Selective oxidation of thiourea with H2O2 catalyzed by [RuIII(edta)(H2O)]-: Kinetic and mechanistic studies

Article abstract of DOI:10.1039/c3dt32737j

Reported here is the first example of a ruthenium complex, [Ru ^III(edta)(H₂O)]^- (edta^4- = ethylenediaminetetraacetate), that catalyzes the oxidation of thiourea (TU) in the presence of H₂O₂. The kinetics and mechanism of this reaction were investigated in detail by using rapid-scan spectrophotometry as a function of both the hydrogen peroxide and thiourea concentrations at pH 4.9 and 25 °C. Spectral analyses and kinetic data clearly support a catalytic process in which hydrogen peroxide reacts directly with thiourea coordinated to the Ru^III(edta) complex. HPLC product analyses revealed the formation of formamidine disulfide (TU₂) as a major product at the end of the catalytic process, however, formation of other products like thiourea dioxide (TUO₂), thiourea dioxide (TUO₃) and sulfate was also observed after longer reaction times. Catalytic intermediates such as [Ru ^III(edta)(OOH)]^2- and [Ru^V(edta)(O)] ^- were evidently found to be non-reactive in catalyzing the oxidation of thiourea by H₂O₂ under the specified conditions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt32737j

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Selective oxidation of thiourea with H O catalyzed by
2
2
III
−
[
Ru (edta)(H O)] : kinetic and mechanistic studies†
Cite this: Dalton Trans., 2013, 42, 4725
2
a,b
b
b
Debabrata Chatterjee,* Sabine Rothbart and Rudi van Eldik*
III
O)]− (edta4− = ethylenediamine-
. The kinetics and
Reported here is the first example of a ruthenium complex, [Ru (edta)(H
2
2 2
tetraacetate), that catalyzes the oxidation of thiourea (TU) in the presence of H O
mechanism of this reaction were investigated in detail by using rapid-scan spectrophotometry as a func-
tion of both the hydrogen peroxide and thiourea concentrations at pH 4.9 and 25 °C. Spectral analyses
and kinetic data clearly support a catalytic process in which hydrogen peroxide reacts directly with
III
thiourea coordinated to the Ru (edta) complex. HPLC product analyses revealed the formation of forma-
Received 16th November 2012,
Accepted 10th January 2013
2
midine disulfide (TU ) as a major product at the end of the catalytic process, however, formation of other
products like thiourea dioxide (TUO
2
), thiourea dioxide (TUO
3
) and sulfate was also observed after longer
DOI: 10.1039/c3dt32737j
reaction times. Catalytic intermediates such as [Ru (edta)(OOH)] and [Ru (edta)(O)]− were evidently
III
2−
V
www.rsc.org/dalton
2 2
found to be non-reactive in catalyzing the oxidation of thiourea by H O under the specified conditions.
available in the literature, albeit there exists a report on the
Introduction
3+
formation of urea via hydrolysis of [Ru(NH
3
)
5
(TU)] in basic
1
5
Thiourea (NH
extensively used in chemistry and biochemistry.
recent past, efficacy of thiourea and 1,3-dimethyl-2-thiourea dation of various substrates, we have undertaken a detailed
2 2
CSNH , TU), a sulfur-containing compound, is aqueous solution. In view of the above and considering the
1
,2
III
In the potential of the Ru -edta/H O system towards effecting oxi-
2
1
2
6
3
(DMTU) in scavenging reactive oxygen intermediates (ROIs),
kinetic and mechanistic investigation of the oxidation of
4
III
and preventing ROI-induced lung injury has been reported. In thiourea by the Ru (edta)/H O system in aqueous solution.
2
2
2 2
addition, it has immense industrial potential. Reactions of This is the first report on the oxidation of thiourea with H O
thiourea with hydrogen peroxide under certain conditions using a ruthenium complex. We report herein the results of
III
produce a powerful reductive bleaching agent, which is routi- kinetic studies of the oxidation of thiourea by the Ru (edta)/
5
–7
nely used in the textile industry. Apart from the mechanism
2 2 2 2
H O system as a function of [TU] and [H O ] at 25 °C and pH
of thiourea oxidation, which is quite complex because of the 4.9. The goal of the present work is to contribute to the
formation of various oxidation products with the oxidation mechanistic understanding of the catalytic oxidation process.
