Kinetics and mechanism of the reactions of superoxochromium(III) ion with biological thiols

Article abstract of DOI:10.1021/jp9809336

The kinetics of the oxidation of three biological thiols (L-cysteine, glutathione, and DL-penicillamine) to their sulfinic and sulfonic acid derivatives by CrOO²⁺ in aqueous perchloric acid and in the presence of 2-propanol have been studied spectrophotometrically with the aid of the initial-rates method. The kinetic order of the oxidant is 2, whereas that of the reductant is not defined. The acidity of the medium has a slight effect on the initial rates (acid catalysis for both L-cysteine and DL-penicillamine and base catalysis for glutathione). An increase of the ionic strength leads to a rise of the initial rate for both L-cysteine and DL-penicillamine, whereas the initial rate for glutathione is insensitive to the ionic strength. The reactions are inhibited by both 2-propanol and dissolved O₂ and catalyzed by Mn²⁺, whereas Ce³⁺ has almost no effect on them. At low 2-propanol concentration and in the absence of Mn²⁺ the initial rate vs temperature plots have a minimum at around 20 °C, whereas in the presence of either concentrated 2-propanol or Mn²⁺ the Arrhenius law is fulfilled. A single mechanism is proposed for the three reactions involving a CrOO²⁺/thiol complex, CrOOH²⁺, and CrO²⁺ as intermediates. The bimolecular rate constants for the reactions of the intermediate CrO²⁺ with L-cysteine and DL-penicillamine at 25.0 °C have been obtained (around 10³ M^-1 s^-1 in both cases). Some kinetic data for the decomposition of CrOO²⁺ in the absence of thiol are also given.

Full text of DOI:10.1021/jp9809336

J. Phys. Chem. A 1998, 102, 5837-5845
5837
Kinetics and Mechanism of the Reactions of Superoxochromium(III) Ion with Biological
Thiols
Joaquin F. Perez-Benito* and Conchita Arias
Departamento de Quimica Fisica, Facultad de Quimica, UniVersidad de Barcelona, Marti i Franques, 1,
08028 Barcelona, Spain
ReceiVed: January 28, 1998; In Final Form: April 27, 1998
The kinetics of the oxidation of three biological thiols (L-cysteine, glutathione, and DL-penicillamine) to their
sulfinic and sulfonic acid derivatives by CrOO2+ in aqueous perchloric acid and in the presence of 2-propanol
have been studied spectrophotometrically with the aid of the initial-rates method. The kinetic order of the
oxidant is 2, whereas that of the reductant is not defined. The acidity of the medium has a slight effect on
the initial rates (acid catalysis for both L-cysteine and DL-penicillamine and base catalysis for glutathione).
An increase of the ionic strength leads to a rise of the initial rate for both L-cysteine and DL-penicillamine,
whereas the initial rate for glutathione is insensitive to the ionic strength. The reactions are inhibited by both
2-propanol and dissolved O2 and catalyzed by Mn2+, whereas Ce3+ has almost no effect on them. At low
2-propanol concentration and in the absence of Mn2+ the initial rate vs temperature plots have a minimum at
around 20 °C, whereas in the presence of either concentrated 2-propanol or Mn2+ the Arrhenius law is fulfilled.
A single mechanism is proposed for the three reactions involving a CrOO2+/thiol complex, CrOOH2+, and
CrO2+ as intermediates. The bimolecular rate constants for the reactions of the intermediate CrO2+ with
L-cysteine and DL-penicillamine at 25.0 °C have been obtained (around 103 M-1 s-1 in both cases). Some
kinetic data for the decomposition of CrOO2+ in the absence of thiol are also given.
Introduction
CrOO2+ seems interesting enough by itself to justify an
investigation of its oxidizing properties. The study of the
Chromium(VI) is a strong carcinogen,1,2 but since it cannot
react directly with DNA at physiological pH,3 it is believed that
one or several of the metabolites (either reactive intermediates
or the final product Cr(III)) generated from its intracellular
reduction must be the actual mutagenic agents.4 Hence, in an
attempt to clarify the mechanism of its carcinogenic action, the
reactions of Cr(VI) with different biological reductants (such
as L-ascorbic acid,5,6 hydrogen peroxide,7,8 or sulfhydryl-
containing biomolecules9,10) under near-physiological pH condi-
tions have received considerable attention lately, and in many
studies, the search for reactive intermediates has been empha-
sized. In particular, the use of difference UV spectroscopy has
reactivity of that species toward thiols was especially appealing
to us in view of the fact that, although CrOO2+ is involved as
an intermediate in the mechanism of the reactions of Cr(VI)
with most organic reductants under aerobic, acidic conditions,19
much more is currently known on its reactions with inorganic
reductants20-23 than with their organic counterparts.24
Experimental Section
Materials and Methods. The water used as solvent was
previously subjected to deionization followed by distillation and
circulation through a Millipore system. The chemical reagents
were of analytical grade, and they were used as acquired from
different sources (Aldrich, Fluka, Merck, and Sigma). The
kinetic runs were followed at 246 nm by means of a Hitachi U
2000 UV-vis spectrophotometer provided with a thermostated
compartment where the 1 cm path length quartz cells were
lodged. Usually, the solution was vigorously stirred to reach
air-saturation conditions. In the experiments where the con-
centration of oxygen had to be decreased or increased, O2- and
Ar-saturated aqueous solutions (from commercial gas bottles
and previously washed) were mixed in the desired proportion.
The concentration of dissolved O2 at the beginning of each
kinetic experiment was calculated from the known solubilities
of this species under O2 and air atmospheres.19,25
allowed the detection of superoxochromium(III) ion (CrOO2+
)
as one of the intermediates involved in the reactions of Cr(VI)
with biological thiols.11 Since CrOO2+ is believed to yield free
superoxide ion (O2-, a species generally considered as extremely
dangerous for living organisms12,13) by decomposition under
neutral-pH conditions,14 the possibility that CrOO2+ might
associate with other species (such as Cr(V)15,16 and Cr(III)15,17
in the mechanism involved in the development of the toxic and
carcinogenic effects of Cr(VI) should be considered.
