Oxyhalogen-sulfur chemistry: Oxidation of taurine by chlorite in acidic medium

Article abstract of DOI:10.1021/jp963067a

The reaction between chlorite and the aminosulfonic acid, taurine, has been studied in neutral to acidic pH. The stoichiometry of the reaction was deduced as 3ClO₂^- + H₂NCH₂CH₂SO₃H + 3H⁺ a?? Cl(H)NCH₂CH₂SO₃H + 2ClO₂ + 2H₂O. The formation of chlorotaurine is rapid and is followed by a slower accumulation of chlorine dioxide. The chlorotaurine disproportionates at low pH to give dichlorotaurine and taurine. There is no appreciable reaction between chlorine dioxide and any of the amine species in solution. The reaction is characterized by an induction period during which the reactive species HOCl and H(OH)NCH₂CH₂SO₃H are formed. This is followed by the autocatalytic production of chlorotaurine and chlorine dioxide. The autocatalysis is mediated through the formation of the intermediate Cl₂O₂, which is typical of the reactions of chlorite. One notable result obtained in this study was that the C-S bond in taurine does not cleave even when subjected to a strong oxidizing agent, HOCl.

Full text of DOI:10.1021/jp963067a

J. Phys. Chem. B 1997, 101, 1207-1214
1207
Oxyhalogen-Sulfur Chemistry: Oxidation of Taurine by Chlorite in Acidic Medium1
Cordelia R. Chinake*
Department of Chemistry, UniVersity of Natal, P.O. Box X01, ScottsVille 3209, South Africa
Reuben H. Simoyi*,†
Department of Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506-6045
ReceiVed: October 7, 1996; In Final Form: December 10, 1996X
The reaction between chlorite and the aminosulfonic acid, taurine, has been studied in neutral to acidic pH.
The stoichiometry of the reaction was deduced as 3ClO2- + H2NCH2CH2SO3H + 3H+ f Cl(H)NCH2CH2SO3H
+ 2ClO2 + 2H2O. The formation of chlorotaurine is rapid and is followed by a slower accumulation of
chlorine dioxide. The chlorotaurine disproportionates at low pH to give dichlorotaurine and taurine. There
is no appreciable reaction between chlorine dioxide and any of the amine species in solution. The reaction
is characterized by an induction period during which the reactive species HOCl and H(OH)NCH2CH2SO3H
are formed. This is followed by the autocatalytic production of chlorotaurine and chlorine dioxide. The
autocatalysis is mediated through the formation of the intermediate Cl2O2, which is typical of the reactions
of chlorite. One notable result obtained in this study was that the C-S bond in taurine does not cleave even
when subjected to a strong oxidizing agent, HOCl.
Introduction
which catalyzes the reactions of hydrogen peroxide with
halogens. The enzyme myeloperoxidase, which is abundant in
Taurine, H2NCH2CH2SO3H, is a sulfur-containing amino acid
found ubiquitously in living organisms and often in concentra-
tions exceeding those of all the other amino acids.2 It is a
crystalline, colorless compound which is soluble in water, but
not in ethanol or ether. At 25 °C it has a pKa of 1.5 and a pKb
of 8.74.3 Taurine can be obtained from natural sources such
as ox bile and molluscs or by organic synthesis.4 Its derivatives
include 2-aminoethanesulfinic acid, 2-guanidoethanesulfonic
acid, 2-hydroxyethanesulfonic acid, 2-ureidoethanesulfonic acid,
and 2-aminoethanethiosulfonic acid.5 Although taurine was first
identified in 1827,6 the question of its functional importance in
plant and animal physiology is still not completely resolved.
Many studies have implicated taurine and its derivatives in
several physiological roles such as (i) osmoregulation in marine
invertebrates, (ii) energy storage in certain worms, (iii) inhibition
of nerve impulses in invertebrate and vertebrate nerve tissue,
(iv) elimination of cholesterol in vertebrates, and (v) regulation
of excitability in various tissues.7 Taurine and isethionic acid,
HO-CH2CH2-SO3H, have been shown to inhibit calcium-
activated respiration in rat liver mitochondria.8 Taurine require-
ments can be met through dietary supplements or through several
biosynthetic pathways in some organisms.2 There is an equi-
librium between newly synthesized taurine and taurine leaving
the cells or being metabolized in other pathways.2
Taurine is involved in the killing of foreign microorganisms
by the immune system.9 The most abundant cells in the human
body’s defensive mechanism are the granulocytes which contain
a multilobed nucleus and cytoplasmic granules containing potent
enzymes.10 There are three types of granulocytes: neutrophils,
eosinophils, and basophils.
