Oxyhalogen-sulfur chemistry: Kinetics and mechanism of oxidation of chemoprotectant, sodium 2-mercaptoethanesulfonate, MESNA, by acidic bromate and aqueous bromine

Article abstract of DOI:10.1021/jp411790v

The oxidation of a well-known chemoprotectant in anticancer therapies, sodium 2-mercaptoethanesulfonate, MESNA, by acidic bromate and aqueous bromine was studied in acidic medium. Stoichiometry of the reaction is: BrO ₃^- + HSCH₂CH₂SO₃H - Br^- + HO₃SCH₂CH₂SO₃H. In excess bromate conditions the stoichiometry was deduced to be: 6BrO ₃^- + 5HSCH₂CH₂SO₃H + 6H⁺ - 3Br₂ + 5HO₃SCH₂CH ₂SO₃H + 3H₂O. The direct reaction of bromine and MESNA gave a stoichiometric ratio of 3:1: 3Br₂ + HSCH ₂CH₂SO₃H + 3H₂O - HO ₃SCH₂CH₂SO₃H + 6Br^- + 6H⁺. This direct reaction is very fast; within limits of the mixing time of the stopped-flow spectrophotometer and with a bimolecular rate constant of 1.95 ± 0.05 × 10⁴ M^-1 s^-1. Despite the strong oxidizing agents utilized, there is no cleavage of the C-S bond and no sulfate production was detected. The ESI-MS data show that the reaction proceeds via a predominantly nonradical pathway of three consecutive 2-electron transfers on the sulfur center to obtain the product 1,2-ethanedisulfonic acid, a well-known medium for the delivery of psychotic drugs. Thiyl radicals were detected but the absence of autocatalytic kinetics indicated that the radical pathway was a minor oxidation route. ESI-MS data showed that the S-oxide, contrary to known behavior of organosulfur compounds, is much more stable than the sulfinic acid. In conditions where the oxidizing equivalents are limited to a 4-electron transfer to only the sulfinic acid, the products obtained are a mixture of the S-oxide and the sulfonic acid with negligible amounts of the sulfinic acid. It appears the S-oxide is the preferred conformation over the sulfenic acid since no sulfenic acids have ever been stabilized without bulky substituent groups. The overall reaction scheme could be described and modeled by a minimal network of 18 reactions in which the major oxidants are HOBr and Br₂(aq).

Full text of DOI:10.1021/jp411790v

Article
pubs.acs.org/JPCA
Oxyhalogen−Sulfur Chemistry: Kinetics and Mechanism of Oxidation
of Chemoprotectant, Sodium 2‑Mercaptoethanesulfonate, MESNA,
by Acidic Bromate and Aqueous Bromine
Risikat Ajibola Adigun,† Morgen Mhike,† Wilbes Mbiya,† Sreekanth B. Jonnalagadda,†,‡
and Reuben H. Simoyi†,‡,*
†Department of Chemistry, Portland State University, Portland, Oregon 97207-0751, United States
‡School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban 4000, South Africa
ABSTRACT: The oxidation of a well-known chemoprotectant
in anticancer therapies, sodium 2-mercaptoethanesulfonate,
MESNA, by acidic bromate and aqueous bromine was
studied in acidic medium. Stoichiometry of the reaction is:
BrO3 + HSCH2CH2SO3H → Br− + HO3SCH2CH2SO3H.
−
In excess bromate conditions the stoichiometry was deduced
to be: 6BrO3 + 5HSCH2CH2SO3H + 6H+ → 3Br2
+
−
5HO3SCH2CH2SO3H + 3H2O. The direct reaction of bromine and MESNA gave a stoichiometric ratio of 3:1: 3Br2 +
HSCH2CH2SO3H + 3H2O → HO3SCH2CH2SO3H + 6Br− + 6H+. This direct reaction is very fast; within limits of the mixing
time of the stopped-flow spectrophotometer and with a bimolecular rate constant of 1.95 0.05 × 104 M−1 s−1. Despite the strong
oxidizing agents utilized, there is no cleavage of the C−S bond and no sulfate production was detected. The ESI−MS data show
that the reaction proceeds via a predominantly nonradical pathway of three consecutive 2-electron transfers on the sulfur center to
obtain the product 1,2-ethanedisulfonic acid, a well-known medium for the delivery of psychotic drugs. Thiyl radicals were
detected but the absence of autocatalytic kinetics indicated that the radical pathway was a minor oxidation route. ESI−MS data
showed that the S-oxide, contrary to known behavior of organosulfur compounds, is much more stable than the sulfinic acid.
In conditions where the oxidizing equivalents are limited to a 4-electron transfer to only the sulfinic acid, the products obtained
are a mixture of the S-oxide and the sulfonic acid with negligible amounts of the sulfinic acid. It appears the S-oxide is the preferred
conformation over the sulfenic acid since no sulfenic acids have ever been stabilized without bulky substituent groups. The overall
reaction scheme could be described and modeled by a minimal network of 18 reactions in which the major oxidants are HOBr
and Br2(aq).
and urothelial toxicity. MESNA has also been found to prevent
contrast medium-induced nephropathy, a common complication
in radiographic procedures.8 Its clinical development and sub-
sequent application as a chemoprotectant was triggered by the
dose-limiting nephrotoxicity associated with most antineoplastic
drugs which make their administration in clinically effective
doses impossible.9−11 MESNA is of particular interest because it
ameliorates associated toxicity without reducing efficacy of the
administered anticancer drug.12−14 Successful phase II clinical
trials have already been undertaken on the prophylactic use of
Mesnex for women receiving a regimen of carboplatin and
ifosfamide for advanced breast cancer carcinomas.14 This use of
MESNA has been without a full analysis of its interactions with
biological proteins or its possible bioactivation. Bioactivation and
analysis of subsequent metabolites is necessary for the derivation
of better nephroprotectant analogues.
INTRODUCTION
■
Renal failure in cancer patients is a common problem in
oncology. This complication is frequently multifactorial in
origin.1 Several antineoplastic agents are potentially nephro-
toxic. Pre-existing renal impairment, combined with admin-
istration of potentially nephrotoxic drugs increase the risk of
nephrotoxicity.2−4 Methotrexate-related renal damage most
frequently occurs with high-dose therapy and can be avoided
by forced alkaline diuresis and administration of folinic acid.4
Cisplatin nephrotoxicity is dose-related and, in the absence of
a chemoprotectant (an agent which provides protection against
toxic effects of a chemotherapy agent), used to be considered
dose-limiting.5 These drugs form an integral part of chemo-
therapy regimens used in cancer treatment. For instance,
cisplatin is used in 50−70% of all cancer patients most often in
combination with other drug(s).6 Most anticancer drugs and
their metabolites are eliminated through the kidneys, thus,
making the kidneys susceptible to injury.7
Sulfur compounds undergo a variety of metabolic reactions,
namely: oxidations, reductions, hydrolysis, and conjugations.15
Sodium 2-mercaptoethanesulfonate, MESNA (trade name
Mesnex),8 is a low molecular weight thiol used clinically as
a chemoprotectant to inhibit anticancer drug-induced neuro-
toxicity, nephrotoxicity, hemorrhagic cystisis, myelo-suppression,
Received: December 1, 2013
Revised: February 5, 2014
Published: February 7, 2014
© 2014 American Chemical Society
2196
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Sulfur in most organic configurations is nucleophilic, and nucleo-
philic atoms are usually susceptible to metabolic activation via
oxidation.16 Thus, the oxidation of sulfur-containing compounds
represents an important aspect of sulfur metabolism. These
oxidations appear to be involved in many cellular functions,
including the reductive degradation of polypeptide hormones
and proteins, regulation of protein synthesis, maintenance of
intracellular redox potential, and protection of the cell from
oxidative damage.17 Biological oxidations of organosulfur
compounds are predominantly through S-oxygenation, in which
there is successive addition of oxygen onto the sulfur center
until it is oxidatively saturated to the +6 oxidation state.18 The
molecular basis for S-oxygenation of sulfur compounds is not
well understood since relatively few studies have addressed this
problem. Although S-oxygenation of xenobiotics by microsomes
supplemented with NADPH and oxygen has been demonstrated
repeatedly, the facile oxidations of sulfur compounds by hydrogen
peroxide and other species of reduced oxygen, such as superoxide
ion, frequently complicate interpretation of in vitro studies.16 In
general, the metabolism of chemically stable compounds depends
on enzyme catalysis, and both the microsomal cytochrome
P-450 system and the flavin-containing monooxygenases19 are
often implicated in S-oxygenations catalyzed by microsomes.