In this article, we present the results found in a kinetic study
of the reactions of CrOO2+ with three biological thiols (L-
cysteine, glutathione, and DL-penicillamine, henceforth desig-
nated generically as RSH) in aqueous perchloric acid. Although
the acidic conditons of our work are not directly relevant to
biological studies (except perhaps in the cases of stomach cells
or tissues under acute-inflammation conditions18), the species
)
Synthesis of Superoxochromium(III) Ion. This species may
be prepared either from Cr(III) or from Cr(VI) as starting
material. In the first method, Cr3+ is reduced to Cr2+ by a Zn/
Hg amalgam under anaerobic conditions and then injected into
an O2-saturated acidic aqueous solution.26 In the second method,
* Corresponding author. Fax: 34 93 4021231. E-mail: j.perez@
dept.qf.ub.es.
S1089-5639(98)00933-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/29/1998

5838 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Perez-Benito and Arias
Cr(VI) is reduced by a suitable two-electron organic reductant
(we chose 2-propanol, 2-PrOH) in an acidic aqueous solution
containing O2. Scott et al.19 have reported the formation of
CrOO2+ as a relatively stable intermediate in the reduction of
-
HCrO4 under those conditions. Although higher yields of
CrOO2+ can be obtained by the first method, it is usually
contaminated by a small amount of Cr(IV) (in the form of
CrO2+), which shows a very high reactivity toward thiols.
Hence, we preferred the second method, since the contaminants
present in this case (excess 2-propanol, acetone, and inert Cr-
(III) complexes) show no reactivity toward either CrOO2+ or
thiols. Moreover, this method can be more easily implemented
than the amalgam method, so that a fresh CrOO2+ solution may
be prepared right before each kinetic run, thus avoiding the
problems related with the aging of the Cr(II) solution used in
the first method.
Figure 1. Dependence of the initial rate on the initial concentration
of CrOO2+ for its decomposition in the absence of thiol (empty triangles,
at [HClO4] ) 1.61 M, [2-PrOH] ) 0.231 M, and [Mn2+] ) 1.30 ×
10-3 M) and for its reactions with the thiols (at [RSH]o ) 9.51 × 10-4
M, [HClO4] ) 1.71 M, [2-PrOH] ) 0.245 M, and [Mn2+] ) 0):
L-cysteine (empty circles), glutathione (filled circles), and DL-penicil-
lamine (filled triangles) in the presence of oxygen (2.60 × 10-4 M) at
25.0 °C. Inset: double-logarithmic plots for the four reactions; slopes:
1.71 ( 0.01 (decomposition), 2.16 ( 0.04 (L-cysteine), 2.08 ( 0.04
(glutathione), and 1.99 ( 0.03 (DL-penicillamine).
Previous to each kinetic experiment, 20 mL of an air-saturated
aqueous solution 1.02 × 10-4 M in Cr(VI) (from potassium
dichromate), 0.261 M in 2-propanol, and 1.83 M in HClO4 were
left to react in a thermostated volumetric flask, and an aliquot
of it was introduced into the spectrophotometer. When the
absorbance at 246 nm indicated that the concentration of
CrOO2+ was at its peak and no Cr(VI) could be detected at
352 nm, the reaction with the thiol was started. To that end, in
a typical experiment, 15 mL of the so-prepared superoxide
solution was mixed with 10 mL of another thermostated aqueous
solution containing the desired concentration of thiol (and, when
necessary, other substances); after stirring, an aliquot was
introduced into the spectrophotometer to follow the kinetics of
the reaction. The method used to prepare the superoxide proved
to be both ready and reproducible; when prepared at 25.0 °C,
the time necessary to obtain the CrOO2+ from Cr(VI) was 6.5
min, and the yield (estimated from the absorbance readings using
the value ꢀ245 ) 7400 M-1 cm-1 for the superoxide27) was 28
( 1%.
not in large excess with respect to CrOO2+. Therefore, the
kinetic data reported in this work were obtained by application
of the initial-rates method, and they are the averages of two
independent, duplicated determinations. A total number of
around 1500 kinetic experiments was performed, and the typical
standard deviation from two duplicated initial rates was (3%.
Results
Reaction Products. The products found from the oxidation
of L-cysteine (RSH) by CrOO2+ in acidic media were L-
cysteinesulfinic acid (RSO2H) and L-cysteic acid (RSO3H), but
no L-cystine (RSSR) could be detected under the conditions of
our experiment. This contrasts with the situation found for other
similar reactions, since thiols are oxidized by most oxidants
(among them, some closely related to CrOO2+, such as Cr(VI),31
Analysis of Products. The organic products of the reactions
between CrOO2+ and two different reductants (L-cysteine and
L-cysteinesulfinic acid) were identified by HPLC. Commercial
samples of L-cysteine, L-cysteinesulfinic acid, L-cysteic acid,
and L-cystine were used as references. All the samples were
subjected to precolumn derivatization of the amino groups with
trinitrobenzenesulfonic acid (TNBS).28-30 The derivatives were
separated on a C18 reversed-phase column, using as eluent an
acetonitrile-aqueous phosphate (10:90, v/v) mixture, the aque-
ous part consisting of a solution 0.01 M in NH4H2PO4 whose
pH was previously adjusted to 3.5 with a few drops of 4 M
a diperoxochromium(IV) complex,32 O2- 33 and a superoxoco-
,
balt(III) complex34) to the corresponding disulfides. However,
the case of hydrogen peroxide is special: although under
ordinary conditions the RSH compounds are oxidized by H2O2
to RSSR,35 when the RSH molecule is bound to cobalt(III) as
a ligand, the oxidation by H2O2 leads to RSO2H,36-38 whereas
when HCO2H is also present in the medium, the oxidation of
RSH by H2O2 leads to RSO3H.39,40 On the other hand, we have
found that under acidic conditions CrOO2+ oxidizes L-cysteine-
sulfinic acid to L-cysteic acid, which is consistent with the fact
that RSO3H is a much more stable oxidation level of the sulfur
atom than RSO2H.36
H3PO4. The eluent was pumped at a flow rate of 1.0 mL min-1
.
The HPLC equipment was constituted by a Kontron 414 pump,
a Kontron 465 autosampler, a Merck-Hitachi D-2500 integrator,
and an Applied Biosystems 1000 S diode-array detector. The
column effluent was monitored at two wavelengths (335 and
430 nm). The identification of the TNBS derivatives of the
reaction products was based on a comparison of both their
corresponding retention times and UV-vis spectra with those
obtained for the commercial compounds used as references.
Kinetic Data. Given the instability of CrOO2+, it decom-
poses even if no reductant is present in the medium.41
Consequently, besides the three CrOO2+-RSH reactions, we
have also studied the kinetics of the slow decomposition of
CrOO2+ in the absence of thiol.
