Food Chemistry 148 (2014) 18–23
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Electrooxidation of morin hydrate at a Pt electrode studied by cyclic voltammetry Anna Masek a,⇑, Ewa Chrzescijanska b, Marian Zaborski a a b
Technical University of Lodz, Institute of Polymer and Dye Technology, Faculty of Chemistry, ul Stefanowskiego 12/16, 90-924 Lodz, Poland Technical University of Lodz, Institute of General and Ecological Chemistry, Faculty of Chemistry, ul Zeromskiego 116, 90-924 Lodz, Poland
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 23 September 2013 Accepted 1 October 2013 Available online 9 October 2013 Keywords: Antioxidants Morin Electrochemical oxidation Cyclic voltammetry
a b s t r a c t The process and the kinetics of the electrochemical oxidation of morin in an anhydrous electrolyte have been investigated using cyclic and differential pulse voltammetry. The oxidation mechanism proceeds in sequential steps related to the hydroxyl groups in the three aromatic rings. The oxidation of the 20 ,40 dihydroxy moiety at the B ring of morin occurs first, at very low positive potentials, and is a one-electron, oneproton irreversible reaction. The rate constant, electron transfer coefficient and diffusion coefficients involved in the electrochemical oxidation of morin were determined. The influence of the deprotonation of the ring B hydroxyl moiety is related to the electron/proton donating capacity of morin and to its radical scavenging antioxidant activity. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polyphenols, flavonoids and tannins are widely distributed in plants, mainly in fruits and vegetables, and contribute to their flavour and colour (Janeiro & Brett, 2005; Lako et al., 2007). Flavonoids constitute a large class of compounds with different activities and contain a number of phenolic hydroxyl groups attached to ring structures that impart antioxidant capacity to these compounds (Cosio, Buratti, Mannino, & Benedetti, 2006; Hopia & Heinonen, 1999; Masek, Zaborski, & Chrzescijanska, 2011; Xu, Luo, Xing, & Chen, 2006; Škerget et al., 2005). Their structure is based on the 15-carbon skeleton of 2-phenylchroman (Jungbluth & Ternes, 2000). Phenol compounds as antioxidants can react in very different ways, including direct reaction with free radicals, scavenging free radicals, intensifying the dismutation of free radicals to compounds with a considerably lower reactivity, chelating pro-oxidative metals (mainly iron) or inhibiting or intensifying the action of many enzymes (Braca et al., 2003; Dai, Miao, Zhou, Yang, & Liu, 2006; Hou, Zhou, Yang, & Liu, 2004). Moreover, they can intensify the action of other antioxidants, e.g., vitamins dissolved in fats and fine molecular substances dissolved in water. Owing to their antioxidant capacity, flavonoids significantly slow down the ageing processes and prevent many diseases, phenomena practically utilised in pharmacology and cosmetology (Dilis, Vasilopoulou, & Trichopoulou, 2007; Lotito & Frei, 2006; Silva
⇑ Corresponding author. Tel.: +48 42 631 32 13; fax: +48 42 636 25 43. E-mail address: [email protected] (A. Masek). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.10.003
et al., 2000). The miscellaneous effects of flavonoids and their synthetic derivatives make it possible to search for new experimental drugs. They may also be used in the chemical industry as natural additives to stabilise polymeric materials (Masek, Zaborski, & Kosmalska, 2011). One of the flavonoids, a derivative of flavone, is morin (20 ,3,40 ,5,7-pentahydroxyflavone) (1). Morin is a natural dye extracted from the Brazilian yellow tree. Owing to the presence of hydroxyl groups, morin can show good antioxidant capacity. Its chemical structure contains a resorcinol group in ring A (with the m-hydroxyls in positions C5 and C7), a catechol group in ring B (with the m-hydroxyls in positions C20 and C40 ) and in position C3 of ring C, a carbonyl group in position C4 in ring C and a C@C bond between carbons C2 and C3 in ring C. As can be deduced, morin presents OH functional groups that can be