Stimulated granulocytes produce oxidizing agents (e.g., H2O2)
and secrete granular proteins, which contribute to their antimi-
crobial, cytotoxic, and cytolytic activities, into the extracellular
medium.9 Each group of cells contains a specific peroxidase
neutrophils, catalyzes the oxidation of Cl- ions by H2O2 to yield
HOCl:
myeloperoxidase/H
H2O2 + Cl-
+8 HOCl + H2O
(R1)
HOCl is capable of destroying a variety of microoganisms and
mammalian cell targets.11 It is also believed to be involved in
the inflammatory response.11 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.12 The most important of such compounds is the
â-amino acid taurine:
HOCl + H2NCH2CH2SO3H f
taurine (Tau)
ClHNCH2CH2SO3H + H2O (R2)
Taurine chloramine
(TauNHCl)
A second chlorine atom can be added to give dichlorotaurine
(TauNCl2), which is a stronger lytic agent.12
The chloramine derivatives can thus continue to moderate
the neutrophil toxicity long after HOCl has been depleted. The
N-Cl derivatives contain oxidized chlorine (Cl+).12 The
toxicity of a particular N-Cl is determined by its overall
structure and not just the N-Cl bond. The reaction of taurine
with HOCl prevents the attack of cellular components by
HOCl.12 Erythrocytes take up and detoxify TauNHCl and
TauNCl2.12 However, at very high concentrations of TauNHCl
and TauNCl2, they both attack the cells by oxidizing heme and
protein sulfhydryl groups, thus inhibiting energy metabolism.12
Recent work by Folkes et al. has looked at the myeloper-
oxidase-catalyzed reactions of HOCl with glutathione, ascorbate,
and taurine.13 Their experimental data on the HOCl-taurine
system confirm that the reaction follows an electrophilic rather
† Author to whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, February 1, 1997.
S1089-5647(96)03067-2 CCC: $14.00 © 1997 American Chemical Society

1208 J. Phys. Chem. B, Vol. 101, No. 7, 1997
Chinake and Simoyi
Figure 1. (a) NMR spectrum of taurine in D2O showing the two triplets from the R- and â- sets of methylene protons. The sharp peak at δ ) 4.67
ppm is from the solvent. (b) Structure of taurine showing the R-, â-, and γ-protons. Due to rapid exchange, γ-protons are not observed.
than a free radical mechanism. 13C-NMR studies of the HOCl-
taurine reaction have shown that N-chlorotaurine is formed and
is stable at pH 7.0.14 The 13C-NMR of [13C]taurine (28% of
[1-13C]taurine + 72% of [2-13C]taurine) shows two methylene
singlets at 49.9 and 37.9 ppm.14 These are assigned to HO3-
SCH2- and -CH2NH2, respectively. When 13C-labeled taurine
and HOCl are mixed, two new peaks are obtained at 53.1
(-CH2NHCl) and 51.3 ppm (HO3SCH2-).14
Although the presence of TauNHCl and TauNCl2 in intact
cells has been demonstrated, their chemical characterisitics and
complete functions are not fully understood. A full understand-
ing of the reactivity of taurine can help in our understanding of
its unique physiological role. We present in this paper a pure
reaction kinetics and mechanism study of the reaction of taurine
with chlorite because there is a need to understand the basic
chemical reaction kinetics of taurine and chlorotaurine. There
are several studies which focus on the physiological role of
taurine, but without a firm grasp of its in vitro reactivity, it is
impossible to decipher the mechanism by which it acts in vivo.
chlorine dioxide formed was measured spectrophotometrically.
1H-NMR spectra of taurine and chlorotaurine were obtained on
a JEOL GX 270 spectrometer using D2O as solvent and internal
standard. Chlorotaurine was quantified by its ability to oxidize
iodide to iodine.19 Kinetics experiments were carried out at
25.0 ( 0.5 °C and an ionic strength of 0.5 M (NaClO4). The
reactions were followed spectrophotometrically by using the
absorbance of ClO2 at 360 nm. The first stage of the reaction
which required stopped-flow techniques was followed on a Hi-
Tech Scientific stopped-flow spectrophotometer. The reaction
was also monitored by following the formation of chlorotaurine
at 250 nm.19 A conventional Perkin-Elmer Lambda 2S UV-
vis spectrophotometer interfaced to a DEC 486/33 DX computer
was used for slower experiments.