7
,8
state of sulfur ranging from −1 to +6,
the physiological
effects of thiourea oxides (products from the oxidation of
thiourea) have drawn attention of researchers to look at the
Experimental
Materials
9
,10
biofunction of these sulfur species.
The significance of thiourea oxidation has been well docu-
III
1
1
K[Ru (Hedta)Cl]·2H O was prepared by following the pub-
mented in a very recent review article, however, the use of a
metal complex as a catalyst in TU oxidation is scanty in the
2
1
7
lished procedure. The micro-analysis and spectral data are in
1
7
1
2–14
good agreement with those reported in the literature. Anal.
calculated for K[Ru (Hedta)Cl]·2H O: C 24.0, H 3.42, N 5.59;
Found. C 23.8, H 3.45, N 5.63. IR, ν/cm : 1720 (free –COOH),
literature.
It is noteworthy that no report on the use of a
III
ruthenium complex as a catalyst in thiourea oxidation is
2
−
1
−
−1
1
2
650 (coordinated –COO ). UV-Vis in H O: λmax/nm (εmax/M
−
1
cm ): 283 (2800 ± 50), 350 sh (680 ± 10). All other chemicals
a
Chemistry and Biomimetics Group, CSIR-Central Mechanical Engineering Research
used were of A.R grade. Doubly distilled H O was used
throughout the experiments.
2
Institute, MG Avenue, Durgapur-713209, India. E-mail: dchat57@hotmail.com;
Fax: +91-343-2546745; Tel: +91-343-2546828
b
Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany. E-mail: vaneldik@chemie.uni-erlangen.de; Instrumentation
Fax: +49-9131-8527387; Tel: +49-9131-8528383
A Perkin-Elmer 240C elemental analyzer was used to collect
microanalytical (C, H, N) data. The UV-Vis and IR (using KBr
†
Electronic supplementary information (ESI) available: UV-VIS spectra, kinetic
traces and HPLC chromatograms. See DOI: 10.1039/c3dt32737j
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pellets) spectra were recorded on Varian Model Cary 100 Bio
and Perkin Elmer Model Lambda 783 spectrophotometers,
respectively. Kinetic measurements were performed in a reac-
tion vessel using a Hellma 661.502-QX quartz suprasil immer-
sion probe attached via optical cables to a 150 W Xe lamp and
a multi-wavelength J&M detector, which records complete
absorption spectra at constant time intervals. The reaction of
III
−
[
Ru (edta)(TU)] with H O was followed at 466 nm, whereas
2 2
III
2−
V
−
the reaction of [Ru (edta)(OOH)]
produced in the reaction of [Ru (edta)(H O )
and [Ru (edta)O]
III
O)]− with H
(
2
2 2
with TU was monitored at 425 and 390 nm, respectively. All
the instruments were thermostated at the desired temperature
III
Fig. 1 UV-Vis spectral changes observed during the oxidation of [Ru (edta)-
(±0.1 °C). The pH of the solutions was measured with a
TU)] (2.0 × 10−4 M) by H
−
O (2.0 × 10 M) at 25 °C and pH 4.9 (1 mM acetate
−2
(
2 2
Mettler Delta 350 pH meter. Acetate and phosphate buffers
were used to adjust the pH of the experimental solutions.
Observed rate constants (kobs) are presented as an average of
several kinetic runs (at least 5–8) and were reproducible
within ±4ꢀ.
buffer). Overall reaction time 120 s.
The analysis of the oxidation products of thiourea was per-
formed using an HPLC system (Waters 600 pump and 600 con-
troller with Waters 2487 dual absorbance detector) equipped
with a C18Nova–Pak column (Waters, 3.9 × 150 mm, bead size
4
μm) pre-equilibrated with 5ꢀ methanol. Elutions were per-
formed using a mobile phase of an HPLC grade water–aceto-
nitrile mixture (70 : 30 v/v) at a flow rate of 0.5 mL min−
1
.
Elution profiles of thiourea, Ru(edta) and H O were obtained
2
2
isocratically in the same solvent at a flow rate of 0.8 mL min−
1
.
