GPx-like activity of selenides and selenoxides: Experimental evidence for the involvement of hydroxy perhydroxy selenane as the active species

Article abstract of DOI:10.1021/ja209570y

The reaction mechanism of the GPx-like oxidation of PhSH with H ₂O₂ catalyzed by selenoxides proceeds via formation of the hydroxy perhydroxy selenane, which is a stronger oxidizing agent than selenoxide. A hydroxy perhydroxy selenan

Full text of DOI:10.1021/ja209570y

Communication
pubs.acs.org/JACS
GPx-Like Activity of Selenides and Selenoxides: Experimental
Evidence for the Involvement of Hydroxy Perhydroxy Selenane as
the Active Species
Vanessa Nascimento,† Eduardo E. Alberto,‡ Daniel W. Tondo,† Daniel Dambrowski,† Michael R. Detty,‡
Faruk Nome,*,† and Antonio L. Braga*,†
†Department of Chemistry, Federal University of Santa Catarina, Florianop
́
olis-SC, Brazil
‡Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
S
* Supporting Information
as the rate-determining step.4e−g The sluggish in situ formation of 2
ABSTRACT: The reaction mechanism of the GPx-like
oxidation of PhSH with H2O2 catalyzed by selenoxides
proceeds via formation of the hydroxy perhydroxy sele-
nane, which is a stronger oxidizing agent than selenoxide.
A hydroxy perhydroxy selenane intermediate was observed
by electrospray ionization mass spectrometry and 77Se
NMR spectroscopy in reactions of selenoxide 8 with
H2O2.The initial velocity of oxidation of PhSH by H2O2
with selenoxide 8 is 4 orders of magnitude higher than that
of 8 without peroxide. Selenoxide 8 is not reduced to
selenide 6 by PhSH in the presence of H2O2. While
electronic substituent effects have minimal impact on the
catalytic performance of selenoxides, chelating groups
increase the rate of catalysis.
accounts for its lower catalytic activity in comparison with the GPx
mimetic ebselen.5 Although a similar mechanism is well-established
for telluride catalysts in the oxidation of thiols with H2O2,6 it has
been shown that in an aqueous environment, selenoxides 2 are in
equilibrium with dihydroxy selenanes 3a and, under acidic con-
ditions, with hydroxyselenonium 3.6b,7 These species can add H2O2
reversibly to form hydroxy perhydroxy selenanes 4, which are the
active oxidizing agents and are much better than selenoxides 2 for
the oxidation of a variety of substrates (Scheme 1, cycle B).8
Reduction of selenoxide 2 to selenide 1 by a thiol and the
reaction of 2 with H2O2 to produce 4 are competing reactions
(Scheme 1). Our previous findings suggested that selenoxides 2
may not be the active oxidants responsible for the GPx-like
activity exhibited by organoselenides.8 To probe the mecha-
nism of action of selenides and selenoxides as GPx mimics, we
examined ebselen 5, selenides 6 and 7, and selenoxides 8−12
(Chart 1) under both stoichiometric and catalytic loadings as
lutathione peroxidase (GPx) is an important selenoen-
G
zyme found in humans that is responsible for the reduc-
tion of toxic peroxides at the expense of glutathione (GSH), an
endogenous thiol.1 The presence and action of peroxides is
linked to a number of diseases, including Alzheimer’s and
Parkinson’s diseases, in a process known as oxidative stress.2
The selenium(II)/(IV) redox cycle plays an important role in
biological systems, especially in sulfide/disulfide redox chemistry.3
Selenides have been studied as GPx mimetics, catalyzing the reduc-
tion of peroxides by a variety of thiols. One proposed catalytic cycle
involves oxidation of selenides 1 to selenoxides 2 by peroxides,
followed by the fast reduction of 2 to 1 by 2 equiv of thiol through
the formation of thioselenurane 2′ (Scheme 1, cycle A).4 In this
Chart 1. Organoselenium Catalysts Employed in This Study
Scheme 1. Oxidation Reactions Promoted by Peroxides
Activated by Selenides 1 or Selenoxides 2
promoters for the reduction of H2O2 using PhSH as the
cofactor.
