Highly efficient and stable catalyst for peroxynitrite decomposition

Article abstract of DOI:10.1139/v01-035

The new cobalt substituted-polyoxometalate K₇[CoAlW₁₁O₃₉]·15H₂O and the simple CoCl₂·6H₂O salt are efficient catalysts for peroxynitrite decomposition. These compounds also catalyze the oxidation of ascorbic acid and the nitration of phenol by peroxynitrite.

Full text of DOI:10.1139/v01-035

792
Highly efficient and stable catalyst for
peroxynitrite decomposition
Yurii V. Geletii, Alan J. Bailey, Jennifer J. Cowan, Ira A. Weinstock, and
Craig L. Hill
Abstract: The new cobalt substituted-polyoxometalate K7[CoAlW11O39]·15H2O and the simple CoCl2·6H2O salt are ef-
ficient catalysts for peroxynitrite decomposition. These compounds also catalyze the oxidation of ascorbic acid and the
nitration of phenol by peroxynitrite.
Key words: peroxynitrite, polyoxometalates, antioxidant.
Résumé : Le nouveau polyoxométalate substitué du cobalt, K7[CoAlW11O39]·15H2O ainsi que le simple sel
CoCl2·6H2O sont des catalyseurs efficaces pour la décomposition du peroxynitrite. Ces composés catalysent aussi
l’oxydation de l’acide ascorbique et la nitration du phénol par le peroxynitrite.
Mots clés : peroxynitrite, polyoxométalates, antioxydant.
[Traduit par la Rédaction] 794
Introduction
Geletii et al.
ported (9, 12, 13); however, they appeared to be unstable
and less active.
The recent interest in peroxynitrite1, a compound known
for many years (1), was inspired by Beckman et al. (2), who
suggested it was formed in vivo from NO and O•2−. Under
physiological conditions, ONOO– is partially protonated to
HOONO (pKa = 6.8), which rapidly (k ~ 1 s–1, pH = 7.0,
25°C) decomposes to highly reactive radical species in a
unimolecular reaction (1–4). Peroxynitrite also reacts with
many compounds in bimolecular reactions, and it is a power-
ful oxidant with substantial toxicity (1–4). There is accumu-
lating evidence for peroxynitrite formation in vivo, which
naturally has led to a search for compounds that can inter-
cept this powerful oxidant and destroy it safely. The discov-
ery of a fast peroxynitrite reaction with myeloperoxidase
(6.2 × 106 M–1 s–1 at pH = 7.2) (5) inspired studies of
peroxynitrite reactions with synthetic, water-soluble Fe- and
Mn-porphyrin complexes (6–11). Reactions of peroxynitrite
with combinations of these metalloporphyrins and antioxi-
dants were found to be even more rapid. Recently, several
nonheme catalysts for peroxynitrite decomposition were re-
Here we report that a simple cobalt salt, CoCl2·6H2O, and
the Co-substituted polyoxometalate, K7[CoIIAlW11O39]·15H2O,
catalytically decompose peroxynitrite at neutral pH (6.5–7.4)
with high turnover numbers. Their activity is comparable to
MnTMPyP, where TMPyP is the dianion of tetrakis(N-methyl-
4′-pyridyl) porphyrin. All species also catalyze ascorbic acid
oxidation and phenolic nitration by peroxynitrite.
Results and discussion
Peroxynitrite decay in the absence and in the presence of
representative catalysts is exponential and follows the rate
law in eq. [1].
[1]
-d[ONOO–]/dt = kobs[ONOO–] = ko[ONOO–] +
kcat[ONOO–][cat]
When kobs values are plotted as a function of catalyst con-
centrations ([cat]) straight lines are obtained. The y-inter-
cepts provide ko and the slopes of each line provide kcat. The
data are summarized in Table 1. The following compounds
were evaluated and found to be minimally active (kcat < 1 ×
103 M–1 s–1): CuCl2·2H2O, FeCl2·4H2O, K9[AlW11O39]·13H2O,
Received September 29, 2000. Published on the NRC
Research Press at http://canjchem.nrc.ca Web site on June 30,
2001.
Na5[AlW12O40]·13H2O,
K7[VIVAlW11O40]·8H2O,
and
Dedicated to Professor Brian R. James on the occasion of his
65th birthday.
K5[CoIIIW12O40]·13H2O (the reproducible value for the
uncatalyzed rate constant, ko = 0.35 ± 0.03 s–1 at pH 7.4).
The stabilities of the three most active catalysts, CoCl2,
K7[CoIIAlW11O39], and MnTMPyP, were evaluated by com-
paring their activities, indicated by experimental kobs values,
before and after reaction with large excesses of peroxynitrite
(for concentrations see Table 2). The resistance of each cata-
lyst to irreversible decomposition was estimated by n =
[NaOONO]d/∆[cat], where [NaOONO]d is the total amount
of peroxynitrite decomposed and ∆[cat] is the amount of the
catalytic activity lost in the catalytic reaction. The latter term
Yu.V. Geletii, A.J. Bailey, J.J. Cowan, I.A. Weinstock, and
C.L. Hill.2 Department of Chemistry, Emory University, 1515
Pierce Drive, Atlanta, GA. 30322, U.S.A.
1The term peroxynitrite is used to refer to peroxynitrite anion
(O=NOO–) and peroxynitrous acid (ONOOH) unless
otherwise indicated. The IUPAC-recommended names are
oxoperoxonitrate (–1) and hydrogen oxoperoxonitrate,
respectively.
2Corresponding author (telephone: (404) 727-6611; fax:
(404) 727-6076; e-mail: chill@emory.edu).
Can. J. Chem. 79: 792–794 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-5/6-792