electrochemically oxidised. The oxidation of the 3’,4’dihydroxy group in ring B can lead to the formation of the corresponding o-quinone. In contrast, the 5,7 dihydroxy group in ring A cannot lead to the formation of the corresponding m-quinone. Additionally, the catechol group in ring B is oxidised more easily than the resorcinol group in ring A (Janeiro & Brett, 2004). Studies have been conducted to evaluate the use morin as an oxidant in the treatment of malignant diseases (Merwid-Lad et al., 2011). The physical and chemical properties of antioxidants, such as their redox potential, the number of electrons transferred and the rate constants of electrode reactions, can be determined by means of electrochemical measurements. These parameters are very relevant not only for evaluating the antioxidant capacity of flavonoids (Hopia & Heinonen, 1999; Hussain & Siddiqa, 2011;
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A. Masek et al. / Food Chemistry 148 (2014) 18–23
60.0
400.0 350.0
I
300.0
30.0
200.0 2
150.0
0.0
2.1. Reagents
-50.0
To assess the electrochemical oxidation mechanism and kinetics for the compound under investigation, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods were used, employing an Autolab analytical unit (EcoChemie, Holland). A threeelectrode system was used for the measurements. Platinum was used as an anode and auxiliary electrode. The electrode potential was measured versus a ferricinium/ferrocene reference electrode (Fc+/Fc) whose standard potential is defined as zero, independent on the solvent used. Prior to the measurements all of the solutions were degassed with argon. During the measurements, an argon blanket was maintained over the solution. The effect of the scan rate on the electrooxidation of morin in an anhydrous medium was assessed. 3. Results and discussion 3.1. The electrochemical behaviours of morin at a Pt electrode on its electrooxidation The electrochemical reactions proceeding at the electrode are characterised by the dependence of the current on the electrode potential. The electrode reactions characterising the electrochemical oxidation of morin hydrate at the platinum electrode were studied by cyclic and differential pulse voltammetry. Selected cyclic and differential pulse voltammograms (with a higher definition) for the electrooxidation of hydroxymorin and the supporting electrolyte are shown in Fig. 1. The half-wave potential of the electrode reaction, as investigated by cyclic voltammetry, corresponds to the peak potential from the differential pulse voltammetry. Within the potential range where the compound oxidation peaks appear, the supporting electrolyte ((C4H9)4NClO4 in acetonitrile – 0.1 mol L1) shows no characteristic peaks other than the charging of the electrical double layer (1, curve – 3). However, a small wave appears in the supporting electrolyte within the potential range from 0.4 to 1.0 V in the voltammograms. This wave can be attributed to the oxidation of perchlorate ions of (C4H9)4NClO4. On the other hand, this wave current is relatively low in comparison with the peak currents attributed to the oxidation of morin. Zieja, GadowskaTrzos, and Stojek (2001) reported that this wave could also be
10.0 0.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Potential (V)
-10.0
Fig. 1. Voltammograms of morin electrooxidation at Pt electrode; 1 – cyclic voltammogram, 2 – differential pulse voltammogram, 3 – cyclic voltammogram recorded in the supporting electrolyte; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile, v = 0.1 V s1.
caused by the oxidation of impurities such as water and other organic substances. From the dependence shown in Fig. 1 (curves 1 and 2) it follows that morin is irreversibly oxidised in at least two steps at potentials lower than those of electrolyte decomposition. The half-wave potential (E1/2) of the first step of hydroxymorin oxidation, as determined by cyclic voltammetry, is 0.69 V. This corresponds to the peak potential from the differential pulse voltammetry measurements; the half-wave potential (E1/2) of the second step is equal to 1.07 V.