There are several biological oxidants in the physiological
environment that are capable of oxidatively bioactivating thiol-
based xenobiotics such as MESNA. These include molecular
oxygen and reduced oxygen species such as superoxide anion
radical, hydrogen peroxide and the hydroxyl radical. Other
oxidants of importance are oxyhalogens HOCl and HOBr and
molecular iodine, I2. Stimulated granulocytes produce oxidizing
agents (e.g., H2O2) and secrete granular proteins into the extra-
cellular medium which contributes to their antimicrobial,
cytotoxic and cytolytic activities.20 Each group of cells contains
a specific peroxidase which catalyzes the reactions of hydrogen
peroxide with halogens. The enzyme myeloperoxidase which is
abundant in neutrophils catalyzes the oxidation of Cl− ions by
H2O2 to yield HOCl:21
Our research group has worked extensively on elucidating
mechanisms of organosulfur reactions that are of direct
relevance to metabolic activation of drugs and other xenobiotics
with anticipation of better understanding of biological activities
and insights into the course of events at the molecular level.24
To this end, we present details of a kinetics and mechanistic
study of interaction of MESNA with acidified bromate and
aqueous bromine following observations from several studies
in our laboratories indicating HOBr to be most significant
−
oxidizing species involved in BrO3 oxidations with resultant
production of bromine (another strong oxidizing species) in
the presence of Br−.25−27 Use of nonphysiological conditions
(low pH) is necessary since bromate is inert and pH 7.4. The
use of such a strong oxidant system and in nonphysiological
concentrations ensures that the oxidation of MESNA is com-
plete in minutes, instead of the several hours it might take in
the physiological environment. This also pushes the oxidation
to the final oxidation products with no lingering intermediates
which might complicate the kinetics. This manuscript focuses on
the mechanism and detection of all possible metabolites in the
oxidation of MESNA. Reaction kinetics were derived in conditions
of equimolar to excess oxidant conditions to discourage sulfur’s
propensity of generating sulfur−sulfur polymerizations.
EXPERIMENTAL SECTION
■
Materials. Sodium bromate, perchloric acid (70−72%),
sodium bromide, bromine, sodium thiosulfate, 5,5-dimethyl-1-
pyroline-N-oxide (DMPO) (Fisher), barium chloride, sodium
2-mercaptoethanesulfonate, DCl, and D2O (Sigma-Aldrich) were
used without further purification. Bromine solutions, being
volatile, were kept capped and standardized spectrophotometri-
cally before each set of experiments. Water used for preparing
reagent solutions was obtained from a Barnstead Sybron
Corporation water purification unit capable of producing both
distilled and deionized water (Nanopure).
Methods. Kinetics of MESNA−bromate and MESNA−
bromine reactions were studied by monitoring the formation
and consumption of bromine’s isolated peak at 390 nm (ε =
142 M−1cm−1). All reactions were carried out at a constant
temperature of 25 0.5 °C and an ionic strength of 1.0 M
using sodium perchlorate. Reactions between MESNA and
bromate were monitored on a Perkin-Elmer Lambda 25 UV/vis
spectrophotometer while bromine−MESNA reactions were
studied using a Hi-Tech Scientific SF-61 stopped-flow
spectrophotometer. Mass spectra of dynamic and product
solutions were taken on a Thermo Scientific LTQ-Orbitrap
Discovery mass spectrometer (San Jose, CA) equipped with
electrospray ionization source operated in both the positive and
negative modes.
Stoichiometric Determinations. For determination of
excess oxidizing power at the end of a reaction, an iodometric
titration method was used in which varying amounts of
bromate were reacted with a fixed concentration of MESNA.
Excess bromate concentrations were then evaluated by addition
excess of acidified iodide and subsequent titration of the
liberated iodine with standard sodium thiosulfate solution.
Product identification. For the bromate−MESNA reac-
tion, Fourier Transform mass spectra (FTMS) of solutions
containing stoichiometric amounts of bromate and MESNA
were taken in the negative mode using electrospray ionization
technique. Intermediates involved in the six-electron oxidation
of MESNA by acidified bromate were observed and identified
by recording mass spectra of the stepwise two-electron
H2O2 + Cl−/Br− + myeloperoxidase/H+
→ HOCl/HOBr + H2O
HOCl and HOBr are capable of destroying a variety of
microoganisms and mammalian cell targets.22 They are also
believed to be involved in the inflammatory response.22 HOCl
is, however, a short-lived oxidant. It has been demonstrated
that HOCl generates longer-lived oxidizing species through
its reaction with nitrogen compounds to yield chlorinated
nitrogen derivatives with a N−Cl bond.23 This might include
the chlorinated derivatives with the α-amino acid taurine:
ClHNCH2CH2SO3H. The chloramine derivatives can thus
continue to moderate the neutrophil toxicity long after HOCl
has been depleted. Human bodies contain a very high con-
centration of Cl− ions, nearly up to 0.1 M, and Br− ions at
0.001 M. It is difficult to exclude HOCl and HOBr in any
meaningful representation of possible oxidation reactions
in vivo. It would appear plausible that HOCl and HOBr will
react rapidly and efficiently with thiols, thiocarbamides, thioamides
and thioethers before any reactivity might take place with inert
amino acids such taurine. Though not a biological oxidant,
precursor of HOBr is BrO3− and it is being used here as an
oxidant because it generates the intermediate that is useful in
following the reaction, with the exact mechanism of the HOBr and
Br2 oxidations being inferred from the overall reaction scheme.
2197
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
oxidation of substrate by aqueous bromine. Products of reactions
were further identified by nuclear magnetic resonance spectro-
scopy (NMR) using a Bruker Avance III (600-MHz 1H)
spectrometer and processed with Bruker Top Spin 2.1 software.
Involvement of free radicals in the MESNA oxidations were
evaluated on a Bruker Biospin e-scan spectrometer designed to
perform electron paramagnetic resonance (EPR) measurements
in the X-band-range.
stoichiometry of the reaction was derived simply from titrating
bromine (buret) into MESNA until the end point was attained
from color observation. The experimentally determined
stoichiometry for this reaction was 3:1:
3Br + HSCH2CH2SO3H + 3H2O
2
→ HO3SCH2CH2SO3H + 6Br− + 6H+
(R2)
NMR Analysis. Figure 2a shows the normal NMR spectrum
of MESNA in D2O which shows the expected set of coupling
triplets from the C−C backbone of MESNA. Each triplet
integrated as two protons. A 1:1 mixture of bromate and
MESNA in DCl shown in Figure 2b displays a single sharp
NMR peak which appears downfield to both of the triplets in
Figure 2a (ppm 3.19 vs 2.78 and 3.12 in Figure 2a). Figure 2c
shows a 1:3 MESNA − bromine mixture NMR spectrum which
also shows the expected single peak due to the methylene
protons. The slight shift in the peak to ppm 3.16 is due to the
fact that in Figure 2b, the solution was run in high acid, while in
part c, the reaction was run in neutral conditions.