Elution was followed at different wavelengths reported for
thiourea and its corresponding oxidation products. The of thiourea by H O . [TU] = 4 × 10 M, [H O ] = 2 × 10 M, 25 °C, pH 4.9
Fig. 2 Kinetic traces recorded at 466 nm for the Ru(edta) catalyzed oxidation
2
0
−4
−2
2
2
2
2
retention times for thiourea (TU), hydrogen peroxide (H
and formamidine disulfide (TU ) at 236 nm were found to be
.62, 2.1, and 2.6 min, respectively, under the selected
2
O
2
)
(1 mM acetate buffer).
2
1
conditions.
species viz. formamidine disulfide (TU
TUO ), thiourea trioxide (TUO ), sulfite and sulfate formed in
the oxidation reaction. It was proposed that the oxidation of
thiourea by H proceeds through the formation of a transi-
ent sufenyl acid (HOSC(NH)NH
2
), thiourea dioxide
(
2
3
2 2
O
Results and discussion
−
1
2
) intermediate (k
at 25 °C). The sufenyl acid intermediate subsequently
The sixth reacts with another molecule of thiourea to yield formamidine
1
= 0.115 M
III
III
−1
20
The K[Ru (Hedta)Cl] complex rapidly converts into the [Ru -
s
1
8,19
(
Hedta)(H
coordination site of the ruthenium complex is occupied either disulfide (k
at low pH by a water molecule or at high pH by a hydroxide disulfide (TU
ion. The pK
values related to the acid-dissociation equilibria at 25 °C). In our case, disappearance of the peak at 466 nm is
of the pendant carboxylic acid arm and the coordinated water attributed to the oxidation of the coordinated TU in
2
O)] complex when dissolved in water.
= 4.5 M−
1
s
−1
at 25 °C). However, formamidine
20
2
−
1
−1
) slowly converts into TUO
(k
3
= 0.0058 M
s
2
2
a
1
8,19
III
−
molecule are 2.4 and 7.6, respectively at 25 °C.
The [Ru (edta)(H
form a sulfur-bound red coloured complex [Ru (edta)(TU)]
[Ru (edta)(TU)] by H
O)] complex rapidly reacts with TU to After complete oxidation of thiourea, the reaction is followed
2
2 2
O under the specified conditions.
III
−
III
−
by the formation of a band at 390 nm and a shoulder at
−1
−1 19
V
−
(
λmax = 466 nm; εmax = 1960 ± 20 M cm ). The second- 425 nm, characteristic for the formation of [Ru (edta)O] and
III
2−
21
order rate constant for the formation of this complex is 2970 ± [Ru (edta)(OOH)] , respectively.
−
1
−1
19
5
0 M
s
at 25 °C and pH 5.0. Addition of an excess of
Fig. 2 shows typical kinetic traces recorded at 466 nm to
III
−
H O to the red solution of [Ru (edta)(TU)] resulted in a demonstrate the effect of the catalyst concentration on the oxi-
2
2
III
−
rapid decay of the band at 466 nm (Fig. 1). It is noteworthy dation of coordinated TU in [Ru (edta)(TU)] . All traces show
that the kinetics of the spontaneous oxidation of thiourea by an initial induction period which increases with decreasing
2
0
hydrogen peroxide was recently studied by Epstein et al.
A
catalyst concentration (Fig. 2) and is ascribed to the rapid
III
−
reversed-phase ion-pair HPLC technique was employed to reformation of the [Ru (edta)(TU)] complex in the presence
monitor the course of the reaction, and quantitatively charac- of an excess of TU following the oxidation of the coordinated
terize the concentrations of a variety of sulfur-containing TU under the specified conditions. Following the induction
4
726 | Dalton Trans., 2013, 42, 4725–4729
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period, the reaction rate (slope of the trace) becomes slower on
III
decreasing the Ru (edta) concentration.
