Investigation of the Interaction of Resveratrol with Free Radical

ISSN 10231935, Russian Journal of Electrochemistry, 2012, Vol. 48, No. 8, pp. 1–7.
© Pleiades Publishing, Ltd., 2014.
Published in Russian in Elektrokhimiya, 2012, published in Elektrokhimiya, 2012, Vol. 48, No. 8, pp. @@@–@@@.
Investigation of the Interaction of Resveratrol
with Free Radical Diphenylpicrylhydrazyl at Different pHs
by Cyclic Voltammetry: Correlation between Antioxidant Activity
and Association Complex Constant1
Ender Biçera, z, Serkan Özdemira, Aytaç Güderb, and Halil Korkmaza
a
Ondokuz Mayιs University, Faculty of Arts and Science, Department of Chemistry, 55139 AtakumSamsun, Turkey
b
Giresun University, Vocational High School of Health Services, Department of Medical Laboratory Techniques,
28200 Giresun, Turkey
Received May 21, 2013
Abstract—The association reaction between resveratrol (RSV) and free: radical 2,2diphenyl1picrylhydra
zyl (DPPH) at different pH values has been firstly investigated by cyclic voltammetry. The voltammetric
experiments showed that the main reductive signal of DPPH gradually decreased with the increasing concen
tration of RSV. The stoichiometry of RSVDPPH complex was determined to be 1 : 1 by means of ampero
metric titration method. Also, the association constants for this molecular complex at pHs of 4, 7.4 and 10
were calculated as 2.55 × 104, 4.96 × 104 and 7.28 × 104 M–1, respectively. At the same time, the antioxidant
activities of RSV at different pHs were determined from its anodic peak potentials on cyclic voltammograms
(CVs). According to the correlation between antioxidant activity and association complex constant of RSV,
the association constants determined by voltammetric DPPH assay can be used as a measurement of the anti
oxidant capability of RSV.
Keywords: resveratrol, voltammetric diphenylpicrylhydrazyl radical assay, interaction, pH effect, association
constant and antioxidant capability
DOI: 10.1134/S1023193514080023
INTRODUCTION
RSV (3,4',5trihydroxytransstilbene, Scheme), is
a naturally occurring phytoalexin synthesized in
response to injury or fungal attack [1]. It has been
found in at least 72 plant species, a number of which
are dietary components, such as mulberries, peanuts,
and grapes [1–3]. Its antioxidant and radical scaveng
ing abilities are also well known [4].
OH
O2N
B
HO
N
A
idant, RSV itself has to undergo oxidation. In a scav
enging reaction a hydrogen atom is donated to the rad
ical that becomes a nonradical. The hydrogen atoms
are supplied by dissociation of the OH groups, a pro
cess that takes place both in scavenging reactions and
electrochemical oxidation [7]. RSV has also shown to
possess an antimutagenic effect [8] and acts as cancer
chemopreventive agent [9]. The transisomer of RSV
is more stable than its cisisomer [10]. Also, the trans
isomer is bibactive and has clear free radicals ability
than the cisisomer [11]. When pH > 10, the stability
of transisomer is poorer [12].
DPPH (Scheme) is an artificial, stable, model
organic radical, which owed to its properties has been
used in numerous studies as a valid means of rapidly
assaying pure antioxidants, antioxidant mixtures and
extracts. Thus DPPH has become the tool of prefer
ence for studies pertaining to the evaluation of radical
scavenging activity [13]. Nevertheless, it was noted
that DPPH is unstable in those cases in which pH < 3
or pH > 12 [14].
In the literature survey [15], the antioxidant activ
ity of RSV was determined by the decreases in the
absorbance value of DPPH at 517 nm using UVVis.
spectroscopy technique. Cyclic voltammetry and
•
N
NO2
O2N
OH
(a)
(b)
Scheme 1. The chemical structures of (a) RSV
and (b) DPPH.
