Indian Journal of Chemistry
Vol. 52A, Aug-Sept 2013, pp. 1019-1025
Synthetic glutathione peroxidase mimics: Effect of nucleophilicity of the
aryl thiol cofactor on the antioxidant activity
Krishna Pada Bhabak, Debasish Bhowmick & Govindasamy Mugesh*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560 012, India
Email: mugesh@ipc.iisc.ernet.in
Received 14 March 2013; accepted 30 April 2013
Catalytic activity of a series of potent amide- and amine-based organoselenium compounds are studied in the presence
of various aromatic thiols having electron donating and electron withdrawing substituents on the phenyl ring. This study
suggests that the antioxidant activities of the synthetic GPx mimics can be significantly increased by the incorporation of a
suitable electron donating group on the phenyl ring of an aromatic thiol.
Keywords: Glutathione peroxidase, Antioxidants, Organoselenium compounds, Aromatic thiols, Peroxide reduction, Thiols,
Enzymatic activity, Enzyme mimics
Glutathione peroxidase (GPx) is a selenocysteine
containing mammalian antioxidant enzyme that
catalyzes the reduction of harmful peroxides in the
presence of glutathione (GSH) as the thiol cofactor
and protects cells/biomolecules from oxidative
damage and inflammation.1 A catalytic mechanism
has been proposed for the enzymatic activity that
involves several redox reactions at the selenium
center of the active site selenocysteine residue. As
shown in Scheme 1, the selenol moiety (E-SeH) at the
active site reacts with peroxide (H2O2) and undergoes
oxidation to generate the selenenic acid (E-SeOH)
with the reduction of peroxides. In presence of thiol
cofactor such as GSH, the selenenic acid produces the
corresponding
selenenyl
sulfide
intermediate
(E-SeSG). Nucleophilic attack of a second molecule
of GSH at the Se-S bond regenerates the active site
with the release of GSH in its oxidized form (GSSG).
However, in the presence of a higher concentration of
peroxide such as in the condition of an ‘oxidative
stress’, the selenenic acid (E-SeOH) may undergo
further oxidation to produce the corresponding
seleninic acid (E-SeO2H) or selenonic acid (E-SeO3H)
intermediates that reduces the catalytic activities.
(Scheme 1).1e-g Therefore, a rapid conversion of the
selenenic acid to selenenyl sulfide and the
regeneration play important roles for the higher
catalytic activity.
Owing to the high importance of GPx for the
mammalian antioxidant defense, during the last few
decades much research attention has been directed
towards mimicing the active site structure of GPx.
Several research groups have been working on the
design and synthesis of small molecule
organoselenium compounds that can mimic the GPx
enzyme activity in the presence of thiol cofactor.2 The
first successful synthetic GPx mimic reported in
the literature was ebselen (1, 2-phenyl-1,2benzisoselenazole-3-(2H)-one), which functions as an
interesting anti-inflammatory drug.3 After the
discovery of ebselen, different types of smallmolecule selenium containing GPx mimics were
discovered as shown in Fig. 1.4-14 These mimics
1020
INDIAN J CHEM, SEC A, AUG-SEPT 2013
Fig. 1Chemical structures of some representative synthetic GPx mimics reported in the literature.
