Effect of Molecular Interactions on Electron-Transfer and Antioxidant Activity of Bis(alkanol)selenides: A Radiation Chemical Study

Article abstract of DOI:10.1002/chem.201601918

Understanding electron-transfer processes is crucial for developing organoselenium compounds as antioxidants and anti-inflammatory agents. To find new redox-active selenium antioxidants, we have investigated one-electron-transfer reactions between hydroxyl (^.OH) radical and three bis(alkanol)selenides (SeROH) of varying alkyl chain length, using nanosecond pulse radiolysis.^.OH radical reacts with SeROH to form radical adduct, which is converted primarily into a dimer radical cation (>Se∴Se<)⁺and α-{bis(hydroxyl alkyl)}-selenomethine radical along with a minor quantity of an intramolecularly stabilized radical cation. Some of these radicals have been subsequently converted to their corresponding selenoxide, and formaldehyde. Estimated yield of these products showed alkyl chain length dependency and correlated well with their antioxidant ability. Quantum chemical calculations suggested that compounds that formed more stable (>Se∴Se<)⁺, produced higher selenoxide and lower formaldehyde. Comparing these results with those for sulfur analogues confirmed for the first time the distinctive role of selenium in making such compounds better antioxidants.

Full text of DOI:10.1002/chem.201601918

DOI: 10.1002/chem.201601918
Full Paper
&
Redox Chemistry
Effect of Molecular Interactions on Electron-Transfer and
Antioxidant Activity of Bis(alkanol)selenides: A Radiation
Chemical Study
Pavitra V. Kumar,[a, b] Beena G. Singh,*[a] Prasad P. Phadnis,[c] Vimal K. Jain,[b, c] and
K. Indira Priyadarsini*[a, b]
Abstract: Understanding electron-transfer processes is cru-
cial for developing organoselenium compounds as antioxi-
dants and anti-inflammatory agents. To find new redox-
active selenium antioxidants, we have investigated one-elec-
Some of these radicals have been subsequently converted
to their corresponding selenoxide, and formaldehyde. Esti-
mated yield of these products showed alkyl chain length de-
pendency and correlated well with their antioxidant ability.
Quantum chemical calculations suggested that compounds
that formed more stable (>Se\Se<)+, produced higher se-
lenoxide and lower formaldehyde. Comparing these results
with those for sulfur analogues confirmed for the first time
the distinctive role of selenium in making such compounds
better antioxidants.
C
tron-transfer reactions between hydroxyl ( OH) radical and
three bis(alkanol)selenides (SeROH) of varying alkyl chain
C
length, using nanosecond pulse radiolysis. OH radical reacts
with SeROH to form radical adduct, which is converted pri-
marily into a dimer radical cation (>Se\Se<)+ and a-{bis-
(hydroxyl alkyl)}-selenomethine radical along with a minor
quantity of an intramolecularly stabilized radical cation.
distinctly different.[6] For example, due to a higher covalent
radius, selenium can interact with its neighboring heteroatom
through nonbonding interactions, which influences the elec-
tron density on the selenium atom, thereby modulating its
ability to participate in the redox reactions.[7,8] We have been
investigating the nature and reactivity of selenium-centered
Introduction
Excessive production of reactive oxygen species (ROS) leads to
loss of redox balance in cells, which is considered to be re-
sponsible for the onset of several chronic diseases like inflam-
mation, cancer, ageing etc., and antioxidants are recommend-
ed to maintain cellular redox homeostasis.[1] Sulfur and seleni-
um compounds, both from natural and synthetic sources, are
finding use as antioxidants and anti-inflammatory agents and
are being explored as a new class of drugs for preventing and
treatment of diseases.[2,3] Selenium is unique in that it is an im-
portant micronutrient and is a constituent of crucial redox-reg-
ulating enzymes like glutathione peroxidase (GPx) and thiore-
doxin reductase (TrxR).[4,5] Preferential application of selenium
as redox modulator emerges from its ability to exhibit a range
of oxidation states and a variety of redox reactions. Although
sulfur and selenium share the same group, their chemistry is
C
radicals generated by hydroxyl ( OH) radical reactions on a vari-
ety of organoselenium compounds ranging from simple sele-
nourea to bifunctional seleno-amino acids like selenomethione
and selenocystine.[9–12] For all the chosen compounds, the OH
C
radical was found to react with the selenium atom; the nature,
stability and reactivity of the resulting selenium radicals are
modulated by nonbonding interactions between selenium and
the neighboring heteroatom.[9–11]
Another important property of selenium compounds is their
ability to exhibit GPx-like activity where it catalyzes the reduc-
tion of hydrogen peroxide, a molecular oxidant and ROS.[13,14]
For a designed organoselenium compound to be an antioxi-
dant, it should not only show GPx-like activity but also exhibit
free-radical reactions and the reaction products of such reac-
tions are recyclable. Previous studies on the antioxidant activi-
ties of both aliphatic as well as aromatic selenium compounds
showed that the free-radical scavenging ability and the GPx ac-
tivity do not correlate as the former proceeds through one-
electron transfer while the latter takes place by two-electron or
oxygen transfer.[9,15] In the present investigation, we propose
for the first time that one-electron-transfer processes in simple
aliphatic bis(alkanol)selenide (SeROH; Scheme 1) can generate
products that are redox cycled and participate in GPx antioxi-
[a] P. V. Kumar, Dr. B. G. Singh, Dr. K. I. Priyadarsini
Radiation and Photochemistry Division
Bhabha Atomic Research Centre, Trombay, Mumbai, 400085 (India)
E-mail: beenam@barc.gov.in
kindira@barc.gov.in
[b] P. V. Kumar, Dr. V. K. Jain, Dr. K. I. Priyadarsini
Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094 (India)
[c] Dr. P. P. Phadnis, Dr. V. K. Jain
Chemistry Division, Bhabha Atomic Research Centre
Trombay, Mumbai, 400085 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601918.
Chem. Eur. J. 2016, 22, 1 – 11
1
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
Scheme 1. Chemical structure of the SeROH compounds.
dant activity. For this, detailed pulse radiolysis studies on the
C
reaction of OH radical with SeROH were carried out, the prod-
ucts were estimated and the results were complimented by
quantum chemical calculations.
Figure 1. Transient spectra obtained at 10 ms after pulse radiolysis of N2O-sa-
turated aqueous solutions of: a) 100 mm, b) 1 mm, and c) 2.5 mm SeEOH at
pH 7. Inset d: the decay traces at: i) 320 nm, and ii) 490 nm obtained during
Results and Discussion
1. Pulse radiolysis studies
C
the reaction of the OH radical with 1 mm SeEOH. Inset e: time-resolved tran-
sient spectra of 50 mm SeEOH obtained at: i) 1 ms, and ii) 20 ms after the
pulse. Absorbed dose, (9.9ꢁ0.2) Gy.
