Effects of metal ions distinguishing between one-step hydrogen- and electron-transfer mechanisms for the radical-scavenging reaction of (+)-catechin

Article abstract of DOI:10.1021/jp026190c

A kinetic study of a hydrogen-transfer reaction from (+)-catechin (1) to galvinoxyl radical (G^·) has been performed using UV-vis spectroscopy in the presence of Mg(ClO₄)₂ in deaerated acetonitrile (MeCN). The rate constants of hydrogen transfer from 1 to (G^·) determined from the decay of the absorbance at 428 nm due to (G^·) increase significantly with an increase in the concentration of Mg²⁺. The kinetics of hydrogen transfer from 1 to cumylperoxyl radical has also been examined in propionitrile (EtCN) at low temperature with use of ESR. The decay rate of cumylperoxyl radical in the presence of 1 was also accelerated by the presence of scandium triflate [Sc(OTf)₃ (OTf = OSO₂CF₃)]. These results indicate that the hydrogen-transfer reaction of (+)-catechin proceeds via electron transfer from 1 to oxyl radicals followed by proton transfer rather than via a one-step hydrogen atom transfer. The coordination of metal ions to the one-electron reduced anions may stabilize the product, resulting in the acceleration of electron transfer.

Full text of DOI:10.1021/jp026190c

J. Phys. Chem. A 2002, 106, 11123-11126
11123
Effects of Metal Ions Distinguishing between One-Step Hydrogen- and Electron-Transfer
Mechanisms for the Radical-Scavenging Reaction of (+)-Catechin
Ikuo Nakanishi,*,† Kentaro Miyazaki,‡,§ Tomokazu Shimada,‡,§ Kei Ohkubo, Shiro Urano,§
Nobuo Ikota,† Toshihiko Ozawa,† Shunichi Fukuzumi,*, and Kiyoshi Fukuhara*,‡
Redox Regulation Research Group, Research Center for Radiation Safety,
National Institute of Radiological Sciences, Inage-ku, Chiba 263-8555, Japan, DiVision of Organic Chemistry,
National Institute of Health Sciences, Setagaya-ku, Tokyo 158-8501, Japan, Department of Applied Chemistry,
Shibaura Institute of Technology, Minato-ku, Tokyo 108-8548, Japan, Department of Material and Life Science,
Graduate School of Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation (JST),
Suita, Osaka 565-0871, Japan
ReceiVed: May 28, 2002; In Final Form: September 10, 2002
A kinetic study of a hydrogen-transfer reaction from (+)-catechin (1) to galvinoxyl radical (G•) has been
performed using UV-vis spectroscopy in the presence of Mg(ClO4)2 in deaerated acetonitrile (MeCN). The
rate constants of hydrogen transfer from 1 to G• determined from the decay of the absorbance at 428 nm due
to G• increase significantly with an increase in the concentration of Mg2+. The kinetics of hydrogen transfer
from 1 to cumylperoxyl radical has also been examined in propionitrile (EtCN) at low temperature with use
of ESR. The decay rate of cumylperoxyl radical in the presence of 1 was also accelerated by the presence of
scandium triflate [Sc(OTf)3 (OTf ) OSO2CF3)]. These results indicate that the hydrogen-transfer reaction of
(+)-catechin proceeds via electron transfer from 1 to oxyl radicals followed by proton transfer rather than via
a one-step hydrogen atom transfer. The coordination of metal ions to the one-electron reduced anions may
stabilize the product, resulting in the acceleration of electron transfer.
Introduction
previously been demonstrated that the effect of Mg2+ on the
