Reduction mechanism of a coordinated superoxide by thiols in acidic media

Article abstract of DOI:10.1039/b918582h

In weakly acidic media ([H⁺], 0.01-0.06 M), 2-mercaptoethanol (mercap, RSH), thioglycolic acid (tga, R′SH) and l-cysteine (cys, R′′SH) reduce the superoxo ligand of the complex ion, {μ-amido-μ-superoxo-bis[tetraamminecobalt(iii)]}⁴⁺ (1) to its corresponding hydroperoxo complex, {μ-amido-μ-hydroperoxo- bis[tetraamminecobalt(iii)]}³⁺ (2). During this act, RSH and R′SH are quantitatively oxidized to their respective disulfides. However, cysteine (R′′SH) is converted to a mixture of ～80% of the disulfide, cystine and ～20% to cystine sulfinic acid. Cystine itself is not a source of the sulfinic acid. Dissolved copper, even at the impurity level, dramatically catalyzes the reaction such that the direct reactions are inaccessible. Nevertheless, the catalyzed path can be masked completely with 0.20 mM dipicolinic acid and it can be determined for the first time that, the direct reactions are first-order in [1], in [total thiol] and in basicity. Rate decreases linearly with increasing mol% of D₂O in the solvent. H-atom (H⁺ + e) transfer from thiols to superoxide in 1 seems logical for the conversion of 1 to 2. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b918582h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Reduction mechanism of a coordinated superoxide by thiols in acidic media†
Ritu Mishra, Subrata Mukhopadhyay and Rupendranath Banerjee*
Received 8th September 2009, Accepted 22nd December 2009
First published as an Advance Article on the web 3rd February 2010
DOI: 10.1039/b918582h
+
In weakly acidic media ([H ], 0.01–0.06 M), 2-mercaptoethanol (mercap, RSH), thioglycolic acid (tga,
R¢SH) and L-cysteine (cys, R¢¢SH) reduce the superoxo ligand of the complex ion,
4
+
{
m-amido-m-superoxo-bis[tetraamminecobalt(III)]} (1) to its corresponding hydroperoxo complex,
3+
{
m-amido-m-hydroperoxo-bis[tetraamminecobalt(III)]} (2). During this act, RSH and R¢SH are
quantitatively oxidized to their respective disulfides. However, cysteine (R¢¢SH) is converted to a
mixture of ~80% of the disulfide, cystine and ~20% to cystine sulfinic acid. Cystine itself is not a source
of the sulfinic acid. Dissolved copper, even at the impurity level, dramatically catalyzes the reaction
such that the direct reactions are inaccessible. Nevertheless, the catalyzed path can be masked
completely with 0.20 mM dipicolinic acid and it can be determined for the first time that, the direct
reactions are first-order in [1], in [total thiol] and in basicity. Rate decreases linearly with
+
increasi g mol% of D
2
O in the solvent. H-atom (H + e) transfer from thiols to superoxide in 1 seems
logical for the conversion of 1 to 2.
Introduction
Experimental
A consequence of oxygen being the ultimate electron acceptor in
Materials
respiration is the generation of reactive oxygen species (ROS),
-
2-Mercaptoethanol (Aldrich) and thioglycolic acid (Aldrich)
solutions were prepared with 99% reagent grade thiols. Sodium
perchlorate solution for maintenance of ionic strength in ki-
such as superoxide (O
2
) and hydrogen peroxide (H
2
2
O ) as
byproducts, exposing biological systems to “oxidative stress”.
ROS are highly damaging since they can inactivate proteins,
damage DNA, and perturb membrane integrity, with long-term
exposure implicated in aging, cancer, and neurodegenerative
12
netic measurements was prepared from NaHCO
3
and HClO .
4
Sodium perchlorate solution was standardized using a cation
exchange resin in the acid form. All other materials used,
including L-cysteine (SRL, India), cystine (SRL, India) and
2,6-pyridinedicarboxylic acid (dipicolinic acid, Aldrich) were of
reagent grade and used without further purification. Thiols were
dissolved just prior to the kinetic studies and fresh solutions were
always used.
1-5
diseases.
Thiol/disulfide couples are important in modulating the redox
6,7
potentials at biological sites. Thiol-containing compounds are
effective antioxidants and act as scavengers for ROS and thus
protect cells from destructive effects of ROS and other free
8-11
radicals.
The reactions of superoxide with thiols therefore
The superoxo complex, m-amido-m-superoxo-bis[tetraammine-
attract much scientific interest.