On the basis of the observed spectral changes reported in
Fig. 1, a series of experiments were performed in which the
concentrations of Ru-edta, H O and thiourea were systemati-
2
2
cally varied, while pH and temperature were kept constant in
order to obtain time-resolved concentration dependences for
the observed reaction steps. Some typical absorbance-time
2 2
traces at 466 nm as a function of [TU] and [H O ] are shown in
Fig. 3 and 4, respectively. The kinetic traces in Fig. 3 and 4 all
exhibit the mentioned induction period due to the rapid refor-
mation of [Ru (edta)(TU)] by the reaction of TU (excess over
III
−
III
−
Fig. 5 Plot of the rate of disappearance of the red [Ru (edta)(TU)] complex
III
−4
2 2
versus [H O ] at 25 °C, pH 4.9 (1 mM acetate buffer), [Ru ] = 2 × 10 M.
III
−
[
Ru (edta)(H
coordinated TU in [Ru (edta)(TU)] (band at 466 nm). The
induction period increases with increasing [TU] (Fig. 3) as a reaction estimated by the maximum slope reached following
2
O)] catalyst complex) following the oxidation of
III
−
III
−
result of the reformation of [Ru (edta)(TU)] that will con- the induction period, increases linearly with increasing H O
2
2
tinue for a longer time in the presence of a larger excess of TU, concentration (Fig. 5).
III
−
III
−
i.e. a longer recycling of the catalyst, [Ru (edta)(H
2
O)] . Fol-
2 2
The observed reaction of [Ru (edta)(TU)] with H O under
lowing the induction period, the rate of the reaction becomes the specified conditions can be accounted for in terms of the
slightly slower on increasing the TU concentration (Fig. 3).
Representative kinetic traces (recorded at 466 nm) for the
rate-law expressed in eqn (1)
III
−
oxidation of coordinate TU in [Ru (edta)(TU)] as a function
of [H O ] are shown in Fig. 4. The induction period clearly
III
ꢀ
III
ꢀ
–
d½Ru ðedtaÞðTUÞ ꢁ=dt ¼ k½Ru ðedtaÞðTUÞ ꢁ½H2O2ꢁ
ð1Þ
2
2
2 2
shortens on increasing the H O concentration as seen in
Fig. 4. Under the specified conditions, the rate of the oxidation
The value of the second-order rate constant (k) estimated
−
1 −1
from the slope of the plot in Fig. 5 is 3.4 M
s
at 25 °C. This
O)] mediated oxidation of
thiourea is approximately thirty times faster than the spon-
III
−
rate constant for the [Ru (edta)(H
2
2
0
taneous oxidation of thiourea by H O . The kinetic data,
2
2
III
−
2
therefore, signify the catalytic role of the [Ru (edta)(H O)]
complex towards effecting oxidation of thiourea under the
specified experimental conditions.
Very recently we have reported on the reactivity of the cataly-
III
2−
tically active intermediate species [Ru (edta)(OOH)]
and
V
−
III
[
Ru (edta)O] that are formed in the reaction of [Ru (edta)-
(H O)] with H O (at higher concentrations up to 50 mM) and
2 2 2
effect substrate oxidation. These intermediates are ruthe-
−
1
6
nium equivalents of Compounds 0 and I much discussed in
2
2,23
V
−
biomimetic oxidation processes.
Since [Ru (edta)O]
Fig. 3 Kinetic traces recorded at 466 nm for the Ru(edta) catalyzed oxidation
−
−4
−2
of thiourea by H
2
O
2
. [Ru(edta)(H
2
O)] = 2 × 10 M, [H
2
O
2
] = 2 × 10 M, [TU] = species are known to effect the oxidation of organic sub-
−4
24
V
−
(2–10) × 10 M, 25 °C, pH 4.9 (1 mM acetate buffer).
strates, we have also examined the reaction of [Ru (edta)O]
preformed by reacting [Ru (edta)(H
III
−
(
2
O)] with H
2
O
2
in a 1 : 1
ratio) with TU (in high excess), and found no immediate spec-
tral changes (see Fig. S1 in ESI†) demonstrating the inability of
V
−
[
Ru (edta)O] to effect the oxidation of TU. Similarly, the spec-
tral studies observed (see Fig. S2 in ESI†) for the reaction of
III
2−
[
Ru (edta)(OOH)] with TU revealed that the decay of the
III
shoulder at 425 nm signifying the conversion of [Ru (edta)-
OOH)] to [Ru (edta)O] is not affected by the presence of
2
−
V
−
(
TU even in excess. This necessarily suggests the non-involve-
III
2−
ment of [Ru (edta)(OOH)] species in the oxidation of TU
under the specified conditions.