RESULTS
■
Stoichiometry. Despite the use of a very strong oxidant,
the sulfur center was not oxidized to sulfate, and remained
tethered to the organic moiety to form a symmetric ethane
disulfonic acid with a strictly 1:1 stoichiometry of bromate to
MESNA:
BrO3− + HSCH2CH2SO3H → Br− + HO3SCH2CH2SO3H
(R1)
The product; 1,2-ethanedisulfonic acid is a well-known
medium for delivery of psychotic and hypnotic drugs such as
chlomethiazole and prochlorperazine.28 It is a very strong acid
with pKa values of −1.46 and −2.06. Results of the iodometric
titration described in the experimental section above are shown
in Figure 1. MESNA concentrations were held constant at
Reaction Kinetics. Formation of bromine was utilized to
follow the reaction although bromine is not a product of the
reaction being studied (stoichiometry R1). Bromine formation
is a secondary reaction that involves excess bromate and the
bromide formed in eq R1. Effectively, three reactions are
proceeding simultaneously in any mixture of acidic bromate
and MESNA: reactions R1, R2 and the BrO3 /Br− reaction,
−
reaction R3:
BrO3− + 5Br− + 6H+ → 3Br2 + 3H2O
(R3)
Thus, in excess bromate conditions, the overall stoichiometry is
a linear combination of reactions R1 and R3 which converts all
the bromide from reaction R1 by oxidizing it to bromine:
6BrO3− + 5HSCH2CH2SO3H + 6H+
→ 3Br + 5HO3SCH2CH2SO3H + 3H2O
(R4)
2
From stoichiometry R4, amount of bromine formed is
determined by initial MESNA concentrations. A spectrophoto-
metric determination of final bromine concentrations in
varying MESNA concentrations proved stoichiometry R4 of
0.60[MESNA]0. The direct bromine−MESNA reaction,
reaction R2, was so much faster than either reactions R1 or
R3 such that production of bromine is an indicator that the
thiol is depleted. Thus, experimental observation of the reaction
spectrophotometrically involved an initial quiescent period
with no bromine formation. This induction period was then
followed by a rapid bromine formation according to reaction
R3. Time taken before bromine formation could then be related
to rate of reaction R1.
−
Figure 1. Plot used for the determination of the BrO3 −MESNA
reaction. MESNA concentrations were constant at 4 mM while
bromate concentrations were varied up to 11 mM. Solutions were
incubated overnight and excess acidified iodide was added and
liberated iodine titrated against standard thiosulfate with freshly
prepared starch as indicator. The intercept, of 4 mM bromate,
indicates complete depletion of bromate with no excess oxidizing
power left in solution and represents the concentration of bromate
needed to completely consume MESNA.
Effect of Bromate. At constant acid and MESNA
concentrations, bromate concentrations were varied at high
−
excess over that needed for stoichiometry R1 ([BrO3 ]0/
4 mM while bromate concentrations were varied from 5 mM
to 11 mM. The thiosulfate titer was recorded for each variation
of bromate and results plotted against the initial bromate con-
centrations. This gave a straight line of intercept (representing
no excess oxidizing power left) at 4 mM bromate. Thus, a
mixture of 4 mM each of MESNA and bromate, after complete
reaction, afforded a product with no excess bromate con-
centrations to afford an iodometric titration. This was the
stoichiometric ratio for the reaction R1. The reaction of
aqueous bromine with MESNA was rapid enough such that the
[MESNA]0 > 20). Some of the experimental traces are shown
in Figure 3a. There was a sharp end of the induction period
and commencement of bromine formation was extremely rapid.
The first 400 s are shown in Figure 3a. Expected bromine
absorbance readings, based on stoichiometry R3 should be in
the regions of 0.85. Thus, at 400 s, reaction R4 would not have
gone to completion. At higher bromate concentrations (see
trace d in Figure 3a), bromine concentrations show an initial
transient peak which then gives way to the final bromine peak.
Since reaction R3 is fast, transient bromine formation would
2198
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Figure 2. (a) Standard NMR spectrum of MESNA in D2O. It shows the standard coupling triplets from the coupling sets of methylene protons with
the protons adjacent to the sulfonic group being slightly more deshielded. (b) NMR spectrum of the final solution from a 1:1 mixture of bromate
and MESNA in DCl. A solution with less than 1:1 bromate: MESNA gave a spectrum with both spectra (a and b). Only a single peak is observed as
the two peaks from spectrum a coalesce and move slightly downfield as expected from the symmetric 1,2-ethylene disulfonic acid. (c) NMR
spectrum from a 1:3 ratio of MESNA to bromine in D2O. There is no trace of the original compound at these ratios with complete formation of the
product.
suggest that one of the oxidation intermediates of MESNA is
relatively stable. A plot of inverse of induction time vs [BrO3 ]0
Direct Br2−MESNA Reaction. This reaction was extremely
rapid, to the point of approaching the limits of the mixing
time of the stopped-flow ensemble of 1 ms. Figure 5 shows
some absorbance traces of the Br2−MESNA reactions taken
at varying initial MESNA concentrations. The reaction is
essentially complete in 30 ms. The reaction showed first order
dependence on both Br2 and MESNA with a bimolecular rate
constant of 1.95 0.05 × 104 M−1 s−1.
−
shown in Figure 3b gives a straight line. This implies that the
rate of the primary reaction of relevance to the point of
bromine production is related to bromate concentrations to the
first power. This would be the rate-determining step. The plot
in Figure 3b also implicitly confirms stoichiometry R1. An
induction period of infinity is when the inverse of the induction
period is 0.00. This occurs when there is just enough bromate
to oxidize MESNA completely with no excess bromate left to
form bromine from the product of reaction R1, Br−. With an
initial concentration of 10 mM MESNA, the plot gives an
intercept value of 10 mM bromate for reaction stoichiometry
for reaction R1.
MECHANISM
■
The general driving force for this reaction is controlled by the
well-established oxyhalogen kinetics which involve a square
dependence term in acid:30−34
H+ + BrO3− ⇄ HBrO3
(R5)
Effect of Acid. Acid is the most important catalyst in most
oxyhalogen-based reactions.29 It is also strongly catalytic in the
bromate−MESNA reaction. Figure 4a shows that the induction
period is rapidly shortened as acid concentrations are increased.
The acid dependence of the induction period was an inverse
square with saturation at high acid concentrations, tending toward
simple inverse dependence (see Figure 4b). Low acid concentra-
tions gave smooth bromine formation after the induction period,
while high acid concentrations gave a hint of oligooscillatory
behavior as that observed in Figure 3a.
HBrO3 + H+ ⇄ H2BrO3+
(R6)
H2BrO3+ + 2e− → HBrO2 + OH−
(R7)
The oxidizing species is the protonated bromic acid. In
standard oxyhalogen kinetics, the two electrons in reaction R7
are provided by Br− which is oxidized to HOBr. Addition of
bromide in conditions of excess bromate concentrations
accelerated the rate of reaction and decreased the induction
periods in Figures 3a and 4a. The resulting oxybromine species,
2199
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Figure 4. (a) Acid dependence of the bromate-MESNA reaction.
Induction time decreases and rate of formation of bromine after
induction period increases with acid. (b) No second order dependence
could be discerned from the data in part a. Acid effect approaches first
order in high acid, and second order for the slower low acid
concentrations.
Figure 3. (a) Bromate dependence of the MESNA−bromate reaction.