The selenides used in our study were prepared by in situ
reduction of the corresponding diselenides followed by reac-
tion with benzyl bromide.9 Selenoxides 8−12, all lacking
β-hydrogens to avoid undesired selenoxide syn elimination,10
were prepared by oxidation of the parent selenides.8d In a first
set of experiments, we carried out preparative reactions using
catalytic cycle, the oxidation of selenides 1 to selenoxides 2,
presumably the working oxidant in the catalytic cycle, was assigned
Received: October 17, 2011
Published: December 4, 2011
© 2011 American Chemical Society
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dx.doi.org/10.1021/ja209570y | J. Am. Chem.Soc. 2012, 134, 138−141

Journal of the American Chemical Society
Communication
Table 1. Preparative Reactions of Oxidation of PhSH with H2O2 Using Stoichiometric Amounts of 6 or 8
a
a
a
b
entry
cat
PhSH
H2O2
6 or 8
v0 (μM min−1
)
1
2
3
none
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.5
0.0
0.5
1.0
1.0
1.0
0.0
0.5
0.5
0.5
0.0
0.5
0.5
0.53 0.01
2.09 0.002
2.02 0.04
(1.85 0.007) × 104
0.54 0.01
6
8
c
4
8
5
6
none
6
8
7.11 0.02
(1.89 0.006) × 104
c
7
a
b
Equivalents in MeOH at 276.8 0.4 K. Final concentrations: PhSH = 0.5 mM; H2O2 = 0.25 or 0.5 mM; 6 or 8 = 0.25 mM. Values of v0 are
c
averages standard deviations for three independent runs. Reaction progress monitored in a stopped-flow apparatus.
stoichiometric amounts of benzyl phenyl selenide 6 or benzyl
phenyl selenoxide 8 as activators of H2O2 in the oxidation of
PhSH in MeOH. Formation of PhSSPh was monitored through
the UV absorption increase at 305 nm.11 Linear increases in
absorbance (k0) were observed in the initial stages of the
reaction and converted to initial velocities (v0) in units of μM
min−1 (Table 1).
selenoxide 2, dihydroxy selenane 3a, hydroxyselenonium 3, and
hydroxy perhydroxy selenane 4 is expected to form. When
selenoxide 8 was treated with H2O2, the 77Se NMR spectrum
showed two signals at δ 882 and 879. The intensity of the signal
at δ 882 decreased with time relative to the selenoxide signal at
δ 879 associated with gas evolution (presumably O2), perhaps
from decomposition of either H2O2 or 4 (δ 882), whose initial
concentration would decrease with time and H2O2 concen-
tration.13 No benzeneseleninic acid (PhSeO2H, 77Se NMR: δ
1218) was detected throughout the experiments, indicating that
selenoxide 8 was robust under the conditions of reaction (see
the SI). These data suggest that selenoxides 2 are not the
functional oxidants for the GPx-like activity of selenides in the
presence of H2O2 and that hydroxy perhydroxy selenane 4 is
more likely the active oxidizing agent (Scheme 1, cycle B).
The performance of organoselenium compounds in a variety
of oxidation reactions using peroxides, including their GPx-like
activity,14 has been shown to be substituent-dependent.8,15 We
next examined selenides 6 and 7 and selenoxides 8−12, which
incorporate electron-donating, electron-withdrawing, or chelat-
ing substituents, as catalysts (0.1 mol % relative to PhSH).
Linear increases in absorbance were observed in the initial
stages of the catalyzed reactions (Figure 1), and the results for
Each equivalent of catalyst 6 or 8 would react with 1 equiv of
H2O2 to produce the corresponding oxidizing agent. This species
would then react with 2 equiv of PhSH. Surprisingly, the reaction
rate for the oxidation of PhSH using selenide 6 and a
stoichiometric amount of H2O2 was essentially identical to that
observed for the reaction using selenoxide 8 in the absence of
H2O2 and just 4-fold higher than that of the control reaction
(entries 1−3). Selenoxide 8 and 1 equiv of H2O2 gave a 104-fold
increase in the rate of oxidation of PhSH to PhSSPh relative to
the same reaction using 8 in the absence of H2O2 or with selenide
6 and H2O2 (entries 2−4). In a second set of experiments, the
amount of H2O2 was increased to 2 equiv relative to the catalyst,
and a 3-fold increase in initial velocity was observed with 6
relative to the reaction with 1 equiv of H2O2 (entries 2 and 6).