Based on the above experimental observations, the oxi-
2 2
dation of thiourea by the Ru(edta)/H O system can be
accounted for in terms of the suggested reaction mechanism
outlined in Scheme 1.
Fig. 4 Kinetic traces recorded at 466 nm for the Ru(edta) catalyzed oxidation
−
−4
−4
of thiourea by H
2
O
2
. [Ru(edta)(H
2 2 2
O) ] = 2 × 10 M, [TU] = 4 × 10 M, [H O ] =
−3
(2–40) × 10 M, 25 °C, pH 4.9 (1 mM acetate buffer).
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4
–6, and the much slower reactions at lower (pH 1–2) or
1
9
higher pH (pH 8.5–9.5) substantiate the above arguments.
Conclusions
In conclusion, we report the first example of a Ru(III) complex
capable of mediating the oxidation of thiourea using H O as
2
2
an oxidant under ambient conditions. The results of the
present study strongly implicate the direct attack of the
III
−
oxidant, H O to the coordinated TU in the [Ru (edta)(TU)]
2
2
Scheme 1 Suggested mechanism for the Ru(edta) catalyzed oxidation of TU
III
complex resulting in the formation a [Ru (edta)S(OH)C(NH)-
with H
2 2
O . Reactions (2), (3) and (4) represent the different reaction steps in the
−
overall catalytic process.
(NH
another molecule of TU to yield formamidine disulfide {(NH2)-
NH)CSSC(NH)(NH )} as an immediate oxidation product.
However, formamidine disulfide undergoes further slow oxi-
As proposed in Scheme 1, oxidation of TU predominantly dation during the prolonged reaction with H O .
2
)] intermediate species, which further reacts with
(
2
2
2
takes place through substrate activation and not by peroxide
activation. The coordinated edta ligand plays a key-role as it
III
−
imparts high lability in [Ru (edta)(H O)] towards water sub-
2
Acknowledgements
stitution through which TU can bind to ruthenium directly
through the S-donor atom, resulting in the formation of a red The authors gratefully acknowledge financial support from
coloured complex (characteristic of sulfur to ruthenium charge the Deutsche Forschungsgemeinschaft and the assistance of
transfer) in a kinetically preferred pathway. Coordination of Dr Alicja Franke. DC is thankful to Prof. Goutam Biswas, Director
TU through the S atom causes activation of the SvC bond of of the Central Mechanical Engineering Research Institute, for
TU, and makes it vulnerable towards direct oxidation by H O . his support of this work. DC is also thankful to Dr Debasish
2
2
In the proposed scheme, reaction (3) is responsible for the oxi- Bhattacharyya of the Indian Institute of Chemical Biology,
dation of coordinated thiourea and is the rate-determining Kolkata for performing HPLC analysis.
step. Under turnover conditions of excess thiourea (excess
III
−
2
over ruthenium catalyst), [Ru (edta)S(OH)C(NH)(NH )] sub-
sequently reacts rapidly with another molecule of thiourea in
Notes and references
reaction (4) to form formamidine disulfide {(NH )(NH)CSSC-
2
III
−
(NH)(NH
2
)} selectively. [Ru (edta)H
2
O] regenerated via reac-
1 W. E. Curtis, M. E. Muldrow, N. B. Parker, R. Barkley,
S. L. Linas and J. E. Repine, Proc. Natl. Acad. Sci. U. S. A.,
1988, 85, 3422.
2 R. C. Sprong, C. J. M. Aarsman, J. F. L. M. van Oirschot and
B. S. van Asbeck, J. Lab. Clin. Med., 1997, 129, 470.
3 M. J. Kelner, R. Bagnell and K. J. Welch, J. Biol. Chem.,
1990, 265, 1306.
tion (4) in turn rapidly binds free TU through reaction (2) to
mediate its further oxidation via reaction (3). Evidence for the
complete consumption of TU and formation of formamidine
disulfide (TU ), an oxidation product of TU via reaction (4),
2
came from HPLC studies carried out immediately following
the disappearance of the red colour (see Fig. S3a in ESI†). It
2
0
has been reported in the literature that the reaction of
4 Y.-L. Lai, H.-D. Wu and C. F. Chen, J. Cardiovasc. Pharma-
col., 1998, 32, 714.