Geletii et al.
Table 1. The rate constants (kcat) of representative
793
Scheme 1.
compounds used to catalyze peroxynitrite decompo-
sition at 25°C.
-OH
CoIII[(ONOO)-]
Catalyst
kcat (1 × 10–4) (M–1 s–1)
.
NO2
MnTMPyP
CoCl2
K7[CoAlW11O39]
2.1 ± 0.2
3.0 ± 0.3
1.9 ± 0.15
-
ONOO
CoIII(OH)
CoIII-O.
Note: For all experiments the pH = 7.0 ± 0.05, 75 mM
phosphate buffer, [ONOO–]o = 300–400 µM, [cat]o = 0–40
µM (0–14 µM for MnTMPyP).
NO. + O2
ONOO- + H+
was calculated as ∆[cat] = [cat]o(kobs – ko′ bs)/(kobs – ko), where
[cat]o is the initial concentration of the catalyst, ko′ bs is the
decomposition rate in the presence of the catalyst that has
o
o
Nitrite formation reaction
o
been pretreated with a large excess of peroxynitrite, and kobs
.
NO. + NO2
N2O3
2NO2–
2H+
and ko are the decomposition rates in the presence and the
absence of the catalyst, respectively.3 The results, presented
in Table 2, show that both K7[CoIIAlW11O39] and MnTMPyP
are substantially more stable than Co2+ (as CoCl2).
The self-decomposition of peroxynitrite at neutral pH is
known to form nitrate and nitrite, and the formation of ni-
trite is accompanied by dioxygen evolution in a 2:1 ratio
(14). The addition of K7[CoIIAlW11O39] increases the yield
of nitrite (from 11 to 23% based on peroxynitrite, if 20 µM
of K7[CoIIAlW11O39] is added at pH 7.4).
+
H2O
stant (kcat, eq. [1]), increases from 1.9 × 104 to 12.7 × 104 M–1
s–1 in the presence of 7.4 mM ascorbic acid at pH 7.4. How-
ever, the rate of K6[CoIIIAlW11O39] reduction by ascorbic
acid is too slow to account for the observed increase in the
total rate of peroxynitrite decomposition. Therefore, ascorbic
acid is likely to reduce the CoIII[(OONO)–] intermediate
complex, thus accelerating the total rate.
Since CoCl2 is more active than K7[CoIIAlW11O39], the
activity of the latter could be attributed to Co2+ ions formed
via dissociation of K7[CoIIAlW11O39] (eq. [2]). However, the
activity of a mixture of CoCl2 and K7[CoIIAlW11O39] was
the same as the sum of the activities of each of the individ-
ual components, suggesting that K7[CoIIAlW11O39] itself is
also an active catalyst.
The nitration of phenols by peroxynitrite has long been
known (17), and Beckman et al. (18) were the first, to our
knowledge, to report a catalytic version of this process. To
assess phenol nitration activity by the Co systems, the yield
and the rate of 3-nitro-4-hydroxyphenylacetic acid (3-NO2-
4-HPA) from the nitration of 4-hydroxyphenylacetic acid (4-
HPA) were quantified. The yield of 3-NO2-4-HPA, and the
rate of its formation, both increase with increasing
K7[CoIIAlW11O39] concentration. Based on the initial
peroxynitrite concentration, the 3-NO2-4-HPA yield was 7%
in the absence of a catalyst and 40% in the presence of
82 µM of K7[CoAlW11O39] (pH 7.4, [4-HPA] = 6.0 mM,
[NaOONO]o = 1.1 mM). However, unlike in the catalyzed
oxidation of ascorbic acid by peroxynitrite, the observed
second-order rate constant (kcat, eq. [1]) does not increase
with 4-HPA concentration. The mechanism of nitration may
involve a fast reduction of CoIII-O• by the phenolic species;
subsequent coupling of the resulting phenoxy radical and
NO•2 gives nitrophenol. Mn- and Fe-porphyrins active in
peroxynitrite decomposition have also been found to cata-
lyze the nitration of phenols (6, 8–11, 19) likely through the
coupling of phenoxy and NO•2 radicals (19).
[2]
[CoII/IIIAlW11O39]7–/6–
Co2+/3+
+
[AlW11O39]9–
Treatment of the starting K7[CoIIAlW11O39] with peroxynitrite
results in its immediate oxidation to K6[CoIIIAlW11O39]
(λmax shifts from 560 to 680 nm). The mechanism in
Scheme 1 (proposed mechanism of peroxynitrite decomposi-
tion catalyzed by K7[CoIIAlW11O39], where CoII represents
[CoIIAlW11O39]7–) is consistent with all the above data and
is based on that proposed for catalysis of peroxynitrite de-
composition by Fe-porphyrins (7, 11) and sulfito-bound
Co(III) (cis-[Co(NH3)4(SO3)2]–) (12).4 The rate-limiting step
is likely to be a homolysis of the O—O bond in a CoIII
peroxonitrito intermediate to form CoIII-O• and NO2• . Previ-
ously, a Co(III) peroxynitrito complex has been isolated and
characterized (15). The reaction is thought to occur by an
outer-sphere mechanism. However, since the lability of
Co(III) increases when its geometry is perturbed from rigor-
ous Oh symmetry (16), as is the case in K7[CoIIAlW11O39],
an inner-sphere mechanism cannot definitely be ruled out.
Further investigations are ongoing.
Experimental
MnTMPyP (ICN Biomedicals Inc.) was standardized in
aqueous solution using ε462 = 1 × 105 M–1 cm–1 (20).
Peroxynitrite was synthesized from H2O2 and nitrite in a
simple flow reactor (21, 22) and its concentration was deter-
The addition of ascorbic acid significantly increases the
rate of peroxynitrite consumption in reactions catalyzed by
K7[CoIIAlW11O39], i.e., the observed second-order rate con-
3 kobs = ko + kcat[cat], see eq. [1].
4 The reaction decelerates at [OH–] = 0.4 M, [NaOONO]o:[cat] ~ 10–15, as a result of catalyst decomposition.
© 2001 NRC Canada

