Cu(II)-catalyzed oxidation of thiols by superoxide ligated to Co III2

Article abstract of DOI:10.1002/poc.2991

Copper(II) dramatically catalyzes the oxidation of thiols by a superoxide bridging two Co^III ions. The catalyzed path overwhelmingly dominates over the uncatalysed path and is first order in the superoxo complex concentration. The first-order rate constants show a first-order dependence in [Cu²⁺], a second-order dependence in [thiol] and linearly varies with [H⁺]^-3. On the basis of observed kinetics reported here, it is proposed that Cu(II) reacts with two thiol molecules to form a Cu ^II(thiol)₂ complex, an electron is transferred from one ligated thiol to the Cu^II center to form Cu^I(thiol) and a thiyl radical. The copper(I)-thiol complex is oxidized by the conjugate base of the title complex to regenerate Cu^II(thiol). A Cu^II/I catalytic cycle is thus believed to be responsible for the observed catalysis. Copyright

Full text of DOI:10.1002/poc.2991

Research Article
Received: 23 January 2012,
Revised: 21 May 2012,
Accepted: 12 June 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.2991
Cu(II)-catalyzed oxidation of thiols by
superoxide ligated to Co2III
Ritu Mishraa*, Rupendranath Banerjeeb and Subrata Mukhopadhyayb
Copper(II) dramatically catalyzes the oxidation of thiols by a superoxide bridging two CoIII ions. The catalyzed path
overwhelmingly dominates over the uncatalysed path and is first order in the superoxo complex concentration. The
first-order rate constants show a first-order dependence in [Cu2+], a second-order dependence in [thiol] and linearly
varies with [H+]ꢀ3. On the basis of observed kinetics reported here, it is proposed that Cu(II) reacts with two thiol
molecules to form a CuII(thiol)2 complex, an electron is transferred from one ligated thiol to the CuII center to form
CuI(thiol) and a thiyl radical. The copper(I)-thiol complex is oxidized by the conjugate base of the title complex to
regenerate CuII(thiol). A CuII/I catalytic cycle is thus believed to be responsible for the observed catalysis. Copyright
© 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: superoxide; 2-mercaptoethanol; thioglycolic acid; L-cysteine; cobalt(III); kinetics
Instrumentation
INTRODUCTION
Elemental analyses (H, N) of complex 1 were performed on a Perkin-Elmer
240 C elemental analyzer. IR spectra were recorded on a Perkin-Elmer RX1
Aliphatic thiols are vulnerable to oxidation by many oxidizing
agents and lead to disulphides, sulphoxide, and so forth,
depending on the thermodynamic strength of the oxidant.[1–6]
The thioredoxins are small proteins that demonstrate a wide
range of redox activity in plants and animals, all involving their
two redox reversible sulfhydryl groups. In biological systems,
thiols are oxidized by flavins, cytochromes, and dehydroascorbic
acid to control the cellular redox potential and prevent oxidative
damage.[7–9] Often trace metal ions, especially Cu2+ ion, catalyze
such reactions.[10,11] Even ubiquitous Cu2+ ions present as
impurities in the solution affect the kinetics.[12]
FTIR spectrum spectrophotometer with the sample prepared as a KBr
pellet in the range 4000–400 cmꢀ1, and Raman spectroscopy was
performed on Scientific NXR FT-Raman MODULE, Thermoelectron
Corporation, USA, Nd:YVO4 laser with 1064 nm excitation. Absorbance
and UV-vis spectra were recorded with a Jasco V-650 spectrophotometer
(Jasco International Co. Ltd., Hachioji, Tokyo, Japan) using 1.00 cm quartz
cells. The kinetics was monitored in situ in an electrically controlled
thermostated (25.0 ꢁ 0.1 ꢂC) cell housing. Acid solutions were standard-
ized by pH metric titration using a Metrohm 736 GP Titrino autotitrator
(Metrohm AG, Switzerland).