C
The reaction of SeROH with OH radical results in the formation
of different types of selenium-centered radicals and these tran-
sients were characterized by monitoring their spectral and ki-
netic properties under different experimental conditions like
pH, proton and SeROH concentration. The results from individ-
ual alcohol are presented below.
lenium-centered radical cation (>Se +) with parent SeEOH, as
C
observed (path IIc and IIIa in Scheme 2) with most sulfur and
selenium radical cations, through two-centered-three-electron
(2cꢀ3e) hemibond formation involving the p-orbital of the (>
Se +) with the p-orbital containing the lone pair of electrons of
C
Bis(2-ethanol)selenide (SeEOH)
the other selenium atom of SeEOH.[9–11] The absorption band at
320 nm may be either due to the formation of (>Se\OH)
(path I in Scheme 2) or selenium-centered radical cation (>
C
Reaction of SeEOH with OH radicals produced a transient that
showed absorption spectrum in the wavelength range of 200
to 600 nm and was found to be different at various concentra-
tions of SeEOH (Figure 1). At 100 mm, the transient spectrum
showed a small peak at 320 nm and another intense absorp-
tion band with a maximum at 490 nm. Increasing the concen-
tration to 1 or 2.5 mm, generated a similar spectral pattern,
however the absorbance at 490 nm increased significantly,
while that at 320 nm was little influenced. The concentration-
dependent absorbance at 490 nm is attributed to the dimer
radical of the type (>Se\Se<)+ formed by the reaction of se-
Se +) (path II a/b in Scheme 2).
C
To distinguish these different species, their decay at 320 and
490 nm was monitored in the presence of phosphate ion
(proton donor). If the species is (>Se\OH), it decays to
+
C
(>Se ) by the loss of water which is expected to be accelerat-
ed at higher proton concentrations.[16,17] To understand this,
the concentration of SeEOH was fixed at 100 mm SeEOH and
ꢀ
the dihydrogenphosphate (H2PO4 ) ion concentration was
varied from 5 to 50 mm. The absorbance at 490 nm increased
C
Scheme 2. Proposed mechanism for the possible reactions of the OH radical with SeROH.
&
&
Chem. Eur. J. 2016, 22, 1 – 11 www.chemeurj.org
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
ꢀ
with increasing H2PO4 concentration, while that at 320 nm did
not change. Thus the absorbance at 320 nm is ruled out to be
from (>Se\OH) radical. To further confirm this, the transient
C
spectrum was recorded for the reaction of SeEOH with OH
radical at pH 1. For this study, the concentration of SeEOH was
kept at 50 mm, so that high proton and low SeEOH concentra-
tion would favor formation of (>Se +). The transient spectrum
C
at pH 1, showed an absorption maximum only at 490 nm,
while the absorption band at 320 nm was not observed (Fig-
ure S1 in the Supporting Information), clearly confirming that
the transient absorbing at 320 nm is neither due to (>Se\OH)
nor (>Se +). On close observation of the decay and formation
C
kinetics of the 320 nm absorbing species as a function of
SeEOH and proton concentration, it can be inferred that this
absorption band is formed very fast (<1 ms) as compared to
the (>Se\Se<)+ radical, and is not the precursor or successor
of (>Se\Se<)+ radical (inset d, Figure 1). The absorbance at
320 nm decayed by following first-order kinetics with a rate
constant of (3.8ꢁ0.2)ꢁ103 sꢀ1, while that at 490 nm decayed
by second-order kinetics with 2k/elvalue of (9.4ꢁ0.3)ꢁ105 sꢀ1
(inset e, Figure 1) indicating radical–radical dismutation reac-
tion. From these results, the 320 nm absorbing species can be
envisaged to be a carbon-centered radical of the type a-(hy-
Figure 2. Transient spectra obtained at 5 ms after pulse radiolysis of N2O-sa-
turated aqueous solutions of: a) 50 mm, b) 250 mm, and c) 1 mm SePOH at
pH 7. Inset: the time-resolved transient spectra of 100 mm SePOH obtained
at: i) 1 ms, and ii) 40 ms after the pulse. Absorbed dose, (9.9ꢁ0.3) Gy.
ꢀ
decays faster in the presence of a proton donor like H2PO4 to
form (>Se +), that is converted to (>Se\Se<)+ radical, by re-
C
action with SePOH. As observed in sulfur-centered radical reac-
tions, (>Se\OH) may also directly react with the parent mole-
cule to give (>Se\Se<)+ radical and OHꢀ. Therefore, the de-
pendence of the absorbance at 370 and 500 nm was also stud-
ied as a function of SePOH concentration. For these studies,
the experiment was conducted in nanopure water, without
any added phosphate ion while the concentration of SePOH
was changed. With increasing SePOH concentration the ab-
sorbance at 370 nm decreased with concomitant increase in
the absorbance at 500 nm (inset d, Figure 3). By comparing the
data with other selenium compounds, the transient absorb-
ance at 500 nm was assigned to a (>Se\Se<)+ radical (paths
IIc and IIIa, Scheme 2).
C
droxylethyl)seleno methyl radical (HOCH2CH2SeCH2 ). Such radi-
cals can be formed by rearrangement of (>Se\OH) at a-posi-
tion to the selenium center as reported for similar sulfur com-
pound (path IV, Scheme 2).[18] In summary the observable tran-
sient spectra of SeEOH at 320 and 490 nm were attributed to
+
C
(HOCH2CH2SeCH2 ) and (>Se\Se<) radicals, respectively.
Bis(3-propanol)selenide (SePOH)
C
Similar to SeEOH, the reaction of OH radical with SePOH pro-
duced a concentration-dependent transient spectrum (plots a–
c, Figure 2). At 50 mm, the transient spectrum is broad in the
range from 200 to 600 nm with the absorption maximum at
around 370 nm. On increasing the concentration of SePOH to
100 mm, clear time-resolved changes in the absorption spectra
were observed (inset, Figure 2). Here, the broad absorption
maximum at 350–370 nm seen at 1 ms, decayed with the for-
mation of new absorption maximum at 500 nm (inset,
Figure 2) at 40 ms after the pulse.
An unusual behavior was observed in SePOH, where along
with the absorbance at 500 nm, an additional peak at 460 nm
was noticed at 250 mm. Like the 500 nm absorbing species, this
460 nm transient too increased with increasing parent concen-
tration and decayed by second-order decay kinetics, but they
On increasing the concentration of SePOH to 250 mm (Fig-
ure 2b), the 370 nm band was absent, while the broad band in
the wavelength range above 400 nm increased, but showed
a split in the band with two maxima at 460 and 500 nm and at
1 mm only increase in the absorbance at both the wavelengths
was observed. To assign these bands, experiments were per-
ꢀ
formed at different concentration of H2PO4 and SePOH. At
100 mm SePOH, the spectrum in nanopure water (plot a,
Figure 3) showed only the transient absorbance at 370 nm,
ꢀ
while in the presence of H2PO4 the 370 nm peak was absent
with formation of 500 nm absorbing species (plot b, Figure 3).
Inset c of Figure 3 shows loss of absorbance at 370 nm corre-
lating with the increase in the 500 nm species and with in-
Figure 3. Transient spectra obtained at 20 ms after pulse radiolysis of N2O-sa-
turated aqueous solutions of 100 mm SePOH at pH 7, in: a) the absence, and
b) presence of 5 mm phosphate buffer. Insets c and d: change in absorbance
at: i) 370 nm, and ii) 500 nm as a function of phosphate buffer (Se-
POH=100 mm) and SePOH concentration (in the absence of buffer), respec-
tively. Absorbed dose, (10.4ꢁ0.3) Gy.
ꢀ
creasing H2PO4 concentration. This indicates that the transient
absorbance at 370 nm is due to the (>Se\OH) radical, which
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
3
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
differ in their half-life (decay pattern shown in Figure S2 in the
Supporting Information) indicating that even though both the
transients are formed by association of the selenium radical
with parent molecule, they appear to have different structures.