S-hydroxylated thiourea {(HO)SC(NH)(NH )} and thiourea results
2
in the formation of the oxidation product TU
preferred manner. In the presence of excess H
undergo further slow oxidation to yield different species such
2
in a kinetically
5 M. A. Arifoglu, W. N. Marmer and R. B. Dudley, Textile Res.,
1992, 62, 94.
6 J. Cagarra, J. Gacen, M. Caro and M. Pepio, J. Soc. Dyers
Colour., 1988, 104, 273.
7 W. Wang, M. N. Schuchmann, H.-P. Schuchmann,
W. Knolle, J. von Sonntag and C. von Sonntag, J. Am. Chem.
Soc., 1999, 121, 238.
8 R. H. Simoyi, I. R. Epstein and K. Kustin, J. Phys. Chem.,
1994, 98, 551.
2
O
2
, TU can
2
2
0
as TUO
depletion of TU
2
.
In the present study we have also noticed complete
with concomitant formation of TUO in the
2
2
HPLC analysis of the sample of the reaction mixture taken
after 1 h (see Fig. S3b in ESI†).
III
It is noteworthy that the rate of formation of the [Ru (edta)-
III
−
(
TU)]− complex in the reaction of [Ru (edta)H O] with TU
2
governs the efficiency of the overall catalytic process. A much
longer induction period was observed at pH 1.6 as compared
9 S. A. Svarovsky, R. H. Simoyi and S. V. Makarov, J. Phys.
Chem. B., 2001, 105, 12634.
to that observed at pH 4.9. A similar kinetic behaviour was 10 A. E. Miller, J. J. Bischoff and K. Pae, Chem. Res. Toxicol.,
also observed at higher pH 9.2 (see Fig. S4 in ESI†). These find-
1988, 1, 169.
ings are consistent with the fact that the rate of the reaction of 11 S. Sahua, P. R. Sahooa, S. Patelb and B. K. Mishraa, J. Sulfur
III
−
[
Ru (edta)H O] with TU reaches a maximum in the pH range
Chem., 2011, 32, 171.
2
4
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1
1
1
1
1
1
2 V. Sharma, W. V. Joshi, F. J. Millero and D. Connor, 18 T. Matsubara and C. Creutz, Inorg. Chem., 1979, 18, 1956.
Environ. Sci. Technol., 1999, 33, 2645. 19 H. C. Bajaj and R. van Eldik, Inorg. Chem., 1988, 27, 4052.
3 E. V. Kudrik, A. K. Theodoridis, R. van Eldik and 20 Q. Gao, G. Wang, Y. Sun and I. R. Epstein, J. Phys. Chem. A,
S. V. Makarov, Dalton. Trans., 2005, 1117–1122. 2008, 112, 5771.
4 N. P. O. Henry, K. Y. Harutyuneran and J. E. Byrd, Inorg. 21 D. Chatterjee, E. Ember, U. Pal, S. Ghosh and R. van Eldik,
Chem., 1979, 18, 197. Dalton Trans., 2011, 40, 10473.
5 D. F. Fairlie, W. A. Wicramsinghe, K. A. Byriel and 22 A. Franke, C. Fertinger and R. van Eldik, Angew. Chem., Int.
H. Taube, Inorg. Chem., 1997, 36, 2242–2243. Ed., 2008, 47, 5238.
6 D. Chatterjee and R. van Eldik, J. Mol. Catal. A: Chem., 23 C. Fertinger, N. Hessenauer-Ilicheva, A. Franke and R. van
012, 355, 61. Eldik, Chem.–Eur. J., 2009, 15, 13435.
7 A. A. Diamantis and J. V. Dubrwaski, Inorg. Chem., 1981, 24 D. Chatterjee, Coord. Chem. Rev., 1998, 168, 273 and refer-
0, 1142. ences cited therein.
2
2
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