3.2. Influence of scan rate and of morin concentration electrooxidation The effect of polarisation rate on the electrooxidation of morin was investigated by cyclic voltammetry with scan rates of 0.01– 0.50 V s1 (Fig. 2). The peak potential and current were determined for the first step of the electrode reaction of morin oxidation, whereas only the peak potential was measured for the second step. In scanning negative potentials, only fluctuations in the current are perceptible, and the accurate localisation of potential peaks is not evident. Two approaches widely used to study the reversibility of electrochemical reactions and to determine whether a reaction rate is controlled by adsorption or diffusion are the analyses of ip on v1/2 and of ln ip on ln v curves. Fig. 3 shows these plots for the first oxidation peak of morin in acetonitrile. For reversible or irre400.0 I
350.0
3
300.0 Current (µA)
2.2. Measurement methods
3
50.0
2. Materials and methods
(1) Acetonitrile (CH3CN) pure p.a. from POCh Gliwice, Poland, (2) Tetrabutylammonium perchlorate (C4H9)4NClO4) from Fluka was used as a supporting electrolyte.
20.0
Current (µA)
40.0 II
100.0
Pure morin hydrate (20 ,3,40 ,5,7-pentahydroxyflavone) was obtained from a commercial source (Sigma–Aldrich) and used as received. The chemicals used for the preparation of the flavonoid solutions were as follows:
50.0
1
250.0 Current (µA)
Nematollahi & Malakzadeh, 2003; Wang, Xu, Zhao, & Hu, 2007; Zhou, Kikandi, & Sadik, 2007) but also for understanding the mechanisms of their oxidation or reduction processes (Janeiro, Corduneanu, & Brett, 2005; Timbola, Souza, Giacomelli, & Spinelli, 2006; Timbola, Souza, Soldi, Pizzolatti, & Spinelli, 2007). One of the parameters suitable for providing information about the antioxidant capacity of flavonoids is the half-wave potential (E1/2). Thus, a flavonoid species that shows a low E1/2 value can be regarded as a good free radical scavenger.
II
250.0
2
200.0
1
150.0 100.0
4
50.0 0.0 -50.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Potential (V)
Fig. 2. Cyclic voltammograms of morin oxidation at Pt electrode; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at various scan rates; curve 1 – v = 0.01 V s1, 2 – v = 0.05 V s1, 3 – v = 0.1 V s1, 4 – v = 0.01 V s1 of the supporting electrolyte (0.1 mol L1 (C4H9)4NClO4 in acetonitrile).
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0.84
A Ep (V) vs Fc+/Fc
0.82
0.80
I peak
0.78
0.76 0.0
0.2
0.4
0.6 v (V s-1)
0.8
1.0
0.84
Ep = {0.0145[ln v (V s-1)]} V + 0.8246 V R² = 0.995
Ep (V) vs. Fc+/Fc
0.83
B
0.81 0.80
I peak
0.78 0.77 -4.0
-2.0 ln v (V s-1)
-1.0
0.0
7.0
Fig. 3. (A) The dependence of the anodic peak current (ip) on the square root of potential scan rate (v); (B) the dependence of anodic peak current on the potential scan rate in double logarithm coordinates for the oxidation of morin at Pt electrode; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile.
C 6.0
ln ip (µA)
versible systems without kinetic complications, ip varies linearly with v1/2, intercepting the origin of the coordinates. However, the dependence of ip on v1/2 is not linear and presents a value different from zero for the linear coefficient if the electrode process is preceded or followed by a homogeneous chemical reaction. Within the polarisation rate range from 0.01 to 0.5 V s1, the electrooxidation peak current depends linearly on the square root of polarisation rate and is described by the following equation:
-3.0
5.0
ln ip = {34.32 (Ep-E0) (V)} µA - 9.02 µA R² = 0.9955 4.0 0
1
0.02
ip ¼ 793:89½v ðV s1 Þ2 lA þ 43:069 lA;R2 ¼ 0:9983
0.04
0.06
(Ep-E0) (V)
This linear fit does not precisely intercept the origin of the coordinates (3 A). This dependence suggests that the electrode process of peak I is diffusion-controlled but can also be preceded by a chemical reaction. On the other hand, the dependence of ln ip on ln v is linear and is described by the following equation:
Fig. 4. (A) The dependence of peak potential (Ep) on the potential scan rate (v) for the oxidation of morin in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at Pt electrode; (B) the dependence of peak potential on the ln v for the oxidation of morin in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at Pt electrode; (C) variation of ln ip versus (Ep E0) for the oxidation of morin in acetonitrile at Pt electrode.