Antioxidant compounds play an important role
against various diseases (e.g., atherosclerosis, chronic
inflammation, cardiovascular disorders and cancer)
and aging processes [5, 6]. In order to act as an antiox
1 The article is published in the original.
z Corresponding author: [email protected] (Ender Biçer).
1
2
ENDER BIÇER et al.
amperometric method were used in a flow system to
measure the “antioxidant capacities” of gallic acid,
catechin, quercetin, caffeic acid and trolox [16].
Electrochemical measurements have advantages
for the determination of antioxidant activity [17] such
as their usage as a rapid proof of the antioxidant capac
ity of a lot of organics. Although voltammetric studies
on antioxidant activities of a number of bioactive com
pounds [18] and flavonoids [19, 20] were present, a
cyclic voltammetric study on the interaction of RSV
with DPPH at different pHs was not reported. There
fore, the purpose of this study is, first, to determine
stoichiometry and binding constants of the interaction
between RSV and DPPH at different pHs by using
cyclic voltammetry. Cyclic voltammetry has been
employed for the determination of the kinetics and
mechanisms of the electrode reactions of both organic
molecules as well as metal ions [21–25]. On the other
hand, in the pH effect on the RSVDPPH interaction,
the selected pH values (4, 7.4 and 10) are close to those
used in the study reported by ElGhorab et al. [26].
EXPERIMENTAL
Reagents
RSV and DPPH were purchased from the Sigma
(SigmaAldrich GmbH, Sternheim, Germany) and
also used without further purification. All reagents
were of analytical grade and were used as received.
Stock solutions (1 × 10–3 M) of RSV and DPPH were
prepared daily by dissolving their accurate amounts in
ethanol and stored in a coloured bottle, at 4°С and
avoiding the exposure to direct light. Dilutions were
done just prior to use.
Buffer Solutions
BrittonRobinson (BR) buffer has prepared by
mixing of 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M
CH3COOH that has been titrated to the desired pH
with 0.2 M NaOH or HCl [27–34]. For the UVVis.
measurement, the solutions under study were buffered
by using a 0.1 M phosphate buffer solution (disodium
hydrogen phosphate anhydrous salt) adjusted to
pH 7.4 with phosphoric acid. At the preparation of
buffer solutions, double distilled and deionized water
was used.
Working Solutions
Ethanol–BR buffer solutions, pHs 4, 7.4 and 10
(1 : 1, v/v), and ethanol–phosphate buffer solution,
pH 7.4 (1 : 1, v/v), were used for voltammetry and
UVVis., respectively.
Apparatus
EG&G PAR Model 384B polarographic analyzer
which is connected to EG&G PARC Model 303A
stand was employed for electrochemical techniques.
The polarographic analyzer was controlled by a laptop
computer with ECDSOFT software [35]. A standard
threeelectrode electrochemical cell (EG&G PARC
Model 303A) was used for all electrochemical experi
ments with a hanging mercury drop electrode
(HMDE) as working electrode, a platinum (Pt) wire as
auxiliary electrode and a saturated silversilver chlo
ride electrode (Ag|AgCl|KClsat) as reference electrode.
All the pH measurements were made with a Jenway
3010 pH meter.
Absorbance spectra were recorded on an UNICAM
UV2100 UVVisible spectrophotometer. The quartz
cuvettes with path length of 1 cm were used.
Voltammetric Procedure
10 mL of the supporting electrolyte solution (BR
buffer solutions with % 50 v/v ethanol at different
pH values), containing an appropriate amount of RSV
or DPPH solution was added to the electrolytic cell.
Then cyclic voltamograms of the samples were
recorded. In addition, the gradually increased
amounts of RSV were transferred into the voltammet
ric cell containing an appropriate amount of DPPH.
Subsequently, cyclic voltamograms of these mixtures
were recorded and the changes in the peak currents
were followed. In this system, there is a reaction
between DPPH and RSV which lowers the amount of
the DPPH in the bulk leading to a decrease in the
DPPH reduction signals. This decrease is directly
dependent on the concentration of the antioxidant in
the bulk solution. The experimental parameters for
cyclic voltammetry are as follows: scan rate, v =
500 mV s–1; working electrode, HMDE; equilibrium
time, 5 s; N2 purge time, 300 s.