There is a rapid and sharp bromine formation after an induction
period in which no bromine is formed. High bromate concentrations
produce bromine so rapidly that it reaches an initial peak which gives
way to a slower bromine formation until, in this case, the full bromine
concentrations are attained at 6 mM (about 0.85 in absorbance). Ten
−
mM [MESNA]0; [BrO3 ]0 = (curve a) 0.20 M; (curve b) 0.25 M;
(curve c) 0.30 M; (curve d) 0.35 M. (b) Plot of inverse of induction
time vs initial bromate concentrations from the data shown in part a.
Intercept represents induction time of infinity, which indicates that
there is no excess bromate to form bromine; indicating the
stoichiometric equivalent. This plot also confirms the 1:1 stoichiom-
etry established from data in Figure 1.
HBrO2 is known to be unstable with respect to HOBr in the
presence of Br−:36
HBrO2 + Br− + H+ ⇄ 2HOBr
(R9)
The major oxidant, if reaction R9 is labile in the forward
direction, would be HOBr:
HOBr + H+ + 2e− ⇄ Br− + H2O
(R10)
HBrO2 and HOBr carry the bulk of the oxidation.35 Since rate-
determining step is reaction R7, the overall kinetics would be
overall fourth order:
The initial part of this reaction system will be controlled by
the sequence of reactions R5, R6, and R8−R10, if one utilizes
the trace amounts of bromide in bromate solutions. The sum of
these reactions shows that the overall stoichiometry of this initial
part of the reaction is the conversion of two bromide ions to
three bromide ions in the form of cubic autocatalysis:37
rate = k0[BrO3−][H+]2[Red]
(1)
Red, the reductant, could be substrate MESNA or, initially, trace
amounts of bromide in the reaction mixture always resident in
any standard bromate solutions. Only catalytic amounts of
bromide are needed in the R5−R8 cascade of reactions
BrO3− + 6H+ + 6e− + 2Br− → 3Br− + 3H2O
(R11)
A slower direct reaction of bromate with substrate, MESNA, can
also produce, initially, bromide, which can be used in reaction
H2BrO3+ + Br− ⇄ HBrO2 + HOBr
(R8)
2200
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
peak for a product with an m/e value of 155.9560 which is
equivalent to the addition of a single atom of oxygen to
MESNA. It also shows a peak for the substrate, MESNA (m/e =
139.9610) and minor peaks for the dimeric MESNA and a
MESNA plus two oxygen atoms adduct (m/e = 138.9534,
172.9585, respectively). Thus, three oxidation products are
obtained with the dominant product being the one oxygen
atom addition to MESNA. The form of this intermediate
cannot be determined from ESI−MS data available. It could be
the sulfenic acid, sulfoxide or the S-oxide. Sulfenic acids are very
unstable and can only be stabilized by bulky substituent groups
as in anthraquinones.41,42 Sulfoxides can only be stabilized
by a ring structure which can enhance the pi-bond stabilization.
Ab initio calculations do not support a terminal sulfoxide
without two electron-donating groups for stabilization.43 Thus,
the oxygen adduct appearing at m/e = 155.9562 should be an
S-oxide (see structure I in Chart 1). Addition of two oxidizing
equivalents (Figure 6b) does not produce predominantly, an
adduct with two oxygen atoms, but still mostly the S-oxide and
the sulfonic acid product (m/e = 188.9537). What would have
been the sulfone or sulfinic acid (HO3SCH2CH2SO2H) appears
to be unstable and disproportionates to the S-oxide and the
sulfonic acid forms.
Known sulfinic acids derived from aminothiols are stabilized
by resonance. Addition of the full 3 oxidizing equivalents the
spectrum (Figure 6c) shows the dominant peak expected for
the product 1,2-ethylene disulfonic acid (m/e = 188.9536) as
well as its monosodium salt (m/e = 210.9356). No peak was
observed for the disodium salt since bromine oxidations were
conducted in neutral to slightly acidic pH. All the acid in this
reaction solution was derived from the stoichiometry. Figure 6c
shows no traces of the dimer nor the sulfinic acid, but does
show small amounts of the S-oxide. We note, in parts a and b of
Figure 6, before the addition of the 6-electron full oxidation
equivalents that take the sulfur center to the sulfonic acid, peaks
for the fully protonated dimeric MESNA at m/e = 280.924.
The precursors for the dimeric species should either be a
condensation-type reaction between an unoxidized thiol and
the S-oxide; or thiyl radicals. The presence of the dimeric
species and the series of oxygen-addition intermediates suggests
at least two pathways for the oxidation of MESNA: successive
2-electron transfers from the sulfur center to the oxybromine
center and a free radical-based initial oxidation pathway.
A series of experiments were run using the X-band EPR
spectrometer to detect any possible radical formation during
the course of the reaction. Figure 7 shows the EPR spectra of
the reaction solution using a DMPO trap. This shows the
presence of an EPR-active adduct whose abundance increases
with MESNA concentrations. All these reactions in excess
bromate indicating that the radical species generated, mediated
by MESNA, are derived from MESNA.
Figure 5. Direct reaction of bromine and MESNA. Variation of
MESNA. The reaction is so fast it is close to the limits of our stopped-
flow. The reaction is essentially over in 20 ms. The 3:1 stoichiometric
ratio could also be graphically determined from the residual bromine
absorbance. The reaction is strictly bimolecular.
R8 to produce the reactive oxidizing oxybromine species HBrO2
and HOBr.
BrO3− + RSH + H+ → HBrO2 + RSOH
(R12)
Reaction R12 can proceed with HBrO2 as the oxidant to
produce HOBr and Br− in consecutive 2-electron transfers. Br−
can then catalyze the standard oxybromine kinetics which
produce the active oxidizing species:38,39
BrO3− + 2H+ + Br− ⇄ HBrO2 + HOBr
(R13)
Reaction R13 is the accepted composite reaction R5 + R6 + R8
that explains second order acid kinetics in all bromate
oxidations.33
Oxidation of MESNA. While the oxybromine kinetics are
reasonably well-understood, not much is known on the thiol
chemistry and sulfur oxidations in general. Thiol oxidations can
proceed via a radical mechanism or through oxygen transfer.40
The thiol group in MESNA is oxidized to the sulfonic acid,
a 6-electron transfer. In general, a monotonic decrease in pH is
observed when aqueous bromine is reacted with MESNA at
neutral pH (stoichiometry R2) from the release of protons and
final formation of the strong 1,2-ethanedisulfonic acid. The
steady pH decrease suggests a sequential formation of the sulfur
oxo-acids all the way to the formation of the sulfonic acid.
It is reasonable to assume that the dominant pathway is via
successive two-electron transitions from sulfur center in the
−2 oxidation state to the +4 state in the sulfonic acid. A series
of experiments were undertaken to prove this (Figure 6a−c).