Kinetic Experiments. The absorbance at 246 nm decreased
continuously as time elapsed for the decomposition of CrOO2+
in the absence of thiol, as well as for its reactions with L-cysteine
and DL-penicillamine. However, in the case of the reaction with
glutathione the absorbance first decreased, passed through a
minimum, and then started to increase slowly. Because of this,
the use of an integrated method in the latter case was judged
unreliable. Moreover, in some of the experiments the thiol was
The initial rates (Vo) of the four reactions led to upward-
concave plots when represented against the initial concentration
of oxidant (Figure 1), and the slopes of the log Vo vs log
[CrOO2+]o plots (Figure 1, inset) all were near 2, although for
the decomposition reaction the apparent kinetic order found for

Reactions of Superoxochromium(III) Ion
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5839
TABLE 1: Effect of the Acidity of the Medium on the
Initial Rates of the CrOO2+-RSH Reactionsa
Vo (10-8 M s-1
)
[HClO4]
(M)b
L-cysteine
glutathione
DL-penicillamine
1.10
1.28
1.46
1.64
1.83
2.01
2.19
2.38
2.56
2.74
3.18 ( 0.04
2.93 ( 0.07
3.04 ( 0.05
3.21 ( 0.12
3.32 ( 0.03
3.50 ( 0.03
3.46 ( 0.10
3.53 ( 0.05
3.73 ( 0.01
3.84 ( 0.06
7.08 ( 0.04
6.96 ( 0.18
6.72 ( 0.06
6.27 ( 0.05
5.88 ( 0.23
5.37 ( 0.08
5.65 ( 0.08
5.53 ( 0.03
5.62 ( 0.10
5.60 ( 0.24
2.22 ( 0.08
2.25 ( 0.08
2.46 ( 0.18
2.22 ( 0.01
2.13 ( 0.10
2.22 ( 0.03
2.32 ( 0.02
2.36 ( 0.05
2.51 ( 0.02
2.61 ( 0.02
a [CrOO2+]o ) 1.71 × 10-5 M, [RSH]o ) 6.09 × 10-4 M, [2-PrOH]
) 0.157 M, [O2]o ) 2.60 × 10-4 M, and 25.0 °C. b The ionic strength
was kept constant with an adequate amount of sodium perchlorate, so
that I ) [HClO4] + [NaClO4] ) 2.74 M.
TABLE 2: Effect of the Ionic Strength of the Medium on
the Initial Rates of the CrOO2+-RSH Reactionsa
Vo (10-8 M s-1
)
[NaClO4]
(M)b
L-cysteine
glutathione
DL-penicillamine
Figure 2. Dependence of the initial rate on the initial concentration
of reductant for the reactions of CrOO2+ (1.71 × 10-5 M) with DL-
penicillamine (up) and L-cysteine (down) at [HClO4] ) 1.10 M, [O2]o
) 2.60 × 10-4 M, and 25.0 °C: (empty squares) [2-PrOH] ) 0.157 M
and [Mn2+] ) 8.86 × 10-4 M; (filled squares) [2-PrOH] ) 0.157 M
and [Mn2+] ) 0; (empty circles) [2-PrOH] ) 0.209 M and [Mn2+] )
0; (filled circles) [2-PrOH] ) 0.261 M and [Mn2+] ) 0; (empty
triangles) [2-PrOH] ) 0.314 M and [Mn2+] ) 0; (filled triangles)
[2-PrOH] ) 0.418 M and [Mn2+] ) 0; (empty diamonds) [2-PrOH] )
0.680 M and [Mn2+] ) 0; (filled diamonds) [2-PrOH] ) 2.77 M and
[Mn2+] ) 0. The initial-rate scales are situated on the left for all the
plots except those corresponding to empty squares (to be read on the
right).
0.00
0.18
0.37
0.55
0.73
0.91
1.10
1.28
1.46
1.64
1.95 ( 0.13
2.10 ( 0.07
2.21 ( 0.01
2.41 ( 0.14
2.40 ( 0.04
2.57 ( 0.10
2.66 ( 0.08
2.91 ( 0.18
3.13 ( 0.20
3.41 ( 0.01
6.91 ( 0.01
7.13 ( 0.32
6.82 ( 0.01
6.78 ( 0.07
6.28 ( 0.82
7.01 ( 0.12
6.89 ( 0.10
6.90 ( 0.07
6.85 ( 0.18
7.01 ( 0.06
1.65 ( 0.07
1.61 ( 0.04
1.68 ( 0.03
1.76 ( 0.01
1.75 ( 0.10
1.86 ( 0.11
1.99 ( 0.01
2.02 ( 0.02
2.14 ( 0.03
2.32 ( 0.07
a [CrOO2+]o ) 1.71 × 10-5 M, [RSH]o ) 6.09 × 10-4 M, [HClO4]
) 1.10 M, [2-PrOH] ) 0.157 M, [O2]o ) 2.60 × 10-4 M, and 25.0
°C. b The values of the ionic strength can be obtained as I ) [NaClO4]
+ 1.10 M.
We have studied the effect of the reducing agent under
different concentrations of Mn2+ and 2-propanol. For both
L-cysteine (Figure 2, down) and DL-penicillamine (Figure 2, up)
the Vo vs [RSH]o plots showed a downward-concave curvature,
which was more pronounced as the concentration of Mn2+
increased and less pronounced as the concentration of 2-propanol
increased, so that at [2-PrOH] ) 2.77 M the plots were
practically linear. On the contrary, in the case of glutathione
(Figure 3), the Vo vs [RSH]o plots in the absence of Mn2+
showed an upward-concave curvature, which was less pro-
nounced as the concentration of 2-propanol increased (again,
at [2-PrOH] ) 2.77 M the plot was practically linear), whereas
in the presence of Mn2+ the plot showed both an upward-
concave (at low [RSH]o) and a downward-concave (at high
[RSH]o) curvature. On the other hand, the reaction between
CrOO2+ and L-cysteinesulfinic acid (Figure 3, inset) seems to
be of first order in the reductant (the Vo vs [RSH]o plot was
linear even at low [2-PrOH]).
The behavior of the reaction of CrOO2+ with glutathione was
also different from that corresponding to the other two thiols
with respect to the effects of both the acidity and the ionic
strength of the medium. At constant ionic strength, an increase
of [HClO4] led to a slight increase of Vo in the cases of L-cysteine
and DL-penicillamine and a slight decrease in the case of
glutathione (Table 1). At constant [HClO4], the addition of
NaClO4 to the solution led to an increase of Vo in the cases of
L-cysteine and DL-penicillamine, whereas in the case of glu-
tathione Vo did not change (Table 2).