Oxidation of 6 would be faster in higher concentrations of H2O2,
producing selenoxide 8, which reacts rapidly with H2O2 to deliver
the active oxidizing agent in the reaction medium. The initial
velocities of the control and selenoxide 8 reactions with different
amounts of H2O2 were shown to be independent of the
concentration of H2O2 (entries 1 vs 5 and 4 vs 7).
While selenoxides 2 are known to oxidize thiols to
disulfides,12 our results clearly indicate that the active oxidizing
agent in the selenide/selenoxide assay as a GPx mimic (in
which an excess of H2O2 is used) is not a selenoxide but is most
likely hydroxyl perhydroxy selenane 4, which appears to be a
much better oxidizing agent than selenoxide. The presence of 4
during the catalytic cycle was established from the following
series of experiments: (i) Products from the reaction between
selenoxide 8 and 2 equiv of PhSH and H2O2 were isolated and
analyzed by 1H NMR spectroscopy. Selenoxide 8 was recovered
in 97% yield after 0.5 h of reaction [see the Supporting
Information (SI)]. (ii) An aliquot of a 1:1 mixture of selenoxide
8 and H2O2 was analyzed by electrospray ionization mass
spectrometry (ESI-MS) in negative ion mode. A peak at m/z
297 for C13H13O380Se in an ion cluster with the isotopic
distribution of Se was detected (Figure S1 in the SI). (iii)
Selenide 6 (77Se NMR: δ 375) was oxidized to selenoxide 8
(77Se NMR: δ 885) with excess H2O2 (5 equiv) in MeOH-d4.
No other selenium species could be detected by 77Se NMR
analysis, although in this environment, the equilibrium between
Figure 1. Initial rates of appearance of PhSSPh using catalytic loadings
of organoselenium catalysts 5−12.
the initial and relative reaction rates are listed in Table 2. All of
the selenoxides screened showed some catalytic activity pro-
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Journal of the American Chemical Society
Communication
moting the oxidation of PhSH by H2O2 (Figure 1). For com-
parison purposes, ebselen 5 was selected as a standard compound
and its relative activity assigned as 1.0 (Table 2, entry 1). The
step is the reaction of 4 with 2 equiv of thiol to produce disulfide
and regenerate the equilibrium among the selenium species 2, 3a,
and 3 to restart the catalytic sequence.18
Once hydroxy perhydroxy selenane 4 is formed, two different
pathways can be proposed for the oxidation of PhSH (Scheme 2).
In path a, PhSH reacts with 4 to produce 13, which reacts with
another equivalent of PhSH to produce PhSSPh and 3a to restart
the catalytic cycle with H2O2. Alternatively, in path b, PhSH would
attack the terminal oxygen of 4, producing 3a and PhSOH (14),
which reacts with another equivalent of PhSH to produce PhSSPh.
While both are plausible paths, the observed effect of substituents
on the catalytic performance of selenoxides 8−10 favors path a.
The attack of thiol in path a develops negative charge on the
a
Table 2. GPx-like Activities of Organoselenium Catalysts 5−12
b
entry
catalyst
v0 (μM min−1
)
Vrel
1
2
3
4
5
6
7
8
5
6
17.18 0.17
5.08 0.14
15.27 0.30
13.92 0.16
14.71 0.13
16.50 0.19
51.77 0.18
19.29 0.19
1.00
0.30
0.89
0.81
0.86
0.96
3.01
1.12
8
9
10
11
12
7
−
terminal oxygen of the −OOH group as OH is lost. In path b,
the formation of 14 involves attack of the PhSH at the terminal
oxygen and develops negative charge via the formation of Se−O−.