This was further confirmed by performing the experiments
[19]
C
with azide (N3 ) radical, a specific one-electron oxidant. The
transient spectrum generated on treatment of 6 mm SePOH
C
with N3 radical at pH 7, showed only one peak with a maxi-
mum at 460 nm and the decay of the absorption-time plot at
both 460 and 500 nm was similar (Figure S3 in the Supporting
C
Information), unlike that observed with the OH radical. From
this it can be inferred that the transient absorbing at 460 nm is
due to the (>Se\Se<)+ radical, while that absorbing at
500 nm may be due to association of the (>Se\OH) adduct
with the parent SePOH. From these studies it can be inferred
that (>Se\OH) absorbing at 370 nm undergoes multiple reac-
tion pathways such as spontaneous dissociation (path IIa,
Scheme 2), acid catalyzed elimination of hydroxide ion (path
IIb, Scheme 2) and reaction with another SePOH molecule
(path IIc, Scheme 2). No detailed decay analysis was attempt-
ed.
Figure 4. Transient spectra obtained at 5 ms after pulse radiolysis of N2O-sa-
turated aqueous solutions of: a) 50 mm, b) 100 mm, and c) 500 mm SeBOH at
pH 7. Absorbed dose, (9.1ꢁ0.2) Gy.
propanol as reference solute (Table 1).[20] Since the OH radical
C
can also participate in H-abstraction from the alkyl groups or
from the OH group of SeROH, it was necessary to assess the
contribution of the radical formed by H-abstraction from
SeROH.
For this, the reaction of SeROH was performed with H atom
at pH 1. The resultant transient spectrum did not show any ab-
sorbance at wavelength >300 nm, indicating that the species
generated by H atom abstraction from the alkyl chain of
SeROH did not absorb in the wavelength region where other
selenium radicals absorb (Figure S6 in the Supporting Informa-
tion).
The stability of the (>Se\Se<)+ radical can be understood
from the equilibrium constant for the (>Se\Se<)+ radical for-
mation, which was calculated according to the procedure de-
scribed earlier.[9] As is evident from Table 1, the equilibrium
constant for the (>Se\Se<)+ radical decreased in the order
SeBOH>SeEOH>SePOH.
Bis(4-butanol)selenide (SeBOH)
As seen in other derivatives, reaction between 50 mm SeBOH
C
and the OH radical gave a transient spectrum in the wave-
length range from 200 to 600 nm, with two absorption
maxima, a weekly absorbing species at 320 nm and a strong
band at 500 nm. The spectra recorded at different concentra-
tions of SeBOH (50, 100 and 500 mm, plots a, b and c, respec-
tively, Figure 4) indicated that the absorbance at 500 nm, in-
creased with increasing SeBOH concentration, due to the (>
Se\Se<)+ radical (paths IIc and IIIa, Scheme 2), and the ab-
C
sorbance at 320 nm remained unchanged. At pH 1, the OH
C
radical reaction with SeBOH (500 mm) produced only the (>
The trichloromethyl peroxyl radicals (CCl3O2 ) radicals are
Se\Se<)+ radical (Figure S4). Further, SeBOH reacted with N3
model peroxyl radicals and can be conveniently generated by
pulse radiolysis.[21] The transient absorption spectra generated
C
to generate a transient absorption spectrum with an absorp-
tion maximum at 500 nm, confirming that the transient is a (>
Se\Se<)+ radical (Figure S5 in the Supporting Information).
On the contrary, the 320 nm absorbing species was formed in
less than 3 ms and its decay is slower than that of the (>
Se\Se<)+ radical absorbing at 500 nm. This result indicates
that the species absorbing at 320 nm is independent of the (>
Se\Se<)+ radical, however, due to a low extinction coeffi-
cient, it was difficult to carry out detailed kinetic studies, and
therefore no attempt was made to assign the nature of the
species.
C
on treatment of CCl3O2 with SeROH showed an absorption
maximum in the range from 480–500 nm similar to that ob-
C
served with the OH radical reaction (Figure S7 in the Support-
ing Information). The bimolecular rate constant for the reaction
C
of CCl3O2 with SeROH was estimated by employing competi-
tion kinetics using 2,2-azino-bis(3-ethylbenzthiazoline-6-sulfo-
nate) (ABTS2ꢀ) as reference solute (Table 1).[22]
2. Oxidizing versus reducing radicals
C
The rate constant for the reaction of SeROH with OH radical
The radicals generated by different reactions of SeROH with
[9,10]
C
was determined by employing competition kinetics using iso-
OH radicals can be both oxidizing and reducing in nature.
C
Table 1. Estimated kinetic parameters for the reaction of OH radical with SeROH compounds.
Compounds
Rate constant (k)ꢁ109 [mꢀ1 sꢀ1
]
(>Se\Se<)+ decay rate 2k/el [sꢀ1]ꢁ105
pH 1 pH 7
Equilibrium constant (K, [mꢀ1])ꢁ104
pH 1 pH 7
C
kSeROH+ OH
kSeROH+CCl3O2·
SeEOH
SePOH
SeBOH
10.0ꢁ0.1
7.3ꢁ0.1
9.7ꢁ0.1
0.48ꢁ0.04
0.88ꢁ0.03
1.13ꢁ0.06
5.5ꢁ0.2
4.2ꢁ0.3
3.7ꢁ0.2
9.4ꢁ0.3
7.9ꢁ0.2
6.5ꢁ0.2
2.0ꢁ0.2
0.8ꢁ0.1
3.5ꢁ0.2
1.1ꢁ0.2
4.2ꢁ0.3
2.8ꢁ0.3
&
&
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
The yield of the oxidizing radicals was estimated by their abili-
Cꢀ
ty to oxidize ABTS2ꢀ to produce ABTS radical, absorbing at
Cꢀ
645 nm. By following the rate of formation of ABTS radical as
a function of ABTS2ꢀ concentration, the bimolecular rate con-
stant for the reaction of (>Se\Se<)+ with ABTS2ꢀ was esti-
mated. For this, N2O-saturated aqueous solutions at pH 7, con-
taining 5 mm SeROH and 10–100 mm ABTS2ꢀ were pulse irradi-
ated (under these conditions the radicals are predominantly of
(>Se\Se<)+ radical type) and by using the extinction coeffi-
Cꢀ
cient of ABTS (e645nm =1.3ꢁ104 mꢀ1 cmꢀ1) and by applying
Schuler’s formula.[23] The radiation chemical yields (G value) of
the (>Se\Se<)+ radicals were estimated and are given in
Table 2. The rate constant for the oxidation of ABTS2ꢀ by (>
Se\Se<)+ radicals formed from SeBOH, SePOH and SeEOH at
pH 7 were estimated and are listed in Table 2.
The (>Se +) and (>Se\OH) radicals are known to undergo
C
deprotonation/dehydration (path IIIb, Scheme 2) to form
carbon-centered radicals of the type a-{bis(hydroxyl alkyl)}sele-
nomethine radical that are reducing in nature (for convenience
these radicals will be hence forth termed as a-reducing radi-
cals).[9,10] The reducing ability of these radicals was followed by
employing different redox systems. For this, initially an N2O-sa-
turated aqueous solution containing 4 mm of SeROH and vary-
Figure 5. a) HPLC chromatogram showing the formation of SeEOHox after ra-
diolytic degradation of an N2O-saturated aqueous solution of 5 mm SeEOH.
b) The calibration curve plotted by analysis of absorption peak area for the
known concentrations of SeEOHox.