The present work deals with the Cu(II)-catalyzed oxidation of few
thiols, viz 2-mercaptoethanol, thioglycolic acid, and cysteine (a para-
digm for the aliphatic thiols) in aqueous acidic media by superoxide
radical ligated to m-amido-m-superoxobis[tetraamminecobalt(III)]
ion (1). In the presence of externally added Cu2+ (10–100 mM), the
catalyzed path dominates overwhelmingly over the uncatalyzed
path, kinetics of the latter had been reported earlier.[13]
Kinetic measurements
Kinetics was determined following decrease in absorbance at 700 nm,
one of the absorption peaks of the superoxo complex (1) at 25.0 ꢂC
and at ionic strength, I = 0.50 M (NaClO4). Copper(II) ion was added in
the concentration range 10–100 mM as its perchlorate salt. This amount
is in overwhelming excess over the copper impurities measured by
Atomic Absorption in the solvent water (2.2 – 2.5 ꢃ 10ꢀ6 M). A large
excess of perchloric acid and thiol over 1 (0.50 mM) in argon-saturated
media was maintained in all the kinetic runs. In the presence of externally
added Cu2+ (much more than present in the reaction media as impurity),
the reaction followed excellent first-order kinetics. The observed first-
order rate constants (ko) were extracted by nonlinear least-squares fitting
of the absorbance (At) versus time (t) data to a standard first-order
exponential decay equation.
EXPERIMENTAL
Materials
2-Mercaptoethanol (mercap) and thioglycolic acid (tga) (Aldrich, Saint
Louis, Missouri, USA) solutions were prepared with 99% reagent grade
thiols. L-cysteine (cys) (SRL, India) was dissolved in water to prepare the
stock solution. Sodium perchlorate solution for the maintenance of ionic
strength in kinetic measurements was prepared from NaHCO3 and HClO4.
All other materials used were of reagent grade and used without further
purification. Thiols were dissolved just prior to the kinetic studies and
fresh solutions were always used. Doubly distilled and then freshly boiled
water was used throughout the studies.
The superoxo complex, m-amido-m-superoxobis[tetraamminecobalt
(III)] tetranitrare (1) was synthesized using a literature process[14] and
recrystallized from 0.5% HNO3. The complex 1 was characterized by
elemental analysis, FTIR and Raman Spectroscopy.
* Correspondence to: Ritu Mishra, Department of Chemistry and Environment,
Heritage Institute of Technology, Kolkata 700 107, India.
E-mail: ritu.mishra@heritageit.edu
a R. Mishra
Department of Chemistry and Environment, Heritage Institute of Technology,
Kolkata 700 107, India
b R. Banerjee, S. Mukhopadhyay
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

R. MISHRA, R. BANERJEE AND S. MUKHOPADHYAY
Stoichiometry
product for all thiols is their disulphides, respectively.[10,19] The
most common one-electron reduction product of 1 in our
reaction condition is the m-amido-m-hydroperoxo-bis[tetraammi-
necobalt(III)}]4+ ion, 2H (Fig. 1).[20,21] A family of spectra for
reactions of 1 with cysteine recorded at different time interval
is shown in Fig. 2, and the final spectrum is also closely similar
in shape and peak positions to those determined for similar
m-hydroperoxo binuclear Co(III) complexes with {N5O} coordi-
nation.[12,20,21] A clean conversion of the superoxo complex 1
to the hydroperoxo complex 2H is therefore anticipated.
The time-resolved spectra of the reaction indicates that the
superoxo ligand of the complex 1 undergoes reduction to its
peroxo moiety, 2, and immediately takes up a proton from the
solution to be converted into protonated peroxo species 2H that
is more stable in the acidic media.[22] The overall plausible
reaction sequence is shown below, where the superoxo and
peroxo species bear their residual charges and RSH denotes all
the three thiols in general.
The complex 1 was allowed to completely react with each of the
mercapto derivatives. The unreacted mercap, tga, or cys were then
quantified spectrophotometrically[15,16] in separate experiments.
RESULTS AND DISCUSSION
Characterization of the complex 1
Elemental analysis: Anal. Calcd for [(NH3)4Co(NH2,O2)Co(NH3)4]
(NO3)4: H, 4.73, N, 33.1. Found: H, 4.63, N, 33.2%. Main IR absorp-
tion bands observed for 1 (KBr Pellet/cmꢀ1) are 3239 (s) and
2098 (w) for n(NH), 1635 (s), 1386 (s) and 1313 (s) for das(NH3)
and 825 (s) for rr(NH3) match well with the reported[17] values
for this complex (n, stretching vibration; das, asymmetric defor-
mation vibration; rr, rocking vibration). The observed Raman
shift of 1074 cmꢀ1 for the O─O stretching also confirms the
presence of the superoxo bridge.[18] Purity of 1 was also checked
by comparing its absorbance at 700 nm (e in Mꢀ1 cmꢀ1, Found
300; reported 306).[14]
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Stoichiometry and the reaction products
The reaction under the kinetic conditions indicated a consump-
tion ratio, Δ[1]: Δ[thiol] = 1:1 (Table S1). It suggests oxidation
4+
H
H
N
CoIII(NH3)4
(NH3)4CoIII
O
O
0.0
5.0x10-5
1.0x10-4
1.5x10-4
2.0x10-4
2.5x10-4
H
(2H)
[mercap]2, M2
Figure 1. Structure of m-amido-m1-hydroperoxo-bis[tetraamminecobalt
(III)}]4+ ion, 2H
Figure 3. Variation of ko with [mercap]2. [1] = 0.50 mM, [H+] = 0.20 M,
[Cu2+] = 5 ꢃ 10ꢀ5 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
0.20
0.18
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
a
b
q
400
450
500
550
600
650
700
750
800
Wavelength(nm)
Figure 2. Time-resolved spectra of 0.50 mM of 1 reacting with 5.0 mM
mercap. [H]+ = 0.2 M, [Cu2+] = 5 ꢃ 10ꢀ5 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC.