Charge development is further from Se in path a and would be
less influenced by the electronic effects of the substituents attached
to it. Moreover, the increased activity of selenoxide 12 bearing an
amino chelating group can be explained by the increased stability
of 3. This nonbonded interaction with the electron lone pair of
nitrogen results in a higher concentration of 3 and consequently
a faster reaction with H2O2 to afford the active oxidizing agent,
hydroxy perhydroxy selenane 4.19
a
Assay conditions: H2O2 (final concentration = 10.4 mM in MeOH),
PhSH (final concentration = 10.0 mM in MeOH), and organo-
selenium catalyst 5−12 (final concentration = 10.0 μM in MeOH) at
b
276.8
0.4 K. Values of v0 were corrected for the uncatalyzed
background reaction and are averages standard deviations for three
independent runs.
initial rate with selenoxide 8 was 3-fold higher than that observed
for selenide 6 (entries 2 and 3). Although selenoxides 8−10 have
aryl substituents with different electronic demands, their catalytic
activities are essentially identical (entries 3−5). Compounds 8−10
were comparable to ebselen 5 as GPx mimics (entry 1).
In summary, we have shown that the GPx-like catalytic
activity of organoselenides does not follow a Se(II)/Se(IV)
redox cycle. Kinetic evidence has revealed that in the presence
of H2O2, selenoxides are converted to hydroxy perhydroxy
selenanes 4, which are kinetically better oxidizing agents than
selenoxides 2. The oxidation of PhSH by stoichiometric amounts
of 4 is 104-fold faster than that observed for selenoxide 2. A
catalytic cycle involving the interconversion of selenoxides 2 to
hydroxy perhydroxy selenanes 4 followed by oxidation of PhSH to
PhSSPh and regeneration of selenoxide 2 is more consistent with
the kinetic, NMR, and MS data presented here. The performance
of the selenoxide catalyst is little affected by electronic demands
at the Se center, but selenoxides containing potential chelating
groups, especially amino groups, appear to be better catalysts.
Oxidation of PhSH with H2O2 is 3-fold faster with selenoxide 12
than with the GPx mimic ebselen 5.
Selenoxides 11 and 12 with chelating substituents had higher
initial rates of reaction than selenoxides 8−10 (Table 2, entries
6 and 7). The X-ray crystal structure of 11 (see the SI for
crystallographic details) revealed a Se···OH distance of 2.60 Å,
which is much shorter than the sum of the van der Waals radii
for O and Se (3.42 Å).16 This noncovalent interaction would
stabilize the corresponding hydroxyselenonium intermediate 3
prior to reaction with H2O2.8d While selenoxide 11 was
comparable to ebselen 5 in reactivity (entry 6 vs 1), selenoxide
12 with a chelating amino group (entry 7) exhibited a 3-fold
higher activity than 5. Selenide 7 (entry 8) was a poorer catalyst
than selenoxide 12 but did display catalytic activity comparable
to that of ebselen 5.
In light of the experimental evidence presented here, a more
consistent catalytic cycle for the GPx-like activity of selenides
and selenoxides is proposed in Scheme 2. Oxidation of selenide
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of catalyst preparation; kinetic studies; ESI-MS, 77Se
NMR, and X-ray data for selenoxide 11. This material is available
free of charge via the Internet at http://pubs.acs.org.
Scheme 2. Revised GPx Catalytic Cycle of Selenides and
Selenoxides
AUTHOR INFORMATION
■
Corresponding Author
albraga@qmc.ufsc.br
ACKNOWLEDGMENTS
■
The authors thank CNPq, INCT-Catal
́
ise, CAPES, and
FAPESC for financial support. E.E.A. and M.R.D. thank the
Office of Naval Research (Award N0014-09-1-0217) for partial
support of this research. Prof. Ernesto S. Lang, Rafael Stieler,
1 with H2O2 generates selenoxide 2, which is the true catalyst in
these systems. In an aqueous solution, 2 adds water or an
intramolecular nucleophile to produce the dihydroxy selenane
3a. Both 2 and 3a would be in equilibrium with hydroxyseleno-
nium 3.17 Subsequently, 3 adds H2O2 reversibly to produce the
final oxidizing agent, hydroxy perhydroxy selenane 4. The final
́
and Barbara Tirloni (UFSM - Brazil) are acknowledged for
elucidation of the X-ray structure of 11. Prof. Gustavo Amadeu
Micke (UFSC - Brazil) is acknowledged for help with ESI-MS
experiments. Dr. Dinesh Sukumaran and Bharathwaj Sathyamoorthy
(SUNY at Buffalo) are acknowledged for help with 77Se NMR
experiments.
140
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Journal of the American Chemical Society
Communication
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