The first experiment involved the addition of an exactly 1:1
ratio of MESNA and aqueous bromine. This will be equivalent
to a two-electron oxidation of the sulfur center. Sulfur, due
to possible polymerizations, can attain several energetically
accessible oxidation states, especially in conditions of excess
reductant. Figure 6a shows an ESI−MS spectrum of the 1:1
MESNA − Br2 reaction mixture after the reaction has been
assumed to have gone to completion (these reactions have
lifetimes of less than a second). The spectrum shows a strong
Radical Pathway and Its Significance. The mechanism
of the generation of radicals from oxybromine chemistry is well-
established. This pathway was derived from studies of the
Belousov−Zhabotinskyi reaction, with the central species being
the autocatalytic bromous acid.35
•
BrO3− + HBrO2 + H+ ⇄ 2BrO2 +H2O
(R14)
•
BrO2 +RSH ⇄ RS•+HBrO2
(R15)
2RS• → RSSR
(R16)
2201
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
RSH is assumed to be the generic thiol, MESNA, and RSSR,
the dimeric species at m/e = 280.9294. Reactions were not run
in strictly anaerobic environments, and so the existence of the
thiyl radicals can unleash a cascade of other radicals, specifically;
superoxide, hydroxyl and hydrogen peroxide.44 If one adds
R14 + 2R15; and recognizing that the reaction is run in excess
bromate, the overall stoichiometry shows that one HBrO2
molecule will produce two HBrO2molecules in quadratic
autocatalysis. The autocatalytic production of bromous acid,
coupled with its lability in reaction R9 should produce a
constant enhancement of the reaction and deliver sigmoidal
kinetics, which is typical for many bromate oxidations.45 No
autocatalysis was observed in this reaction and hence the radical
pathway should not be the dominant route.
Effect of Acid. There was no observation of purely
second order kinetics as expected from oxybromine kinetics.
Figure 6. continued
2202
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Figure 6. (a) ESI−MS spectrum of a reaction mixture of 1:1 MESNA−Br2. The strongest peak is for the S-oxide at m/e 155.9560 apart from the
substrate itself at 139.9610. Reaction is in slightly acidic medium, and thus shows both the protonated and unprotonated MESNA. A small peak
for the dimeric species with an S −S bond can be seen at m/e 138.9534. There is a substantial peak for the singly charged dimeric species at
280.925. Some sulfinic acid can be seen at m/e 172.9585. This is essentially a mixture of metabolites in excess substrate which indicates that the
S-oxide is the most stable intermediate at these conditions. (b) ESI−MS spectrum derived from addition of two equivalents of bromine. The
sulfinic acid, which should be most abundant at these conditions, retains the same low abundance as in spectrum a; with the S-oxide and sulfonic
acid being most abundant. The sulfinic acid (S(II)), if formed, rapidly disproportionates to the 0 and +4 oxidation states. The peak for MESNA
has been reduced substantially from part a. (c) Addition of the full 3 equiv of bromine as according to the expected stoichiometry. MESNA peak
disappears completely, living a large peak for the product, 1,2-ethylene disulfonic acid peak at m/e 188.9536 and a smaller peak for its
monosodium salt at m/e 210.9356. There is no evidence of the sulfinic acid at these conditions; instead, a small and observable peak is still
present for the S-oxide.
Chart 1. Structure I
The complexity was derived from the thiol center, which is
nucleophilic and would be protonated in highly acidic
environments:
+
HO3SCH2CH2SH + H+ → HO3SCH2CH2SH2
,
Kb
If one considers the protonated thiol to be relatively inert, then
the rate of reaction is
k0[BrO3−][H+]2[HO3SCH2CH2SH]T
rate =
1 + Kb[H+]
where [HO3SCH2CH2SH]T is the total of the protonated and
unprotonated MESNA. Rate equation supports first order
kinetics at high acid concentrations but expected second order
kinetics at mild to low acid concentrations.
Overall Reaction Scheme. Both the oxidation product of
MESNA and reduction product of bromate have been satis-
factorily established. Contrary to most organosulfur com-
pounds in the presence of a strong oxidant, the C−S bond does
not cleave and no sulfate production is observed. The C−S
bond is strengthened by the presence of electron-withdrawing
groups on the carbon adjacent to the C−S bond.46−49 Hence
oxidation of cysteamine can only proceed as far as taurine with
Figure 7. Formation of radicals, mediated by MESNA.
2203
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
a
Table 1. Relevant Reactions for the Bromate−MESNA Reaction in Acidic Medium
reaction no.
M1
reaction
kf; kr
BrO3− + RSH + H+ → HBrO2 + RSOH
HBrO2 + RSH → HOBr + RSOH
HOBr + RSH → Br− + H+ + RSOH
BrO3− + 2H+ + Br− ⇄ HBrO2 + HOBr
0.473
M2
M3
M4
1.05 × 105
1.53 × 104
2.5; 1.31 × 104
HBrO2 + Br− + H+ ⇄ 2HOBr
HOBr + Br− + H+ ⇄ Br2 + H2O
HOBr + RSH → RSOH + Br− + H+
HOBr + RSOH → RSO2H + Br− + H+
M5
M6
2.01 × 109; 2.0 × 10−5
8.9 × 109; 110
M7
M8
1.53 × 104
3.34 × 106
HOBr + RSO2H → RSO3H + Br− + H+
Br2(aq) + RSH + H2O → RSOH + 2Br− + 2H+
Br2(aq) + RSOH + H2O → RSO2H + 2Br− + 2H+
Br2(aq) + RSO2H + H2O → RSO3H + 2Br− + 2H+
RSO2H + RSO2H ⇄ RSOH + RSO3H
RSOH + RSH ⇄ RSSR + H2O
RSSR + HOBr + H2O → 2RSOH + Br− + H+
RSSR + Br2 + 2H2O → 2RSOH + 2Br− + 2H+
BrO3− + HBrO2 + H+ ⇄ 2BrO2·+H2O
BrO2•+RSH ⇄ RS•+HBrO2
M9
2.06 × 104
1.95 × 104
15.5
M10
M11
M12
1.76 × 102
M13
M14
M15
2.24 × 104
85.7; 2.20 × 104
68.0
M16
M17
M18
M19
8.66
1.1 × 10−4; approximately 0
1.0 × 102
1.0 × 103; 5.0 × 10−1
2RS• → RSSR
a
Forward and reverse rate constants are given, separated by a semicolon. Except for reactions involving water, the units of kinetics rate constants
derived from the reactions’ molecularity. Those with a single kinetics parameter were assumed to be essentially irreversible.
the C−S bond intact. Other stable sulfonic acids include
trichlorosulfonic and methylaminosulfonic acid.
oxidizing species; HBrO2 and HOBr. Reaction M6 was
studied by temperature-jump spectrophotometry and
favors aqueous bromine in the strongly acidic environ-
ment utilized for this study.50
The combination of oxyhalogen kinetics and oxidation
kinetics of the thiol group can be combined into the set of
16 reactions shown in Table 1. It is the simplest scheme that
can be derived which can still adequately describe the reaction.
These reactions, even if they also appear in the mechanism,
above, have been relabeled M1−M16 (Table 1). They can be
divided into 6 separate groups based on their function. To
simplify the scheme, it is assumed that the major oxidants in
the reaction mixture are HOBr and Br2. If, as is known, that
reaction M5 is rapid, then no loss in accuracy with respect to
the description of the reaction occurs if one ignores HBrO2 as
a viable oxidant, though one would expect it to contribute to
the oxidation.
(c) Oxidation by HOBr: Reactions M7−M9. This is a
cascade of oxidations of MESNA, through its two
intermediates, by HOBr. [Note that M3 has been
deliberately repeated as M7 so as to exhaust the oxidation
reactions of HOBr in this section]. Oxidation by HOBr is
through an oxygen atom transfer and is considered to be
quite rapid. Figures 3a and 4a suggest the presence of a
stable intermediate and the ESI−MS data unequivocally
establishes this as the S-oxide. Hence the rate of oxidation
of the S-oxide will be slower than the rates for the thiol
and for the sulfinic acid.
(a) Initiation Reactions: M1−M3. In the absence of
adequate amounts of bromide in reaction mixture, the
three initial set of reactions produce enough bromide to
catalyze the rest of the reaction. The bromide produced
in reaction M3 can then be used in M4 to start the well-
known cascade of oxybromine kinetics with second order
dependence in acid and first order in bromate. In reactions
with added bromide, reactions M1 − M3 can be removed
with no loss in accuracy of the proposed mechanism.