(a): spectrum of pure complex shown in red; (b)–(q): spectra of reaction
mixtures stated above at 120, 300, 420, 600, 780, 960, 1140, 1320, 1500,
1680, 1860, 2100, 2340, 2580, 2820 and 3060 s, respectively
0.0
5.0x10-5
1.0x10-4
1.5x10-4
2.0x10-4
2.5x10-4
[TGA]2, M2
Figure 4. Variation of ko with [TGA]2. [1] = 0.50 mM, [H+] = 0.60 M,
[Cu2+] = 5 ꢃ 10ꢀ5 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
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J. Phys. Org. Chem. (2012)

CU(II)-CATALYZED OXIDATION OF THIOLS BY SUPEROXIDE LIGATED TO CO2III
5x106
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
4x106
3x106
2x106
1x106
0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
0
200
400
600
[H+]-3, M-3
800
1000
[cys]2, M2
Figure 5. Variation of ko with [cys]2. [1] = 0.50 mM, [H+] = 0.20 M, [Cu2
+] = 1 ꢃ 10ꢀ4 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
Figure 8. Variation of ko/([Cu2+][cys]2) with 1/[H+]3. [1] = 0.50 mM, [cys] =
5.0 mM, [Cu2+] = 1 ꢃ 10ꢀ4 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.4x106
1.2x106
1.0x106
8.0x105
6.0x105
4.0x105
2.0x105
0.0
0
100
200
300
400
500
600
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
1/[H+]3, M-3
[Cu2+], M
Figure 6. Variation of ko/([Cu2+][mercap]2) with 1/[H+]3. [1] = 0.50 mM,
[mercap] = 5.0 mM, [Cu2+] = 5 ꢃ 10ꢀ5 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
Figure 9. Variation of ko with [Cu2+]. [1] = 0.50 mM, [mercap] = 5.0 mM,
[H+] = 0.20 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
3.5x106
3.0x106
2.5x106
2.0x106
1.5x106
1.0x106
5.0x105
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0
5
10
15
20
25
30
35
40
0.0
2.0x10-5
4.0x10-5
6.0x10-5
8.0x10-5
1.0x10-4
[H+]-3, M-3
[Cu2+], M
Figure 7. Variation of ko/([Cu2+][TGA]2) with 1/[H+]3. [1] = 0.50 mM,
[TGA] = 5.0 mM, [Cu2+] = 5 ꢃ 10ꢀ5 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
Figure 10. Variation of ko with [Cu2+]. [1] = 0.50 mM, [TGA] = 5.0 mM,
[H+] = 0.60 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
J. Phys. Org. Chem. (2012)
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R. MISHRA, R. BANERJEE AND S. MUKHOPADHYAY
2 14þ þ 2 RSH ! 2 23þ þ RSSR þ 2 Hþ
(1)
(2)
effects has any contribution towards the observed acid depen-
dence in the reaction,[23] we have measured first-order rate
constants (ko) in presence of LiClO4 in the entire acidity range
(Tables S2–S4). The ko values thus found nicely match (within
5%) with the ko values obtained when the ionic strength was
maintained with NaClO4. We thus believe that the observed
pH-dependence originates from the properties of the redox
partners and not due to any kind of medium effects. From these
observations, we propose the reaction sequence shown in
Scheme 1.