ln ip ¼ 0:4129ln v ðV s1 ÞlA þ 6:6772 lA;R2 ¼ 0:9955
electrochemical reaction at the electrode surface. The value of the electron transfer coefficient for the reaction can be obtained from the following Eq. (1) (Bard & Faulkner, 2001; Harrison & Khan, 1970; Nicholson & Shain, 1964):
(Fig. 3B). The slope of this fit is 0.412, which indicates that the process is only controlled by diffusion. A slope close to 0.5 is expected for diffusion-controlled electrode processes, and a slope close to 1.0 is expected for adsorption-controlled processes (Bard & Faulkner, 2001; Brett & Brett, 1993; Kissinger & Heineman, 1996; Timbola et al., 2007). Fig. 4 A depicts the dependence of EpI on the scan rate for the electrooxidation of morin in acetonitrile under the same conditions as those shown in Fig. 2. For a reversible electrochemical reaction, the peak potential is independent of v. Therefore, we can conclude that the heterogeneous electronic transfer at peak I is irreversible or that there is a homogeneous chemical reaction following each
Ep ¼
RT ln v þ const 2bnb F
ð1Þ
where Ep – peak potential (V), R – universal gas constant (8.314 J K1 mol1), F – Faraday constant (96,487 C mol1), T – Kelvin temperature (298 K), bnb – anodic transfer coefficient, v – scan rate (V s1). This equation is valid for a totally irreversible diffusion-controlled process. Using the dependence of the anodic peak potential on the neperian logarithm of the potential sweep rate (Fig. 4B), a
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peak currents on the square root of the potential sweep rates (Fig. 3A) and the Randles–Sevcik Eq. (5) (Bard & Faulkner, 2001): 1
1
1
ip ¼ ð2:99 105 Þðbnb Þ2 C D2 v 2
ð6Þ 2
1
where D is the diffusion coefficient (cm s ). The diffusion coefficient for morin was calculated to be (5.45 ± 0.05) 106 cm2 s1. The diffusion coefficient calculated from Hayduk and Laudie’s Eq. (6) is 5.79 106 cm2 s1 (Schranke, Murphy, Doucette, & Hintze, 1999):
D¼
Fig. 5. (A) Cyclic voltammograms of morin oxidation at Pt electrode for various concentration; curve 1 – c = 76.4 ppm, 2 – c = 151.2 ppm, 3 – c = 310.8 ppm, 4 – c = 627.2 ppm, 5 – recorded in the supporting electrolyte (0.1 mol L1 (C4H9)4NClO4 in acetonitrile); (B) dependence of the anodic peak current on morin concentration, v = 0.01 V s1.