Figure 3. Dependence of the initial rate on the initial concentration
of reductant for the reaction of CrOO2+ (1.71 × 10-5 M) with
glutathione at [HClO4] ) 1.10 M, [O2]o ) 2.60 × 10-4 M, and 25.0
°C: (squares) [2-PrOH] ) 0.157 M and [Mn2+] ) 8.86 × 10-4 M;
(triangles) 0.157 M and [Mn2+] ) 0; (circles) [2-PrOH] ) 0.680 M
and [Mn2+] ) 0; (diamonds) [2-PrOH] ) 2.77 M and [Mn2+] ) 0.
The [RSH]o scale is situated on the bottom for all the plots except
those corresponding to squares (to be read on the top). Inset: analogous
plot for the reaction of CrOO2+ with L-cysteinesulfinic acid at [2-PrOH]
) 0.157 M and [Mn2+] ) 0, the other experimental conditions
remaining the same as above.
CrOO2+ was a little lower, in good agreement with the rate
law proposed for the same reaction by Bakac and Espenson.20

5840 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Perez-Benito and Arias
Figure 4. Dependence of the initial rate on the concentration of
2-propanol for the decomposition of CrOO2+ (1.71 × 10-5 M) in the
absence of thiol at [HClO4] ) 1.10 M, [O2]o ) 2.60 × 10-4 M, and
25.0 °C. Right inset: detail of the former plot at low alcohol
concentrations. Left inset: effect of 2-propanol on the initial rate of
decomposition in the presence of Mn2+ (8.86 × 10-4 M), the other
experimental conditions remaining the same as above.
Figure 6. Effect of the Cr(IV)-trapping agents Mn2+ (up) and Ce3+
(down) on the initial rate of disappearance of CrOO2+ (1.71 × 10-5
M) for its decomposition in the absence of thiol (empty triangles) and
for its reactions with the thiols L-cysteine (6.09 × 10-4 M, empty
circles), glutathione (3.10 × 10-4 M, filled circles), and DL-penicillamine
(6.09 × 10-4 M, filled triangles) at [HClO4] ) 1.10 M, [2-PrOH] )
0.157 M, [O2]o ) 2.60 × 10-4 M, and 25.0 °C. The initial-rate scales
are situated on the left for all the plots except those corresponding to
empty triangles (to be read on the right).
TABLE 3: Effect of the Cr(IV)-Trapping Agentsa
reductant
Mn(II)b
Ce(III)b
decompositionc
L-cysteined
9.5 ( 0.4
3.0 ( 0.2
20.0 ( 1.8
6.1 ( 0.3
11.4 ( 0.3
0.94 ( 0.09
1.16 ( 0.07
1.16 ( 0.04
glutathionee
DL-penicillamined
a [CrOO2+]o ) 1.71 × 10-5 M, [HClO4] ) 1.10 M, [2-PrOH] )
0.157 M, [O2]o ) 2.60 × 10-4 M, and 25.0 °C. b Ratio between the
initial rate in the presence of 8.82 × 10-4 M Cr(IV)-trapping agent
(either Mn2+ or Ce3+) and the initial rate in the absence of that agent.
c [RSH]o ) 0. d [RSH]o ) 6.09 × 10-4 M. e [RSH]o ) 3.10 × 10-4 M.
forms of MnSO4 and Ce(NO3)3, respectively), the CrOO2+
-
RSH reactions were much more sensitive toward Mn2+ (Figure
6, up) than toward Ce3+ (Figure 6, down). In fact, Mn2+
strongly catalyzed the decomposition of CrOO2+ as well as its
reactions with the three thiols, whereas Ce3+ was a potent
catalyst for the decomposition, a weak catalyst for the reactions
with both glutathione and DL-penicillamine, and a weak inhibitor
for the reaction with L-cysteine (Table 3).
Figure 5. Effect of inhibitors on the initial rate of the reactions of
CrOO2+ (1.71 × 10-5 M) with thiols (6.09 × 10-4 M) at [HClO4] )
1.10 M and 25.0 °C. Up: effect of 2-propanol on the reaction with
glutathione at [O2]o ) 2.60 × 10-4 M. Down: effect of 2-propanol on
the reactions with L-cysteine (circles) and DL-penicillamine (triangles)
at [O2]o ) 2.60 × 10-4 M. Inset: effect of oxygen on the reaction
with L-cysteine at [2-PrOH] ) 0.157 M.
The effect of temperature on these reactions was rather
unusual. In the absence of Cr(IV)-trapping agents, and at low
[2-PrOH], only the decomposition of CrOO2+ followed the
For the decomposition reaction in the absence of Mn2+, an
increase of the alcohol concentration led to a definite increase
of Vo at high [2-PrOH] (Figure 4), whereas at low [2-PrOH] a
slight decrease of Vo was found (Figure 4, right inset); however,
when Mn2+ was also present, no effect of [2-PrOH] on Vo could
be detected (Figure 4, left inset). The CrOO2+-RSH reactions
were inhibited both by 2-propanol, the effect being stronger for
the case of glutathione (Figure 5 up) than for L-cysteine and
DL-penicillamine (Figure 5, down), and by dissolved oxygen
(Figure 5, inset).
Arrhenius law (Figure 7, inset), whereas for the three CrOO2+
-
RSH reactions the Vo vs T plots in the range 10-45 °C showed
both a decreasing and an increasing stretch, with a minimum at
around 20 °C (Figure 7). The latter three plots could be fitted
by nonlinear least-squares to a double-exponential law (eq 1,
where A′1 and A′2 include the initial concentrations), so that
two apparent activation energies, one negative (Ea,1) and the
other positive (Ea,2), were obtained for each CrOO2+-RSH
reaction. However, the Vo vs T plots were uniformly increasing
either when Mn2+ was present in the solution (Figure 8, up) or
when the experiments were done at very high [2-PrOH] (Figure
Manganese(II) and cerium(III) ions are efficient trapping
agents for Cr(IV).42-44 We found that, whereas the decomposi-
tion reaction was very sensitive toward both ions (used in the

Reactions of Superoxochromium(III) Ion
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5841
Figure 7. Effect of temperature on the initial rate of disappearance of
CrOO2+ (1.85 × 10-5 M) for its decomposition in the absence of thiol
(empty triangles) and for its reactions with the thiols (3.13 × 10-4 M)
L-cysteine (empty circles), glutathione (filled circles), and DL-penicil-
lamine (filled triangles) at [HClO4] ) 1.10 M, [2-PrOH] ) 0.157 M,
and [O2]o ) 2.60 × 10-4 M. The three upper curves are the best fits to
double-exponential Arrhenius laws (eq 1) for the experimental data
corresponding to the reactions with L-cysteine (Ea,1 ) - 54 ( 9 kJ
mol-1, Ea,2 ) 40 ( 4 kJ mol-1), glutathione (Ea,1 ) - 42 ( 8 kJ mol-1
,
Ea,2 ) 66 ( 2 kJ mol-1), and DL-penicillamine (Ea,1 ) - 53 ( 3 kJ
mol-1, Ea,2 ) 44 ( 2 kJ mol-1). Inset: Arrhenius plot for the
decomposition of CrOO2+ under the above conditions (Ea ) 51 ( 1 kJ
mol-1).