Spectroscopic Procedure
UVVis spectra were used to testify the formation
of the molecular complex of DPPH with RSV. To
study the interaction between RSV and DPPH, the
increasing amounts of a ethanolic solution of resvera
trol was added to 0.05 mM DPPH in phosphate buffer
solution of pH 7.4 (with % 50 v/v ethanol). The mix
tures were incubated at room temperature for 30 min.
The reaction was then followed by observing the
changes in the absorbance of DPPH at 517 nm in a
spectrophotometer.
RESULTS AND DISCUSSION
Voltammetric Behavior of RSV and Its Interaction
with DPPH
The voltammetric behaviors of RSV and DPPH
at a SMDE were studied in BR buffer solutions
(with % 50 v/v ethanol and pHs of 4, 7.4 and 10) by
using cyclic voltammetry. Figure 1 shows the cyclic
voltammograms (CVs) for a 9.90 × 10–6 M solution of
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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INVESTIGATION OF THE INTERACTION OF RESVERATROL WITH FREE RADICAL
RSV. As can be seen in Fig. 1, RSV gave an oxidation
peak under these experimental conditions. The coun
terpart of oxidation process of RSV is well seen at
physiological and basic pHs. In addition, the anodic
peak potential of RSV shifted to more positive poten
tials with increasing pH value (Fig. 1 and Table 1). Its
anodic wave is accompanied by a reduction wave,
which indicates that the electrode reaction is reversible
character. RSV has an aromatic ring with a reactive
hydroxyl group and has a similar chemical structure as
the flavonoids. The oxidation of flavonoids is related
mainly to the hydroxyl group in its B ring, and the
resorcinol group in ring A presents less electroactively
[36, 37]. The reversible peak couple, detected in this
experiment should correspond to the redox reaction of
hydroxyl group in Bring of RSV. According to the
proposed mechanism of RSV [38], an intermediate of
a free radical forms.
The higher negative oxidation potentials of RSV at
higher pH values (see Table 1) demonstrate that the
oxidation process becomes easier. In the literature, the
oxidation potentials measured by cyclic voltammetry
(CV) have been used to compare the antioxidant
strength of compounds such as phenolic acids, fla
vonoids, cinnamic acids, etc. [18, 39–43]. It was
reported that low oxidation potentials where associ
ated with a greater facility or strength of a given mole
cule for the electrodonation and, thus, to act as anti
oxidant [18]. Moreover, the voltammetric method was
used for the determination of the antioxidant capabil
ity in the same manner as DPPH radical scavenging
because of the correlation found between oxidation
potentials and antiradical power (ARP) [18]. It was
determined that the phenolic compound with a lower
positive oxidation potential (Ер,а) had higher ARP
value [18]. According to this property, it can be said
that the antioxidant activity of RSV increases by
increasing pH value. To support this opinion, in this
study, the binding constant values between RSV and
DPPH at different pHs were first calculated by volta
mmetric measurements owing to the electrochemical
activity of DPPH in the protic medium on mercury
electrode.
Figure 2 shows the CV curves of 3.85 × 10–5 M
DPPH at different pH values in the absence (curve а)
and presence (curves b, c and d) of RSV. In the situa
tion that RSV is absence, DPPH in acidic medium
(BR buffer of pH 4) exhibits three irreversible
cathodic peaks at –0.386, –0.454 and –0.634 V,
respectively.
The electrochemical properties of DPPH and
some nitrocompounds in solid state and solution
phase [44–52] were studied. These irreversible peaks
may be assigned to the reductions of the nitro groups
with different environment (ortho and para) and the
hydrazyl radical, respectively. At physiological and
basic pHs, DPPH exhibits two irreversible peaks.