hydrogen-transfer rates from NADH (dihydronicotinamide
adenine dinucleotide) analogues to aminoxyl or nitrogen radicals
provides a reliable criterion for distinguishing between the one-
step hydrogen atom transfer and the electron-transfer mecha-
nisms.14
We report herein the effect of metal ions, such as Mg2+ and
Sc3+, on the rates of hydrogen transfer from (+)-catechin (1)
to oxyl radical species, such as galvinoxyl and cumylperoxyl
radicals. Cumylperoxyl radical, which is much less reactive than
alkoxyl radicals, is known to follow the same pattern of relative
reactivity with a variety of substrates.15-17 The detailed kinetic
studies provide valuable mechanistic insight into the mechanism
of the antioxidative reactions of 1: whether the reaction between
1 and oxyl radical species proceeds via one-step hydrogen atom
transfer or via electron transfer.
Catechins contained in green tea are a class of bioflavonoids
that show significant antioxidative activity.1-9 It has been
suggested that catechins trap radical species by hydrogen atom
transfer from its phenolic OH group on the B ring.10,11 However,
little is known about the elementary steps of hydrogen-transfer
reactions from catechins to radical species. There are two
possibilities in the mechanism of hydrogen-transfer reactions
from phenolic anitioxidants to radical species, i.e., a one-step
hydrogen atom transfer or electron transfer followed by proton
transfer.12 Kusu et al. have recently reported the quantitative
relationship between the antioxidative activity of catechins and
their oxidation potentials determined by flow-through column
electrolysis.13 The galloylated catechins having more negative
oxidation potentials than those of nongalloylated catechins show
more efficient antioxidative activity on NADPH-dependent
microsomal lipid peroxidation than the nongalloylated cat-
echins.13 On the other hand, based on the gas-phase bond
dissociation enthalpy and ionization potential calculated by
density functional method, Wright et al. have concluded that in
most hydrogen-transfer reactions of phenolic antioxidants,
hydrogen-atom transfer will be dominant rather than electron
transfer.12 Thus, it is still not clear whether the hydrogen-transfer
reaction of catechins proceeds via a one-step hydrogen atom
transfer or electron transfer followed by proton transfer. It has
Experimental Section
Materials. (+)-Catechin (1) was purchased from Sigma.
Galvinoxyl radical (G•) was obtained commercially from
Aldrich. Mg(ClO4)2 and acetonitrile (MeCN; spectral grade)
were purchased from Nacalai Tesque, Inc., Japan, and used as
received. Di-tert-butyl peroxide was obtained from Nacalai
Tesque Co., Ltd., and purified by chromatography through
alumina which removes traces of the hydroperoxide. Cumene
was purchased from Wako Pure Chemical Ind. Ltd., Japan.
Scandium trifluoromethanesulfonate, Sc(OTf)3 (OTf ) OSO2-
CF3, 99%) was obtained from Pacific Metals Co., Ltd. (Taiheiyo
Kinzoku). Propionitrile (EtCN) used as solvent was purified and
dried by the standard procedure.18
* Corresponding authors.
† National Institute of Radiological Sciences, Japan.
‡ National Institute of Health Sciences, Japan.
§ Shibaura Institute of Technology, Japan.
Osaka University.
10.1021/jp026190c CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/19/2002