Figure 8. Effect of temperature on the initial rate of disappearance of
CrOO2+ (1.85 × 10-5 M) for its decomposition in the absence of thiol
(empty triangles) and for its reactions with L-cysteine (empty circles),
glutathione (filled circles), and DL-penicillamine (filled triangles) at
[HClO4] ) 1.10 M and [O2]o ) 2.60 × 10-4 M. Up: [RSH]o ) 3.13
8, down), so that good-enough Arrhenius linear plots were
obtained for the four reactions (Figure 8, insets).
× 10-4 M (for L-cysteine and DL-penicillamine) and 6.03 × 10-5
M
(for glutathione), [2-PrOH] ) 0.157 M, and [Mn2+] ) 1.77 × 10-3 M.
Down: [RSH]o ) 3.13 × 10-4 M, [2-PrOH] ) 1.20 M, and [Mn2+] )
0. Insets: respective Arrhenius plots; the activation energies for the
reactions in the presence of Mn2+ are 61 ( 1 (decomposition), 25 (
1 (L-cysteine), 34 ( 1 (glutathione), and 54 ( 2 (DL-penicillamine) kJ
mol-1, whereas for the reactions in the absence of that ion the activation
energies are 63 ( 1 (decomposition), 43 ( 2 (L-cysteine), 44 ( 2
Ea,1
RT
Ea,2
RT
V ) A′ exp -
+ A′ exp -
(1)
(
)
(
)
o
1
2
An intriguing fact was the finding that, in some of the
experiments, the value of Vo corresponding to 25.0 °C (measured
up to four times, to exclude the possibility of an accidental error)
was notably high when compared with those corresponding to
the other temperatures. This peculiar effect was observed for
the reactions of CrOO2+ with both L-cysteine and glutathione
at low [2-PrOH], either in the absence (Figure 7) or in the
presence (Figure 8, up) of Mn2+, but it disappeared at high
[2-PrOH] (Figure 8, down). On the contrary, this effect was
not observed either for the decomposition of CrOO2+ or for its
reaction with DL-penicillamine under any experimental condi-
tions. When necessary, the point corresponding to 25.0 °C was
excluded from the calculations of the activation parameters. The
activation enthalpies and entropies associated with the four
reactions under the different experimental conditions studied
are given in Table 4.
(glutathione), and 50 ( 2 (DL-penicillamine) kJ mol-1
.
effect at high [O2]o, which is again coherent with the rate law
proposed for the decomposition reaction by Bakac and Espen-
son.20
In an attempt to explore the possible interactions between
CrOO2+ and DNA, some kinetic runs were done in the presence
of adenine, but we found no effect of that molecule on either
the decomposition of CrOO2+ or its reaction with L-cysteine.
Hence, the possibility of a direct reaction between CrOO2+ and
adenine has to be excluded (at least in acidic media).
Some kinetic runs were done to study the possible interference
provoked by the Cr(III) present at the beginning of the reactions
as a consequence of the method used to synthesize the CrOO2+
.
It should be mentioned that at low temperature (10.0 °C) the
absorbance vs time plots presented in some of the experiments
an unusual profile (Figure 9). At high [2-PrOH] the plots
showed a diphasic feature (especially for the case of the reaction
with glutathione), with the absorbance decreasing at first very
rapidly and then more slowly. On the other hand, in the
presence of Mn2+ the plots showed a sigmoidal profile
(especially for the case of the reaction with DL-penicillamine),
so that the rate vs time plots were bell shaped (Figure 9, inset).
To that end, some experiments with a variable [Cr(III)]o
(obtained from the reduction of Cr(VI) by 2-propanol in aqueous
perchloric acid, allowing the solution to age for several days
until all the CrOO2+ had been converted into Cr(III)) were
performed. We could not find any noticeable effect of Cr(III)
on the initial rate of the CrOO2+-RSH reactions, which is
indeed consistent with the fact that Cr(III) is present in aqueous
solution in the form of substitution-inert complexes.22 The UV-
vis spectrum indicated that other chromium species capable of
reacting with thiols (Cr(VI), Cr(V), or Cr(IV)) were not present
in noticeable amounts as contaminants when the kinetic runs
were started. In particular, contamination by Cr(IV) (which may
be important when the CrOO2+ solution is prepared in the
Additional Experiments. A few experiments were done to
study the effect of dissolved O2 on the decomposition of CrOO2+
in the absence of thiol, the results indicating that O2 strongly
inhibits that reaction at very low [O2]o, but it has almost no

5842 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Perez-Benito and Arias
TABLE 4: Activation Parametersa
is indeed a minor effect if compared with the dramatic catalysis
that Cu2+ provokes on a parallel reaction, the oxidation of thiols
by a superoxocobalt(III) complex.34 Especially interesting is
the finding that the CrOO2+-RSH reactions were not inhibited
by concentrations of Cu2+ up to 1 × 10-4 M (unless incubated
with the thiol while the reactants were thermostated, due to the
oxidation of RSH to RSSR by O2 catalyzed by that metal
ion47,48), since it allows casting aside the mechanisms in which
b
reductant
∆H°,q (kJ mol-1
)
∆S°,q (J K-1 mol-1
)
decompositionc
L-cysteined,e
48 ( 1
-57 ( 7
-44 ( 8
-55 ( 3
37 ( 3
64 ( 2
42 ( 2
60 ( 1
41 ( 2
41 ( 2
47 ( 2
58 ( 1
22 ( 1
31 ( 1
51 ( 2
-71 ( 2
-417 ( 26
-364 ( 29
-413 ( 11
-93 ( 11
-1 ( 7
-77 ( 6
-29 ( 3
-84 ( 5
-78 ( 5
-65 ( 5
-19 ( 2
-126 ( 2
-95 ( 3
-31 ( 7
glutathioned,e
DL-penicillamined,e
L-cysteined,f
glutathioned,f
DL-penicillamined,f
decompositiong
L-cysteineh
-
O2 plays an active role as either initiator or propagator of a
chain reaction.