Bromide role is only to catalyze the reaction. As the reaction
proceeds, it is produced autocatalytically (reaction R11).
(b) Standard Oxybromine Reactions: M4−M6. This is a
well-known series of reactions which have been studied
extensively in the analysis of the Belousov−Zhabotinsky
reaction.35 Bromate, itself is quite inert, and has to be
primed through reaction M4 to generate the reactive
(d) Oxidation by Aqueous Bromine: M10−M12. In this acidic
environment, reaction M6 suggests formation of Br2 will
be strongly favored over HOBr, and thus the bulk of the
oxidation should be borne by Br2. Thus, reaction M10 is
essentially diffusion-controlled with reaction M11 slower.
(e) Disproportionation Reaction: M13. Reaction M13 is
essential. The spectrum in Figure 6b shows that, despite
having enough oxidizing power for a 4-electron oxidation
of the sulfur center to the sulfinic acid, the system prefers
to disproportionate into a 2-electron oxidation and a
6-electron oxidation. There is never a viable accumulation
of the sulfinic acid. The sulfonic acid can only be formed
through the sulfinic acid which rapidly disproportionates.
(f) Possible Dimer Formation and Reaction: M14−M16.
The spectrum in Figure 6a shows a low-level formation
of the dimer. One viable route for formation of a dimer
2204
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Figure 8. (a) Modeling the experimental data shown in Figure 3a, (bromate dependence). Interference from tribromide precludes a correct
prediction of the rate of increase in absorbance after the induction period. As expected, the model always underestimates. The final bromine
concentrations and their attainment always coincide with experiments. (b) Simulations of the intermediate species (apart from bromine) which could
not be experimentally determined. No sigmoidal kinetics is observed in the consumption of MESNA, suggesting the dominance of the 2-electron
oxidation pathway. The S-oxide coexists with bromine since it is relatively stable and its further oxidation relatively slower.
(apart from through free radicals, vide infra) is via a
reversible condensation-type reaction between the S-oxide
(sulfenic acid) and the unreacted thiol (M14). The dimer
can be consumed via the reverse reaction of M13 in the
presence of excess oxidant with further oxidation of the
S-oxide and the thiol by HOBr and Br2(aq). These
oxidants are so powerful that they are capable of oxidizing
the dimer directly (reactions M15 and M16).
superoxide radicals whose DMPO adduct spectra are
well-known. Since the radical intensity is mediated by
MESNA, it suggests that these are thiyl radicals and are
not derived from bromine dioxide. These radicals are
deactivated through the formation of the dimeric species
which are evident in the spectra a and b in Figure 6.
Dimer formation is also through reactions such as M14.
Both mechanisms operate simultaneously because dis-
proportionation reactions, such as those responsible for
the products in spectrum b in Figure 6 can only occur
through sulfur oxo-acid formations (part f, above), while
Figure 7 clearly shows radical formation.
(g) Contributions from Free Radicals. Figure 7 shows the
formation of free radicals. The hyperfine splitting of
the ESR and the 1:1:1 splitting of the peaks does not
implicate the well-known hydroxyl radicals nor the
2205
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
Computer Simulations. The mechanism shown in Table 1
was modeled through the SIMKINE software designed and
developed by Jonnalagadda.51,52 Reactions involving radicals,
M17−M19, were not utilized for this model since their
inclusion did not improve the quality of the fit. Well-known
literature values were utilized and fixed for reactions M4−M6.
The kinetics constant for reaction M10 was derived in this
study and also fixed. The rest were optimized for best fit
through a subroutine imbedded in the SIMKINE software
Oxychlorine and oxybromine species exist in the physiological
environment as a direct result accumulation of reactive oxygen
species such as hydrogen peroxide. Superoxide and peroxides, as
well, can oxidize MESNA. As a coinjectant with most anticancer
drugs;8 it is important to examine the metabolites it might
generate and their possible effect in the physiological environ-
ment. This study shows that MESNA might act as an antioxidant
and would moderate reactive oxygen species toxicities. A similar
compound, taurine, is also known to moderate reactive oxygen
species toxicity, but through the formation N-bromo- and
N-chloro-derivatives.56−59 The noncleavage of the C−S bond is
important: the cleavage of the C−S bond before the oxidative
saturation to the +6 oxidation state is known to initiate a cascade
of genotoxic reactive oxygen species that are known to cause
DNA damage.44
program,53 subject to some constraints: kM10 > kM11 < kM12
.
This constraint was from the experimental observation that the
S-oxide had a special stability associated with it. One reaction
not included in the table but relevant in excess bromate
environments involved the formation of tribromide:
Br (aq) + Br− ⇄ Br −(aq); Keq = 17
2
3
(R17)
AUTHOR INFORMATION
■
Without a mass balance relationship that links all bromine
species (both bromine and bromide are continuously produced
and consumed, with bromate as a virtual inexhaustible source of
bromine), the system ends up with two equations while having
three unknowns. Apart from a dynamic iterative procedure at
each time interval, concentrations of bromine and tribromide
could never be obtained analytically and unambiguously.
While absorptivity of bromine at 390 nm is 142 M−1 cm−1,
that of tribromide is 1006 M−1 cm−1 at the same wavelength.54
Experimental data are based on bromine absorbance, indicating
that the model will always underestimate absorbance readings
in environments containing bromine and bromide due to
equilibrium R17 which produces a higher-absorbing species.
Figure 8a shows the superimposition of experimental and
simulated curves. As expected, the model shows a much slower
increase in absorbance after the induction period because the
model utilizes only bromine absorbances, while the exper-
imental traces result from both bromine and tribromide
absorbances. Figure 8b is a plot of the simulations of all
those intermediate species which we could not experimental
access. Trace e of bromine is inserted for comparison. There
are four features of Figure 8b that are relevant: (a) bromide
concentrations rise to a maximum just before the end of the
induction period, thus delivering a large ratio of tribromide to
bromine at the start of the reaction, giving the very sharp
absorbance increase that is experimentally observed. (b) the
S-oxide is very stable and rises to very high transient concentra-
tions. Thus, when the induction period ends, although the
parent thiol would have been depleted, there will still be
substantial concentrations of the S-Oxide. This, then, explains
what appears to be olligooscilatory behavior in bromine
concentrations with a transient peak after the induction period.
(c) HOBr, as expected, never rises to any substantial levels
through the whole reaction. It is too reactive. (d) There is no
sigmoidal decay in the rate of depletion of MESNA (trace a).
This is important. If the free radical route was dominant, then
HBrO2 will be the autocatalytic species and classic sigmoidal
autocatalytic kinetics would be observed.32,55 It would
strengthen, then, the argument that the 2-electron sulfur oxo
acid route is dominant, and dimer formation mainly through
reaction M14.
Corresponding Author
*(R.H.S.) E-mail: rsimoyi@pdx.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Research Grant Number CHE
1056366 from the National Science Foundation and a Research
Professorship allocation awarded to R.H.S. from the University
of KwaZulu-Natal.
REFERENCES
■
(1) Sahni, V.; Choudhury, D.; Ahmed, Z. Chemotherapy-Associated
Renal Dysfunction. Nat. Rev. Nephrol. 2009, 5, 450−462.
(2) Hanigan, M. H.; Townsend, D. M.; Cooper, A. J. Metabolism of
Cisplatin to a Nephrotoxin. Toxicology 2009, 257, 174−175.
(3) Negishi, K.; Noiri, E.; Doi, K.; Maeda-Mamiya, R.; Sugaya, T.;
Portilla, D.; Fujita, T. Monitoring of Urinary L-Type Fatty Acid-
Binding Protein Predicts Histological Severity of Acute Kidney Injury.