Results
Stoichiometry. The stoichiometry of the reaction in per-
chloric acid and excess chlorite was determined to be
3ClO2- + 3H+ + H2NCH2CH2SO3H f
Experimental Section
Cl(H)NCH2CH2SO3H + 2H2O + 2ClO2 (R3)
Materials. The following analytical grade chemicals were
used without further purification: sodium perchlorate, perchloric
acid (72%), potassium iodide, sodium thiosulfate (Fisher), D2O,
and taurine (Aldrich). Technical grade sodium chlorite (Aldrich,
81%) was recrystallized from an ethanol/water mixture. Re-
crystallized solutions of chlorite were standardized iodometri-
cally by adding excess acidified iodide and titrating the liberated
iodine against standard sodium thiosulfate.16 Chlorine dioxide
was prepared by oxidizing potassium chlorate in a sulfuric acid/
oxalic acid mixture and then stored in perchloric acid at 4 °C.17
It was standardized spectrophotometrically by using its molar
absorptivity coefficient of 1265 M-1 cm-1 at 360 nm.18
Methods. The stoichiometric determinations were carried
out by mixing various ratios of chlorite and taurine and leaving
them overnight in stoppered volumetric flasks. The amount of
2-
Qualitative tests for Cl- and SO4 were negative.20 A test
for glycine, NH2CH2COOH, using Sorensen’s test for amino
acids was negative, indicating that the C-S bond was not
cleaved.21 Identification of chlorotaurine was by UV-vis and
NMR spectroscopy.15,19 The NMR spectrum of taurine gave
two triplets centered at 3.05 and 3.22 ppm at pH 4.5 (Figure
1a). The triplet at δ ) 3.05 ppm is from the R-protons while
the triplet at 3.22 ppm is from the â-protons. Due to rapid
exchange with the solvent, the γ-protons (on nitrogen) are not
observed. The labeled structure of taurine is shown in the inset
of Figure 1a.
The product’s NMR spectrum was obtained after an incuba-
tion period of up to 2 days. Figure 2a shows the spectrum of
the product solution. This spectrum shows three peaks: two

Oxyhalogen-Sulfur Chemistry
J. Phys. Chem. B, Vol. 101, No. 7, 1997 1209
Figure 2. (a) NMR spectrum of the reaction products of the ClO2--taurine reaction in excess ClO2-. The spectrum was taken after a 24 h
incubation period. The triplets from the two methylene peaks are unchanged. The singlet peak at δ ) 1.68 ppm is from the N-bonded proton. (b)
Structure of chlorotaurine derived from the NMR spectrum in Figure 2a.
triplets and a singlet in the ratio 2(t):2(t):1(s). There is a very
slight shift in the two triplets from the original taurine spectrum.
The NMR spectrum in Figure 2a confirms the structure of
chlorotaurine as that shown the inset of in Figure 2a. The single
γ-proton is the one that is observed at δ ) 1.68 ppm. This
singlet disappeared in highly acidic solutions. The spectral data
verify the results obtained by qualitative analysis which gave
negative tests for amino acids, chloride, and sulfate. Taurine
is thus a very special molecule in which the C-S bond is not
easily cleaved. Its reactivity, which is centered on the nitrogen
atom, gives it its special physiological importance.13
On prolonged standing and in acidic environments, TauNHCl
disproportionated to give taurine and dichlorotaurine:14
2Cl(H)NCH2CH2SO3H f
H2NCH2CH2SO3H + Cl2NCH2CH2SO3H (R4)
The final product of the oxidation of taurine is TauNHCl.
The final absorbance of ClO2 at 360 nm gives the value expected
from the stoichiometric equation, reaction R3.
TauNHCl has a maximum absorbance at 250 nm with a molar
absorptivity of 398 M-1 cm-1 19
By scanning the spectrum at
.
regular intervals, between 200 and 500 nm, activity was
observed at 250 and 360 nm (Figure 3). The peak at 360 nm
(chlorine dioxide) grew steadily with time. The absorbance peak
at 250 nm (TauNHCl) rose rapidly in the initial stages and was
then slowly depleted, but not to the background value. This
peak could not decay to zero because there is a small
contribution to the absorbance at 250 nm from ClO2. Within
the pH range of 4-7, TauNHCl was quantitatively formed
according to the stoichiometry of reaction R3. Below pH 4.00
there was a mixture of both chlorotaurine and dichlorotaurine.