include heterocyclic compounds having Se-N
covalent bond (1-4), various diselenides (5-9),
monoselenides (10-12), cyclic selenenate ester (13)
and spirodiazaselenurane (14). Based on their
reactivity towards thiols and peroxides, different
catalytic mechanisms have been proposed for various
GPx mimics. Particularly, the diaryl diselenides
having basic amino group in the close proximity to
the selenium center such as (5-7) were found to mimic
the GPx activity very effectively, mainly due to the
presence of the amino group, which plays crucial
roles in the catalytic mechanism.7b Notably, these
diselenides follow the catalytic mechanism similar to
the native GPx enzyme involving selenol, selenenic
acid and selenenyl sulfide as intermediates.1e-g
In contrast to the amine-based mimics, ebselen and
sec-amide-based analogues exhibit relatively poor
antioxidant activity due to the extensive thiol exchange
reaction at selenenyl sulfide intermediate.15-17 Due to
the higher electrophilicity of selenium than sulfur, the
incoming thiol preferably attacks at the selenium
center at Se-S bond in the selenenyl sulfide
intermediate, preventing the regeneration of active
selenol species for the catalysis. The undesired thiol
exchange reaction is further enhanced by the presence
of Se···O/ Se···N nonbonded interactions. These thiol
exchange reactions are more pronounced when
aliphatic/aryl thiols are used as cofactor instead of
natural cofactor GSH. For example, while ebselen has
been shown to exhibit reasonably good catalytic
activity with natural thiol GSH as the cofactor, it
shows very poor antioxidant activity in the presence
of aryl/benzyl thiol for the reduction of peroxides.15-18
This was further supported by our studies on the
detailed catalytic activities of ebselen and its
analogues in the presence of different thiols and
peroxides.17 While the activities were much different
in the presence of GSH and PhSH, the activities were
almost identical when different peroxides such as
H2O2, Cum-OOH and t-BuOOH were used.17 A
number of different approaches were employed to
overcome this complication with thiol exchange
reaction with various modifications in the synthetic
mimics to efficiently reduce peroxides.16,19,20 In
addition to this, it was thought worthwhile to study
the influence of the electronic effects of various
BHABAK et al.: EFFECT OF NUCLEOPHILICITY ON ANTIOXIDANT ACTIVITY OF GPx MIMICS
functional groups at the aromatic ring of PhSH on the
antioxidant activities of well-known GPx mimics.
Therefore, in the present study, we report the
influence of different electron donating and
withdrawing substituents on the aromatic ring in
PhSH towards its nucleophilic reactivity.
Materials and Methods
General procedure
n-Butyllithium (nBuLi) was purchased from Acros
Chemical Co. (Belgium). Methanol was obtained
from Merck and dried before use. All other chemicals
were of the highest purity available. All the reactions
were carried out under nitrogen with use of standard
vacuum-line techniques. Because of the unpleasant
odors and toxic nature of several of the reaction
mixtures involved, most manipulations were carried
out in a well-ventilated fume hood. Et2O and THF
was dried over sodium metal with benzophenone.
Thin layer chromatography analyses were carried out
on pre-coated silica gel plates (Merck), and the spots
were visualized with UV radiation. Column
chromatography was performed on glass columns
loaded with silica gel or on automated flash
chromatography systems (Biotage) with use of
preloaded silica cartridges. 1H (400 MHz),
13
C (100.56 MHz), and 77Se (76.29 MHz) NMR
spectra were obtained on a Bruker 400 MHz NMR
spectrometer. Chemical shifts are cited with respect to
SiMe4 as internal (1H and 13C) and Me2Se as external
(77Se) standard. Mass spectral studies were carried out
on a Bruker Daltonics Esquire 6000 plus mass
spectrometer with ESI-MS mode analysis.
Compounds (1), (5), (15)-(23) were synthesized
following the literature methods.3,7a, 8,17,21,22
Determination of GPx-like activity by HPLC method
GPx-like activity was carried out by high
performance
liquid
chromatography
(HPLC)
consisting of a 2695 separation module and a 2996
photodiode-array detector and a fraction collector.
The assays were performed in 1.8 mL sample vials
and a built-in autosampler was used for sample
injection. In this assay, mixtures containing a 1:2
molar ratio of thiol and peroxide in methanol at room
temperature (22 oC) were used as model systems.