11124 J. Phys. Chem. A, Vol. 106, No. 46, 2002
Nakanishi et al.
Figure 1. Spectral changes observed upon addition of 1 (1.5 × 10-4
M) to a deaerated MeCN solution of G• (8.4 × 10-6 M) at 298 K
(Interval: 10 s). Inset: plot of the pseudo-first-order rate constant (kobs
vs the concentration of 1.
)
Figure 2. Plot of kHT vs [Mg2+] in the hydrogen transfer from (a) 1 to
G• and (b) 2 to G• in the presence of Mg(ClO4)2 in deaerated MeCN at
298 K.
Spectral and Kinetic Measurements. Typically, an aliquot
of (+)-catechin (1; 2.0 × 10-2 M) in deaerated MeCN was
added to a quartz cuvette (10 mm i.d.) which contained
galvinoxyl radical (G•; 8.4 × 10-6 M) and Mg(ClO4)2 (0.1 M)
in deaerated MeCN (3.0 mL). This led to a hydrogen-transfer
reaction from 1 to G•. UV-vis spectral changes associated with
this reaction were monitored using an Agilent 8453 photodiode
array spectrophotometer. The rates of hydrogen transfer were
determined by monitoring the absorbance change at 428 nm
due to G• (ꢀ ) 1.43 × 105 M-1 cm-1). Pseudo-first-order rate
constants 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 to the absorbance at
the reaction time and the final absorbance, respectively) were
linear until three or more half-lives with the correlation
coefficient F > 0.999.
takes place to give catechin radical (1•) and hydrogenated G•
(GH) (eq 1).19
Kinetic measurements for the hydrogen-transfer reactions
between 1 and cumylperoxyl radical were performed on a JEOL
X-band spectrometer (JES-ME-LX) at 203 K. Typically, pho-
toirradiation of an oxygen-saturated propionitrile (EtCN) solu-
tion containing di-tert-butyl peroxide (1.0 M) and cumene (1.0
M) with a 1000 W mercury lamp resulted in formation of
cumylperoxyl radical (g ) 2.0156), which could be detected at
low temperatures. The g values were calibrated by using an
Mn2+ marker. Upon cutting off the light, the decay of the ESR
intensity was recorded with time. The decay rate was accelerated
by the presence of 1 (1.0 × 10-4 M). Rates of hydrogen transfer
from 1 to PhCMe2OO• were monitored by measuring the decay
of the ESR signal of PhCMe2OO• in the presence of various
concentrations of 1 in EtCN at 203 K. Pseudo-first-order rate
constants were determined by a least-squares curve fit using a
microcomputer. The first-order plots of ln(I - I∞) vs time (I
and I∞ are the ESR intensity at time t and the final intensity,
respectively) were linear for three or more half-lives with the
correlation coefficient, F > 0.99. In each case, it was confirmed
that the rate constants derived from at least five independent
measurements agreed within an experimental error of (5%.
The decay of the absorbance at 428 nm due to G• obeyed
pseudo-first-order kinetics when the concentration of 1 was
maintained at more than 10-fold excess of the G• concentration.
The observed pseudo-order-rate constant (kobs) increases linearly
with an increase in the concentration of 1 (inset of Figure 1).
From the slope of the linear plot of kobs vs the concentration of
1 is determined the second-order rate constant (kHT) for the
hydrogen transfer from 1 to G• at 298 K as 2.3 × 102 M-1 s-1
.
The 1• thus formed is known to be unstable and undergo
disproportionation to produce the parent 1 and an o-quinone-
type oxidized product of 1.20
If the hydrogen transfer from 1 to G• involves an electron-
transfer process as the rate-determining step, the rate of hydrogen
transfer would be accelerated by the presence of magnesium
ion.14 This is checked by examining an effect of Mg2+ on the
hydrogen transfer rate from 1 to G•. When Mg(ClO4)2 is added
to the 1-G• system, the rate of hydrogen transfer from 1 to G•
was significantly accelerated. The kHT value increases linearly
with increasing Mg2+ ion concentration as shown in Figure 2a
(black circles). Thus, the hydrogen transfer may proceed via
electron transfer from 1 to G•, which is accelerated by the
presence of Mg2+, followed by proton transfer from 1•+ to G-
to yield 1• and GH as shown in Scheme 1. In such a case, the
coordination of Mg2+ to the one-electron reduced species of G•
(G-) may stabilize the product, resulting in the acceleration of
electron transfer.
Results and Discussion
Effect of Mg2+ on Hydrogen Transfer from (+)-Catechin
to Galvinoxyl Radical. Upon addition of (+)-catechin (1) to a
deaerated acetonitrile (MeCN) solution of galvinoxyl radical
(G•), the absorption band at 428 nm due to G• disappeared
immediately as shown in Figure 1. This indicates that hydrogen
transfer from one of the OH groups on the B ring of 1 to G•
Hydrogen Transfer from (+)-Catechin to Cumylperoxyl
Radical. Direct measurements of the rates of hydrogen transfer