13:26 105
l1:4 v 0:589 o
ð7Þ
where l is the viscosity of the solvent (centipoises) and mo is the molar volume (cm3 g1 mole1). The effect of morin concentration on the electrode reaction of the substrate was investigated in the range from 76.4 ppm (0.255 mmol dm3) to 627.2 ppm (2.08 mmol dm3). Cyclic voltammetry and a dependence of oxidation peak current on the concentration is presented in Fig. 5. This dependence is linear in all the concentration range of morin. An increase in the concentration of morin causes significant increase in the peak current. The linear dependence up to morin concentration of (ip = {0.5492[c(ppm)]}lA–15.58724 lA, R2 = 0.9985)) can be applied in determination of morin concentration at Pt electrodes. The peak currents of the I and II steps of morin electrooxidation increased with increasing concentration of the substrate, halfwave potential (E1/2) of the morin electrooxidation slightly shifts towards more positive values. The half-wave potential of the I step (E1/2) is 0.72 ± 0.08 V, and that of the II is 1.27 V. The calculated anode transition coefficient (bnb) is 0.88 ± 0.05, and the heterogeneous rate constant of step I of morin electrooxidation at the half-wave potential (kbh) is (2.84 ± 0.05) 104 cm s1 (Galus, 1994). 3.3. Morin oxidation processes
value of 0.89 for the electron transfer coefficient (bnb) was obtained for peak I. To obtain information on the rate-determining step, the Tafel slope b was determined using the following equation for a totally irreversible diffusion-controlled process (Harrison & Khan, 1970; Yaghoubian et al., 2011):
Ep ¼ b=2ln v þ constant
ð2Þ
Based on Eq. (2), the slope of the dependence of Ep on ln v is b/2, where b represents the Tafel slope. The slope of Ep vs. ln v plot is o Ep/o (ln v) and was found to be 14.5 mV in this work (Fig. 4B); thus, b = 2 14.5 mV = 29 mV. From the dependence given in Eq. (3) (Bard & Faulkner, 2001):
ip ¼ 0:227 FAC exp½ðbnb f ÞðEp E0 Þ
ð3Þ
where: ip – peak current (A), f = F/RT, A – electrode area (cm2), C⁄ – bulk concentration of morin (mol cm3), E0 – formal potential of an electrode (V). This equation allowed to obtain a dependence:
ln ip ¼ ln a ðbnb f ÞðEp E0 Þ
ð4Þ 0
The dependence of ln ip on (Ep–E ) is linear (Fig. 4C) and is described by the following equation:
ln ip ¼ 34:32ðEp E0 ÞðVÞlA 9:02 lA;R2 ¼ 0:9955
The determined parameters are confirmed by quantum-chemical calculations. The distribution of electron charges in the investigated molecules is not uniform and determines the reactivity of particular positions (Fig. 6A) (Kolos, 1978). The energy of the highest filled orbital (EHOMO – ionisation potential) determines the ease of giving up electrons and indicates the site that is most susceptible to oxidation. The highest electron density in the morin molecule is observed in rings A and B, suggesting the ease of oxidation of the hydroxyl groups in these rings. The oxidation mechanism of hydroxymorin in subsequent electrode steps and its antioxidant activity are connected with the number of hydroxyl groups and their position in the three aromatic rings of this compound. Morin presents conjugation between rings A and B. It has different pharmacophores, including the moiety in ring B and the three hydroxyl groups in rings A and C. At the most positive potential (the first electrode stage), the hydroxyl group of ring B is oxidised and one electron and one proton are exchanged. The hydroxyl group at position 3 at ring C should be oxidised afterwards. The current of peak I is very high compared with the current of peak II, in agreement with the higher radical scavenging activity associated with the oxidation of the ring B moiety. Based on the electroanalytical investigations and literature data, one can deduce the mechanism of hydroxymorin oxidation (Fig. 6B) (Janeiro & Brett, 2004; Wang, Zhang, Tong, Xu, & Yang, 2011).
ð5Þ 4. Conclusions
and a slope of 34.32. The calculated anodic transfer coefficient (bnb) is equal to 0.88. The diffusion coefficient for morin was calculated on the basis of the slopes of the linear dependence of the anodic
Owing to its antioxidant capacity, morin is an important flavonoid. In our study, the electrochemical behaviour of morin at a Pt
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A. Masek et al. / Food Chemistry 148 (2014) 18–23
A
B
HO
B
O
HO
A
_ O
OH
C
B
O
HO
+
- e, - H
A OH
C
OH
O
O
B
O
A
C
O
OH
O
O OH
O
HO HO
or
C O
O
B
O
A
A
OH
O
B
O
HO
- e, - H
C
O
HO
OH
O
HO Tautomerization
.