Runs with and without catalyst were carried out under
identical conditions. Periodically, aliquots were
injected onto the reversed-phase column (Princeton
C18 column, 4.6×150 mm, 5 µm) and eluted with
methanol and water (85:15). The concentrations of the
1021
disulfide (RSSR) produced in the reaction were
determined at 254 nm (315 nm for p-NO2C6H4SH)
with the aid of calibration plots of pure disulfides
(RSSR) as an external standard. The plots for t1/2 were
obtained by sigmoidal curve fitting. The concentration
of thiol and H2O2 used in the assay were 1.0 mM and
2.0 mM, respectively. The catalyst concentration used
was 10 µM (except compounds 23 and 24. 10 µM and
5 µM concentrations were used for compounds
23 and 24 with different PhSH and thiols,
respectively).
Results and Discussion
Considering the presence of glutamine (Gln) and
aspartate (Asp) residues in the close proximity of
selenocysteine (Sec) center at the active site of GPx,23
most of the synthetic mimics were designed having an
amino or amide group close to the selenium center. A
number of compounds having much improved
antioxidant activities as compared to the existing amine
and amide-based synthetic GPx mimics have been
reported in last few years from our group. For example,
the antioxidant potency of the amide-based mimics
could be improved by replacing the sec-amide group
with the corresponding tert-amide counterpart,21
whereas the activity of the benzylamine-based mimics
could be increased further either by the incorporation
of a methoxy group on the phenyl ring or by replacing
the tert-amino group with a sec-amino group.8,22
Keeping all these developments in mind, we have
chosen some mimics such as (1), (5) and (16-24) as
model GPx mimics (Fig. 2) to study the electronic
effect of different substituents on the phenyl ring of
PhSH towards GPx activities in the presence of H2O2
as substrate.3,7a,8,17,21,22 Several electron donating groups
(-OMe, -OH, -Me and -tBu) and electron withdrawing
groups (-Cl and -NO2) are used at the para-position of
the phenyl ring of PhSH. The disulfides corresponding
to the thiols (25)-(31) (Fig. 2) were synthesized
following the literature method.24 The pure disulfides of
known concentrations were used to make independent
calibration plots to determine the antioxidant activities
of the mimics. During the assay, the formation of
disulfide from the corresponding thiol in the presence of
different selenium compounds was monitored by
reverse-phase HPLC method. The amount of
corresponding disulfide produced in each reaction was
determined and the time required for 50% conversion of
thiols into disulfides (t1/2 values) was calculated from the
peak areas at different time intervals using calibration
plots for the respective disulfides.
1022
INDIAN J CHEM, SEC A, AUG-SEPT 2013
As shown in Table 1, the t1/2 values for all the
selenium compounds (1), (5) and (16)-(24) were
found to be lower than the control values in the
presence of selenium compounds (supplementary
data). For a better and simplified comparison, relative
activities of all the compounds were calculated with
respect to the control reactions (in the absence of any
selenium compound) as shown in Table 2. It is clear
from Tables 1 and 2 that ebselen (1) and its analogues,
(16) and (17), exhibited very poor catalytic activities in
the presence of all the aromatic thiols used in the
Fig. 2Chemical structures of the selenium compounds (1), (5)
and (16)-(24) used in the present study for the reduction of H2O2
in the presence of aromatic thiols (25)-(31).