10.2 and 36.9 min, respectively, while the corresponding selen-
oxides were eluted at 2.8, 3.2 and 3.8 min, respectively. Using
the calibration graphs, the yields of SeEOHox, SePOHox and Se-
BOHox were estimated to be (0.20ꢁ0.02), (0.26ꢁ0.02) and
(0.29ꢁ0.03) mmolJꢀ1, respectively. Generally, selenoxides with
the reactive b-methylene group are unstable and undergo syn-
elimination, thus direct estimation of SeROHox may involve
some errors.[6,13] Therefore, an indirect method using dithio-
thretol (DTT) conversion to oxidized DTT (DTTox) by SeROHox
was employed, where the yield of DTTox formed is equivalent
to the SeROHox concentration.
ing concentration of (10–100 mm) methyl viologen (MV2+) was
+
C
pulse radiolyzed and the formation of MV at 605 nm was
monitored.[24]
The reaction system containing SeEOH did not show any
signal at 605 nm, while that for SePOH and SeBOH formed
+
C
MV with a similar rate constant (within experimental limita-
tions; Table 2). This confirmed that the a-reducing radicals
from SeEOH are much less reducing than those from SePOH
and SeBOH. A similar reaction pattern was observed with duro-
quinone (DQ) and thionine (Th2+). The yield of the reducing
radicals was estimated from the yield of these radicals using
their reported extinction coefficient and are listed in Table 2
along with their bimolecular rate constants.[25,26]
DTT and DTTox were observed at 6.8 and 10.6 min (Figur-
es S9–S11 in the Supporting Information), and the estimated
radiation chemical yield of SeEOHox, SePOHox and SeBOHox was
(0.23ꢁ0.02), (0.29ꢁ0.02) and (0.31ꢁ0.03) mmolJꢀ1, respective-
ly. The yields of SeROHox estimated by both direct and indirect
method are similar, indicating their stable nature. The results
further signify that the yield of selenoxide increases with in-
creasing alkyl chain length.
3. Product analysis
The (>Se\Se<)+ radicals formed in the above reactions may
undergo disproportionation to form selenoxide (SeROHox; path
V, Scheme 2) as one of the products.[27] Therefore, formation of
SeROHox was analyzed in these reactions (Figure 5 and Figure
S8 in the Supporting Information). The HPLC chromatogram in-
dicated that SeEOH, SePOH and SeBOH were eluted at 4.4,
It has been reported that during the reaction of bis(2-etha-
C
nol)sulfide (SuEOH) with the OH radical, formaldehyde (HCHO)
is one of the major products with radiation chemical yield of
ꢀ1
C
approximately 0.27 mmolJ , which is 45% of the initial OH
C
Table 2. Rate constants and yield of oxidizing and reducing radicals formed during reaction of the OH radical with SeROH compounds.
SeEOH
SePOH
SeBOH
Cꢀ
reaction with oxidizing radical (ABTS /ABTS2ꢀ) (E0 = +0.67 V)
yield [mmolJꢀ1
rate constant (k) [mꢀ1 sꢀ1
yield [mmolJꢀ1
rate constant (k) [mꢀ1 sꢀ1
yield [mmolJꢀ1
rate constant (k) [mꢀ1 sꢀ1
]
0.22ꢁ0.03
0.34ꢁ0.02
0.31ꢁ0.03
]
]
]
]
(6.0ꢁ0.3)ꢁ109
0.11ꢁ0.02
(3.6ꢁ0.2)ꢁ109
0.13ꢁ0.02
(4.1ꢁ0.2)ꢁ109
0.13ꢁ0.01
C
reaction with reducing radical
Th2+/Th +(E= +0.06 V vs. NHE)
]
(7.4ꢁ0.2)ꢁ108
no reaction
(2.6ꢁ0.3)ꢁ109
0.13ꢁ0.01
(1.9ꢁ0.1)ꢁ109
0.15ꢁ0.04
Cꢀ
DQ/DQ (E=ꢀ0.26 V vs. NHE)
]
(7.6ꢁ0.3) x 108
0.14ꢁ0.02
(2.0ꢁ0.2)ꢁ109
0.17ꢁ0.02
MV2+/MV (E=ꢀ0.44 V vs. NHE)
yield [mmolJꢀ1
]
no reaction
+
C
rate constant (k) [mꢀ1 sꢀ1
(9.4ꢁ0.5)ꢁ108
(1.4ꢁ0.2)ꢁ109
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
radical yield. Anticipating a similar reaction, with the analogous
selenium compound (path IV, Scheme 2), experiments were
performed to detect HCHO and any other aldehydes in the
form of hydrazone and were quantified by using HPLC, where
dinitrophenylhydrazine (DNPH) and DNPH-HCHO were eluted
at 5.1 and 7.4 min, respectively.[28] Using calibration plot for hy-
drazone, the yield of HCHO was estimated to be (0.11ꢁ
0.01) mmolJꢀ1 for SeEOH, which corresponds to approximately
Table 3. Antioxidant parameters of SeROH compounds.
SeEOH SePOH
SeBOH
GPx activity initial velocity (u) [mmolsꢀ1
DNA protection [%][a] (SeROH=1 mm)
]
3.8ꢁ0.2 5.5ꢁ0.2 6.3ꢁ0.3
43ꢁ3 64ꢁ5 72ꢁ5
[a] P<0.01 versus radiation control, calculated with the Student’s t-test.
C
18% of the initial OH radical yield (Figure S12 in the Support-
ing Information). In case of SePOH and SeBOH the yield of
HCHO was negligible (<0.005 mmolJꢀ1) and was less than 2%,
Supporting Information).[31] At a fixed concentration of SeROH
(0.1 mm), the decay rate at 345 nm increased with an increase
in alkyl chain length and followed the pattern SeBOH>
SePOH>SeEOH (Table 3).
C
of the total OH radical. This unusually lower yield of HCHO in
SeEOH as compared to SuEOH suggests facile formation and
higher stability of the (>Se\Se<)+ radical as compared to (>
S\S<)+. In fact it was earlier reported by Bobrowski et al. that
even at 0.1m SuEOH, no (>S\S<)+ radical was observed at
neutral pH.[29] The other products generated from the decay of
a-{bis(hydroxyl alkyl)}selenomethine radical could not be quan-
tified. The details of the reaction mechanism are depicted in
Scheme 2.
5. Quantum chemical calculations
To compliment the experimental results obtained from pulse
radiolysis, energetics for the possible reaction pathways during
C
the OH radical reaction with SeROH was calculated by employ-
ing quantum chemical calculations at the B3LYP/6-31+G(d,p)
level (Becke nonlocal model and Lee–Yang–Parr exchange-cor-
relation functionals).[32,33] Optimized ground-state geometry of
SeROH belongs to the C1 point group and did not show any
interaction between Se and the oxygen atom of the hydroxyl
group. In all SeROH compounds, the highest occupied molecu-
lar orbital (HOMO) was found to be localized on the selenium
4. Antioxidant activity
The stability and nature of a one-electron oxidized transient
should reflect in their antioxidant activity, therefore the ability
of SeROH to protect DNA from g-radiation-induced damage
C
C
was estimated. OH-radical induced strand breaks in DNA leads
atom; therefore an electrophile like the OH radical would
to transformation of super-coiled double stranded DNA to
linear and circular forms.[30] In the presence of compounds that
react mainly on the Se atom. The HOMO values of SeROH com-
pounds are summarized in Table S1 in the Supporting Informa-
tion. Similarly, the geometry of all the possible transients
C
have the ability to scavenge OH radicals, the percentage con-
C
version of the intact DNA to other forms should reduce. There-
formed during the OH radical reaction with SeROH was opti-
C
fore, in the present study, the effect of SeROH on OH-radical
mized at the same calculation level. From the estimated
energy of the optimized structures of SeROH and their respec-
tive transients, the change in energy for all the possible reac-
tions was calculated (Table 4 and Schemes S1–S3 in the Sup-
porting Information). The initial step for the formation of the
induced DNA damage was assayed. Figure S13A–C in the Sup-
porting Information shows the gel images of the electropho-
retic pattern of the DNA samples treated with 8 Gy in the ab-
sence and presence of varying concentration (0.1–1 mm) of
SeEOH, SePOH and SeBOH. From the images it is clear that all
the three compounds showed protection towards radiation-in-
duced DNA damage in a concentration-dependent manner.
