled HPMTS was used to increase the test concentration
and subsequently enabled the positive identification of hydro-
lytic degradation products via mass spectrometry.) The three
replicates of each HPMTS solution (pH 5, 7, and 9) were placed
in a circulating water bath (Brinkman Lauda RMS-20) de-
signed to maintain a constant temperature of 25 °C. At each
sampling interval, aliquots of test solution were taken from
each volumetric flask and analyzed by HPLC and liquid
scintillation counting (LSC). Aseptic techniques were used
during test setup and sampling for HPLC and LSC measure-
ments.
HP LC. Two HPLC systems were developed for analysis of
HPMTS and its hydrolytic degradation products: reversed
phase (C8 column) and ion exclusion (ORH-801 organic acid
column) chromatography. The reversed phase system was
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1898 J. Agric. Food Chem., Vol. 44, No. 7, 1996
Mao et al.
F igu r e 4. EI (A) and ammonia NCI (B) mass spectra of product 2.
radioactivity to that measured by LSC (Beckman LS 5000 TD
or LS 1801 scintillation counter).
study. For pH 5 replicates, a mean mass balance of 110
( 5% (n ) 33) was obtained. For pH 7 and 9 replicates,
mean mass balances of 96 ( 4% (n ) 21) and 95 ( 2%
(n ) 25) were obtained, respectively. These adequate
material mass balances suggested that, under the test
conditions, HPMTS and its hydrolytic degradation
products were not volatile and did not adsorb to the
glass walls of test vessels.
Kin etics of HP MTS Hyd r olysis. The hydrolysis of
HPMTS at 25 °C was found to be pseudo-first-order and
highly pH dependent. In the pH 5 test solution, the
mean HPMTS concentration declined from 89.8 to 60.8
mg/L, with a mean of 54.1% of the parent compound
remaining after a period of 30 days. In the pH 7 test
solution, the mean HPMTS concentration declined from
78.3 to 12.2 mg/L, with a mean of 12.6% of the parent
compound remaining after a period of 24.5 h. In the
LC/MS. LC/MS analysis was conducted on a Hewlett-
Packard Model 5989A MS engine equipped with a particle
beam interface. Full scan electron impact ionization (EI) mass
spectra were obtained using 70 eV of electron energy. Nega-
tive ion chemical ionization (NCI) was also used to analyze
selected samples using either methane or ammonia as reagent
gas. The ion source and analyzer temperatures were 250 and
125 °C for all analyses. The separation was achieved on a
Nucleosil C18 (5 µm, 150 × 2 mm) column at ambient temper-
ature. The mobile phase was 20/80/0.1 acetonitrile/water/
acetic acid at 0.4 mL/min.
RESULTS AND DISCUSSION
Ma ter ia l Ma ss Ba la n ce. The measurement of total
14C residue by LSC showed acceptable material mass
balance for all test replicates during the course of the
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pH Dependent Hydrolysis of HPMTS
J. Agric. Food Chem., Vol. 44, No. 7, 1996 1899
pH 9 test solution, the mean HPMTS concentration
declined from 72.7 to 12.6 mg/L, with a mean of 13.5%
of the parent compound remaining after a period of
approximately 15 min. HPLC column recoveries aver-
aged 96 ( 5%, 98 ( 7%, and 99 ( 3% for pH 5, 7, and
9 samples, respectively. The kinetics data are sum-
marized in Table 1.
Id en tifica tion of Hyd r olytic Degr a d a tion P r od -
u cts. The identification of hydrolytic degradation prod-
ucts produced in a greater than 10% yield was conducted
using both chromatographic (based on retention times)
and spectroscopic (LC/MS) techniques. Four degrada-
tion products were observed in test solutions at all three
pH values (Figure 1). The formation and decline of
these degradants are graphically presented in Figure
2. Among them, products 2 and 3 exceeded 10% of
initial HPMTS concentration and were identified. Prod-
uct 2 was a major product of HPMTS hydrolysis and
appeared to follow the hydrolysis kinetics of HPMTS.
Product 3 appeared to be a secondary reaction product
rather than a product from the initial hydrolysis.
Products 1 and 4 were impurities present in the radio-
labeled HPMTS at the start of the study, and their
concentrations increased very little with time during the
experiment. The kinetics of these two products indi-
cated that they were not hydrolytic degradation prod-
ucts of HPMTS. Since they were radiolabeled impuri-
ties, structural identifications were not attempted.
