Electron-transfer mechanism in radical-scavenging reactions by a vitamin E model in a protic medium

Article abstract of DOI:10.1039/b416572a

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH*) or galvinoxyl radical (GO*) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO₄)₂ in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.

Full text of DOI:10.1039/b416572a

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A R T I C L E
Electron-transfer mechanism in radical-scavenging reactions by a
vitamin E model in a protic medium
Ikuo Nakanishi,*a,b Tomonori Kawashima, Kei Ohkubo, Hideko Kanazawa, Keiko Inami,
a,c
b
c
d
d
e
e
a
Masataka Mochizuki, Kiyoshi Fukuhara, Haruhiro Okuda, Toshihiko Ozawa,
f
b
a
Shinobu Itoh, Shunichi Fukuzumi* and Nobuo Ikota*
a
Redox Regulation Research Group, Research Center for Radiation Safety, National Institute
of Radiological Sciences, Inage-ku, Chiba, 263-8555, Japan. E-mail: nakanis@nirs.go.jp;
Fax: +81-43-255-6819; Tel: +81-43-206-3131
b
c
d
e
f
Department of Material and Life Science, Graduate School of Engineering, Osaka University,
SORST, Japan Science and Technology Agency (JST), Suita, Osaka, 565-0871, Japan
Department of Physical Pharmaceutical Chemistry, Kyoritsu University of Pharmacy,
Minato-ku, Tokyo, 105-8512, Japan
Division of Organic and Bioorganic Chemistry, Kyoritsu University of Pharmacy, Minato-ku,
Tokyo, 105-8512, Japan
Division of Organic Chemistry, National Institute of Health Sciences, Setagaya-ku, Tokyo,
158-8501, Japan
Department of Chemistry, Graduate School of Science, Osaka City University, Sumiyoshi-ku,
Osaka, 558-8585, Japan
Received 11th November 2004, Accepted 29th November 2004
First published as an Advance Article on the web 11th January 2005
•
•
The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH ) or galvinoxyl radical (GO ) by a vitamin E
model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO in
)
4 2
de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH
proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen
atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron
oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN
(
0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.
Introduction
to investigate the effects of metal ions on radical-scavenging
reactions in various solvents with different polarity.
9
Recently, much attention has been paid to the mechanisms of
radical-scavenging reactions of phenolic antioxidants, such as
vitamin E (a-tocopherol) and flavonoids, with regard to the
development of chemopreventive agents against oxidative stress
and associated diseases. There are two mechanisms for the
radical-scavenging reactions of phenolic antioxidants: a one-
step hydrogen atom transfer from the phenolic OH group; and
an electron transfer followed by a proton transfer. Metal ions
are a powerful tool that can be used to distinguish between these
two mechanisms, since electron-transfer reactions are known to
We report herein that the scavenging reactions of 2,2-diphenyl-
•
•
1
-picrylhydrazyl radical (DPPH ) or GO by the vitamin E
model 1H in de-aerated methanol (MeOH) proceed via an
electron transfer mechanism rather than via a one-step hy-
drogen atom transfer, which has been observed in de-aerated
MeCN. Effects of bases on the radical-scavenging rates were
also examined, to clarify whether the actual electron donor is
1–3
−
1
H or the corresponding phenolate anion 1 in MeOH. Different
mechanisms in protic and aprotic solvents are discussed based
on kinetic, electrochemical, and EPR data obtained in this
study, providing valuable and fundamental information about
the radical-scavenging mechanism of phenolic antioxidants.
4
be significantly accelerated by their presence. In fact, we have
recently reported that scavenging reactions of the galvinoxyl
•
radical (GO ) and the cumylperoxyl radical by (+)-catechin in
aprotic media, such as acetonitrile (MeCN) and propionitrile,
proceed via an electron transfer from (+)-catechin to the radicals
(
which is significantly accelerated by the presence of metal ions,
Experimental
2
+
3+
5,6
such as Mg and Sc ) followed by a proton transfer. On the
other hand, no effect of Mg on the hydrogen-transfer rate from
2
+
Materials
a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), to
2
,2,5,7,8-Pentamethylchroman-6-ol (1H) was purchased from
•
2
,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl radical (DOPPH )
Wako Pure Chemical Ind. Ltd., Japan. 2,2-Diphenyl-
1
•
•
or GO in de-aerated MeCN has been observed, indicating
that the radical-scavenging reactions of 1H in MeCN proceed
via a one-step hydrogen atom transfer rather than via electron
-picrylhydrazyl radical (DPPH ) and galvinoxyl radical
•
(
GO ) were commercially obtained from Aldrich. Tetra-n-
butylammonium perchlorate (Bu
4
NClO ), used as a supporting
4
7,8
transfer. However, the effects of solvents on the mechanism
of radical-scavenging reactions of phenolic antioxidants have
yet to be clarified. Leopoldini et al. have reported that the
bond dissociation enthalpies for O–H bonds and the adiabatic
ionization potentials for phenolic antioxidants, calculated with
use of density functional theory, do not follow the same trends in
electrolyte for the electrochemical measurements, was purchased
from Tokyo Chemical Industry Co., Ltd., Japan, recrystallized
from ethanol, and dried under vacuum at 313 K. Mg(ClO
4
2
) and
methanol (MeOH; spectral grade) were purchased from Nacalai
Tesque, Inc., Japan and used as received. Pyridine and 2,6-
lutidine were commercially obtained from Wako Pure Chemical
2
gas, water and benzene. Thus, it is of considerable importance
10
Ind. Ltd., Japan and purified by the standard procedure.
6
2 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 2 6 – 6 2 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5