19
cobalt(III)] tetranitrare (1) was synthesized by a literature process
Aliphatic thiols react with metal-coordinated superoxide
2
+
and recrystallized from 0.5% HNO . Complex 1 was characterized
3
through a trace metal ion, especially Cu ion catalyzed mech-
1
2,13
by elemental analysis, FTIR and Raman spectroscopy.
anisms, such that the direct oxidation process is inaccessible.
2
+
Even ubiquitous Cu ions present as impurities in the solution
affect the kinetics. However, by using a suitable chelating ligand,
14-17
the catalyzed path can be completely suppressed.
In the present
Instrumentation
study, we report the kinetics of the uncatalyzed, direct oxidation of
three thiols, viz., 2-mercaptoethanol (mercap, RSH), thioglycolic
Elemental analyses (H, N) of complex 1 were performed on a
Perkin-Elmer 240C elemental analyzer. IR spectra were recorded
on a Perkin-Elmer RX1 FT-IR spectrum spectrophotometer
with the sample prepared as a KBr pellet in the range 4000–
18
acid (tga, R¢SH) and L-cysteine (cys, R¢¢SH) with an authentic su-
peroxo complex, m-amido-m-superoxo-bis[tetraamminecobalt(III)]
ion (1), in aqueous acidic media.
-
1
4
00 cm and Raman spectroscopy was performed on a Scien-
tific NXR FT-Raman MODULE, Thermoelectron Corporation,
USA, Nd:YVO laser with 1064 nm excitation. Absorbance and
4
UV-vis spectra were recorded with a JascoV-1700 spectrophotome-
ter using 1.00 cm quartz cells. The kinetics was monitored in situ in
an electrically controlled thermostated (25.0 ± 0.1 C) cell housing.
Acid solutions were standardized by pH metric titration using a
Department of Chemistry, Jadavpur University, Kolkata, 700 032, India.
E-mail: rupenju@yahoo.com
◦
†
Electronic supplementary information (ESI) available: Fig. S1–S5 and
Tables S1–S5. See DOI: 10.1039/b918582h
Metrohm 736 GP Titrino autotitrator.
2
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2
+
Detection of Cu
Stoichiometry
2
+
The presence of Cu ion in the reaction blank (without the
complex) was qualitatively tested by dithiooxamide (rubeanic
Complex 1 was allowed to completely react with each of the
mercapto derivatives. The unreacted mercap, tga or cys was then
20
22,23
acid) and was further quantified with a Perkin-Elmer Atomic
Absorption Spectrophotometer, A-Analyst 200.
quantified spectrophotometrically
in separate experiments. For
reactions with cysteine, the unreacted cysteine along with cystine
sulfinic acid was additionally determined by titration of the
24
product mixture with aqueous sodium nitrite. By difference,
cystine sulfinic acid produced, if any, was estimated. No at-
tempt was made to determine sulfinic acid for mercap and tga
oxidations.
Suppression of the catalyzed path
The catalytic effect of adventitious metal ions could be completely
suppressed by the addition of 2.0 mM of dipicolinic acid (dipic).
Most probably, only Cu among all adventitious metal ions
appreciably catalyzes the reaction. We therefore probed the direct
reactions between the mercapto derivatives and 1 with the use
of 2.0 mM dipic. We also occasionally removed Cu by passing
water-scrubbed H S from a Kipp’s apparatus through the blank
reaction (without complex) for kinetic studies. The blank reaction
was then boiled to expel dissolved H S and centrifuged to remove
2
+
Reactivity of cystine
The disulfide, cystine was identified as a major oxidation product
for cysteine. It was necessary to examine the reactivity of cystine
towards 1. A mixture of 2.0 mM dipicolinic acid, 10.0 mM cystine,
2
+
2
0
.05 M HClO
4
maintained at an ionic strength 0.5 M (NaClO ) at
4
◦
25.0 C was purged with Ar. Then, 1 (1.0 mM) was added. Any
2
change in the absorbance of 1 was noted for over 4000 s (vide infra,
Results and discussion”).
the solid materials containing Cu(II) after standing for a few
hours. We further verified spectrophotometrically that such a low
concentration of the sequestering agent, dipic, does not degrade
complex 1. Atomic absorption spectroscopy (AAS) quantified an
“
Results and discussion
2
+
-6
impurity level of [Cu ] = 2.2 ¥ 10 M in the blank reaction media.