present study. The poor catalytic activities of ebselen
and its analogues in the presence of an aromatic thiol
such as PhSH has been reported earlier.16,17 Almost
similar activities were also observed for the sec-amidebased diselenides (18) and (19) corresponding to
ebselen analogues (16) and (17). The poor activities of
sec-amide-based compounds are mainly due to the
presence of strong Se···O non-bonded interactions that
lead to an extensive thiol exchange reaction at the
selenenyl sulfide intermediate as reported previously.21
Interestingly, the Se···O interactions could be reduced
by the replacement of sec-amide group with the
corresponding tert-amide counterparts having higher
catalytic activities.21 The relatively higher activity of
the tert-amide substituted diselenides (20) and (21) is
clearly observed with all the thiols used in the present
study (Tables 1 and 2). Compound (20) with methyl
substituent on amide nitrogen group showed the
highest activity among all the amide-based selenium
compounds screened in the present study. The higher
activity for the tert-amide-based diselenides as
compared to the sec-amide-based compounds is mainly
due to the reduction of thiol exchange reactions.21 The
relative activities of different amide-based compounds
were also found to be dependent on the nucleophilicity
of thiols used in the present study. Aromatic thiols such
as (26)-(29) having electron donating groups at the
para-position are more nucleophilic than the thiols
with electron withdrawing groups such as (30) and
(31). As expected, all the amide-based compounds (1)
and (16-21) exhibited much higher relative activities in
Table 1Reduction of H2O2 by different selenium compounds (1), (5) and (16)-(24) in the presence of different thiols (25)-(31)
t1/2 (min)a
25
26
27
28
29
30
31
Control
763.0±19.6
788.0±26.8
1016.0±31.2
756.0±22.7
730.0±45.9
415.0±9.1
571.0±3.5
664.0±4.9
509.0±9.8
641.0±7.7
542.0±24.0
589.0±9.8
323.0±4.9
536.0±36.0
1
663.0±10.6
572.0±9.9
660.0±11.8
642.0±17.3
542.0±2.8
308.0±22.6
483.0±13.5
16
633.0±21.2
634.0±2.8
662.0±19.7
778.0±19.7
672.0±16.2
338.0±8.4
544.0±15.9
17
466.0±2.1
303.0±0.7
363.0±2.1
399.0±11.3
377.0±8.5
269.0±2.8
541.0±9.5
18
454.0±3.5
333.0±19.6
381.0±3.2
438.0±26.1
389.0±14.1
313.0±14.8
576.0±13.4
19
160.0± 1.5
93.6±6.5
117.0±9.1
145.0±9.0
114.3±3.2
129.0±4.3
464.0±4.3
20
239.0±11.6
199.5±2.7
201.0±7.7
223.0±7.7
180.0±5.6
185.0±10.8
501.0±8.4
21
23.9±1.6
10.2±0.5
15.1±0.9
20.7±1.4
18.3±1.1
5.2±0.4
7.4±0.6
5
29.5±1.7
18.2±0.9
23.6±0.1
30.0±1.1
29.9±1.7
18.2±0.5
19.9±2.4
22
6.6±0.1
10.0±0.8
12.2±1.3
11.1±0.5
10.9±0.8
6.5±0.1
26.6±0.2
23
30.3±1.7
24.1±1.0
22.5±2.1
17.4±1.1
28.9±1.3
19.8±1.7
53.6±4.1
24
a
The reactions were carried out in MeOH at 22 °C. Catalyst: 10.0 µM [except compounds (23)-(24)]; ArSH: 1.0 mM; H2O2: 2.0 mM. The
control reactions were performed under identical conditions in the absence of any selenium compound. A lower concentration of
compounds (23) and (24) (5.0 µM) was used as the conversion was too fast to be measured at 10.0 µM concentrations in presence of the
various thiols except PhSH. [In presence of PhSH, 10.0 µM concentration of the catalysts (23) and (24) were used].
Comp.
BHABAK et al.: EFFECT OF NUCLEOPHILICITY ON ANTIOXIDANT ACTIVITY OF GPx MIMICS
1023
Table 2Relative activities of compounds (1), (5) and (16)-(24) in the presence of thiols (25)-(31) as compared to the control reactions
Comp.
Control
1
16
17
18
19
20
21
5
22
23
24
25
1.00
1.14
1.15
1.20
1.63
1.68
4.76
3.19
31.92
25.86
115.6
25.18
26
1.00
1.54
1.37
1.24
2.60
2.36
8.42
3.94
77.25
43.29
157.6
65.66
27
1.00
1.58
1.54
1.53
2.79
2.66
8.68
5.05
67.28
43.05
166.5
88.34
the presence of thiols (26) and (29) than their activities
in the presence of (30) and (31) (Table 2). While a
maximum rate for all the amide-based compounds was
observed in the presence of thiol (27) with 4-hydroxy
group, the lowest activity was observed when thiol (31)
with electron withdrawing 4-nitro group was
employed.