The concentration of TauNCl2 increased in higher acid con-
centration. The dichloroamine when formed absorbs at 300 nm.
Reaction Dynamics. In acidic environments the reaction of
chlorite and taurine shows three stages. The first is a very short
Figure 3. Spectral scans at 60 s intervals of the ClO2--taurine reaction.
Activity is observed only at λ ) 250 and 360 nm. Absorbance at 250
nm shows a rapid increase followed by a slow decrease while a
monotonic increase in absorbance is observed at 360 nm (ClO2):
[ClO2-]o ) 4.0 × 10-3 M, [taurine]o ) 1.0 × 10-3 M, [H+]o ) 0.025
M.
induction period during which neither chlorotaurine nor chlorine
dioxide is formed (see Figure 4a). The induction period is about
0.05 s. TauNHCl is formed just after this induction period.
The second is a rapid formation of chlorine dioxide and
TauNHCl, and the third is a slower accumulation of chlorine
dioxide and a very slow depletion of TauNHCl (at low pH and
-
excess ClO2 conditions). Further increase in the absorbance
of ClO2 (after depletion of taurine) is due to pure oxyhalogen

1210 J. Phys. Chem. B, Vol. 101, No. 7, 1997
Chinake and Simoyi
conditions. Figure 4b shows a set of absorbance traces at high
acid concentrations. There is no initial rapid accumulation of
ClO2 under these conditions because of the high acid concentra-
tion, but rather ClO2 continues to be formed slowly over a long
period of time.
Data at 250 nm. Figure 5a shows the effect of acid on the
formation of chlorotaurine at 250 nm. The initial rapid
formation of TauNHCl can be observed as well as the
subsequent very slow consumption of TauNHCl (trace a). As
the acid is increased, formation of TauNHCl is suppressed.
Figure 5b shows the effect of taurine on the formation of
chlorotaurine at 250 nm, but in this case no acid has been added
and taurine is in overwhelming excess over chlorite. Here, the
formation of TauNHCl is nearly exponential. A test for pseudo-
first-order kinetics failed because both [H+] and [ClO2-] were
reaction variables. Figure 5c shows the slow depletion of
TauNHCl under the conditions used in Figure 3. The change
in absorbance is within a very small range (0.41 > absorbance
> 0.32) due to contribution from ClO2 at 250 nm. Determining
the contribution to the absorbance solely from TauNHCl is
complicated by the lack of a mass balance equation between
ClO2 and TauNHCl. An approximation can be made by
determining the absorbance at t∞ and equating it solely to ClO2
absorbance. Assuming second-order kinetics (reaction R4) and
no interference from the acid, a rate constant for the depletion
of TauNHCl was evaluated as 0.395 ( 0.05 M-1 s-1
.
The rate of depletion of TauNHCl gave complex dependen-
cies on the initial concentrations of taurine, chlorite and acid.
The initial rate of consumption of TauNHCl increased with
initial taurine concentration (see Figure 6a). A log-log plot
of initial rate (of consumption of TauNHCl) vs [ClO2-]o gave
a straight line of slope slightly greater than 1 (Figure 6b).
Another log-log plot of initial rate vs acid concentration gave
a straight line of slope 0.33 (R2 ) 0.99) (see Figure 6c). The
mechanism of such a complex acid catalysis can best be
understood from the following reaction network:
The depletion of taurine by protonation pushes the equilibrium
for reaction R4 to the right, thus facilitating disproportionation.
Figure 4. (a) Effect of chlorite on the initial absorbance of chlorine
dioxide at 360 nm in conditions of excess taurine. A slower increase
in absorbance follows this initial rapid increase: [taurine]o ) 0.04 M,
[H+]o ) 1.0 × 10-5 M, [ClO2-]o: (a) 0.001, (b) 0.0009, (c) 0.0008,
Mechanism
The data in Figure 4a show that there is a rapid formation of
chlorine dioxide after a short induction period. On prolonged
standing and in mildly acidic conditions, ClO2- decomposes to
give ClO2 according to the following stoichiometry:22
(d) 0.0007, (e) 0.0006, (f) 0.0005 M. (b) Effect of the addition of H+
-
on the formation of chlorine dioxide over a longer time span.: [ClO2
]
o
) 0.004 M, [taurine]o ) 0.001 M, [H+]o ) (a) 0.04, (b) 0.03, (c) 0.02
M.