glutathioneh
DL-penicillamineh
decompositioni
L-cysteinej
Discussion
The finding that, under some experimental conditions, two
exponential functions are clearly involved in the Vo vs T plots
corresponding to the CrOO2+-RSH reactions (eq 1) indicates
that at least two different reaction pathways are involved in the
mechanism.
glutathionek
DL-penicillaminej
a [CrOO2+]o ) 1.85 × 10-5 M, [HClO4] ) 1.10 M, and [O2]o )
2.60 × 10-4 M. b The activation entropies are associated with the
respective pseudo-second-order rate constants (k2 ) Vo/[CrOO2+]o ) and
2
For the first pathway we propose a direct attack of the thiol
on the superoxide (eqs 2-4). The formation of an oxidant-
reductant complex is supported by the following findings: (i)
Provided that the reaction depicted in eq 2 is exothermic (as
happens with the formation of many complexes), the value of
∆H° associated with the equilibrium constant KI would be
negative, which is consistent with the fact that the energy (Figure
7) and enthalpy (Table 4) of activation for this pathway are
negative. (ii) Since it can be assumed that the formation of the
complex implies a notable increase of order in the system, the
value of ∆S° associated with KI must also be negative, which
is consistent with the fact that a negative activation entropy with
an unusually high absolute value was obtained for this pathway
(Table 4). (iii) The fast initial phase observed in the absorbance
vs time plots under some particular experimental conditions
(Figure 9) may be interpreted as the time that must elapse for
the complex to be in quasi-equilibrium with the reactants. This
would be observed only when the reaction was slow enough
(at very high [2-PrOH]) and when the value of KI was high
enough (at low temperature). Under all the other experimental
conditions the quasi-equilibrium situation would be attained
during the process of mixing the reactants.
are referred to the 1 M standard state. c [RSH]o ) 0, [2-PrOH] ) 0.157
M, and [Mn2+] ) 0. d [RSH]o ) 3.13 × 10-4 M, [2-PrOH] ) 0.157
M, and [Mn2+] ) 0. e First reaction pathway (eq 1). f Second reaction
pathway (eq 1). g [RSH]o ) 0, [2-PrOH] ) 1.20 M, and [Mn2+] ) 0.
h [RSH]o ) 3.13 × 10-4 M, [2-PrOH] ) 1.20 M, and [Mn2+] ) 0.
i [RSH]o ) 0, [2-PrOH] ) 0.157 M, and [Mn2+] ) 1.77 × 10-3 M.
j [RSH]o ) 3.13 × 10-4 M, [2-PrOH] ) 0.157 M, and [Mn2+] ) 1.77
× 10-3 M. k [RSH]o ) 6.03 × 10-5 M, [2-PrOH] ) 0.157 M, and
[Mn2+] ) 1.77 × 10-3 M.
KI
\
CrOO2+ + RSH {
} [CrOO-HSR]2+
(2)
Figure 9. Absorbance vs time plots for two kinetic runs corresponding
to the reactions of CrOO2+ (1.85 × 10-5 M) with the thiols (3.13 ×
10-4 M) glutathione (circles, [2-PrOH] ) 1.20 M and [Mn2+] ) 0)
and DL-penicillamine (triangles, [2-PrOH] ) 0.157 M and [Mn2+] )
1.77 × 10-3 M) at [HClO4] ) 1.10 M, [O2]o ) 2.60 × 10-4 M, and
10.0 °C. The time and absorbance scales are situated on the bottom
and on the left for the plot corresponding to circles, whereas for the
plot corresponding to triangles the scales are situated on the top and
on the right, respectively. The dotted line is an extrapolation obtained
with a third-degree polynomial fit of the absorbance vs time data
belonging to the second phase of the kinetic run corresponding to the
reaction with glutathione. Inset: rate vs time plot for the kinetic run
corresponding to the reaction with DL-penicillamine.
kII
[CrOO-HSR]2+ + CrOO2+ slow8
CrOOH2+ + RSO2- + Cr3+ (3)
CrOOH2+ + RSH f CrO2+ + RS• + H2O
(4)
Although the kinetic data found in this work do not provide
any insight on the nature of the complex formed in eq 2, we
might envisage it as a [RSCrOOH]2+ complex, with the
hydrogen atom bonded to the terminal oxygen atom of the
superoxide and the thiyl group bonded to the chromium atom.
After the collision of that complex with a second CrOO2+ ion,
the thiyl group of the former might combine with the superoxide
group of the latter to form sulfinate ion, liberating hydroper-
oxochromium(III) and chromium(III) ions (eq 3).
We believe that the part of the mechanism responsible for
the second exponential function in eq 1 starts with the
decomposition of the superoxide without the intervention of the
thiol (eqs 5-8). According to Bakac et al.,14 this process takes
place by the combination of a unimolecular (eq 5) and a
bimolecular (eq 7) pathway.
absence of alcohol) can be excluded in view of the high
concentration of 2-propanol (0.261 M) used to obtain the
superoxide and the value of the bimolecular rate constant
reported by Scott et al.19 for the reduction of Cr(IV) by
2-propanol (12.0 M-1 s-1, t1/2 ) 0.2 s).
Given that Cu2+ is known to catalyze the dismutation of the
superoxide radicals,45,46 we also studied the possible effect of
that metal ion on these kinetic systems. We found that Cu2+
(in the form of CuSO4) did not affect the decomposition of
CrOO2+ in the absence of thiol, whereas in the case of its
reaction with L-cysteine we found a very slight catalysis, which

Reactions of Superoxochromium(III) Ion
J. Phys. Chem. A, Vol. 102, No. 29, 1998 5843
TABLE 5: Some Relevant Reduction Potentialsa
CrOO2+ (eq 12) rather than by dimerization.52 It should be
considered that, albeit the second-order rate constant for the
latter process is expected to be higher than the one associated
with the former, this disadvantage might be easily overcome
by the very high concentration of CrOO2+ compared with that
of RS•.
redox couple
E°(V)
references
CrOO2+/CrOOH2+
CrOOH2+/CrO2+
CrO2+/Cr3+
+1.03
+2.09b
+1.71c
+1.33d
22, 49
22, 49
22, 49
50, 51
RS•/RSH
a The values for the chromium redox couples were measured at 25
°C, and the value for the thiyl/thiol couple was measured at 23 ( 2
°C. b This value must be considered as a superior limit (the real value
might be a little lower). c This value must be considered as an inferior
limit (the real value might be a little higher). d The value is rather
insensitive to the nature of the substituent (R).