794
Can. J. Chem. Vol. 79, 2001
Table 2. The observed reaction rate constants before (k°obs) and after (k′obs) reaction of the cata-
lyst with a large excess of peroxynitrite, and n values.
kobs (s–1) a
o
ko′ bs (s–1) a
(kobs – ko′ bs)/(kobs – ko)b
nc
o
o
Catalyst
CoCl2
K7[CoAlW11O39]
MnTMPyP
1.85 ± 0.07
1.33 ± 0.05
1.10 ± 0.04
1.33 ± 0.07
1.30 ± 0.05
1.09 ± 0.04
0.3
<0.03
<0.03
~3 × 103
>3 × 104
>3 × 104
Note: All experiments carried out at 25°C. Pretreatment conditions: [NaOONO]o = 24 mM, [cat]o = 25 µM,
pH 6.8, 0.15 M phosphate buffer.
a[NaOONO]o = 0.4 mM, [cat]o = 15 µM, pH 7.4, 0.11 M phosphate buffer.
bko = 0.35 ± 0.03 s–1.
cn = [NaOONO]d/∆[cat]; see text for details.
mined by UV–vis spectroscopy, ε302 = 1.7 × 103 M–1 cm–1
(23).
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The
new
transition-metal-substituted
POM
(α-
K7[CoAlW11O39]·15H2O) was prepared by metallation of the
lacunary complex α-K9[AlW11O39]·15H2O (24) by
Co(NO3)2·6H2O.
A
10
g
portion of K9[AlW11O39]
(3.05 mmol) was stirred in water (100 mL), and a solution of
Co(NO3)2·6H2O (0.88 g, 3.05 mmol, in 10 mL water) was
added dropwise to the slurry via a pipette. The reaction mix-
ture was stirred in a hot water bath (70–80°C) until the solu-
tion became clear. The resulting solution was dark pink
(pH 6.2), and after cooling to room temperature, a red pow-
der deposited. The product was recrystallized from a mini-
mum of warm (60°C) water.5
Kinetic experiments were studied using a SF-61 stop flow
instrument (Hi-Tech Scientific, U.K.). A buffer solution with
the reactants and peroxynitrite diluted in aqueous NaOH
were mixed and the absorbance at 302 nm was measured.
The first-order reaction rate constants were determined using
standard software (KISS 5.1 for Macintosh computer). The
yields of nitrite and 3-NO2-4-HPA were determined after a
small amount of highly concentrated (~0.6 M) peroxynitrite
solution was mixed with a buffer solution containing all
other ingredients. Yields of nitrite were determined as previ-
ously described (25) and 3-NO2-4-HPA was quantified from
its absorbance at 430 nm after adjusting the pH to 10–11
(18).
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Acknowledgements
We thank Dr. Eric A. Boring for the preparation of
K5[CoW12O40]·13H2O, and Professor W. H. Koppenol and
Dr. R. Kissner for helpful discussions. This work was
funded by the National Science Foundation (to CLH) and in
part by INTAS (grant 99–209, to YVG).
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5 Yield: 7.31 g (74%). IR (KBr pellet) (cm–1): 952 (m), 935 (s), 891 (s), 798 (m), 697 (m), 533 (w), 486 (w). Anal. calcd. for
K7[CoAlW11O39]·15H2O: H 0.92, Al 0.82, Co 1.80, K 8.35, W 61.73; found: H 0.91, Al 0.88, Co 1.73, K 8.16, W 62.26.
© 2001 NRC Canada