5ClO2- + 4H+ f 4ClO2(aq) + Cl- + 2H2O (R6)
chemistry involving the oxychlorine species only. Reaction
mixtures of taurine and ClO2 did not show any change in ClO2
absorbance even on prolonged standing, indicating that there is
no appreciable reaction between ClO2 and taurine.
Data at 360 nm. The reaction was monitored at both 250
nm (TauNHCl) and 360 nm (ClO2). Figure 4a shows the effect
of chlorite on the first two stages of the reaction at 360 nm and
in excess chlorite. This shows the very short induction period
followed by the rapid formation of ClO2. The start of the slower
ClO2 formation can be detected about 0.5 s into the reaction in
traces a and b. Higher [ClO2-]o gave higher ClO2 concentra-
tions as predicted by the stoichiometry of reaction R3. This
initial rapid formation of ClO2 is more pronounced in low acid
It has been demonstrated that Cl-, which is always present as
an impurity in ClO2- solutions,23 is directly responsible for the
-
decomposition of ClO2 via HOCl at low pH conditions:
ClO2- + Cl- + 2H+ f 2HOCl
(R7)
Reaction R7 is, however, considered to be quite slow and would
not be effective on the time scale of Figure 4a.
-
Thus, it is the interaction of ClO2 and taurine in the initial
stages which produces the reactive intermediate HOCl:

Oxyhalogen-Sulfur Chemistry
J. Phys. Chem. B, Vol. 101, No. 7, 1997 1211
Figure 5. (a) Effect of acid on the absorbance activity at 250 nm. Major contributions to absorbance at 250 nm are from chlorotaurine. Acid
inhibits the rapid formation of chlorotaurine: [ClO2-]o ) 0.005 M, [taurine]o ) 0.04 M, [H+]o ) (a) 4.0 × 10-6, (b) 6.0 × 10-6, (c) 8.0 × 10-6
,
(d) 10.0 × 10-6 M. (b) Effect of [taurine]o on the rapid formation of chlorotaurine within the first second of the reaction. The absorbance started
decreasing after about 10 s into the reaction. No acid was added: [ClO2-]o ) 0.0005 M, [taurine]o ) (a) 0.060, (b) 0.050, (c) 0.040, (d) 0.030, (e)
0.020, (f) 0.010 M. (c) Depletion of chlorotaurine at 250 nm after the initial rapid increase shown in Figure 3: [ClO2-]o ) 4.0 × 10-3 M, [taurine]o
) 1.0 × 10-3 M, [H+]o ) 0.025 M.
ClO2- + H2NCH2CH2SO3H + H+ f
then converted to the more stable chlorotaurine:
H(OH)NCH2CH2SO3H + HOCl (R8)
RNHOH + Cl- + H+ f RNHCl + H2O (R10)
If reaction R8 is rate-determining, then the rate of reaction will
Addition of reactions R8, R9, and R10 gives the observed
be
stoichiometry of the following reaction:
-
-
-d[ClO2 ]/dt ) kR8[ClO2 ][H2NCH2CH2SO3H][H+] (1)
3ClO2- + H2NCH2CH2SO3H + 3H+ f
-
HOCl produced in reaction R8 will rapidly oxidize ClO2 to
Cl(H)NCH2CH2SO3H + 2ClO2(aq) + 2H2O (R3)
ClO2:22
Reactions R9 and R10 are fast and are not rate-determining.23
Reaction R8 is a composite of three reactions: two protonation
reactions (reactions R11 and R12) and an oxidation reaction
(reaction R13):
2ClO2- + HOCl + H+ f 2ClO2 + Cl- + H2O (R9)
The intermediate RNHOH24 (where R is -CH2CH2SO3H) is

1212 J. Phys. Chem. B, Vol. 101, No. 7, 1997
Chinake and Simoyi
a
b
c
Figure 6. (a) Effect of taurine on the depletion (disproportionation) of chlorotaurine at λ ) 250 nm. Higher initial taurine concentrations gave
higher transient chlorotaurine concentrations: [ClO2-]o ) 4.0 × 10-3 M, [H+]o ) 0.02 M. (b) A log-log plot of the initial rate of depletion of
chlorotaurine vs the initial chlorite concentration: [taurine]o ) 1.0 × 10-3 M, [H+]o ) 0.02 M. (c) A log-log plot of the initial rate of depletion
of chlorotaurine vs the acid concentration: [ClO2-]o ) 4.0 × 10-3 M, [taurine]o ) 1.0 × 10-3 M.