RS• + CrOO2+ f RSO2- + Cr3+
(12)
Manganese(II) ion is a good one-electron reductant for Cr-
(IV).42,44 Thus, eq 13 can account for the strong catalytic effect
that those ions exert on both the decomposition of CrOO2+ and
its reactions with thiols (Figure 6, up), since that process avoids
the two-electron reduction of Cr(IV) by the alcohol (eq 10) and
the subsequent regeneration of the reactant (eq 11).
kIII
CrOO2+ slow8 Cr2+ + O2
(5)
kIV
Cr2+ + CrOO2+ 98 [CrOOCr]4+
(6)
(7)
(8)
kIX
9
8 Cr3+ + Mn3+ + 2OH- (13)
CrO2+ + Mn2+ + H2O
kV
2CrOO2+ slow8 [CrOOCr]4+ + O2
Assuming that the [CrOO-HSR]2+ complex is in quasi-
equilibrium with the reactants and that all the other intermediates
involved are in steady state, the rate law that can be deduced
from the proposed mechanism for the reaction in the presence
of manganese(II) ion is given by eq 14 (where [CrOO2+]T is
the total superoxide concentration, either free or complexed by
the thiol), whereas the rate law for the reaction in the absence
of that ion can be obtained as a particular case of eq 14 simply
by taking [Mn2+] ) 0.
[CrOOCr]4+ f 2CrO2+
Figure 7 shows that the values of the initial rate corresponding
to the decomposition of the superoxide in the absence of thiol
are too low when compared with the values corresponding to
the rate-increasing branch of the Vo vs T plots. Thus, the
decomposition cannot explain by itself the second exponential
function of eq 1. However, when the thiol is present in the
medium, it can react with the oxochromium(IV) ion in a one-
electron reduction process (eq 9), thus preventing its two-
electron reduction by 2-propanol (eq 10),19,44 and the subsequent
regeneration of the superoxide caused by the intervention of
dissolved oxygen (eq 11).19,26 Hence, through eq 9 the thiol
exerts an amplification effect on the decomposition of the
superoxide.
2K1kII[CrOO2+ 2[RSH]
]
T
d[Cr3+
]
V )
)
+
dt
1 + 2KI[RSH]
]
2{2kIIIkIV + kVIII[O2](K1kII[RSH] + 2kV)}(2kVI[RSH] + kIX[Mn2+])
T
[CrOO2+
kVIII[O2](1 + 2KI[RSH])(kVI[RSH] + kVII[2-PrOH] + kIX[Mn2+])
(14)
Equation 14 is consistent with the profiles found for the Vo
vs [RSH]o plots corresponding to the different thiols. Ef-
fectively, eq 14 predicts that the Vo vs [RSH]o plot consists of
a first stretch showing an upward-concave curvature at low thiol
concentrations (due to the existence of a quadratic term in [RSH]
in the numerator of the second addend of the rate law) followed
by another stretch showing a downward-concave curvature at
higher thiol concentrations, eventually reaching a saturation limit
when the thiol concentration is high enough. Provided that we
assume that the value of KIkII associated with the reaction with
glutathione is much higher than those for L-cysteine and DL-
penicillamine, the upward-concave stretch will be perceptible
only in the former case (Figure 3), whereas in the latter two
cases the plots will show only a downward-concave curvature
(Figure 2). Moreover, eq 14 also predicts that the downward-
concave profile is less pronounced as [2-PrOH] increases
(because the term kVII[2-PrOH] in the denominator of the second
addend of the rate law becomes more important than the term
kVI[RSH]) and, provided that the concentration of alcohol present
kVI
9
CrO2+ + RSH
8 Cr3+ + RS• + OH-
(9)
kVII
CrO2+ + (CH3)2CHOH 8 Cr2+ + (CH3)2CO + H2O (10)
kVIII
Cr2+ + O2
8 CrOO2+
(11)
We have proposed that the reaction of the thiol with either
hydroperoxochromium(III) (eq 4) or oxochromium(IV) (eq 9)
ions takes place by a direct hydrogen atom transfer, thus
involving the formation of thiyl free radicals. On the contrary,
we propose (in accordance with the second-order found for
CrOO2+, Figure 1) that the reaction with the superoxide cannot
take place by direct hydrogen-atom transfer from the thiol to a
single superoxochromium(III) ion (leading to CrOOH2+ and
RS•), but by the intervention in the process of two CrOO2+ ions
(eq 3). These assumptions are supported by the values of the
reduction potentials given in the literature22,49-51 (Table 5), since,
according to them, the transfer of a hydrogen atom from RSH
to either CrOOH2+ or CrO2+ is thermodynamically favorable
(at least under standard conditions), whereas the hydrogen atom
transfer to a single CrOO2+ ion is unfavorable. The process
depicted in eq 3, however, is likely to provide a more favorable
route for the reduction of the superoxide, since the relatively
stable sulfinate ion (RSO2-), instead of the very energetic thiyl
free radical (RS•), is formed.
in the medium is high enough, more pronounced as [Mn2+
]
increases (because the term kIX[Mn2+] in the numerator of the
second addend of the rate law becomes more important than
the term 2kVI[RSH]). It should be noticed that, according to
eq 14, manganese(II) ion behaves as a catalyst for the reactions
only when the concentration of alcohol is high enough (since
then the effect of the term kIX[Mn2+] in the numerator will be
more important than the effect of the same term in the
denominator). Given that under the experimental conditions
of our work manganese(II) ion did behave as a catalyst (Figure
6, up), that condition was actually fulfilled.
