5584 J. Phys. Chem. A, Vol. 108, No. 26, 2004
Darkwa et al.
TABLE 1: Full Mechanisma
M6) which can disproportionate to thiosulfinates in the absence
of excess oxidant.30 Further oxidation of the sulfenic acid will
give the sulfinic acid and finally cysteic acid. Our experimental
data gave a lower-limit rate constant for direct reaction of
L-cysteinesulfinic acid and chlorine dioxide of kM18 ) 210 (
no.
reaction
M1 ClO2- + H+ h HClO2; Ka-1
M2 OCl- + H+ h HOCl
M3 RCH2SH + H+ h [RCH2SH2]+; Kb
15 M-1 s-1
.
M4 RCH2SH + ClO2- f RCH2SOH + OCl-
This mechanism is effectively a combination of the four
oxidants in the reaction mixture, HClO2, ClO2-, HOCl, and
ClO2(aq) with the four reductants, RCH2SH, (RCH2S-)2, RCH2-
SOH, and RCH2SO2H. Apart from the protolytic equilibria and
adduct formations (e.g., reactions M12, M14, M16, and M18),
all reactions involving the oxidation of a sulfur center were
assumed irreversible. The sulfur-sulfur reactions, M27 and
M28, are important for stoichiometric consistency in the
presence of excess reductant. Under such conditions, the organo-
sulfur species would disproportionate such that the more stable
sulfinic and sulfonic acids are the dominant products. The
possibility also exists for the formation of various thiosulfinates
in excess reductant conditions.
M5 RCH2SH + HClO2 f RCH2SOH + HOCl
M6 (RCH2S-)2 + ClO2- + H2O f 2RCH2SOH + OCl-
M7 RCH2SOH + ClO2- f RCH2SO2H + OCl-
M8 RCH2SO2H + ClO2- f RCH2SO3H + OCl-
M9 RCH2SH + HOCl f RCH2SOH + H+ + Cl-
M10 RCH2SOH + HOCl f RCH2SO2H + H+ + Cl-
M11 RCH2SO2H + HOCl f RCH2SO3H + H+ + Cl-
M12 (RCH2S-)2 + ClO2(aq) + H2O h (RCH2S)2‚ClO2
M13 (RCH2S)2‚ClO2 + ClO2(aq) + H2O f 2 RCH2SOH + HClO2
M14 RCH2SH + ClO2(aq) h [RCH2SClO2]-‚H+
M15 RCH2SClO2- + ClO2(aq) + H2O f RCH2SOH + 2ClO2- + H+
M16 RCH2SOH + ClO2(aq) h [RCH2S(O)ClO2]-‚ H+
M17 RCH2S(O)ClO2- + ClO2(aq) + H2O f RCH2SO2H + 2ClO2- + H+
M18 RCH2SO2H + ClO2(aq) h [RCH2S(O2)ClO2]-‚ H+
M19 RCH2S(O2)ClO2- + ClO2(aq) + H2O f RCH2SO3H + 2ClO2- + H+
M20 ClO2- + HOCl + H+ h Cl2O2 + H2O
Computer Simulations. We utilized a unique approach to
modeling the dynamics of the oxidation of cysteine by chlorite.
We modeled the simplest system first, the reaction with the
smallest number of intermediates: the cysteinesulfinic acid-
chlorine dioxide system. The kinetics parameters derived from
this calculation were used to model the cysteine-chlorine
dioxide reaction. The full reaction scheme, chlorite-cysteine,
was then finally modeled using the data derived from the other
two calculations. This approach was possible because we had
established that the oxidation of cysteine passed through the
sulfinic acid before proceeding to cysteic acid.