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•
Spectral and kinetic measurements
Since the phenoxyl radical of 1H (1 ) generated in the reaction
of 1H with radicals readily reacts with molecular oxygen (O ),
from the phenolic OH group of 1H to DPPH takes place to
• •
•
produce 1 (eqn. (1)). The absorption band of 1 was shifted
7,8
from 423 nm in MeCN to 427 nm in MeOH. Such a shift in
2
•
the absorption band of 1 may be due to a stronger solvation of
reactions were carried out under strictly de-aerated conditions.
A continuous flow of Ar gas was bubbled through a MeOH
•
1
in MeOH than in MeCN.
•
−5
solution (3.0 mL) containing DPPH (4.8 × 10 M) and
Mg(ClO (0–0.3 M) in a square quartz cuvette (10 mm id) with
)
4 2
a glass tube neck for 10 min. Air was prevented from leaking into
neck of the cuvette with a rubber septum. Typically, an aliquot
−
2
of 1H (2.0 × 10 M), which was also in de-aerated MeOH, was
added to the cuvette with a microsyringe. This led to a reaction
•
of 1H with DPPH . UV-vis spectral changes associated with
the reaction were monitored using an Agilent 8453 photodiode
•
array spectrophotometer. The rates of the DPPH -scavenging
reactions of 1H were determined by monitoring the absorbance
•
4
−1
−1
change at 516 nm due to DPPH (e = 1.13 × 10 M cm ) using
a stopped-flow technique on a UNISOKU RSP-1000-02NM
spectrophotometer. The pseudo-first-order rate constants (kobs
)
were determined by a least-squares curve fit using an Apple
Macintosh personal computer. The first-order plots of ln(A–
A ) vs. time (A and A are denoted as the absorbance at the
∞ ∞
reaction time and the final absorbance, respectively) were linear
until three or more half-lives with the correlation coefficient q >
•
0
.999. The reaction of 1H with GO was carried out in the same
(
1)
manner and the rates were determined from the absorbance
•
5
−1
−1
•
change at 428 nm due to GO (e = 1.32 × 10 M cm ). The
rate constants of the reactions in the presence of base (pyridine
or 2,6-lutidine) were determined in the same manner.
The rate of the DPPH -scavenging reaction of 1H was
measured by monitoring the decrease in absorbance at 516 nm
due to DPPH using a stopped-flow technique. The decay of the
absorbance at 516 nm due to DPPH obeyed pseudo-first-order
•
•
Electrochemical measurements
kinetics when the concentration of 1H ([1H]) was maintained
at more than a 10-fold excess of the DPPH concentration. The
pseudo-first-order rate constants (kobs) increase with increasing
•
The cyclic voltammetry (CV) and second-harmonic alternating
1
1–16
current voltammetry (SHACV)
formed on an ALS-630A electrochemical analyzer in de-aerated
MeOH containing 0.10 M Bu NClO as a supporting electrolyte.
measurements were per-
[
1H], exhibiting first-order dependence on [1H]. From the slope
of the linear plot of kobs vs. [1H], the second-order rate constant
HT) was determined for the radical-scavenging reaction as
4
4
(
1
k
k
The Pt working electrode (BAS) was polished with BAS polish-
ing alumina suspension and rinsed with acetone before use. The
counter electrode was a platinum wire. The measured potentials
were recorded with respect to an Ag/AgNO
electrode. The E1/2 values (vs. Ag/AgNO
3
−1
−1
.07 × 10
M
s , in de-aerated MeOH at 298 K. The
HT value thus obtained in de-aerated MeOH is significantly