Probably, the first and second reduction peaks of
DPPH are overlapped with increasing pH, so these
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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3
(a)
I, nA
20
–20
–60
–100
–140
–1.2
–1.6
–0.8
–0.4
–0.8
–0.4
(b)
60
20
–20
–60
–100
–140
–1.2
–1.6
(c)
40
0
–40
–80
–120
–1.4
–1.8
–1.0
–0.6
E, V vs. Ag/AgCl/KClsat
Fig. 1. Cyclic voltamograms of 9.90 × 10–6 M RSV in BR
buffer solutions (with % 50 v/v ethanol). (a) pH 10;
(b) pH 7.4; (c) pH 4. Other experimental conditions: scan
rate, v = 500 mV s–1; working electrode, HMDE; equilib
rium time, 5 s; N2 purge time, 300 s.
peaks transform to the onepeak form. With increasing
pH, the peak potentials of DPPH shift to more nega
tive values (see Fig. 2). This means that Н3O+ ions are
involved in the reduction process of DPPH. The last
peak of DPPH is a main peak because of the fact that
Table 1. The anodic peak potentials of free RSV at different
pHs
No. 8
pH
Ep,a, V (vs. Ag/AgCl/KClsat)
4.0
–1.303
7.4
–1.453
10.0
–1.494
2012
4
ENDER BIÇER et al.
(a)
I, nA
a
500
300
g
g
100
–100
–0.4
–0.8
–1.2
–1.6
–1.2
–1.6
(b)
d
a
700
500
d
300
100
–100
–0.4
–0.8
1/C RSV = K [ ( 1 – A )/ ( 1 – ( I/I 0 ) ) ] – K,
(c)
1800
a
1400
g
1000
g
600
200
1 2
–200
–0.4
–0.8
it always appears on the voltamograms, obtained at all
pH values. So, this peak was chosen to investigate the
interaction RSV and DPPH in solution phase.
The peak currents of DPPH were decreasing with
increasing concentrations of RSV. But, peak poten
tials of DPPH in the presence of RSV are depending
on the pH value of the medium. At both acidic and
basic pHs, peak potentials of DPPH shifted to more
negative values whereas peaks of DPPH shifted to
more positive potentials at physiological pH (Fig. 2).
It showed that RSV affected the electron transfer pro
cess of DPPH. The reason of the decrease of peak cur
rent was that the apparent diffusion coefficient
decreased [53] and also the apparent concentrations of
electroactive species decreased [54].
The positive peak potential shift accompanied with
the decrease in the peak currents of DPPH is due to its
intercalation into the RSV molecule at physiological
pH. However, the displacement of the peaks of DPPH
in the negative going direction may be linked with the
electrostatic interaction of DPPH with RSV molecule
at pHs 4 and 10 [55].
The current titrations were performed by keeping
the constant concentration of DPPH while varying the
concentrations of RSV. The current titration equation
for a 1 : 1 association
3
–1.2
–1.6
E, V vs. Ag/AgCl/KClsat
Fig. 2. Cyclic voltamograms of 3.85 × 10–5 M DPPH in
BR buffer solutions of (a) pH 4 (with % 50 v/v ethanol) in
the presence of (a) 0; (b) 9.52 × 10–6; (c) 1.42 × 10–5;
(d) 1.89 × 10–5; (e) 2.35 × 10–5; (f) 2.80 × 10–5 and
(g) 3.26 × 10–5 M RSV; (b) pH 7.4 (% 50 v/v ethanol) in
the presence of (a) 0; (b) 1.89 × 10–5; (c) 4.59 × 10–5 and
(d) 5.45 × 10–5 M RSV; (c) pH 10 (% 50 v/v ethanol) in the
presence of (a) 0; (b) 1.42 × 10–5; (c) 2.35 × 10–5;
(d) 3.26 × 10–5; (e) 4.15 × 10–5; (f) 5.02 × 10–5 and
(g) 6.47 × 10–5 M RSV. Other experimental conditions are
as described in Fig. 1. Note that as shown in Fig. 2C, with
increasing RSV concentrations in the presence of 3.85 ×
10–5 M DPPH several undefined anodic (1, 2 and 3 on the
curves e–g for [RSV] ≥4.15 × 10–5 M) and cathodic peaks (at
about 1.12 V on the curves c–g for [RSV] ≥2.33 × 10–5 M)
are seen.