+
OH OH
O
Fig. 6. (A) Electron density and probable sites in morin molecule susceptible electrooxidation; (B) proposed oxidation mechanism of morin.
electrode was investigated. The investigated flavonoid is irreversibly oxidised in at least two electrochemical steps. In the first electrochemical step, the hydroxyl group of ring B is oxidised and one electron and one proton are exchanged. In the second electrode step, the hydroxyl groups of ring C are oxidised at a more positive potential and successive electrons and protons are exchanged. Based on the experimental results, the electrochemical reaction mechanisms of morin at Pt were proposed. Corresponding electrochemical parameters were calculated, including the diffusion coefficient of morin, the transfer coefficient and the rate constant. It is helpful for us to understand the kinetics of morin. The data obtained are consistent with available knowledge and suggest that voltammetric studies on mechanically transferred solids may provide a convenient method for elucidating the electrochemical oxidation mechanisms of this type of compound in anhydrous media. References Bard, A. J., & Faulkner, L. R. (2001). Electrochemical methods, fundamentals and applications (2nd ed.). New York: John Wiley & Sons, pp. 236, 503, 709. Braca, A., Fico, G., Morelli, I., De Simone, F., Tomè, F., & De Tommasi, N. (2003). Antioxidant and free radical scavenging activity of flavonol glycosides from different Aconitum species. Journal of Ethnopharmacology, 86, 63–67. Brett, C. M. A., & Brett, A. M. O. (1993). Electrochemistry: Principles, methods, and applications. New York: Oxford University Press, p. 427.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Electrooxidation of morin hydrate at a Pt electrode studied by cyclic voltammetry Anna Masek a,⇑, Ewa Chrzescijanska b, Marian Zaborski a a b
Technical University of Lodz, Institute of Polymer and Dye Technology, Faculty of Chemistry, ul Stefanowskiego 12/16, 90-924 Lodz, Poland Technical University of Lodz, Institute of General and Ecological Chemistry, Faculty of Chemistry, ul Zeromskiego 116, 90-924 Lodz, Poland
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 23 September 2013 Accepted 1 October 2013 Available online 9 October 2013 Keywords: Antioxidants Morin Electrochemical oxidation Cyclic voltammetry
a b s t r a c t The process and the kinetics of the electrochemical oxidation of morin in an anhydrous electrolyte have been investigated using cyclic and differential pulse voltammetry. The oxidation mechanism proceeds in sequential steps related to the hydroxyl groups in the three aromatic rings. The oxidation of the 20 ,40 dihydroxy moiety at the B ring of morin occurs first, at very low positive potentials, and is a one-electron, oneproton irreversible reaction. The rate constant, electron transfer coefficient and diffusion coefficients involved in the electrochemical oxidation of morin were determined. The influence of the deprotonation of the ring B hydroxyl moiety is related to the electron/proton donating capacity of morin and to its radical scavenging antioxidant activity. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Polyphenols, flavonoids and tannins are widely distributed in plants, mainly in fruits and vegetables, and contribute to their flavour and colour (Janeiro & Brett, 2005; Lako et al., 2007). Flavonoids constitute a large class of compounds with different activities and contain a number of phenolic hydroxyl groups attached to ring structures that impart antioxidant capacity to these compounds (Cosio, Buratti, Mannino, & Benedetti, 2006; Hopia & Heinonen, 1999; Masek, Zaborski, & Chrzescijanska, 2011; Xu, Luo, Xing, & Chen, 2006; Škerget et al., 2005). Their structure is based on the 15-carbon skeleton of 2-phenylchroman (Jungbluth & Ternes, 2000). Phenol compounds as antioxidants can react in very different ways, including direct reaction with free radicals, scavenging free radicals, intensifying the dismutation of free radicals to compounds with a considerably lower reactivity, chelating pro-oxidative metals (mainly iron) or inhibiting or intensifying the action of many enzymes (Braca et al., 2003; Dai, Miao, Zhou, Yang, & Liu, 2006; Hou, Zhou, Yang, & Liu, 2004). Moreover, they can intensify the action of other antioxidants, e.g., vitamins dissolved in fats and fine molecular substances dissolved in water. Owing to their antioxidant capacity, flavonoids significantly slow down the ageing processes and prevent many diseases, phenomena practically utilised in pharmacology and cosmetology (Dilis, Vasilopoulou, & Trichopoulou, 2007; Lotito & Frei, 2006; Silva