2
3þ þ Hþ ! 2H4þ
Kinetics and mechanism
The title complex 1 suffers practically no decomposition in under
the reaction conditions. In the presence of excess [thiol] over [1]
and externally added Cu2+, absorbance of 1 reduces with time
and the process followed excellent first-order kinetics. A family
of time-resolved spectra describing such changes is shown in
Fig. 2, and a typical first-order plot is displayed in Fig. S1. The
evaluated first-order rate constants, ko, showed a square depen-
dence on [thiol] (Figs. 3–5; Tables S2–S4), linear dependence on
[H+]ꢀ3 (Figs. 6–8; Tables S2–S4), and a first-order dependence on
[Cu2+] (Figs. 9–11; Tables S2–S4). To verify if specific medium
Deprotonation of the two thiol molecules in presence of Cu2+
accounts for the two [H+]ꢀ2 dependence observation. Another
inverse dependence on [H+] could only come from 1. We
propose dissociation of an H+ ion from one of the coordinated
NH3 molecules in 1, thus forming a conjugate base [(NH3)5Co
(O2)Co(NH3)4(NH2)]4+ (1-H) as the exclusive kinetically reactive
species(Equation 3).[24,25] The superoxo coordinated to 1 has
no dissociable proton, and the observed inverse proton depen-
dence could not be traced at the superoxo moiety in 1. Cu(II)
reacts with thiols to form [CuII(RSꢀ)2] complex species (Equations
4 and 5); an electron is transferred from one of the ligated
thiolate anions to the CuII center to form [CuI(RSꢀ)] and a thiyl
radical (Equation 6). The copper(I)-thiol complex thus formed is
oxidized by 1 at the rate step to produce [CuII(RSꢀ)]+ that again
reacts with another molecule of RSH to regenerate [CuII(RSꢀ)2]
(Equation 5). Two of the thiol radicals condense to form the
corresponding disulfide product (Equation 9).
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
The Scheme 1 leads to Equation (10), assuming reaction
shown in Equation (6) is fast and quantitatively complete[26,27]
(see Supporting Information).
Â
Ã
2
k2K1K2K-H Cu2þ ½RSHꢄ
ko ¼
(10)
3
½Hþꢄ
0.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
5.0x10-4
[Cu2+], M
Figure 11. Variation of ko with [Cu2+]. [1] = 0.50 mM, [cys] = 5.0 mM,
[H+] = 0.20 M, I = 1.0 M (NaClO4), T = 25.0 ꢂC
Equation (10) abides all the kinetic dependences with k2K1K2K-H
=
2.38 (ꢁ0.1) ꢃ 103 sꢀ1 for mercap, 9.12 (ꢁ0.5) ꢃ 104 sꢀ1 for tga
Scheme 1. Reaction Sequence of Cu(II) catalyzed oxidation of thiols by 1
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CU(II)-CATALYZED OXIDATION OF THIOLS BY SUPEROXIDE LIGATED TO CO2III
and 4.44 (ꢁ0.3) ꢃ 103 sꢀ1 for cysteine at 25.0 ꢂC. The values suggest
that mercap and cysteine react at comparable rates with 1 in pres-
ence of Cu2+, but tga reacts at a much faster rate.
[7] I. Peler, C. E. Felder, O. Man, I. Silman, J. L. Sussman, Proteins 2004,
54, 20–40.
[8] J. C. Bardwell, Mol. Microbiol. 1994, 14, 199–205.
[9] N. Kharasch, A. S. Arora, Phosphorus and Sulfur 1976, 2, 1–50.
[10] S. Shinkai, S. Yamada, R. Ando, T. Kunitake, Bioorg. Chem. 1980, 9,
238–247.
[11] M. Takhashi, M. Takano, K. Asada, J. Biochem. 1981, 90, 87–94.
[12] R. Munday, C. M. Munday, C. C. Winterbourn, Free Rad. Biol. Med.
2004, 36, 757–764.
The substitution-inert CoIII centers of the CoI2II superoxo
complex (1) cannot be reduced by an inner-sphere path.
Nevertheless, increased [H]+ decelerated the rate, a feature
unusual for an outer-sphere reaction of a CoIII complex.[28–30]
This can be simply explained through equilibria (3), (4), and (5).
Deprotonation of 1 (Equation 3) increases its reactivity, whereas
metal-assisted deprotonation (Equations 4 and 5) of thiols
increases their reactivity. Solutions of Cu+ in the presence of
excess RSꢀ result in the formation of [(CuI)(ꢀSR)] complexes.[31]
In addition, reduction of CuII to CuI by thiols are well-known
reactions (Equation 6).[24,32,33] Although Cu(I) is not stable in
aqueous solution,[34] a number cationic and anionic complexes
of Cu(I) are found to be stable in aqueous solution.[35,36] In fact,
complexes of glutathione and Cu(I) have been suggested to be
intermediates in cellular metabolism and storage of Cu(I).[37]
Thus, the striking catalytic effect exhibited by Cu(II) in the
experiments is mainly attributed to the fact that Cu(II) is
reduced to the unipositive state Cu(I) by organic thiols under
the experimental conditions, and also that the reactivity of
Cu(I) is greatly enhanced when bonded with chelating thiol
ligands. Thus, the Scheme 1 proposed here seems logical for
explaining the Cu2+-ion catalyzed oxidation of aliphatic thiols
by metal-bound superoxide.