Am. J. Pathol. 2009, 174, 1154−1159.
(4) Pectasides, D.; Pectasides, E.; Papaxoinis, G.; Xiros, N.;
Kamposioras, K.; Tountas, N.; Economopoulos, T. Methotrexate,
Paclitaxel, Ifosfamide, and Cisplatin in Poor-Risk Nonseminomatous
Germ Cell Tumors. Urol. Oncol. 2010, 28, 617−623.
(5) Links, M.; Lewis, C. Chemoprotectants - A Review of Their
Clinical Pharmacology and Therapeutic Efficacy. Drugs 1999, 57,
293−308.
(6) Dyson, P. J.; Sava, G. Metal-Based Antitumour Drugs in the Post
Genomic Era. Dalton Trans. 2006, 1929−1933.
(7) de Jonge, M. J. A.; Verweij, J. Renal Toxicities of Chemotherapy.
Semin. Oncol. 2006, 33, 68−73.
(8) Ludwig, U.; Riedel, M. K.; Backes, M.; Imhof, A.; Muche, R.;
Keller, F. MESNA (Sodium 2-Mercaptoethanesulfonate) for Pre-
vention of Contrast Medium-Induced Nephrotoxicity. Clin. Nephrol.
2011, 75, 302−308.
(9) Miller, R. P.; Tadagavadi, R. K.; Ramesh, G.; Reeves, W. B.
Mechanisms of Cisplatin Nephrotoxicity. Toxins 2010, 2, 2490−2518.
(10) Patel, J. M. Metabolism and Pulmonary Toxicity of Cyclo-
phosphamide. Pharmacol. Ther. 1990, 47, 137−146.
(11) Nicolao, P.; Giometto, B. Neurological Toxicity of Ifosfamide.
Oncology 2003, 65, 11−16.
(12) Brock, N.; Pohl, J.; Stekar, J.; Scheef, W. Studies on the
Urotoxicity of Oxazaphosphorine Cytostatics and Its Prevention 0.3.
Profile of Action of Sodium 2-Mercaptoethane Sulfonate (Mesna).
Eur. J. Cancer Clin. Oncol. 1982, 18, 1377−&.
(13) Scheef, W.; Klein, H. O.; Brock, N.; Burkert, H.; Gunther, U.;
Hoeferjanker, H.; Mitrenga, D.; Schnitker, J.; Voigtmann, R.
Controlled Clinical Studies With An Antidote Against the Urotoxicity
CONCLUSION
■
This short kinetics study has shown that MESNA can be
oxidatively bioactivated in the physiological environment;
though not as rapidly and as quantitatively shown in this study.
2206
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
of Oxazaphosphorines - Preliminary-Results. Cancer Treat. Rep. 1979,
63, 501−505.
(34) Szalai, I.; Oslonovitch, J.; Forsterling, H. D. Oscillations in the
Bromomalonic Acid/Bromate System Catalyzed by [Ru(Phen)
(3)](2+). J. Phys. Chem. A 2000, 104, 1495−1498.
(35) Mrakavova, M.; Melichercik, M.; Olexova, A.; Treindl, L. The
Autocatalytic Reduction of Ferriin by Malonic Acid With Regard to
the Ferroin-Catalyzed Belousov-Zhabotinsky Reaction. Collect. Czech.
Chem. Commun. 2003, 68, 23−34.
(36) Sirimungkala, A.; Forsterling, H. D.; Dlask, V.; Field, R. J.
Bromination Reactions Important in the Mechanism of the Belousov-
Zhabotinsky System. J. Phys. Chem. A 1999, 103, 1038−1043.
(37) Petrov, V.; Scott, S. K.; Showalter, K. Excitability, Wave
Reflection, and Wave Splitting in A Cubic Autocatalysis Reaction-
Diffusion System. Philos. Trans. R. Soc. London, Ser. A: Math., Phys. Eng.
Sci. 1994, 347, 631−642.
(14) Katz, A.; Epelman, S.; Anelli, A.; Gorender, E. F.; Cruz, S. M.;
Oliveira, R. M.; Marques, L. A. A Prospective Randomized Evaluation
of Three Schedules of Mesna Administration in Patients Receiving an
Ifosfamide-Containing Chemotherapy Regimen: Sustained Efficiency
and Simplified Administration. J. Cancer Res. Clin. Oncol. 1995, 121,
128−131.
(15) Park, S. B.; Howald, W. N.; Cashman, J. R. S-Oxidative Cleavage
of Farnesylcysteine and Farnesylcysteine Methyl Ester by the Flavin-
Containing Monooxygenase. Chem. Res. Toxicol. 1994, 7, 191−198.
(16) Del Corso, A.; Cappiello, M.; Mura, U. Thiol Dependent
Oxidation of Enzymes: the Last Chance Against Oxidative Stress. Int. J.
Biochem. 1994, 26, 745−750.
(38) Bala, D.; Treindl, L. Temperature and Oxygen Effect on
Oscillations of the Belousov-Zhabotinsky Reaction With 2-Oxopenta-
nedioic Acid As Substrate. Collect. Czech. Chem. Commun. 2000, 65,
1839−1847.
(39) Pullela, S. R.; Cristancho, D.; He, P.; Luo, D. W.; Hall, K. R.;
Cheng, Z. D. Temperature Dependence of the Oregonator Model for
the Belousov-Zhabotinsky Reaction. Phys. Chem. Chem. Phys. 2009, 11,
4236−4243.
(17) Organisciak, D. T.; Darrow, R. M.; Jiang, Y. I.; Marak, G. E.;
Blanks, J. C. Protection by Dimethylthiourea Against Retinal Light
Damage in Rats. Invest. Ophthalmol. Visual Sci. 1992, 33, 1599−1609.
(18) Chinake, C. R.; Simoyi, R. H. New Experimental-Data on the
Chlorite-Thiourea Reaction. J. Phys. Chem. 1993, 97, 11569−11570.
(19) Ziegler, D. M. Recent Studies on the Structure and Function of
Multisubstrate Flavin-Containing Monooxygenases. Annu. Rev. Phar-
macol. Toxicol. 1993, 33, 179−199.
(20) Thomas, E. L.; Grisham, M. B.; Jefferson, M. M.
Myeloperoxidase-Dependent Effect of Amines on Functions of
Isolated Neutrophils. J. Clin. Invest 1983, 72, 441−454.
(21) Thomas, E. L.; Bozeman, P. M.; Jefferson, M. M.; King, C. C.
Oxidation of Bromide by the Human Leukocyte Enzymes Myeloper-
oxidase and Eosinophil Peroxidase. Formation of Bromamines. J. Biol.
Chem. 1995, 270, 2906−2913.
(22) Jesaitis, A. J.; Dratz, E. A. The Molecular Basis of Oxidative
Damage by Leukocytes; CRC Press: Boca Raton, FL, 1992.
(23) Thomas, E. L.; Grisham, M. B.; Melton, D. F.; Jefferson, M. M.
Evidence for a Role of Taurine in the in Vitro Oxidative Toxicity of
Neutrophils Toward Erythrocytes. J. Biol. Chem. 1985, 260, 3321−
3329.
(24) Kapungu, G. P.; Rukweza, G.; Tran, T.; Mbiya, W.; Adigun, R.;
Ndungu, P.; Martincigh, B.; Simoyi, R. H. Oxyhalogen-Sulfur
Chemistry: Kinetics and Mechanism of Oxidation of Captopril by
Acidified Bromate and Aqueous Bromine. J. Phys. Chem. A 2013, 117,
2704−2717.