It is known that the amine-based diselenides exhibit
much higher antioxidant activities than the
corresponding amide substituted GPx mimics.7a,7b,8 A
number of tert-amine based diselenides with very high
antioxidant activities have been reported in the
literature having very weak or no Se···N non-bonded
interactions.7c,8 For example, the tert-amine-based
diaryl diselenide (5) that possesses reasonably weak
Se···N interaction exhibits moderate antioxidant
activity.7a,8 A dramatic enhancement in the antioxidant
activity was observed for compounds such as (7), (8)
and (23) having different substitutions. It should be
noted that all these modifications led to a weaker or no
Se···N non-bonded interaction resulting in much less
or no thiol exchange reactions.7c,8,22 As expected, the
tert-amine substituted compounds (5) and (22)
exhibited significantly high activity in the presence of
all the aryl thiols, (25)−(31), used in the present study.
As shown previously, an enhancement in the catalytic
activity was observed for the corresponding sec-aminebased compounds (23) and (24). For example, in the
presence of thiophenol as cofactor, the Me-substituted
amide-based compounds (16), (18) and (20) exhibited
poor catalytic activities and only a slight increase in the
activity (3-fold) is observed upon the conversion of
sec-amide group (18) to the corresponding tert-amide
analogue (20). However, a dramatic enhancement is
observed upon the conversion of amide group to the
Relative activity
28
1.00
1.39
1.17
0.97
1.89
1.73
5.21
3.39
36.52
25.2
136.2
86.89
29
1.00
1.23
1.35
1.08
1.94
1.87
6.40
4.05
39.89
24.41
135.18
50.34
30
1.00
1.28
1.34
1.22
1.54
1.32
3.21
2.24
79.81
22.80
127.69
41.92
31
1.00
1.06
1.18
1.05
1.05
0.99
1.23
1.14
77.16
28.6
42.93
21.46
corresponding amino group. As shown in Table 2,
tert-amine-based diselenide (5) exhibited 6.7 times
higher activity than the corresponding tert-amide
analogue (20) and the sec-amine substituted diselenide
(23) showed almost 70 times higher activity as
compared to the corresponding sec-amide-based
diselenide (18). A similar increase in the activity is also
observed for the isopropyl substituted amide and amine
compounds.
As observed for the amide-based compounds, the
enhancement of the catalytic activity was found to be
dependent on the nucleophilicity of aromatic thiols.
However, the nucleophilicity of aromatic thiol has a
more pronounced impact on the catalytic activity of
amine-based GPx mimics than that of amide substituted
compounds. In general, all the amine-based diselenides
exhibited higher activities in the presence of thiols with
electron donating substituents on the phenyl ring. For
example, while the relative activity of compound (5) in
the presence of PhSH was ~32 times, almost 77, 67, 36
and 40 times higher activity was observed in the
presence of thiols with -OMe, -OH, -Me, and -tBu
substitutions, respectively, at the para-position of the
phenyl ring (Table 2). In addition to the electron
donating substituents, a much higher relative activity of
(5) such as 80- and 77-fold respectively was observed in
the presence of the thiols (30) and (31) having electron
withdrawing (–Cl and –NO2) groups on the phenyl ring.