The percentage protection for a given concentration of seleni-
um compounds was calculated by considering the damage
caused by radiation as 100% and calculating the ratio of intact
DNA with respect to DNA control. The IC50 value, that is, the
concentration required to protect DNA from radiation-induced
damage by 50% was estimated to be 0.60 and 0.58 mm for
SePOH and SeBOH, respectively, while for SeEOH, the IC50
value was >1 mm (Figure S13D in the Supporting Informa-
tion).
C
(>Se\OH) radical (path I, Scheme 2) on reaction of the OH
radical with SeROH was endothermic. Conversion of the (>
Se\OH) radical to (>Se +) can be either by self-dissociation
C
(path IIa, Scheme 2) or by acid-catalyzed dehydration (path IIb,
Scheme 2). The calculations, as given in Table 4, indicate that
the acid catalysis is energetically more favorable than the
spontaneous dissociation of (>Se\OH). The results also show
+
C
that >Se derived from SePOH and SeBOH can acquire stabili-
zation through formation of five and six-membered rings, re-
spectively, by the interaction of the lone pair of the oxygen
+
C
with the selenium center (path IIIc, Scheme 2). Further, >Se
,
can decay by two competing reactions: 1) formation of the
2cꢀ3e bonded (>Se\Se<)+ radical (path IIIa, Scheme 2), and
2) irreversible loss of proton to form a-reducing radicals (path
At a fixed concentration of 1 mm, the respective DNA pro-
tection exhibited by SeEOH, SePOH and SeBOH are listed in
Table 3.
IIIb, Scheme 2). As seen from Table 4, conversion of (>Se +) to
C
The antioxidant activity of SeROH compounds was also eval-
uated in terms of their GPx-like activity, by monitoring their re-
action with hydrogen peroxide, using glutathione (GSH)–gluta-
thione disulfide (GSSG)–nicotinamide adenine dinucleotide
phosphate (NADPH) coupled assay, measuring the initial veloci-
ty (u) for the decay of NADPH at 345 nm (Figure S14 in the
the (>Se\Se<)+ radical is exothermic while that to a-reduc-
ing radical is endothermic, indicating that the former reaction
is more favorable compared to the latter. On close examination
of the optimized structure of (>Se +) derived from SeEOH, it
C
can be observed that the distance between the selenium
center and the hydrogen atom present at the a-carbon atom
&
&
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
C
Table 4. Calculated energy changes for different reactions involving OH radicals with SeROH.
Transients reactions
B3LYP energy change (DE, kcalmolꢀ1
)
SeEOH
SePOH
SeBOH
SuEOH
SuPOH
SuBOH
C
>Se+ OH!(>Se\OH)
3.49
4.51
7.94
ꢀ89.60
349.56
115.63
173.81
ꢀ82.45
350.53
109.45
134.58
ꢀ96.72
347.11
120.31
258.89
ꢀ98.71
340.79
115.98
3 Oþ
(>Se\OH) H
>Se + +2H2O
ꢀ82.53
ꢀ90.56
C
ƒƒ!
(>Se\OH)!>Se + +OHꢀ
356.02
340.78
C
H
2 O
+
+
>Se
>Se
a-reducing radical+H O+
115.03
107.84
C
C
ƒ!
3
>Se
(>Se\Se<)+
ꢀ14.89 (6.81)[a]
ꢀ11.57 (2.29)[a]
ꢀ16.68 (ꢀ3.78)[a] 151.68 (150.77)[a] 122.21 (120.98)[a] 236.37 (235.22)[a]
ƒ!
3 H2O
2(>Se\Se<)+
>Se=O+
18.45 (ꢀ24.95)[a]
20.87 (ꢀ6.85)[a]
23.62 (ꢀ2.18)[a]
34.57 (32.75)[a]
39.37 (36.91)[a]
36.70 (34.44)[a]
ƒƒ!
3 >Se+2H3O+
[a] The values in the parenthesis are BSSE (for dimer radical cation) corrected.
is less than that of the sum of their van der Waals radii, indicat-
ing the presence of nonbonding interactions.
though stable thermodynamically, is generally not observed in
transient absorption studies. The (>Se\Se<)+ radical decayed
by radical–radical reaction, where the rate constant was higher
for SeEOH compared to SePOH and SeBOH. Subsequent to the
radical-radical reactions, they undergo hydrolysis to form Se-
ROHox as the major product.[27] The estimated yields under the
radiolysis conditions were 40.3, 49.0 and 51.6% of the total
The energy of such nonbonded interaction (Enb) can be ex-
trapolated by performing the NBO analysis of the orbitals on
the (>Se +) radical.[34] Enb represents the perturbation induced
C
by the delocalization of the electron density in a bond and the
value can be directly correlated to the strength of interaction.
The NBO analysis indicated that the s-orbital of a-CꢀH inter-
acts with the s* orbital centered at the Se atom with Enb of
1.61 kcalmolꢀ1. In case of SePOH and SeBOH, the s-orbital of
a-CꢀH interacts with the s* orbital of SeꢀO bond with Enb of
1.41 and 1.32 kcalmolꢀ1, respectively.
C
yield of the OH radical, respectively, for SeEOHox, SePOHox and
SeBOHox. If SeROHox is formed only by the radical–radical decay
of (>Se\Se<)+, its yield should always be less than 50% of
C
the yield of OH. Marginally higher yields for SePOH and
SeBOH indicate that other pathways such as hydrolysis of radi-
cal cations and reaction with radiolytically generated hydrogen
peroxide may contribute to its formation.
Discussion
A strikingly interesting observation for selenium as com-
pared to similar sulfur compounds is that selenium compounds
are nearly quantitatively converted to selenoxides while no
sulfoxides could be reported with sulfur analogues.[29] This
property plays an important role in imparting catalytic antioxi-
dant activity in selenium compounds.
Initial research in the design of selenium antioxidants was con-
centrated mainly on GPx-like activity, where the reaction with
molecular oxidants like hydroperoxides was crucial.[13,14] How-
ever, recent research recognizes the necessity to know the role
of one-electron transfer processes in the antioxidant activity,
where involvement of free radical oxidants is important. With
this aim, our group has been investigating the reactions of
The (>Se +), which is in equilibrium with the (>Se\Se<)+
C
radical, can undergo proton loss to form a-reducing radical.
The reducing ability of such radicals derived from SeBOH is
also higher as compared to those derived from SePOH and
SeEOH. This may be the consequence of the presence of the
electron-donating alkyl hydroxyl moiety attached to the a-
carbon atom. The (>Se\OH) radical derived from SeEOH, like
the analogous sulfur radical, can undergo a Barton-type reac-
tion to form HCHO. However, the estimated yield of HCHO was
found to be four times lower than that for SuEOH.[18] This indi-
cates that the (>Se\OH) radical of SeEOH preferentially forms
(>Se\Se<)+, due to higher stabilization of this species as
seen from quantum chemical calculations. In case of SePOH
and SeBOH, no detectable amount of HCHO was observed as
their corresponding (Se\OH) adducts form less stable seven-
and eight-membered ring species, respectively. Formation of
carbon-centered radicals and aldehydes can lead to increased
oxidative stress through the formation of peroxyl radicals and
Schiff reactions with amino acids, respectively.[2,10,35] The negli-
gible contribution from these species in case of selenium com-
pounds as compared to their sulfur analogues further ascer-
tains that selenium is a better antioxidant than sulfur.