[13] R. Mishra, S. Mukhopadhyay, R. Banerjee, Dalton Trans. 2010, 39,
2692–2696.
[14] R. Davies, M. Mori, A. G. Sykes, J. A. Weil, Inorg. Synth. 1982, 12,
206–208.
[15] J. P. Danehy, V. J. Elia, Anal. Chem. 1972, 44, 1281–1284.
[16] O. Folin, J. M. Looney, J. Biol. Chem. 1922, 51, 421–434.
[17] M. Mori, J. A. Weil, M. Ishiguro, J. Am. Chem. Soc. 1968, 90,
615–621.
[18] C. G. Barraclough, G. A. Lawrance, P. A. Lay, Inorg. Chem. 1978, 17,
3317–3322.
[19] R. C. Kapoor, O. P. Kachhwaha, B. P. Sinha, J. Phys. Chem. 1969, 73,
1627–1361.
[20] A. B. Hoffman, H. Taube, Inorg. Chem. 1968, 7, 1971–1976.
[21] M. Mori, J. A. Weil, J. Am. Chem. Soc. 1967, 89, 3732–3744.
[22] R. Davies, M. Mori, A. G. Sykes, J. A. Weil, Inorg. Synth. 1982, 12,
204–205.
[23] A. G. Sykes, J. A. Weil, Prog. Inorg. Chem. 1970, 13, 1–106.
[24] R. Mishra, S. Mukhopadhyay, R. Banerjee, Dalton Trans. 2009,
5469–5473.
[25] K. Mandal, R. Banerjee, Dalton Trans. 2012, 41, 2714–2719.
[26] A. J. Miralles, R. E. Armstrong, A. Haim, J. Am. Chem. Soc. 1977, 99,
1416–1420.
[27] Y. Kurimura, Bull. Chem. Soc. Jap. 1973, 46, 2093–2096.
[28] A. V. Kachur, C. J. Kocha, J. E. Biaglowa, Free Rad. Res. 1998, 28,
259–269.
Acknowledgements
[29] L. Pecci, G. Montefoschi, G. Musci, D. Cavallini, Amino Acids 1997, 13,
355–367
The work was carried out with the financial assistance from
Council of Scientific and Industrial Research (New Delhi, India)
and Department of Science and Technology (New Delhi, India).
[30] S. P. Ghosh, S. K. Saha, R. N. Bose, J. W. Reed, M. C. Ghosh, E. S. Gould,
Inorg. Chem. 1993, 32, 2261–2264.
[31] G. A. Bagiyan, S. A. Grachev, N. V. Soroka, Izv. Akad. Nauk SSSR,
Ser. Khim. 1975, 435, Bull. Acad. Sci. USSR, Div. Chem. Sci. 1975,
24 (Engl. Transl.)].
[32] R. C. Smith, V. D. Reed, Phosphorus, Sulfur, and Silicon 1994, 90,
REFERENCES
147–154.
[1] T. J. Wallace, J. Org. Chem. 1966, 31, 1217–1221.
[2] H. Firouzabadi, N. Iranpoor, F. Kiaeezadeh, J. Toofan, Tetrahedron
1986, 42, 719–725.
[3] A. McKillop, D. Koyunçu, A. Krief, W. Dumont, P. Renier, M. Trabelsi,
Tetrahedron Lett. 1990, 31, 5007–5010.
[4] A. R. Ramesha, S. Chandrasekaran, J. Org. Chem. 1994, 59, 1354–1357.
[5] W. S. Allison, Acc. Chem. Res. 1976, 9, 293–299.
[6] P. Nagy, M. T. Ashby, J. Am. Chem. Soc. 2007, 129, 14082–14091.
[33] S. K. Ghosh, S. K. Saha, M. C. Ghosh, R. N. Bose, J. W. Reed, E. S. Gould,
Inorg. Chem. 1992, 31, 3358–3362.
[34] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, John
Wiley & Sons, New York, 1988, 757.
[35] M. F. Khan, J. R. J. Sorensen, J. Inorg. Biochem. 1991, 41, 221–234.
[36] J. R. J. Sorensen, Prog. Med. Chem. 1989, 26, 437–568.
[37] J. H. Freedman, M. R. Ciriolo, J. Peisach, J. Biol. Chem. 1989, 264,
5598–5604.
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