+
the basic moiety on taurine and its pKb. Reaction R12 will be
H2NCH2CH2SO3H + H+ h H3 NCH2CH2SO3H
Ka
1
significant below a pH of 2.5, which is the pKa of chlorous
acid.23
(R11)
The rate of reaction, from rate-determining step R13, becomes
H+ + ClO2- h HClO2
Ka2
(R12)
-
-
-d[ClO2 ]/dt ) kR13Ka1[ClO2 ][H2NCH2CH2SO3H][H+]
followed by:
(2)
which is the same as eq 1.
+
ClO2- + H3 NCH2CH2SO3H f
The monochlorotaurine does not undergo any further oxida-
tion except disproportionation as in reaction R4.14 The estab-
lishment of reaction R8 as the rate-determining step leads to
the deduction that rates of formation of monochlorotaurine and
chlorine dioxide are controlled by eq 1.
H(OH)NCH2CH2SO3H + HOCl (R13)
HOCl then rapidly reacts with taurine to form chlorotaurine:
HOCl + H2NCH2CH2SO3H f
d[RNHCl]/dt ) 1/2d[ClO2]/dt )
Cl(H)NCH2CH2SO3H + H2O (R2)
-
k′[ClO2 ][H2NCH2CH2SO3H][H+] (3)
Protolytic reaction R11 will dominate over reaction R12 due to
where k′ ) kR13Ka1.

Oxyhalogen-Sulfur Chemistry
J. Phys. Chem. B, Vol. 101, No. 7, 1997 1213
Figure 7. Schematic diagram showing the various reactions involved in the chlorite-taurine reaction. ClO2(aq) acts as a Cl(IV) sink as it undergoes
no further reaction with any other species in solution except Cl-. Tau represents taurine, RNHCl is taurine monochloramine, RNHCl2 is taurine
dichloramine, and, RNHOH is the hydroxylamine formed from taurine while NR stands for no reaction.
TABLE 1: Mechanism of the Reaction between Taurine and
Chlorite in Acidic Medium
Role of Acid. Acid has a complex role in the oxidation of
taurine. The protonation equilibria of reactions R11 and R12
have opposite effects on the rate of reaction. With reaction R13
as the rate-determining step, it can be assumed that protonation
of taurine will enhance the reaction. The total Cl(III) species
[Cl(III)]T will be given by
M1 ClO2- + HOCl + H+ h Cl2O2 + H2O
M2 HOCl + Cl- + H+ h Cl2 + H2O
M3 ClO2- + Cl- + 2H+ h 2HOCl
M4 Cl2O2 + ClO2- h 2ClO2 + Cl-
M5 ClO2- + H+ h HClO2
M6 H2NCH2CH2SO3H + H+ h [H3NCH2CH2SO3H]+
-
M7 ClO2- + [H3NCH2CH2SO3H]+
f
[Cl(III)]T ) [HClO2] + [ClO2 ]
(4a)
H(OH)NCH2CH2SO3H + HOCl
M8 H(OH)NCH2CH2SO3H + Cl- f Cl(H)NCH2CH2SO3H + OH-
M9 H2NCH2CH2SO3H + HOCl f Cl(H)NCH2CH2SO3H + H2O
M10 2ClHNCH2CH2SO3H f
and
-
[ClO2 ] ) [Cl(III)]T/{1 + Ka2[H+]}
(4b)
Cl2NCH2CH2SO3H + H2NCH2CH2SO3H
Global Reaction Mechanism. The full reaction mechanism
is shown in the schematic sketch in Figure 7. Reaction starts
from left to right in the sketch. The initiation reaction is reaction
R8, which is at the extreme left of the sketch. The global
reaction characteristics are determined by the reactivities of the
intermediate products of reaction R8, RNHOH and HOCl. A
very fast reaction ensues which forms oxidatively inert ClO2
from the reaction of HOCl with excess ClO2-. This explains
the very rapid ClO2 formation (see Figure 4a) after a very short
induction period. The HOCl concurrently also reacts with
taurine to form chlorotaurine, while Cl- formed from reaction
R9 reacts with the intermediate hydroxylamine RNHOH to also
give the more stable chlorotaurine. The stoichiometry of the
overall reaction is satisfied by the first two sets of reactions
shown in the schematic diagram. The third set of reactions
shows the disproportionation of chlorotaurine as well as the
relatively slow oxyhalogen reaction, reaction R7. The third set
shows that ClO2 is a sink for oxyhalogen species as no
appreciable reactions were observed between ClO2 and taurine
and HOCl and RNHCl. The regeneration of taurine from
chlorotaurine can trigger reaction R8 and the whole process
repeats itself.