C
free radicals, like OH, with several functionalized selenium
compounds using pulse radiolysis technique. In the present in-
vestigation we have focused on bis(alkanol)selenides (SeROH)
of three different alkyl chain lengths, SeEOH, SePOH and
SeBOH.
C
The results indicate that the initial reaction of OH radical
takes place at the selenium center, producing three different
types of radicals like hydroxyselenouranyl radical (>Se\OH),
selenium centered radical cation (>Se +), dimer radical cation
C
(>Se\Se<)+ and a-{bis(hydroxyl alkyl)}selenomethine radical
(a-reducing radical). The relative reactivity and yield of the
transients depend upon the alkyl chain length.
The (>Se\Se<)+ radical from all the three SeROH exhibits
broad transient absorption with maxima in the range of 460 to
500 nm. The Keq value for (>Se\Se<)+ radical formation,
which also indicates their relative stability, was higher for
SeBOH as compared to SeEOH or SePOH. The lower value of
Keq for SePOH is also due to the formation of (>Se +) radical,
C
which can acquire stabilization through the formation of a ki-
netically stable five-membered ring. Such radicals in case of
SeEOH would form an unstable four-membered ring, while
that for SeBOH result in a six-membered ring. The latter, al-
The rate constants for the reaction of SeROH with peroxyl
radicals and their relative GPx activity showed linear correlation
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
with their HOMO energy levels (TableS1 in the Supporting In-
formation). This along with the results reported for many other
selenium compounds, prompted us to believe that, the antioxi-
dant activity is initiated mainly by a single-electron-transfer re-
action, a process which is not considered very important by
many as compared to the two-electron transfer or oxygen-
transfer reactions.
sulfur compounds with the ꢀa methylene group leads to elon-
gation and easy cleavage of the CꢀH bond resulting in higher
radiolytic degradation as compared to analogous selenium
compounds. The higher a-reducing radical seen in sulfur com-
pounds is also due to its lower affinity to form (>S\S<)+ rad-
ical. All these calculations support the experimentally observed
pulse radiolysis results and the products formed.
These compounds were also tested for in vitro antioxidant
activity by means of protecting plasmid pBR322 DNA from g-
radiation induced damage. All compounds showed protection
at high (millimolar) concentrations. This may be due to direct
Conclusions
C
The bis(alkanol)selenides react with the OH radical to form (>
C
scavenging of OH radical, which may not be of great signifi-
Se\Se<)+ radicals primarily, which depending on the alkyl
chain length undergo transformation to produce selenoxides
(SeROHox) as an important reaction products. Earlier it was pre-
sumed that the formation of selenoxide occurs only by oxygen
atom transfer, our results confirm that the selenide–selenoxide
conversion can be initiated by a one-electron oxidation pro-
cess. The experimental results combined with the calculations
proved that selenium compound with higher HOMO energy
level, formed more stable (>Se\Se<)+ radicals and produced
higher yields of SeROHox. Such compounds also exhibit better
antioxidant activity, a property which is clearly not observed in
analogous sulfur compounds. The SeROHox formed during
these reactions can be reduced back to SeROH in the presence
of reducing agents like thiols, thereby imparting catalytic activ-
ity. The results therefore provide a unique example of how
probing single-electron-transfer processes in real time scales,
can be utilized to design selenium-based antioxidants, where
the free radical as well GPx-like activity can be correlated with
synergistic effect.
cance. However, in the cellular systems, with the presence of
high amounts of thiols, one would expect reversible reduction
of selenoxides by the thiols, providing catalytic antioxidant
mechanism. The observations from antioxidant studies confirm
that, SeBOH which gives higher yield of SeBOHox, exhibits
better ability to scavenge ROS. In order to understand this sig-
nificantly different activity of selenium over sulfur, we made
detailed analysis of the nature of radicals and their structures
by quantum chemical calculations at B3LYP/6-31+G(d,p) level.
It is well established that the sulfur/selenium centered radical
cations are stabilized by forming 2cꢀ3e bonds. In this, the orbi-
tal with the lone-pair electron (either the suitable heteroatom
like N or O or another sulfur/selenium atom) mix with another
orbital containing an unpaired electron on sulfur/selenium to
form a hemibond.[36] In such hemibond formation, there are
two electrons in the bonding orbital and one electron in the
antibonding orbital, which tends to cause mutual repulsion be-
tween the two bonding atoms. In selenium, the orbitals are
more diffused as compared to sulfur, thereby reducing the or-
bital overlap and repulsion and thus imparting higher stability
to selenium-centered radicals. Also, the higher covalent radius
in selenium reduces steric congestion at the selenium center
as compared to sulfur. These unique features allow the seleni-
um atom to readily form the (>Se\Se<)+ radical to an extent
Experimental Section
Materials
GSH, GSSG, glutathione reductase (GR), NADPH, ABTS2ꢀ, DTT, DTTox,
MV2+, sodium azide and DQ (>99%) were purchased from Sigma–
Aldrich. DNPH was purchased from Sisco Research Laboratories
and recrystallized from ethanol prior to use. HPLC grade acetoni-
trile, trifluoroacetic acid (TFA) and HCHO were purchased from
Advent, India. All the other chemicals and reagents were of
“Analar” grade and were used as such. The solutions were freshly
prepared for each experiment in nanopure water with a conductivi-
ty of 0.1 mScmꢀ1, obtained from a milipore water purification
system. The pH of the solutions was adjusted using monosodium
phosphate (NaH2PO4), disodium phosphate (Na2HPO4·2H2O) and
perchloric acid (HClO4).
that other intermediates like (>Se\OH) and (>Se +) are not
C
observed. For the same reasons, sulfur analogues do not show
(>S\S<)+ radical formation even at a concentration of
0.1m.[29] The quantum chemical calculations support these ob-
servations, where the calculated enthalpy change for formation
of (>Se\Se<)+-type radical is exothermic, while it is endo-
thermic for (S\S<)+ radicals.
In sulfur compounds the (>S +)-type radical undergoes de-
C
protonation, which is facilitated in molecules where the p-orbi-
tal of the oxidized sulfur overlaps efficiently with the s-orbital
of the CꢀH bond.[36] As the orbitals present on selenium are
more diffused than sulfur, a poor overlap with the s-CꢀH orbi-
tal takes place leading to lower deprotonation. This was clearly
observed in the NBO analysis, where the interaction between
the chalcogen atoms with the neighboring CꢀH bond is higher
in case of sulfur as compared to selenium compounds. On
The SeROH were synthesized by the reported method (see the
Supporting Information).[37] The corresponding selenoxide was syn-
thesized by treating SeROH with 1.5 equivalents of hydrogen per-
oxide (H2O2) and characterized by NMR spectroscopy (1H, 13C and
77Se; Figures S14–S16). The characterization details of SeROH are
given in the Supporting Information. NMR spectra were recorded
on a Bruker Avance-II 300 MHz spectrometer operating at 300.13
close examination of the optimized structure of (>S +) of
C
1
(1H) and 57.25 MHz (77Se{1H}). H NMR chemical shifts were relative
SuEOH, it can be observed that the distance between the
to internal DMSO peak (d=2.49 ppm). The 77Se{1H} NMR chemical
shifts were relative to external diphenyl diselenide (Ph2Se2) in
CDCl3 (d=463.0 ppm relative to Me2Se (0 ppm)). UV–visible ab-
sorption studies were measured on a JASCO V-630 spectrophotom-
sulfur center and the hydrogen atom present at the a-carbon
atom is less than that for (>Se +) of SeEOH. The Enb value is
C
1.5-times higher as compared to SeEOH. Higher Enb values in
&
&
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
eter. High performance liquid chromatographic (HPLC) measure-
ments were carried out on a ELICO HL 460 HPLC system. Depend-
ing upon the experimental requirement, two different 60Co g-radia-
tion sources were employed, that is, for the DNA nicking assay,
a low dose rate source operating at 1 Gyminꢀ1 was used, while for
other experiments, 60Co g-source with a dose rate of 40 Gyminꢀ1
was used.