2
+
Such an impurity level of Cu ion catalyzes the conversion of cys,
tga and mercap so much so that the catalyzed reaction interferes
with the study of the direct reaction. For example, the reaction
deviated from first-order kinetics (Fig. S1, ESI†) in the absence of
Characterization of complex 1
Elemental analysis: Anal. Calc. for [(NH
3
)
4
Co(NH
2
2
,O )Co-
(
NH
IR absorption bands observed for 1 (KBr Pellet/cm ) are 3239 (s)
and 2098 (w) for n(NH), 1635 (s), 1386 (s) and 1313 (s) for das(NH )
3
)
4
](NO
3
)
4
: H, 4.73, N, 33.1. Found: H, 4.63, N, 33.2%. Main
-
1
21
dipic. Calculations using known equilibrium constants for the
reactions of dipic with H and Cu demonstrate that even at 0.05
+
2+
3
25
2
+
and 825 (s) for r
this complex (n, stretching vibration; das, asymmetric deformation
vibration; r rocking vibration). The observed Raman shift of
074 cm for the O–O stretching (Fig. S3, ESI†) also confirms the
r
(NH
3
) match well with the reported values for
M HClO
[Cu(dipic)
order kinetics could be observed in the presence of 2.0 mM dipic
Fig. S2, ESI†). Table S1 (ESI†) further demonstrates catalysis by
4
, 2.0 mM dipic practically sequesters Cu completely
2
-
2+
11
(
2
]/[Cu ] = 3.78 ¥ 10 ). Furthermore, excellent first-
r
-
1
1
(
26
2
+
presence of the superoxo bridge. Purity of 1 was also checked by
comparing its absorbance at 700 nm (e in M cm , found 300;
reported 306).
Cu and its suppression with 2.0 mM of dipic.
-
1
-1
19
Kinetic measurements
Reaction order
All the kinetic runs were conducted in acid perchlorate media
In the presence of dipicolonic acid (dipic), the reactions of 1 with
any of the thiols used here were found to be first-order in [1] (at
either of the measuring wavelengths, vide infra), [thiol] (Fig. 1(a),
Table S2, ESI†) and basicity of the media (Fig. 1(b), S4, S5,
Tables 1 and S3, ESI†).
(
0.01–0.06 M) where the ionic strength for kinetic studies was
adjusted to 0.50 M (NaClO ). All solutions were prepared in
doubly distilled, deionized and then freshly boiled water. Complex
has two visible absorption peaks at 700 and 474 nm. Kinetics
4
1
was determined following a decrease in absorbance at 700 nm
and occasionally at 474 nm. A large excess of perchloric acid and
any one of the mercapto derivatives over 1 (0.50 mM) along with
Table 1 Second-order rate constants for the reaction of [1] (0.5 mM)
with thiols (5.0 mM) at different acidities, [dipicolinic acid] = 2.0 mM, I =
◦
2
.0 mM dipicolinic acid were used. Purified argon gas was passed
0.50 M (NaClO
4
), T = 25.0 C
for ~5 min through the reacting solutions prior to kinetic studies.
Under these conditions, self-decomposition of 1 is too insignificant
and reactions with the mercapto derivatives followed excellent
k¢/M s-1
-1
+
[H ]/M
mercap
tga
cys
first-order kinetics. Observed first-order rate constants (k
extracted by non-linear least-squares fitting of the absorbance (A
versus time (t) data to a standard first-order exponential decay
equation. The reported first-order rate constants (k ) displayed
in Tables S2 and S3 (ESI†) are the averages of at least three
independent measurements and the average coefficient of variation
o
) were
0
0
0
.010
.015
.020
0.80 ± 0.05
0.54 ± 0.03
0.36 ± 0.02
0.24 ± 0.01
0.18 ± 0.01
0.15 ± 0.01
0.12 ± 0.004
1.98 ± 0.12
1.30 ± 0.06
1.00 ± 0.06
0.70 ± 0.04
0.52 ± 0.03
0.42 ± 0.02
0.34 ± 0.02
2.48 ± 0.15
1.66 ± 0.11
1.24 ± 0.07
0.82 ± 0.04
0.62 ± 0.03
0.49 ± 0.02
0.38 ± 0.02
t
)
o
0.030
0
0
0
.040
.050
.060
(
CV) for these measurements were within 5%.
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4+
Fig. 2 Structure ofm-amido-m
ion, 2.