(25) Darkwa, J.; Mundoma, C.; Simoyi, R. H. Reactivity and
Oxidation of DL-Cysteine by Some Common Oxidants. J. Chem. Soc.,
Faraday Trans. 1998, 94, 1971−1978.
(40) Chikwana, E.; Otoikhian, A.; Simoyi, R. H. Oxyhalogen-Sulfur
Chemistry: Kinetics and Mechanism of Oxidation of Guanylthiourea
by Acidified Bromatel. J. Phys. Chem. A 2004, 108, 11591−11599.
(41) Bruice, T. C.; Sayigh, A. B. The Structure of Anthraquinone-1-
Sulfenic Acid (Fries’ Acid) and Related Compounds. J. Am. Chem. Soc.
1959, 81, 3416−3419.
(42) Bruice, T. C.; Markiw, R. T. The Synthesis of a Disulfenic Acid.
Anthraquinone-1,4-Disulfenic Acid. J. Am. Chem. Soc. 1957, 79, 3150−
3154.
(43) Jenks, W. S.; Matsunaga, N.; Gordon, M. Effects of Conjugation
and Aromaticity on the Sulfoxide Bond. J. Org. Chem. 1996, 61, 1275−
1283.
(44) Makarov, S. V.; Mundoma, C.; Svarovsky, S. A.; Shi, X.;
Gannett, P. M.; Simoyi, R. H. Reactive Oxygen Species in the Aerobic
Decomposition of Sodium Hydroxymethanesulfinate. Arch. Biochem.
Biophys. 1999, 367, 289−296.
(45) Chinake, C. R.; Simoyi, R. H. Kinetics and Mechanism of the
Complex Bromate-Iodine Reaction. J. Phys. Chem. 1996, 100, 1643−
1656.
(46) Chinake, C. R.; Mundoma, C.; Olojo, R.; Chigwada, T.; Simoyi,
R. H. Oxyhalogen-Sulfur Chemistry: Kinetics and Mechanism of the
Oxidation of a Bunte Salt 2-Aminoethanethiolsulfuric Acid by
Chlorite. Phys. Chem. Chem. Phys. 2001, 3, 4957−4964.
(47) Chinake, C. R.; Simoyi, R. H. Oxyhalogen-Sulfur Chemistry:
Oxidation of Taurine by Chlorite in Acidic Medium. J. Phys. Chem. B
1997, 101, 1207−1214.
(48) Morakinyo, M. K.; Chikwana, E.; Simoyi, R. H. Oxyhalogen-
Sulfur Chemistry - Kinetics and Mechanism of the Bromate Oxidation
of Cysteamine. Can. J. Chem.Rev. Can. Chim. 2008, 86, 416−425.
(49) Mundoma, C.; Simoyi, R. H. Oxyhalogen-Sulfur Chemistry -
Oxidation of 2-Aminoethanethiolsulfuric Acid by Iodate in Acidic
Medium. J. Chem. Soc., Faraday Trans. 1997, 93, 1543−1550.
(50) Kustin, K.; Eigen, M. Disproportionation Kinetics of Halogens
Studied by Temperature-Jump Spectrophotometry. J. Am. Chem. Soc.
1962, 84, 1355−1359.
(51) Jonnalagadda, S. B.; Pramasur, N.; Shezi, M. N. ‘Simkine’ for
Simulation of Kinetics Involving Complex Reaction Mechanisms.
Comput. Biol. Chem.: Elsevier: Amsterdam, 2003, 27, 147−152.
(52) Mzolisi, N. S.; Jonnalagadda, S. B. Simkine 2 - Versatile Software
for the Computation of Kinetic Profiles of Intricate Chemiscal
Reaction Mechanisms. S. Afr. J. Chem. 2006, 59, 82−85.
(53) Mzolisi, N. S.; Jonnalagadda, S. B. Easy to Use Program
‘Simkine 3’ for Simulating Kinetics Profiles of Intricate Chemical
Systems and Optimizing Predictable Rate Coefficients Therein. Bull.
Chem. Soc. Ethiop. 2012, 26, 267−277.
(26) Chigwada, T. R.; Chikwana, E.; Ruwona, T.; Olagunju, O.;
Simoyi, R. H. S-Oxygenation of Thiocarbamides. 3. Nonlinear Kinetics
in the Oxidation of Trimethylthiourea by Acidic Bromate. J. Phys.
Chem. A 2007, 111, 11552−11561.
(27) Otoikhian, A. A.; Simoyi, R. H. Kinetics and Mechanism of
Oxidation of N, N ′-Dimethylaminoiminomethanesulfinic Acid by
Acidic Bromate. J. Phys. Chem. A 2008, 112, 8569−8577.
(28) Balldin, J.; Alling, C.; Gottfries, C. G.; Lindstedt, G.; Langstrom,
G. Changes in Dopamine Receptor Sensitivity in Humans After Heavy
Alcohol Intake. Psychopharmacology (Berlin) 1985, 86, 142−146.
(29) Sortes, C. E.; Faria, R. B. Revisiting the kinetics and mechanism
of bromate-bromide reaction. J. Braz. Chem. Soc. 2001, 12, 775−779.
(30) Ajibola, R. O.; Simoyi, R. H. S-Oxygenation of Thiocarbamides
IV: Kinetics of Oxidation of Tetramethylthiourea by Aqueous Bromine
and Acidic Bromate. J. Phys. Chem. A 2011, 115, 2735−2744.
(31) Lopez-Cueto, G.; Ostra, M.; Ubide, C. New Way of Application
of the Bromate-Bromide Mixture in Kinetic Analysis. Anal. Chim. Acta
2001, 445, 117−126.
(32) Noyes, R. M.; Field, R. J.; Thompson, R. C. Mechanism of
Reaction of Bromine(V) With Weak One-Electron Reducing Agents. J.
Am. Chem. Soc. 1971, 93, 7315−7316.
(33) Noyes, R. M. Chemical Oscillations and Instabilities. 39. A
Generalized Mechanism for Bromate-Driven Oscillators by Bromide. J.
Am. Chem. Soc. 1980, 102, 4644−4649.
2207
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208

The Journal of Physical Chemistry A
Article
(54) Simoyi, R. H.; Epstein, I. R. Systematic Design of Chemical
Oscillators. 40. Oxidation of Thiourea by Aqueous Bromine:
Autocatalysis by Bromide. J. Phys. Chem. 1987, 91, 5124−5128.
(55) Jonnalagadda, S. B.; Chinake, C.; Simoyi, R. H. Kinetics and
Mechanism of the Reaction Between Thymol Blue and Bromate in
Acidic Medium. J. Chem. Soc., Faraday Trans. 1995, 91, 1635−1640.
(56) Green, T. R.; Fellman, J. H.; Eicher, A. L.; Pratt, K. L.
Antioxidant Role and Subcellular Location of Hypotaurine and
Taurine in Human Neutrophils. Biochim. Biophys. Acta 1991, 1073,
91−97.
(57) Green, T. R.; Fellman, J. H.; Eicher, A. L. Myeloperoxidase
Oxidation of Sulfur-Centered and Benzoic Acid Hydroxyl Radical
Scavengers. FEBS Lett. 1985, 192, 33−36.
(58) Kearns, S.; Dawson, R., Jr. Cytoprotective Effect of Taurine
Against Hypochlorous Acid Toxicity to PC12 Cells. Adv. Exp. Med.
Biol. 2000, 483, 563−570.
(59) Learn, D. B.; Fried, V. A.; Thomas, E. L. Taurine and
Hypotaurine Content of Human Leukocytes. J. Leukocyte Biol. 1990,
48, 174−182.
2208
dx.doi.org/10.1021/jp411790v | J. Phys. Chem. A 2014, 118, 2196−2208