DNA nicking assay
C
OH-radical induced DNA damage was studied by separating super
coiled pBR322 DNA from the linear form by using gel electrophore-
sis.[30] For this, 1 mL of pBR322 (250 ngmLꢀ1) was mixed with differ-
ent concentrations of SeROH and the final volume was made up to
20 mL. This solution was exposed to g-radiation with a total dose of
8 Gy using a 60Co g-source. Reaction samples were mixed with 4 mL
of bromophenol loading dye (6ꢁ). An equal volume (20 mL) of
each sample was loaded on a 1.5% agarose gel stained with ethid-
ium bromide and subjected to electrophoresis. Gel images were
taken using a UV transilluminator and analyzed with the GeneTools
software (Version 08-3d.3.SynGene). The data given are mean ꢁ
S.E.M. from two independent experiments; each one assayed in
triplicate. The statistical significance of the results was checked
with the Student t-test.
Pulse radiolysis studies
Pulse radiolysis studies were carried out by using a 7 MeV electron
beam with 100 ns pulse width.[26] Thiocyanate dosimeter (aerated
aqueous solution of 10 mm KSCN, Ge 475 nm=2.59ꢁ10ꢀ4 m2 Jꢀ1
)
was used to estimate the absorbed dose and an average dose of
9 Gy per pulse was used for all experiments, unless otherwise
stated.[38] The reaction with OH radical was carried out in N2O satu-
C
rated solution, where e’aq is quantitatively converted to OH radical
C
to give the final GOH of 0.6 mmoleJꢀ1 [39]
.
The one-electron oxidation
C
C
GPx activity
reactions were studied by using N3 radical, a specific one-electron
C
oxidant generated by treatment of OH with 0.1m sodium azide
GPx activity of SeROH compounds was calculated by using the
NADPH-GSH-GSSG coupled assay.[30] Briefly, 2 mm H2O2 was added
to 1 mm GSH solution containing 0.3 mm NADPH, 5unitsmLꢀ1 GR
and 0.1 mm of SeROH dissolved in pH 7.4 phosphate buffers. The
reaction was monitored by following a decrease in absorbance of
NADPH at 340 nm as a function of time.
C
(NaN3). CCl3O2 radicals were generated by radiolysis of aerated
aqueous solution containing 48% isopropanol and 4% carbon tet-
rachloride at neutral pH. Under the experimental condition, the ra-
diochemical yields (G) of N3 and CCl3O2 radicals were 0.69 and
0.64 mmolJꢀ1, respectively.[40,41] The bimolecular rate for the reac-
C
C
C
tion of SeROH with CCl3O2 radical was estimated by employing
competition kinetics using ABTS2ꢀ as standard (e645nm =1.35ꢁ
104 mꢀ1 cmꢀ1).
Quantum chemical calculations
High performance liquid chromatographic (HPLC) analysis
The geometry of the proposed transients were optimized in gas-
eous phase by extensive variation in initial confirmation of the
transients at B3LYP/6–31+G(d,p) (Becke non-local model and Lee–
Yang–Parr exchange correlation functionals) level. B3LYP is a hybrid
functional and gives vibrational force fields, frequencies, and spec-
tra, as well as thermochemical properties, with better accuracy.[42]
6–31+G(d,p) is a well explored basis set attached with double dif-
fusion functions for better results on reaction chemistry calcula-
tions of organic molecules involving free or lone pairs of electrons.
The most stable transient structures were further optimized in
water using polarizable continuum (PCM) solvent density (SMD)
model.[43] SMD model includes calculation of cavitation and disper-
sion–repulsion energies and particularly useful when calculating
free energy of solvation for a molecule going from a gas phase to
solvent phase. The optimized structures were verified as global
minima structures by performing the frequency calculation (see
the Supporting Information). The energetics of the reactions was
calculated by estimating the difference in the zero-point corrected
B3LYP energy of the products and the reactants. All these calcula-
tions were performed by adopting the GAMESS suite of programs
on a PC-based LINUX cluster platform.[44] Visualization of the geom-
etry and relevant molecular orbitals was carried out by following
the Chemmissian software (version V4.38). The B3LYP energy ob-
tained for dimer radical cations of selenium and sulfur compounds
was corrected for BSSE (basis-set superposition error) using coun-
ter-poise method. The BSSE correction values were 0.91, 1.23 and
1.15 kcalmolꢀ1 for SuEOH, SuPOH and SuBOH dimer radical cat-
ions, respectively. Similarly BSSE correction values for SeEOH,
SePOH and SeBOH were 21.7, 13.86 and 12.9 kcalmolꢀ1, respective-
ly. To estimate the strength of non-bonding interactions (Enb), natu-
ral bond order (NBO) analysis was carried out at the B3LYP/6–31+
G (d,p) level by using a Gaussian09.[45]
C
The products obtained on OH radical induced oxidative degrada-
tion of SeROH were identified and their yield was estimated by
using HPLC. For this, N2O saturated aqueous solutions of 5 mm
SeROH were irradiated using 60Co g-source with a dose of 0.83 kGy.
To avoid further reaction of the products, the irradiation dose was
set in such a way that not more than 10% of reactants underwent
radiolysis. HCHO and selenoxide were the anticipated products ob-
tained during radiolysis and were separated by isocratic method
on a Prontosil 120-5-C18 reverse phase column and detected with
an absorption detector.
The yield of HCHO was estimated by treating it with DNPH to form
DNP-HCHO hydrazone which was detected by measuring its ab-
sorbance at 345 nm.[28] The radiated solutions (150 mL) were mixed
with 50 mL DNPH (4 mm dissolved in 10 mm HCl) and stirred for
5 min. An aliquot (20 mL) of this reaction mixture was injected in
the HPLC and eluted with acetonitrile/water (60:40; v/v) mixture
containing 0.1% TFA as mobile phase. The standard calibration
curve was obtained from known concentration of HCHO derivat-
ized with DNPH.
Selenoxide was estimated by two methods. In the direct method,
the yield of the selenoxide formed during radiolysis was separated
by HPLC using an acetonitrile/water mixture in the ratio 5:95 (v/v)
with 0.1% TFA as eluent, monitored at 240 nm. The yield was cal-
culated from the calibration curve obtained from injecting known
concentration of pure selenoxides. In the second method, the se-
lenoxide formed was estimated by reaction with DTT, monitoring
the formation of DTTox, eluted and monitored by using similar ex-
perimental conditions used in the direct method. The yield of
DTTox is equivalent to the amount of selenoxide formed and was
estimated from the standard calibration curve obtained for DTTox.
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
[26] S. N. Guha, P. N. Moorthy, K. Kishore, D. B. Naik, K. N. Rao, Proc. Indian
Acad. Sci. Chem. Sci. 1987, 99, 261–271.
Acknowledgements
[27] C. Schoneich, A. Aced, K. D. Asmus, J. Am. Chem. Soc. 1993, 115, 11376–
11383.
[28] A. Soman, Y. Qiu, Q. Chan Li, J. Chromatogr. Sci. 2008, 46, 461–465.