larger than that determined in de-aerated MeCN (4.35 ×
3
(0.01 M) reference
) were converted
2
−1 −1
7
1
0 M s ). A similar result has been reported by Litwinienko
3
19
and Ingold. Intermolecularly hydrogen-bonded phenolic OH
groups of hydrogen-bond accepting solvents, such as alcohols,
are known to be essentially unreactive against radicals. Thus,
17
to those vs. SCE by adding 0.29 V. All electrochemical
measurements were carried out at 298 K under 1 atm Ar.
19
the enhanced kHT value in MeOH suggested that the reaction
EPR measurements
mechanism in MeOH may be different from that in MeCN. The
•
Typically, an aliquot of a stock solution of 1H (2.0 × 10− M) in
2
GO -scavenging rate constant by 1H in de-aerated MeOH has
de-aerated MeOH was added to the EPR sample tube (0.8 mm
also been determined in a same manner by monitoring the
•
•
id) containing a de-aerated MeOH solution of DPPH (2.0 ×
decrease in absorbance at 428 nm due to GO as 2.54 ×
−
4
3
−1
−1
10
M) with a microsyringe under 1 atm Ar. EPR spectra of
10 M s , which is slightly smaller than that in de-aerated
3
•
−1 −1
the phenoxyl radical 1 produced in the reaction between 1H
MeCN (3.32 × 10 M s ).
•
and DPPH were taken on a JEOL X-band spectrometer (JES-
RE1XE). The EPR spectra were recorded under non-saturating
microwave power conditions. The magnitude of modulation was
chosen to optimize the resolution and the signal-to-noise ratio
of the observed spectra. The g values and the hyperfine splitting
constants were calibrated with a Mn marker. Computer
simulation of the EPR spectra was carried out using Calleo
ESR Version 1.2 program (Calleo Scientific Publisher) on an
Apple Macintosh personal computer.
Effect of magnesium ion on the rates of radical scavenging
reactions
If the radical-scavenging reactions of 1H involve an electron-
transfer process as the rate-determining step, the rates of radical
2
+
5,6
scavenging would be accelerated by the presence of metal ions.
This was investigated by examining the effect of Mg(ClO
4
2
) on
the radical-scavenging rates by 1H in de-aerated MeOH. When
•
Mg(ClO
4
)
2
is added to the 1H–DPPH system in de-aerated
•
MeOH, the rate of DPPH -scavenging reaction by 1H was
Results and discussion
significantly accelerated. Such an acceleration was not observed
•
7
for the DPPH -scavenging reaction by 1H in MeCN. The kHT
Radical-scavenging reactions of the vitamin E model in
de-aerated MeOH
2
+
value increases linearly with increasing Mg concentration
[Mg ]) as shown in Fig. 1a. A similar acceleration effect of
Mg has been observed for the GO -scavenging reaction by
1H in de-aerated MeOH (Fig. 1b). Thus, the radical-scavenging
reactions in de-aerated MeOH may proceed via an electron
transfer from 1H to DPPH or GO , which is accelerated by
the presence of Mg , followed by proton transfer from 1H
to DPPH or GO as shown in Scheme 1. In such a case,
2
+
(
•
2+
•
Upon addition of 1H to a de-aerated MeOH solution of DPPH ,
the absorption band at 516 nm due to DPPH disappeared
immediately, accompanied by an appearance of the absorption
band at 427 nm. Since the absorption band at 427 nm is
•
•
•
•
2+
•+
diagnostic of the phenoxyl radical derived from 1H (1 ) in
18
−
−
MeOH, this spectral change indicates that hydrogen transfer
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 2 6 – 6 2 9
6 2 7