M21 Cl2O2 + ClO2- h 2 ClO2(aq) + Cl-
M22 RCH2SH + Cl2O2 + H2O f RCH2SOH + 2HOCl
M23 RCH2SOH + Cl2O2 + H2O f RCH2SO2H + 2HOCl
M24 RCH2SO2H + Cl2O2 + H2O f RCH2SO3H + 2HOCl
M25 RCH2SOH + HClO2 f RCH2SO2H + HOCl
M26 RCH2SO2H + HClO2 f RCH2SO3H + HOCl
M27 2RCH2SOH h RCH2SO2H + RCH2SH
M28 RCH2SOH + RCH2SO3H h 2 RCH2SO2H
a Legend: RCH2SH, cysteine (“R” represents the asymmetric center
on cysteine); (RCH2S-)2, cystine; RCH2SOH, cysteinesulfenic acid;
RCH2SO2H, cysteinesulfinic acid; RCH2SO3H, cysteic acid.
Cysteinesulfinic Acid-Chlorine Dioxide. Table 2 shows the
very short and abbreviated mechanism used to simulate the
oxidation of the sulfinic acid by chlorine dioxide. Since our
experimental data have shown no strong acid dependence for
this specific reaction, we could eliminate reactions M1 and M3
from the mechanism. Autocatalysis was not observed in the
consumption of chlorine dioxide, and thus we did not have to
include the autocatalytic production of HOCl. Reaction P1 was
assumed to be bimolecular, as deduced from our kinetics data,
even though it is written as termolecular. The only kinetics
parameters that had to be guessed were those for P2 and P3.
Kinetics parameters for P1 were estimated from this study, and
those for P4 were derived from the literature.34 The model was
insensitive to values of kP2 and kP3 as long as they were not
rate-determining. Figure 11A shows the reasonably good fit
obtained using this very simple mechanism.
In eq 2, [Cl(III)]T represents the total chlorine(III) species and
is given by the relation
-
[Cl(III)]T ) [HClO2] + [ClO2 ]
(3)
In the limit of low acid concentrations, e.g., 8 > pH > 3.0, the
terms in the denominator as well as the last term drop out giving
Rate ) kM4[RCH2SH]0[Cl(III)]T
(4)
In highly basic pH conditions, higher than physiological pH,
the thiolate anion asserts itself, giving a different mechanism
and also possibly different oxychlorine products. This range was
not studied in this report.
Cysteine-Chlorine Dioxide. The modeling of this system
was merely an extension of that for the oxidation of the sulfinic
acid. We could now utilize the kinetics parameters deduced from
the model in Table 2. The mechanism used to simulate the
cysteine-chlorine dioxide reaction is shown in Table 3. The
observation of acid dependence in Figure 8A meant that we
had to include equilibria Q14 and Q15. Since acid inhibition
was observed below pH 2, we assumed that equilibrium Q15
was more important. This proved to be the case in our
simulations. In this model the protonated thiol was assumed to
The autocatalytic production of chlorine dioxide is fueled by
the composite reaction M20 + M21 coupled with reactions
M22-M24 that contain autocatalytic production of HOCl. The
mechanism in Table 1 acknowledges three main oxidizing
species in the reaction mixture: ClO2-, HOCl, and ClO2(aq).
The intermediate we propose, Cl2O2, is only introduced to justify
the observed autocatalysis. For the autocatalysis to prevail, the
reactions of this intermediate, M22-M24, should be slower than
the reactions involving HOCl, M9-M11. The initial oxidation
of cystine should give two sulfenic acid molecules (reaction
TABLE 2: Cysteinesulfinic Acid-Chlorine Dioxide Mechanism
no.
reaction
kf; kr
P1
P2
P3
P4
2ClO2(aq) + RCH2SO2H + H2O f 2HClO2 + RCH2SO3H
HClO2 + RCH2SO2H f RCH2SO3H + HOCl
RCH2SO2H + HOCl f RCH2SO3H + H+ + Cl-
2HClO2 + HOCl h 2ClO2(aq) + H+ + Cl- + H2O
210
350
5 × 103
1.01 × 106; 1 × 102