[29] K. Bobrowski, G. L. Hug, B. Marciniak, B. Miller, C. Schoneich, J. Am.
Chem. Soc. 1997, 119, 8000–8011.
[30] R. Ragagopalan, K. Wani, N. G. Huigol, T. V. Kagiya, C. K. Krishnannair, J.
Radiat. Res. 2002, 43, 153–159.
[31] M. Maiorinoa, A. Roveria, F. Ursini, C. Gregolin, J. Free Radicals Biol. Med.
1985, 1, 203–207.
The authors thank the Computer Center, BARC, for providing
ANUPAM parallel computing facility. P.K. acknowledges CSIR for
the senior research fellowship. The authors acknowledge Dr.
D. K. Maity, HBNI, for initial training in computations and Dr.
Ayesha Khan, Chemistry Department, Savitribai Phule Pune
University, India for her help in Gaussian calculations.
[32] O. Makogon, R. Flyunt, T. Tobien, S. Naumov, M. Bonifacic, J. Phys. Chem.
A 2008, 112, 5908–5916.
[33] B. K. Sarma, G. Mugesh, ChemPhysChem 2009, 10, 3013–3020.
[34] M. Iwaoka, H. Komatsu, T. Katsuda, S. Tomoda, J. Am. Chem. Soc. 2004,
126, 5309–5317.
[35] P. A. Grimsrud, H. Xie, T. J. Griffin, D. A. Bernlohr, J. Biol. Chem. 2008, 283,
21837–21841.
[36] K. D. Asmus, in Radiation Chemistry: Present Status and Future Trends,
(Eds.: C. D. Jonah, B. S. M. Rao), Elsevier, Amsterdam, 2001, pp. 341–
394.
Keywords: antioxidants · pulse radiolysis · radical reactions ·
selenides · selenoxides
[1] C. Winterbourne, Nat. Chem. Biol. 2008, 4, 278–286.
[2] E. E. Battin, J. L. Brumaghim, Cell Biochem. Biophys. 2009, 55, 1–23.
[3] G. Hardy, I. Hardy, Nutrition 2004, 20, 590–593.
[4] L. Flohe, W. A. Gunzler, H. H. Schock, FEBS Lett. 1973, 32, 132–134.
[5] V. N. Gladyshev, K. T. Jeang, T. C. Stadtman, Proc. Natl. Acad. Sci. USA
1996, 93, 6146–6151.
[37] S. Dey, N. Ghavale, A. S. Hodage, V. K. Jain, G. Kedarnath, L. B. Kumb-
hare, P. P. Phadnis, P. C. Parashiva, K. I. Priyadarsini, A. Wadawale, Bhabha
Atomic Research Centre, document BARC/2009/I/003.
[38] G. V. Buxton, C. C. Stuart, J. Chem. Soc. Faraday Trans. 1995, 91, 279–
281.
[39] G. V. Buxton, Q. G. Mulazzani, in Radiation: Chemical Techniques, Wiley-
VCH, Weinheim, Germany, 2001, pp. 503–557.
[40] R. Zhao, J. Lind, G. Merenyi, T. E. Eriksen, J. Am. Chem. Soc. 1998, 120,
2811–2816.
[41] X. Shen, J. Lind, T. E. Eriksen, J. Phys. Chem. 1989, 93, 553–557.
[42] P. J. Stephens, F. J. Devlin, J. Phys. Chem. 1994, 98, 11623–11627.
[43] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B 2009, 113,
6378–6396.
[44] M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H.
Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su, T. L. Windus, M.
Dupuis, J. A. Montgomery, J. Comput. Chem. 1993, 14, 1347–1358.
[45] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Na-
katsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G.
Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Ha-
segawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E.
Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Ra-
ghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakr-
zewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Dan-
iels, ꢂ. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussi-
an, Inc., Wallingford CT, 2009.
[6] L. A. Wessjohann, A. Schneider, M. Abbas, W. Brandt, Biol. Chem. 2007,
388, 997–1006.
[7] A. J. Mukherjee, S. S. Zade, H. B. Singh, R. B. Sunoj, Chem. Rev. 2010, 110,
4357–4416.
[8] B. G. Singh, E. Thomas, S. N. Sawant, K. Takahashi, K. Dedachi, M.
Iwaoka, K. I. Priyadarsini, J. Phys. Chem. A 2013, 117, 9259–9265.
[9] B. G. Singh, S. A. Nadkarni, V. K. Jain, K. I. Priyadarsini, RSC Adv. 2015, 5,
66621–66627.
[10] B. Mishra, A. Sharma, S. Naumov, K. I. Priyadarsini, J. Phys. Chem. B 2009,
113, 7709–7715.
[11] B. Mishra, L. B. Kumbhare, V. K. Jain, K. I. Priyadarsini, J. Phys. Chem. B
2008, 112, 4441–4446.
[12] B. Mishra, D. K. Maity, K. I. Priyadarsini, H. Mohan, J. P. Mittal, J. Phys.
Chem. A 2004, 108, 1552–559.
[13] F. Kumakura, B. Mishra, K. I. Priyadarsini, M. Iwaoka, Eur. J. Org. Chem.
2010, 440–445.
[14] L. Orian, S. Toppo, Free Radical Biol. Med. 2014, 66, 65–74.
[15] B. Mishra, K. I. Priyadarsini, H. Mohan, G. Mugesh, Bioorg. Med. Chem.
Lett. 2006, 16, 5334–5338.
[16] B. G. Singh, E. Thomas, F. Kumakura, K. Dedachi, M. Iwaoka, K. I. Priya-
darsini, J. Phys. Chem. A 2010, 114, 8271–8277.
[17] M. Bonifacic, I. Stefanic, G. L. Hug, D. A. Armstrong, K. D. Asmus, J. Am.
Chem. Soc. 1998, 120, 9930–9940.
[18] C. Schoneich, K. Bobrowski, J. Am. Chem. Soc. 1993, 115, 6538–6547.
[19] Z. B. Alfassi, R. H. Schuler, J. Phys. Chem. 1985, 89, 3359–3363.
[20] W. Wang, M. N. Schuchmann, H. P. Schuchmann, W. Knolle, J. von Sonn-
tag, C. von Sonntag, J. Am. Chem. Soc. 1999, 121, 238–245.
[21] T. N. Das, K. I. Priyadarsini, J. Phys. Chem. 1994, 98, 5212–5278.
[22] B. S. Wolfenden, R. L. Willson, J. Chem. Soc. Perkin Trans. 1 1982, 11,
805–812.
[23] R. H. Schuler, A. L. Hartzell, B. Behar, J. Phys. Chem. 1981, 85, 192–199.
[24] S. Solar, W. Solar, N. Getoff, J. Holcman, K. Sehested, J. Chem. Soc. Fara-
day Trans. 1982, 78, 2467–2477.
Received: April 25, 2016
[25] D. Meisel, G. Czapski, J. Phys. Chem. 1975, 79, 1503–1509.
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
&
Redox Chemistry
P. V. Kumar, B. G. Singh,* P. P. Phadnis,
V. K. Jain, K. I. Priyadarsini*
&& – &&
Effect of Molecular Interactions on
Electron-Transfer and Antioxidant
Activity of Bis(alkanol)selenides: A
Radiation Chemical Study
Roots radicals: Functionalized alkyl se-
lenides react with hydroxyl radical to
form dimer radical cation, which is con-
verted to a selenoxide that is catalytical-
ly recycled by thiols. Comparing these
results with those for sulfur analogues
confirmed for the first time the distinc-
tive role of selenium in making such
compounds better antioxidants.
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