(1)
where CRSV is the concentration of RSV (in BR
buffer), К is the association constant, I and I0 are the
current of the last peak of DPPH with and without
RSV and А is the proportionality constant. If Eq. (1)
corresponds well to the experimental data, this may
suggest that the complex of RSV with DPPH is a 1 : 1
association complex.
From the experimental data, linear equations of
1/[RSV] versus 1/[1 – (I/I0)] were obtained with lin
ear regression constant, r2 (Table 2), revealing that the
association of DPPH with RSV has a 1:1 stoichiome
try. Also, the stoichiometric value of the RSV–DPPH
complex was determined by the amperometric titra
tion of DPPH with RSV (Fig. 3). Figure 3 shows the
change in current of the last peak of DPPH with
increasing the mole ratio of RSV to DPPH. As can be
seen in Fig. 3, the intercept value (stoichiometry of
DPPH–RSV complex) was 0.96 which is very close to
the assuming value (1 : 1) according to Eq. (1). How
ever, the association constants of DPPH to RSV at dif
ferent pHs were calculated from Eq. (1) (Table 2).
It is well known that RSV forms 1 : 1 complexes
with some biological active compounds (i.e., fibrino
gen, hydroxypropylβcyclodextrin, collagen, the
apolactoferrin) [57–60]. As similar to RSVDPPH
system (Table 2), the binding constants of fibrinogen
and apolactoferrin with RSV were determined as
about 104 М–1 [57, 60]. Moreover an examination of
Table 2 reveals that the association constant of the
RSV–DPPH complex increases with increasing
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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INVESTIGATION OF THE INTERACTION OF RESVERATROL WITH FREE RADICAL
5
Table 2. The current titration equations and association constants of DPPH• with RSV at different pHs as determined by
cyclic voltammetry
pH
Equation
K, M–1
log(K/M–1)
r2
4.0
7.4
10.0
1/CRes (M–1) = (27843/[1 – (I/I0)]) – 25487
1/CRes (M–1) = (11496/[1 – (I/I0)]) – 49601
1/CRes (M–1) = (90867/[1 – (I/I0)]) – 72802
2.55 × 104
4.96 × 104
7.28 × 104
4.406
4.695
4.862
0.994
0.981
0.997
pH value. At basic medium, the greater association
ability of RSV may be sourced from its greater ARP
under this experimental condition. At the same time,
the determined association constants (Table 2) are
well matched with the antioxidant activities of RSV,
depicted from Table 1 for different pHs. Ер,а values of
RSV (see Table 1) were done to obtain more insight in
the mechanism underlying the increase in its associa
tion constant value with increasing pH. The actual
mechanism for the antioxidant action of the deproto
nated forms of RSV can probably be hydrogen atom
donation [61]. Therefore, Table 1 lists the Ер,а values of
RSV and its deprotonated anionic species as a measure
for the ease of hydrogen atom donation from the weak
est remaining OH moieties in the anion form. The
observed pH dependence of the anodic peak potential
and association constant values of RSV might be
attributed to an effect of the pH on the deprotonation
of the OH moieties [62, 63].
Since RSV has three hydroxyl groups (pKa1 = 6.4,
pKa2 = 9.4 and pKa3 = 10.5 [4, 64]), it exists as different
molecular species depending upon the pH of the
medium. On the basis of a comparison of the pKa val
ues of RSV to its pHdependent antioxidant force (as
directly to anodic potential or the association con
stant), it is concluded that a significant increase in the
antioxidant activity of RSV is related to deprotonation
of its hydroxyl moieties. Consequently, upon deproto
nation, RSV has better antioxidant property.
