Enhanced radical-scavenging activity of naturally-oriented artepillin C derivatives

Article abstract of DOI:10.1039/b715973k

More than two-fold augmentation in the radical-scavenging activity of artepillin C could be achieved via altering the O-H bond dissociation enthalpy of artepillin C by means of structural modifications. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715973k

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Enhanced radical-scavenging activity of naturally-oriented artepillin C
derivatives{
Sushma Manda,a Ikuo Nakanishi,*ab Kei Ohkubo,b Yoshihiro Uto,c Tomonori Kawashima,b Hitoshi Hori,c
Kiyoshi Fukuhara,d Haruhiro Okuda,d Toshihiko Ozawa,ae Nobuo Ikota,f Shunichi Fukuzumi*b and
Kazunori Anzai*a
Received (in Cambridge, UK) 16th October 2007, Accepted 14th November 2007
First published as an Advance Article on the web 28th November 2007
DOI: 10.1039/b715973k
(2H–6H) (Scheme 1) and their enhanced scavenging activity
More than two-fold augmentation in the radical-scavenging
activity of artepillin C could be achieved via altering the O–H
bond dissociation enthalpy of artepillin C by means of
structural modifications.
?
towards cumylperoxyl radical (PhCMe2OO ). PhCMe2OO , which
?
?
is less reactive than RO , is known to follow the same pattern of
relative reactivity with a variety of substrates.8 The structure–
activity relationship is also discussed based on the results obtained
in this study, providing a valuable insight into the development of
antioxidants stronger than the naturally occurring ones.
Reactive oxygen species (ROS) and other free radicals have been
implicated as pathological mediators in many clinical disorders.
The reactivity of the most frequently encountered active free
2
Synthesis of 2H–6H was based on regioselective C-prenylation
of corresponding ortho-substituted phenols according to our
established procedure9 (see ESI,{ S1). This indicates that the
?
?
radicals, such as superoxide anion (O2 ), hydroxyl radical ( OH),
?
?
?
alkyl radical (R ), alkoxy radical (RO ), peroxyl radical (ROO ),
?
?
?
nitric oxide (NO ) and lipid peroxyl radical (LOO ), varies, but
some may cause severe damage to biological molecules, especially
to DNA, lipids and proteins. However, several attempts including
the use of naturally occurring and chemically synthesized
antioxidants have been made to find out the possible ways for
scavenging of these free radicals1 and synthetic attempts have also
been made in the last two decades to develop more potential
antioxidants.2 The plant-derived phenolic compounds have
attracted much attention due to their limited or zero toxicity in
in vivo systems. Artepillin C [3-{4-hydroxy-3,5-bis(3-methyl-2-
butenyl)phenyl}-2(E)-propenoic acid] (1H), a major component
(.5%) of Brazilian propolis,3 has been reported to show
antioxidative activity4 alongside other important biological
activities.5 Recently, we reported the free radical-scavenging
activity of artepillin C and also discussed the possible scavenging
mechanism.6 Since O–H bond dissociation enthalpy (DHT) is
known to regulate the antioxidative potency in phenolic com-
pounds,7 synthetic approaches towards the lowering of DHT
by structural modifications of artepillin C (1H) may result in
remarkable changes in its antioxidative activity. We report herein
the synthesis of five naturally-oriented artepillin C derivatives
artepillin C derivatives could efficiently scavenge PhCMe2OO .
In the presence of the artepillin C derivatives the decay rate of
?
PhCMe2OO follows pseudo-first-order kinetics. The pseudo-first-
order rate constant (k) exhibits first-order dependence with
respect to the concentration of the artepillin C derivatives. From
the slopes of the linear plots were determined the second-order rate
constants (kobs) for the reaction between artepillin C derivatives
?
and PhCMe2OO in EtCN at 203 K. The kobs value for the
aRadiation Modifier Team, Heavy-Ion Radiobiology Research Group,
Research Center for Charged Particle Therapy, National Institute of
Radiological Sciences (NIRS), Inage-ku, Chiba 263-8555, Japan.
E-mail: nakanis@nirs.go.jp; Fax: +81-43-255-6819;
Tel: +81-43-206-3131
bDepartment of Material and Life Science, Graduate School of
Engineering, Osaka University, SORST, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan
cFaculty of Engineering, The University of Tokushima, Tokushima
770-8506, Japan
dDivision of Organic Chemistry, National Institute of Health Sciences
(NIHS), Setagaya-ku, Tokyo 158-8501, Japan
eDepartment of Health Pharmacy, Yokohama College of Pharmacy,
Yokohama 245-0066, Japan
fSchool of Pharmacy, Shujitsu University, Okayama 703-8516, Japan
{ Electronic supplementary information (ESI) available: Detailed proce-
dures for the synthesis of 2H–6H. See DOI: 10.1039/b715973k
Scheme 1
626 | Chem. Commun., 2008, 626–628