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to 1H• may stabilize the product, resulting in the acceleration
+
of the initial electron-transfer process. In the presence of a
large amount of a strong Lewis acid, such as Mg(ClO
4
2
) , no
deprotonation of 1H occurs in MeOH.
Solvent effect on the one-electron oxidation potential of the
vitamin E model
The solvent effect on the one-electron oxidation potential
0
ox
(
E ) of 1H was examined by cyclic voltammetry (CV) and
second-harmonic alternating current voltammetry (SHACV)
2
+
•
Fig. 1 Plots of kHT vs. [Mg ] in the reaction of 1H with (a) DPPH and
11–16
measurements.
reported that the one-electron oxidation of a-tocopherol itself
occurs at about 0.97 V vs. SCE in MeCN (0.25 M Bu NPF
Very recently, Williams and Webster have
•
(
b) GO in de-aerated MeOH at 298 K.
4
6
)
21
based on the detailed electrochemical analyses. A similar cyclic
voltammogram was observed for the electrochemical oxidation
of 1H in MeCN (0.1 M Bu
4
NClO
4
) (data not shown), from
0
which was determined the E value (vs. SCE) of 1H in MeCN as
ox
0
.97 V. On the other hand, the CV wave of 1H in MeOH (0.1 M
Bu
4
NClO
4
) was irreversible. Thus, SHACV measurement was
0
0
carried out to determine the E value of 1H in MeOH. The E
ox
ox
value (vs. SCE) of 1H in MeOH (0.1 M Bu
from the intersection of an SHACV wave (Fig. 3), is located at
.63 V, which is significantly more negative than the value in
4
NClO
4
), determined
0
Scheme 1 Radical-scavenging reaction by 1H via an electron transfer
in MeOH.
0
MeCN (0.97 V). Such a negative shift of the Eox value in MeOH
as compared to that in MeCN may be ascribed to a stronger
solvation of 1H in MeOH than in MeCN. Thus, the ease of
one-electron oxidation of 1H in MeOH as compared to in MeCN
2
+
−
−
•+
the coordination of Mg to DPPH or GO may stabilize the
product, resulting in the acceleration of the electron transfer.
may result in the difference in the radical-scavenging mechanism.
Effect of base on the rates of radical scavenging reactions
In protic media, such as alcohols and water, 1H may be in
−
equilibrium with the corresponding phenolate anion 1 , which
is a much stronger electron donor as compared to the parent
20
−
1
H. In such a case, 1 may act as an electron donor rather than
the parent 1H in MeOH.
In order to clarify an actual electron donor in MeOH, the
effect of base on the radical-scavenging rates of 1H was exam-
•
ined. The addition of pyridine to the 1H–DPPH system results
•
in a significant increase in the rate of the DPPH -scavenging
reaction by 1H. The kHT value increases with increasing pyridine
concentration to reach a constant value as shown in Fig. 2.
When pyridine is replaced by 2,6-lutidine, a stronger base than
pyridine, the limiting kHT value is larger than that in the case
of pyridine, as shown in Fig. 2. If the rate of acceleration is
due to the deprotonation of the phenolic OH group of 1H in
the presence of base, the limiting kHT value should be the same
regardless of the basicity of pyridines. The different limiting kHT
values between pyridine and 2,6-lutidine in Fig. 2 suggest that
Fig. 3 SHACV of 1H recorded at the scan rate of 4 mV s− on Pt
1
4 4
working electrode in de-aerated MeOH (0.1 M Bu NClO ) at 298 K.
−
little deprotonation occurs to produce 1 and that the actual
electron donor is the parent 1H rather than 1− in MeOH, as
EPR spectrum of the phenoxyl radical derived from the vitamin
E model in de-aerated MeOH
shown in Scheme 1. In such a case, the coordination of pyridines
The EPR detection of radical species derived from 1H would
provide valuable information about the solvation of the radical
22,23
•
species.
The EPR spectrum of 1 in de-aerated MeOH at
2
98 K is shown in Fig. 4a. It should be noted that the g value of
•
the EPR spectrum of 1 in MeOH (2.0040) is apparently smaller
than that in MeCN (2.0047). The observed hyperfine structure
7
in Fig. 4a is well reproduced by the computer simulation (Fig. 4b)
with four hyperfine splitting constants (hfc) listed in Table 1.
•
7
Table 1 also shows the hfc values of 1 in MeCN. All the hfc
values in MeOH are also smaller than those in MeCN. The
•
smaller g value of the EPR spectrum of 1 as well as the
smaller hfc values in MeOH than those in MeCN indicates
•
that the stronger solvation of 1 may occur in MeOH than
•
+
in MeCN. Although the EPR spectrum of 1H could not
•
be observed because of the fast deprotonation to produce 1
•
+
•
(Scheme 1), stronger solvation of 1H may also occur in MeOH
than in MeCN, resulting in the ease of one-electron oxidation of
1H in MeOH than in MeCN.
Fig. 2 Plot of kHT vs. [base] for the reaction of 1H with DPPH in
the presence of pyridine (black circles) or 2,6-lutidine (white circles) in
de-aerated MeOH at 298 K.
6
2 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 6 2 6 – 6 2 9

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•
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•
•
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This work was partially supported by a Grant-in-Aid for
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6 2 9