While the tert-amine-based diselenides (5) and (22)
showed higher activities in the presence of substituted
thiols (26-31) than in presence of PhSH, the sec-amine
substituted diselenides (23) and (24) exhibited lower
relative activities in the presence of thiols (30) and (31)
having electron withdrawing –Cl and –NO2 groups,
respectively. These discrepancies are probably due to the
1024
INDIAN J CHEM, SEC A, AUG-SEPT 2013
higher basicity of the sec-amino group than that of
tert-amine analogues. Owing to the lower pKa of the
thiol group in (30) and (31), the incoming thiol
approaching the sec-amine-based compounds may
undergo deprotonation in the presence of sec-aminebased diselenides to the corresponding thiolates
that are stabilized by the electron withdrawing 4-Cl
and 4-NO2 substituents, decreasing the nucleophilicity
of the thiolate for catalysis. As the –NO2 group
is much more electron withdrawing than the
–Cl group, the relative activity of sec-amine-based
compounds (23) and (24) was the lowest in the
present of thiol (31).
To understand the effect of different aromatic
thiols on the catalytic activity of GPx mimics, we
have chosen the most active mimic (23) in the present
study as a representative molecule. The formation of
disulfides from different thiols in the presence of
compound (23) was plotted as a function of time as
shown in Fig. 3. It is clear from the plot that the
compound exhibited much higher activities in the
presence of thiols with electron donating groups. The
maximum catalytic activity was observed in the
presence of 4-hydroxy substituted thiophenol (27).
Similar to (23), all other mimics in the present study
exhibited maximum activity in the presence of thiol
(27). While PhSH is commonly used as an aromatic
thiol cofactor for the determination of GPx activity of
the synthetic mimics, the present study suggests
4-hydroxythiophenol (27) to be a better co-substrate for
future studies. Furthermore, the catalytic activities of
the known active GPx mimics that are screened in the
presence of PhSH may exhibit much higher rate in the
presence of 4-hydroxythiophenol, making them highly
potent GPx mimics.
Conclusions
In the present study, the GPx mimetic activity of a
series of amide- and amine-based organoselenium
compounds has been investigated in the presence of
various aromatic thiols with electron donating and
electron withdrawing substituents on the phenyl ring.
This is the first study that describes the importance of
electronic contribution of various substituents on
phenyl ring of aromatic thiols towards GPx activity of
synthetic mimics. This study reveals that the
antioxidant activities can be significantly increased by
the incorporation of a suitable electron donating group
such as 4-hydroxyl moiety on the phenyl ring of an
aromatic thiol. The higher GPx activity is probably
due to the increase in the nucleophilic character of the
thiol in the presence of an electron donating
substituent. The electronic effect of para-substituents
is more pronounced for the GPx activity of aminebased mimics than for that of the corresponding
amide-based compounds. This study further indicates
that 4-hydroxythiophenol acts as a better thiol
cofactor than PhSH for screening new synthetic
mimics in future.
Supplementary Data
Supplementary data associated with this article,
i.e., Tables S1-S84, are available in the electronic
form at http://www.niscair.res.in/jinfo/ijca/IJCA_52A(89)1019-1025_SupplData.pdf.
References
Fig. 3Catalytic reduction of H2O2 by catalyst (23) in the
presence of thiols (25)-(31). The formation of disulfide was
followed by a reverse-phase HPLC and the conversion (%) was
calculated from the corresponding calibration plot. [Assay cond.
selenium catalyst: 5.0 µM; thiol: 1.0 mM; H2O2: 2.0 mM in
MeOH at 22 °C. 10 µM of (23) was used in the assay in the
presence of thiophenol (25)].
1 (a) Flohé L, Günzler E A & Schock H H, FEBS Lett, 32
(1973) 132; (b) Rotruck J T, Pope A L, Ganther H E,
Swanson A B, Hafeman D G & Hoekstra W G, Science, 179
(1973) 588; (c) Epp O, Ladenstein R & Wendel A, Eur J
Biochem, 133 (1983) 51; (d) Selenium in Biology and
Human Health edited by Burk R F, (Springer, New York)
1994; (e) Mugesh G, du Mont W-W, Chem Eur J, 7 (2001)
1365; (f) Jacob C, Giles G I & Giles N M, Angew Chem, Int
Ed, 42 (2003) 4742; (g) Roy G, Sarma B K, Phadnis P P &
Mugesh G, J Chem Sci, 117 (2005) 287.