UVvis Absorption Spectra for Confirmation
of RSV–DPPH Complex
In order to investigate the binding reaction of RSV
to DPPH, the UVVis. spectroscopy has also been
used. Figure 4 shows the changes on the UVVis
absorption spectra of DPPH with the addition of RSV.
In the absence of RSV, DPPH exhibited two absorp
tion peaks at 325 and 517 nm, respectively. When RSV
and DPPH are mixed, the spectrum shows significant
differences from that of free DPPH (Fig. 4). Espe
cially, the absorbance at 517 nm of DPPH gradually
decreased against increase of RSV concentration in
the solution. However, the absorption band at 325 nm
decreased (Figs. 4a–4j) and then increased
(Figs. 4k–4u) and also shifted to shorter wavelength
Absorbance
4
3
u
2
a
I, nA
a
(I)
500
k
j
1
u
300
0
200
(II)
300
100
0
0.4
0.8
1.2
Mole ratio of RSV to DPPH radical
Fig. 3. The plot of cyclic voltammetric current for main
peak of 3.85 × 10–5 M DPPH in BR buffer solution of
pH 4 (with % 50 v/v ethanol) versus the mole ratio of RSV
to DPPH. Note that I and II are straight lines, used to find
the intersection point.
RUSSIAN JOURNAL OF ELECTROCHEMISTRY
Vol. 48
400
500
600
700
800
Wavelength, nm
Fig. 4. UVVis. spectra of 5 × 10–2 mM DPPH in phos
phate buffer solution of pH 7.4 (with % 50 v/v ethanol) in
the presence of (a) 0; (b) 6.25 × 10–4; (c) 1.25 × 10–3;
(d) 1.875 × 10–3; (e) 2.5 × 10–3; (f) 3.125 × 10–3; (g) 3.75 ×
10–3; (h) 6.25 × 10–3; (i) 7.5 × 10–3; (j) 8.75 × 10–3; (k) 1 ×
10–2; (l) 1.125 × 10–2; (m) 1.25 × 10–2; (n) 1.5 × 10–2;
(o) 1.75 × 10–2; (p) 2 × 10–2; (q) 2.25 × 10–2; (r) 2.875 ×
10–2; (s) 3.25 × 10–2; (t) 3.625 × 10–2 and (u) 4 × 10–2 mM
RSV (incubation time: 30 min).
No. 8
2012
6
ENDER BIÇER et al.
on increasing concentration of RSV. It is well known
that RSV has two characteristic peaks near 305 and
315 nm, respectively [65]. Therefore, the increases in
the peak intensity at 325 nm and blue shifting
(Figs. 4k–4u) may be based on the formation of
dimeric RSV molecule according to Eq. (4). On the
other hand, the hypochromism at 517 nm in the pres
ence of RSV were suggested to be due to a binding
reaction of RSV molecule to DPPH.
The Proposed Mechanism
The peak current of DPPH decreases in the pres
ence of RSV. Probably, this case can be produced by
donation of hydrogen from antioxidant RSV to DPPH
for the formation of stable nonradical DPPH–H.
These decreases in the current indicate that RSV has a
radical scavenger property. According to the stoichi
ometry of the association complex in this study, the
following reaction of RSV (ROH) which can donate a
hydrogen atom probably takes place and so, the
reduced form of DPPH (DPPH–H) is generated:
DPPH• + ROH
•
ROH–DPPH
RO• + RO•
ROH–DPPH•,
(2)
R•,
(3)
(4)
DPPH–H +
RO–OR (fast).
CONCLUSIONS
In conclusion, our study showed that the antioxi
dant capacities of RSV at different pHs as deduced
from its oxidation peak potentials on the CVs could be
correlated with the association constants determined
using a voltammetric DPPH assay. The pH values in
the reaction medium significantly changed the antiox
idant ability of RSV and also its association force (or
scavenging activity as indirect) against DPPH as a sta
ble radical. The antioxidant activity and association
constant of RSV at basic medium are higher than those
of physiological and acidic mediums.
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