Given that we could not detect any disulfide in the reaction
product mixture, we propose that most of the thiyl radicals
(formed in eqs 4 and 9) migth disappear via oxidation by

5844 J. Phys. Chem. A, Vol. 102, No. 29, 1998
Perez-Benito and Arias
42-44
In the absence of Mn(II), according to the mechanism
proposed, the intermediate Cr(IV) disappears by reacting with
either 2-propanol or the thiol. Since the bimolecular rate
constant for the Cr(IV)/2-propanol reaction has been reported
by Scott et al.19 (kVIII ) 12.0 ( 0.4 M-1 s-1), from the study of
the competition between the alcohol and the thiol it is possible
in principle to obtain the value of the bimolecular rate constant
for the Cr(IV)/RSH reaction. A nonlinear least-squares fit of
the Vo vs [2-PrOH] experimental data to eq 14 was excellent
for both L-cysteine and DL-penicillamine (Figure 5, down),
yielding for the rate constant kVI the values (0.97 ( 0.05) ×
103 and (1.09 ( 0.07) × 103 M-1 s-1, respectively. Thus, the
reactivity of Cr(IV) toward those thiols is almost 100 times
higher than its reactivity toward 2-propanol, but 100 times lower
Cr(IV) very much like Mn2+ (eq 16),
the effect of Ce3+
on the initial rate of the CrOO2+/RSH reactions is almost
negligible when compared with that of Mn2+ (Table 3). A
possible explanation would be given by the well-known ability
of Ce4+ to react with hydroperoxochromium(III) ion regenerat-
ing the reactant (eq 17).14,20,22 Thus, the inhibition caused by
eq 17 would compensate for the catalysis caused by eq 16
(similar to that produced by Mn2+).
CrO2+ + Ce3+ + H2O f Cr3+ + Ce4+ + 2OH- (16)
Ce4+ + CrOOH2+ f Ce3+ + CrOO2+ + H+ (17)
On the contrary, most of the Mn(III) generated when Mn(II)
acts as a Cr(IV)-trapping agent might disappear by dismutation
(eq 18),53 instead of by reaction with CrOOH2+. The transient
colloidal manganese dioxide formed as a product of the latter
reaction might explain the autocatalysis observed when the
CrOO2+/RSH reactions were performed at low temperature and
in the presence of Mn2+ (Figure 9, inset), since colloidal MnO2
is known to act as an autocatalyst for many permanganate
reactions,54,55 and the decrease of temperature would favor the
adsorption of the reactants on the colloid surface.
than its reactivity toward Mn2+ 44
On the contrary, the Vo vs
.
[2-PrOH] experimental data corresponding to glutathione (Figure
5, up) did not fit well to eq 14. The reason for that unfulfillment
is that for glutathione the decrease of the initial rate as the
alcohol concentration increased was not gradual, as happened
with the other two thiols, but very sudden for [2-PrOH] < 0.3
M and much less pronounced at higher concentrations. One of
the possible explanations for this behavior might be that in the
case of the CrOO2+/glutathione reaction the intermediate Cr-
(IV) is not in steady state (as assumed in the deduction of eq
14) unless the concentration of alcohol in the medium is high
enough to compensate for the rapid reduction of CrOO2+ to
2Mn3+ + 2H2O ) Mn2+ + MnO2 + 4H+
(18)
CrO2+
.
The effect of the acidity of the medium on the CrOO2+/RSH
reactions is rather slight, since an increase of 2.5 times in
[HClO4] results in changes in Vo of around 20% (Table 1). The
effect for the case of glutathione (base catalysis) is of different
sign than for L-cysteine and DL-penicillamine (acid catalysis).
The reaction with glutathione differs also from the other two
reactions with respect to the effect of the ionic strength (Table
2), as well as with respect to other features of the reaction. This
difference is probably caused by the peptidic nature of the
glutathione molecule, with more functional groups than the
relatively simple amino acids L-cysteine and DL-penicillamine.
The finding that L-cysteic acid is one of the reaction products
from the oxidation of L-cysteine by CrOO2+ can be easily
explained by eq 15. Actually, we have observed (Figure 3,
inset) that L-cysteinesulfinic acid is capable of reducing the
superoxide and that L-cysteic acid is the major organic product.
Although eq 15 is certainly not an elementary step, the
mechanism corresponding to this side reaction is not required
for the present study, since L-cysteinesulfinate ion is expected
to behave as a quasi-stable intermediate (the sulfinic acids
derived from L-cysteine and glutathione are relatively stable
On the other hand, our experimental results indicate that in
the mechanism corresponding to the decomposition of CrOO2+
in acidic media and in the absence of thiol CrO2+ (but not
CrOOH2+) is involved as an intermediate, given that the process
is strongly catalyzed by both Mn2+ and Ce3+ (Table 3).
However, our results do not exclude the possibility that under
less acidic conditions CrOOH2+ might also be involved in the
mechanism, as proposed by Bakac et al.14
Finally, it should be noticed that for the deduction of the rate
law (eq 14) we have considered just the steps that we think to
be predominant. However, there might also be other minor steps
involved (such as the reactions of Cr2+ with CrO2+ and
[CrOOCr]4+ 14
, for instance). Moreover, it cannot be discarded
that another reaction pathway involving as slow step the
unimolecular decomposition of the superoxide/thiol complex (eq
19) instead of its reaction with a superoxochromium(III) ion
(eq 3) might also have an important contribution to the global
reaction.
[CrOO-HSR]2+ f Cr2+ + RSO2H
(19)
toward oxidation,39 which precludes the possibility of RSO2
-
Nevertheless, the relatively small effect caused by Ce3+ when
compared with that caused by Mn2+ (Table 3) seems to indicate
that CrOOH2+ is actually involved as an intermediate in the
mechanism of the CrOO2+/RSH reactions, and so, it is more
consistent with eq 3 than with eq 19.
being in steady state). Thus, its reaction with the superoxide
will not affect the value of the initial rate ([RSO2-]o ) 0).
2CrOO2+ + 3RSO2- + 5H+ ) 2Cr3+ + 3RSO3H + H2O
(15)
Acknowledgment. The authors are indebted to Prof. James
H. Espenson (Iowa State University) for helpful discussions,
and to Dr. Isidre Casals and Maria Reixach (Scientific-Technical
Services of the University of Barcelona) for their assistance with
the HPLC analysis of the reaction products.
The finding that the initial rate for the decomposition of
CrOO2+ is affected by the concentration of 2-propanol at [Mn2+
]
) 0, but not when the value of [Mn2+] is high enough (Figure
4), indicates that the alcohol does not react directly with CrOO2+
but only with CrO2+
.
The inhibition by both 2-propanol and dissolved O2 (Figure
5) and the catalysis by Mn2+ (Figure 6, up) can be taken as
strong evidence of CrO2+ being involved as an intermediate in
the mechanism of the CrOO2+/RSH reactions. However,
although Ce3+ is known to be an efficient trapping agent for
Supporting Information Available: Values of the initial
rates for the decomposition of superoxochromium(III) ion and
for its reactions with L-cysteine, glutathione, and DL-penicil-
lamine at various temperatures in the range 283-318 K under
different concentrations of 2-propanol and manganese(II) ion

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