1
-hydroperoxo-bis[tetraamminecobalt(III)}]
1
.6 : 1 (Table S4, ESI†), was found for cys, suggesting oxidation
product for cys is not only cystine, the disulfide; but some portions
of cysteine are also “overoxidized” – probably, to cystine sulfinic
15,29,30
acid.
The reaction stoichiometries suggest that mercap and thiogly-
collic acid are quantitatively oxidized to their respective disulfides
and that cysteine is oxidized ~80% to the disulfide, cystine;
and the rest ~20% is oxidized parallel to cystine sulfinic acid,
+
HOOCCH(NH
3
)CH
2
SO H (Table S5, ESI†). Cystine however,
2
was found unreactive on the time scale of the oxidation of cysteine
and therefore, cannot be a source of the sulfinic acid.
Stoichiometric observations for cysteine require parallel reac-
tions, (eqn (1) and (2)). Reaction stoichiometry for mercap and
tga can be described by eqn (3) alone.
2
1 + 2 R¢¢SH → 2 2 + R¢¢SS R¢¢
1 + R¢¢SH + 2 H
O → 4 2 + R¢¢SOOH
1 + 2 RSH (or R¢SH) → 2 2 + RSSR (or R¢SSR¢)
(1)
(2)
(3)
4
2
2
Eqn (2) may further be amplified as a series of reactions (2a) to
2c), where (2a) + (2b) + (2c) leads to eqn (2).
(
∑
2
1 + 2 R¢¢SH → 2 2 + 2 R¢¢S
(2a)
(2b)
(2c)
+
Fig. 1 (a) Variation of k
o
with [thiol]. [1] = 0.50 mM, [H ] = 0.05 M for
∑
2
1 + 2 R¢¢S + 2 H
2
O → 2 R¢¢SOH + 2 2
H + R¢¢SH
cys, 0.03 M for mercap and tga, [dipicolinic acid] = 2.0 mM, I = 0.50 M
◦
+
(
[
NaClO
4
), T = 25.0 C. (b) Variation of k
o
with 1/[H ]. [cys] = 5.0 mM,
1] = 0.50 mM, [dipicolinic acid] = 2.0 mM, I = 0.5 M (NaClO
4
), T =
2 R¢¢SOH → R¢¢SO
2
◦
2
5.0 C.
Effect of basicity on reaction rate and the proposed reaction
Several groups have studied the kinetics of oxidation of
Spectral studies: reaction product of 1
The most common one-electron reduction product of 1 is
12,13,31,32
4
+
thiols,
but the presence and strong catalytic influence of
the m-amido-m-hydroperoxo-bis[tetraamminecobalt(III)}] ion, 2
2
+
25
Cu complicates the proposed mechanistic interpretations.
In most of the situations, increased [H ] strongly accelerates
outer-sphere reductions of superoxo complexes. The Co
(
Fig. 2). A family of spectra for reactions of 1 with any of the
+
mercapto derivatives recorded at different times maintain three
isosbestic points at 420, 520 and 575 nm and the final spectrum is
also closely similar in shape and peak positions (Fig. 3) to those
determined for similar m-hydroperoxo binuclear Co(III) complexes
33
III
2
superoxo complex 1 is also a coordinatively saturated low-spin
substitution-inert species that cannot be reduced by an inner-
+
2
7,28
sphere path. Nevertheless, here an increased [H] decelerated the
with {N
5
O} coordination.
A clean conversion of the superoxo
rate. In addition, the rate decreased linearly with increasing mol%
complex 1 to the hydroperoxo complex 2 is therefore anticipated.
of D O in the solvent (vide infra, solvent isotope effect).
2
Inactivation of 1 at rapid equilibrium to form a kinetically
Stoichiometry and the reaction products
+
dead-end species 1H by H could thus be the only source of the
+
-1
Measurements of the amount of unreacted reducing agents,
[H ] -dependent rate associated with an electroprotic reaction,
+
mercap and tga indicated a consumption ratio, D[1]: D[thiol] = 1 : 1
in which the –SH group transfers a H ion simultaneously with
(
Table S4, ESI†). But a smaller consumption ratio, D[1]: D[cys] =
an electron (eqn (4)–(7)). The thiols selected can neither give up
2
694 | Dalton Trans., 2010, 39, 2692–2696
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+
Fig. 3 Spectra at different times of 0.50 mM of 1 reacting with 5.0 mM cys. [H ] = 0.05 M, [dipicolinic acid] = 0.20 mM, I = 0.50 M (NaClO
4
), T =
◦
2
5.0 C. (a): Spectrum of the pure complex (b)–(n): spectra of reaction mixtures at 25, 50, 100, 150, 220, 320, 400, 500, 640, 820, 1050, 1475, 1650 s,
respectively.