Identification of Product 3. To identify the two major
degradants (products 2 and 3), a pH 9 sample (collected
at the test termination) was extracted with ethyl
acetate. The extract (containing HPMTS and products
2 and 3) was concentrated and analyzed by HPLC and
LC/MS. Product 3 was identified to be bis(2-hydroxy-
propyl) disulfide by comparisons to the synthesized
reference standard using reversed phase HPLC, ion
exclusion HPLC, and particle beam LC/MS. EI mass
spectra of HPMTS and product 3 are shown in Figure
3.
Identification of Product 2. EI and ammonia NCI
mass spectra of product 2 are shown in Figure 4. To
determine if product 2 was a thiol, a pH 9 (replicate 2)
sample was reacted with hydrogen peroxide and the
reaction mixture was monitored using ion exclusion
chromatography. In the presence of an oxidizing agent
(i.e. H2O2) thiols would be oxidized to disulfides and
subsequently change their retention profiles. Results
from this experiment were negative, indicating that
product 2 was not a thiol. On the basis of the LC/MS
data, the chemical structure of product 2 was proposed
to be bis(2-hydroxyisopropyl) disulfide. This structure
was consistent with the retention profiles of product 2
on both chromatographic systems since bis(2-hydroxy-
isopropyl) disulfide is a primary alcohol (diol) which was
more polar than bis(2-hydroxypropyl) disulfide (a sec-
ondary alcohol).
F igu r e 5. Proposed hydrolytic degradation pathway of HP-
MTS.
form thiols, which in turn were oxidized to disulfides.
The formation of propylene epoxide and resultant thiols
would be accelerated under basic conditions, which was
consistent with the kinetics of hydrolysis. With propy-
lene epoxide as the intermediate, possible degradation
products could also include 1,2-propanediol and an
isomer of bis(2-hydroxyisopropyl) disulfide.
ACKNOWLEDGMENT
We thank Dr. Paul Fackler and Ms. Pamela Lincoln
of Springborn Laboratories, Inc., for support and in-
volvement in this project. We also thank Dr. William
Rosen of the University of Rhode Island for technical
assistance.
LITERATURE CITED
Mabey, W.; Mill, T. Critical review of hydrolysis of organic
compounds in water under environmental conditions. J .
Phys. Chem. Ref. Data 1978, 7, 383-415.
Smith, A. E.; Aubin, A. J . Degradation of [14C]amidosulfuron
in aqueous buffers and in an acidic soil. J . Agric. Food Chem.
1993, 41, 2400-2403.
Szeto, S. Y. Determination of kinetics of hydrolysis by high-
pressure liquid chromatography: application to hydrolysis
of the ethylene glycol butyl ether ester of triclopyr. J . Agric.
Food Chem. 1993, 41, 1118-1121.
Szeto, S. Y.; Burlinston, N. E.; Rathe, J . E.; Oloffs, P. C.
Kinetics of hydrolysis of the carboximide fungicide vinclo-
zolin. J . Agric. Food Chem. 1989, 37, 523-529.
U.S. EPA. Pesticide Assessment Guidelines, Subdivision N
Chemistry: Environmental Fate; Series 161-1 Hydrolysis
Studies; PB83-153973, EPA 540/9-82-021; U.S. Government
Printing Office: Washington, DC, Oct 1982.
Con clu sion s. The hydrolysis of HPMTS was deter-
mined to follow pseudo-first-order kinetics. The hydro-
lytic degradation was highly pH dependent, and kinetic
half-lives were determined to be 50 days, 9 h, and 6 min
at pH 5, 7, and 9, respectively. Two major and at least
two minor hydrolysis products were observed. The two
major degradation products were identified to be bis-
(2-hydroxypropyl) disulfide and bis(2-hydroxyisopropyl)
disulfide. On the basis of these degradation products,
a hydrolytic degradation pathway is postulated (Figure
5). HPMTS was postulated to undergo an intramolecu-
lar reaction to form a reactive intermediatespropylene
epoxide. Propylene epoxide would react with HS- to
Received for review October 31, 1995. Revised manuscript
received March 11, 1996. Accepted April 23, 1996.X This
project was funded by Buckman Laboratories International,
Inc.
J F950722N
X Abstract published in Advance ACS Abstracts, J une
1, 1996.
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