2 (a) Mugesh G & Singh H B, Chem Soc Rev, 29 (2000) 347;
(b) Mugesh G, du Mont W W & Sies H, Chem Rev, 101
(2001) 2125.
3 (a) Müller A, Cadenas E, Graf P & Sies H, Biochem
Pharmacol, 33 (1984) 3235; (b) Wendel A, Fausel M,
BHABAK et al.: EFFECT OF NUCLEOPHILICITY ON ANTIOXIDANT ACTIVITY OF GPx MIMICS
4
5
6
7
8
9
10
Safayhi H, Tiegs G & Otter R, Biochem Pharmacol, 33
(1984) 3241.
(a) Reich H J & Jasperse C P, J Am Chem Soc, 109 (1987)
5549; (b) Jacquemin P V, Christiaens L E, Renson M J;
Evers M J & Dereu N, Tetrahedron Lett, 33 (1992) 3863.
Erdelmeier I, Tailhan-Lomont C & Yadan J-C, J Org Chem,
65 (2000) 8152.
Back T G & Dyck B P, J Am Chem Soc, 119 (1997) 2079.
(a) Wilson S R, Zucker P A, Huang R R C & Spector A, J
Am Chem Soc, 111 (1989) 5936; (b) Iwaoka M & Tomoda S,
J Am Chem Soc, 116 (1994) 2557; (c) Mugesh G, Panda A,
Singh H B, Punekar N S & Butcher R J, Chem Commun,
(1998) 2227; (d) Mugesh G, Panda A, Singh H B, Punekar N S
& Butcher R J J Am Chem Soc, 123 (2001) 839; (e) Sarma B K
& Mugesh G, Inorg Chem, 45 (2006) 5307.
Bhabak K P & Mugesh G, Chem Eur J, 14 (2008) 8640.
Sun Y, Mu Y, Ma S, Gong P, Yan G, Liu J, Shen J & Luo G,
Biochim Biophys Acta, 1743 (2005) 199.
(a) Back T G & Moussa Z, J Am Chem Soc, 124 (2002)
12104; (b) Back T G, Moussa Z & Parvez M, Angew Chem,
Int Ed, 43 (2004) 1268.
1025
11 Phadnis P P & Mugesh G, Org Biomol Chem, 3 (2005) 2476.
12 Zade S S, Singh H B & Butcher R J, Angew Chem, Int Ed, 43
(2004) 4513.
13 Kuzma D, Parvez M & Back T G, Org Biomol Chem, 5
(2007) 3213.
14 Sarma B K, Manna D, Minoura M & Mugesh G, J Am Chem
Soc, 132 (2010) 5364.
15 Kice J L & Purkiss D W, J Org Chem, 52 (1987) 3448.
16 Sarma B K & Mugesh G, J Am Chem Soc, 127 (2005) 11477.
17 Bhabak K P & Mugesh G, Chem Eur J, 13 (2007) 4594.
18 Sarma B K & Mugesh G, Chem Eur J, 14 (2008) 10603.
19 Sarma B K & Mugesh G, Org Biomol Chem, 6 (2008) 965.
20 Haenen G R, Rooij B M De, Vermeulen N P & Bast A, Mol
Pharmacol, 37 (1990) 412.
21 Bhabak K. P & Mugesh G, Chem Asian J, 4 (2009) 974.
22 Bhabak K P & Mugesh G, Chem -Eur J, 15, (2009) 9846.
23 Ursini F, Maiorino M, Brigelius-Flohé R, Aumann K-D,
Roveri A, Schomburg D & Flohé L, Methods Enzymol, 252
(1995) 38.
24 Kawaguchi A W, Sudo A & Endo T, J Polym Sci Part A,
Polym Chem, 50 (2002) 1457.