One-Step Hydrogen- and Electron-Transfer Mechanisms
J. Phys. Chem. A, Vol. 106, No. 46, 2002 11125
SCHEME 1
SCHEME 2
Figure 3. Plot of kHT vs [Mg2+] (white circles) and [Sc3+] (black
circles) in the hydrogen transfer from 1 to cumylperoxyl radical in the
presence of Mg(ClO4)2 or Sc(OTf)3 in EtCN at 203 K.
SCHEME 3
In contrast to the case of Mg2+, the rate of hydrogen transfer
from 1 to cumylperoxyl radical is enhanced significantly by
the presence of Sc(OTf)3 (OTf ) triflate). Scandium ion has
recently been reported to be the most effective promoter among
various metal ions in metal ion-promoted electron-transfer
reactions.30 The kHT values increase with increasing the scan-
dium ion concentration, as shown in Figure 3. The acceleration
effect of Sc3+ can be ascribed to the strong binding of Sc3+ to
PhCMe2OO-, which results in a decrease in the free energy of
the electron transfer as shown in Scheme 3. Thus, the electron
transfer from 1 to cumylperoxyl radical becomes energetically
feasible in the presence of Sc3+. It should be noted that there is
no interaction between cumylperoxyl radical and Sc3+, since
the g value of cumylperoxyl radical in the presence of Sc3+ is
the same as the value in its absence.
Direct Hydrogen Atom Transfer vs Electron Transfer.
Judging from the one-electron oxidation potential of 1 (E°ox vs
SCE ) 0.12 V),13 which is higher than the one-electron
reduction potential of G• (E°red vs SCE ) 0.05 V), the free
energy change of electron transfer from 1 to G• is positive [∆G°et
(in eV) ) e(E°ox - E°red) > 0, where e is the elementary charge],
and thereby the electron-transfer step is endergonic. In such a
case, the overall rate of hydrogen transfer (kHT), which consists
of electron-transfer and proton-transfer steps, would be slower
than the initial electron-transfer rate (ket). The maximum ket value
is evaluated from the ∆G°et value by eq 2, where it is assumed
that the activation free energy (∆Gqet) is equal to ∆G°et (no
additional barrier is involved), Z is the frequency factor taken
as 1 × 1011 M-1 s-1, and kB is the Boltzmann constant.31
from (+)-catechin (1) to cumylperoxyl radical were performed
in propionitrile (EtCN) at 203 K by means of ESR. The
photoirradiation of an oxygen saturated EtCN solution contain-
ing di-tert-butylperoxide and cumene with a 1000 W mercury
lamp results in formation of cumylperoxyl radical, which was
readily detected by ESR. The cumylperoxyl radical is formed
via a radical chain process shown in Scheme 2.21-25 The
photoirradiation of ButOOBut results in the homolytic cleavage
of the O-O bond to produce ButO•,26,27 which abstracts a
hydrogen from cumene to give cumyl radical, followed by the
facile addition of oxygen to cumyl radical. The cumylperoxyl
radical can also abstract a hydrogen atom from cumene in the
propagation step to yield cumene hydroperoxide, accompanied
by regeneration of cumyl radical (Scheme 2).28,29 In the
termination step, cumylperoxyl radicals decay by a bimolecular
reaction to yield the corresponding peroxide and oxygen
(Scheme 2).28,29 When the light is cut off, the ESR signal
intensity decays, obeying second-order kinetics due to the
bimolecular reaction in Scheme 2.
In the presence of 1, the decay rate of cumylperoxyl radical
after cutting off the light becomes much faster than that in the
absence of 1. The decay rate in the presence of 1 (1.0 × 10-4
M) obeys pseudo-first-order kinetics. This decay process is
ascribed to the hydrogen transfer from 1 to cumylperoxyl radical
(Scheme 2). The pseudo-first-order rate constants increase with
increasing 1 concentration to exhibit first-order dependence on
[1]. From the slope of the linear plot of kobs vs the concentration
of 1 is determined the second-order rate constant (kHT) for the
hydrogen transfer from 1 to cumylperoxyl radical as 6.0 × 102
M-1 s-1 in EtCN at 203 K.
ket ) Z exp(-∆G°et/kBT)
(2)
Figure 4 shows a plot of log kHT versus ∆G°et, calculated by
eq 2 (denoted by ET line). The kHT value of the 1-G• system
(black circle) is much smaller than the corresponding maximum
ket value. This indicates that the hydrogen transfer proceeds via
electron transfer followed by proton transfer as shown in Scheme
1 rather than a direct one-step hydrogen atom transfer. The
hydrogen transfer from the vitamin E model, 2,2,5,7,8-pentam-
ethyl-6-chromanol (2), to G• (eq 3) is known to proceed via a
Mg2+ shows little acceleration effect on the rate of hydrogen
transfer from 1 to cumylperoxyl radical as shown in Figure 3.

11126 J. Phys. Chem. A, Vol. 106, No. 46, 2002
Nakanishi et al.
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shown in Figure 4 (white circle). This confirms that the
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Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (No. 11228205)
from the Ministry of Education, Culture, Sports, Science and
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