The rate of reaction now becomes:
kR13Ka1[Cl(III)]T
1 + Ka2[H+]
d[RNHCl]
)
[H2NCH2CH2SO3H][H+] (5)
dt
At low acid, where Ka2[H+] , 1, eq 5 simplifies to eq 1. At
high acid concentrations, eq 5 predicts that acid should have a
reduced effect on the rate of reaction. Experimental results have
shown that there is indeed a reduced but not complete loss of
acid effect on the reaction rate as the acid concentration
increases. This is also due to the fact that the reaction
+
HClO2 + H3 NCH2CH2SO3H f
H(OH)NCH2CH2SO3H + HOCl + H+ (R14)
is not completely inert, but merely much slower than reaction
R12.
Formation of N-chlorotaurine. Chlorotaurine is formed quite
rapidly at the beginning of the reaction (see data at λ ) 250
nm in Figures 5a,b). Its consumption, however, is relatively
slow (Figure 5c) and is controlled by the pH of the solution.
Experimental results show that this regeneration of taurine is
also affected by the [ClO2-]/[Taurine] ratio. Formation of
chlorotaurine is quantitative (with no subsequent formation of
dichorotaurine) in low ratios (i.e., excess taurine over chlorite).
In high ratios and low pH, taurine, mono-, and dichlorotaurine
coexist.
The full set of reactions that controls this reaction is shown
in Table 1. The first five reactions are pure oxyhalogen
reactions, and the last five reactions involve reactions of the
amino acid.
-
Nonlinearities. ClO2 oxidations have a well-established
pathway which generates nonlinearities in the form of autoca-
talysis and clock mechanisms.25 The production of ClO2 and

1214 J. Phys. Chem. B, Vol. 101, No. 7, 1997
Chinake and Simoyi
ethane-thiolsulfuric Acid (AETSA) by Bromate in Acidic Medium. J. Chem.
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by the reaction system to generate the reactive species HOCl
and RNHOH. The autocatalytic nature of the formation of ClO2
and chlorotaurine can easily be explained by the formation of
the intermediate species Cl2O2 which leads to HOCl autoca-
talysis:26
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ClO2- + HOCl + H+ h Cl2O2 + H2O
Cl2O2 + 2e- + H+ f 2HOCl
(R15)
(R16)
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The reductant in reaction R16 can be taurine, chlorotaurine, or
-
ClO2
.
Conclusion
The oxidation of taurine makes interesting comparison with
the oxidation of thiocarbonyls which we have previously
studied.27 In thiourea, for example, the sulfur in the CdS bond
is successively oxidized to the sulfonic acid -C-SO3H, with
the final step being the cleavage of the C-S bond to give sulfate,
H+, and large heat evolution. This step has been shown to be
slow and was expected in the reactions of taurine. The work
reported in this paper establishes that the C-S bond in taurine
is not readily cleaved. Instead, all the chemical reactivity is at
the nitrogen center. The sulfonc acid moeity has been impli-
cated in cation transport in tissues.8,15 TauNHCl is known to
protect tissues against oxidative toxicity.12 Thus, a study of
the fundamental reaction kinetics such as undertaken here can
only contribute to a better understanding of the biological mode
of action of sulfur amino acids.
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551.
Acknowledgment. This work was supported by NSF Grant
CHE-9632592 awarded to R.H.S.
References and Notes
(1) Part 22 in the series Nonlinear Dynamics in Chemistry Derived
from Sulfur Chemistry. For part 21, see: Darkwa, J.; Mundoma, C.; Simoyi,
R. H. Oxyhalogen-Sulfur Chemistry: Nonlinear Oxidation of 2-Amino-