-
+
34
+
(
ꢀ
RSH ꢀ RS + H ; pK
a
~8–10) nor accept a proton (RSH + H
the parent complex, 1 is found overwhelmingly more reactive over
its protonated form 1H. This observation, we presume, refutes a
simple electron transfer and suggests an electroprotic mechanism,
where an electron and a proton are simultaneously transferred
from a thiol to the superoxo ligand in 1. Thus 2, the hydroperoxo
analogue of 1 is formed (eqn (5)). In contrast to 1, 1H has no room
to accommodate one further proton from the thiol at the rate step
of an electroprotic transfer and hence is non-reactive.
+
35
RSH ; pK ~ -7)
2
+
K
+ H ꢂꢀ ꢀꢀ ꢀꢁꢀꢀ 1H
(
(
4)
5)
1
k
•
1
+ RSH  → 2 + RS
∑
RS → RSSR (fast)
(6)
(7)
∑
RS → RSO
2
H (fast)
The thiol forms are kinetically inactive for outer-sphere electron
17
+
transfer reactions. The present study indicates that the thiol form
can become kinetically active through an electroprotic path.
Eqn (4)–(7) signifies that in acidic media H scavenges 1 to form
kinetically inactive 1H; residual 1 via an electroprotic path oxidizes
a mercapo derivative mainly to the corresponding disulfide and
also to sulfinic acid in the case of cysteine.
Formation of thiyl radical (RS ) from oxidation of thiols, their
subsequent, rapid dimerization and a rapid parallel reaction to
Solvent isotope effect
∑
We observed a significant retardation in k
o
values when solvent
o
o
36
H O is enriched with D O (k D2O/k
= 0.46 ± 0.0014), for the
sulfinic acid are well-known facets.
The derived rate law for the eqn (4)–(7) is eqn (8).
2
2
H2O
+
reduction of 1 (0.50 mM) by mercap (5.0 mM) at [H ] = 0.03
◦
M, I = 0.5 M (NaClO
4
), T = 25.0 C). Moreover, the plot of k
o
+
k
o
= k[thiol]/(1 + K[H ])
(8)
versus mol% of D
2
O in the solvent media is found to be linear
+
-1
(Fig. 4, r > 0.98). This supports an electroprotic mechanism,
However, plots of k
and S4, ESI†) and also k
Fig. 1(a)) were found to be linear with statistically insignificant
o
versus [H ] at a fixed thiol (Fig. 1(b), S3
37
with one proton transfer at the rate step. Indeed, an inverse
solvent kinetic isotope effect, k /k < 1, could be expected if 1H
o
versus [thiol] plots at a particular acidity
(
H
D
were the kinetically active species. This supports our presumed
intercept and thus eqn (8) is reduced to eqn (9).
+
k
o
= k[thiol]/K[H ]
(9)
The slopes of the straight lines in Fig. 1(a) yielded the composite
3
constant k/K. The 10 k/K values thus obtained for the thiols
decrease in the order:
L-Cysteine (25.1 ± 1.5) < thioglycolic acid (19.2 ± 1.1) < 2-
mercaptoethanol (8.03 ± 0.05).
The slopes of the linear plots of k versus [thiol] at different
o
-
1
-1
acidities yielded the second-order rate constants (k¢, M s ) for
the oxidation of the thiols (Table 1).
Generally, protonation of a parent complex greatly enhances
its kinetic activity as an oxidant. In the present work, however,
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Engineering and Science University, Shibpur, Howrah 711 103,
West Bengal for recording Raman spectrum of complex 1.
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+
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1
1
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38
Free superoxide is known to be a strong base; (pK ~ -4.88 )
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39
1
9 R. Davies, M. Mori, A. G. Sykes and J. A. Weil, Inorg. Synth., 1982,
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2
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+
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2
III
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2
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4
, (NH ) ,
3
4
III 33
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4
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2+
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-1
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1
7, 3317.
◦
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-
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3
41
<
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2
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3
3
3
3
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Acknowledgements
The work was carried out with the financial assistance from
CSIR (New Delhi) and DST (New Delhi). The award of a JRF
UGC, New Delhi) to R. M. is gratefully acknowledged. We also
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4
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(
thankfully acknowledge the Department of Chemistry, Bengal
2
696 | Dalton Trans., 2010, 39, 2692–2696
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