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Table 1 Rate constants (kobs) for scavenging of cumylperoxyl radical
by 1H–6H in EtCN at 203 K, energy difference values (DHT) between
phenoxyl radicals and phenols with reference to 1H, and ionization
potential (IP) values determined by density functional theory (DFT)
calculations
Compound kobs/mol21 dm3 s21 DHT/kcal mol21 IP/kcal mol21
1H
2H
3H
4H
5H
6H
4.9 6 102
5.7 6 102
1.5 6 102
5.5 6 102
9.4 6 102
1.2 6 103
0
162.4
152.3
164.9
154.3
162.3
153.2
22.7
0.5
22.1
24.2
26.0
3
PhCMe2OO scavenging by 6H (1.2 6 10 mol21 dm3 s21) is the
largest among the examined artepillin C derivatives and is much
larger than that for (+)-catechin (5.0 6 102 mol21 dm3 s21), which
is one of the strongest antioxidants.14,15 The rate constants of the
?
Fig. 2 Plot of log kobs vs. calculated energy difference values (DHT
)
?
PhCMe2OO -scavenging reactions by other derivatives have also
between phenoxyl radicals and phenols with reference to 1H.
been determined and are listed in Table 1. All the artepillin C
derivatives, except 3H, could afford significantly larger kobs values
than 1H (4.9 6 102 mol21 dm3 s21). The kobs value of 3H was
found to be smaller than that of 1H, and this may be due to the
absence of an electron-donating alkene group in 3H. The kobs
value increases with an increasingly electron-rich environment
in the molecule.
It was also found that the kobs values for the artepillin C
derivatives were linearly correlated with the corresponding energy
difference values, equal to DHT, (Table 1) between the phenoxyl
?
?
radicals (1 –6 ) and the phenols determined by density functional
theory (DFT) calculations16 as shown in Fig. 2. On the other hand,
as shown in Fig. 3, such a linear correlation cannot be observed
between the kobs values and the ionization potential (IP) calculated
Direct measurements of the rate of the reaction between
?
artepillin C derivatives and PhCMe2OO were performed in
?
by DFT (Table 1). These results suggest that the PhCMe2OO -
propionitrile (EtCN) at 203 K by means of electron paramagnetic
?
scavenging reaction by the artepillin C derivatives may proceed via
resonance (EPR). PhCMe2OO is formed via a radical chain
process as shown in Scheme 1.10 The photoirradiation of
a one-step hydrogen atom transfer rather than via an electron
?
transfer oxidation of the artepillin C derivatives by PhCMe2OO .
ButOOBut results in the homolytic cleavage of the O–O bond to
Furthermore, enhancement in the antioxidative activity of the
artepillin C derivatives can be explained by the fact that electron-
donating (ED) groups reduce the DHT and electron-withdrawing
(EW) groups have the reverse effect.17,18 By comparing the radical-
scavenging efficiency of a series of artepillin C derivatives, we
found the same trend and can conclude that the two structural
factors are important for the antioxidative potency of the artepillin
C derivatives, i.e., the additional presence of ED substituents and
the absence of EW groups. In fact, the kobs value for 6H, which is
2.4 times larger than that of 1H, can be explained by the presence
of two electron-donating groups, methoxy and hydroxypropenyl,
in the molecule. Similarly, it can be explained why 3H is least
t
produce Bu O , which abstracts a hydrogen atom from cumene
11
?
to give cumyl radical, followed by the facile addition of molecular
?
oxygen to cumyl radical. PhCMe2OO can also abstract a
hydrogen atom from cumene in the propagation step to yield
cumene hydroperoxide, accompanied by regeneration of cumyl
radical (Scheme 1).12 In the termination step, PhCMe2OO decays
?
by a bimolecular reaction to yield the corresponding peroxide and
molecular oxygen (Scheme 1).13 In the presence of the artepillin C
?
derivatives, the decay rate of PhCMe2OO was much faster than
that in their absence (Fig. 1).
?
Fig. 1 Time course changes of the EPR signal intensity of PhCMe2OO
in the absence and presence of 6H (1.3 6 1024 mol dm23) in EtCN at
?
203 K. Inset: EPR spectrum of PhCMe2OO .
Fig. 3 Plot of log kobs vs. calculated ionization potentials (IP) of 1H–6H.
Chem. Commun., 2008, 626–628 | 627
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PhCMe2OO scavenging activity.
In conclusion, structural modification of artepillin C, resulting
in the decline of DHT, leads to the enhancement of cumylperoxyl-
scavenging activity. This is explained by the fact that 6H, having
six-fold lower DHT as compared to artepillin C, showed more than
?
two-fold higher PhCMe2OO -scavenging activity than artepillin C.
Such an augmentation in radical-scavenging efficiency may have
implications for reducing the excessive amount of radical scavenger
used in in vitro as well as in in vivo studies.
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628 | Chem. Commun., 2008, 626–628
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