⇑ Corresponding author. Tel.: +48 42 631 32 13; fax: +48 42 636 25 43. E-mail address: [email protected] (A. Masek). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.10.003
et al., 2000). The miscellaneous effects of flavonoids and their synthetic derivatives make it possible to search for new experimental drugs. They may also be used in the chemical industry as natural additives to stabilise polymeric materials (Masek, Zaborski, & Kosmalska, 2011). One of the flavonoids, a derivative of flavone, is morin (20 ,3,40 ,5,7-pentahydroxyflavone) (1). Morin is a natural dye extracted from the Brazilian yellow tree. Owing to the presence of hydroxyl groups, morin can show good antioxidant capacity. Its chemical structure contains a resorcinol group in ring A (with the m-hydroxyls in positions C5 and C7), a catechol group in ring B (with the m-hydroxyls in positions C20 and C40 ) and in position C3 of ring C, a carbonyl group in position C4 in ring C and a C@C bond between carbons C2 and C3 in ring C. As can be deduced, morin presents OH functional groups that can be electrochemically oxidised. The oxidation of the 3’,4’dihydroxy group in ring B can lead to the formation of the corresponding o-quinone. In contrast, the 5,7 dihydroxy group in ring A cannot lead to the formation of the corresponding m-quinone. Additionally, the catechol group in ring B is oxidised more easily than the resorcinol group in ring A (Janeiro & Brett, 2004). Studies have been conducted to evaluate the use morin as an oxidant in the treatment of malignant diseases (Merwid-Lad et al., 2011). The physical and chemical properties of antioxidants, such as their redox potential, the number of electrons transferred and the rate constants of electrode reactions, can be determined by means of electrochemical measurements. These parameters are very relevant not only for evaluating the antioxidant capacity of flavonoids (Hopia & Heinonen, 1999; Hussain & Siddiqa, 2011;
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A. Masek et al. / Food Chemistry 148 (2014) 18–23
60.0
400.0 350.0
I
300.0
30.0
200.0 2
150.0
0.0
2.1. Reagents
-50.0
To assess the electrochemical oxidation mechanism and kinetics for the compound under investigation, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods were used, employing an Autolab analytical unit (EcoChemie, Holland). A threeelectrode system was used for the measurements. Platinum was used as an anode and auxiliary electrode. The electrode potential was measured versus a ferricinium/ferrocene reference electrode (Fc+/Fc) whose standard potential is defined as zero, independent on the solvent used. Prior to the measurements all of the solutions were degassed with argon. During the measurements, an argon blanket was maintained over the solution. The effect of the scan rate on the electrooxidation of morin in an anhydrous medium was assessed. 3. Results and discussion 3.1. The electrochemical behaviours of morin at a Pt electrode on its electrooxidation The electrochemical reactions proceeding at the electrode are characterised by the dependence of the current on the electrode potential. The electrode reactions characterising the electrochemical oxidation of morin hydrate at the platinum electrode were studied by cyclic and differential pulse voltammetry. Selected cyclic and differential pulse voltammograms (with a higher definition) for the electrooxidation of hydroxymorin and the supporting electrolyte are shown in Fig. 1. The half-wave potential of the electrode reaction, as investigated by cyclic voltammetry, corresponds to the peak potential from the differential pulse voltammetry. Within the potential range where the compound oxidation peaks appear, the supporting electrolyte ((C4H9)4NClO4 in acetonitrile – 0.1 mol L1) shows no characteristic peaks other than the charging of the electrical double layer (1, curve – 3). However, a small wave appears in the supporting electrolyte within the potential range from 0.4 to 1.0 V in the voltammograms. This wave can be attributed to the oxidation of perchlorate ions of (C4H9)4NClO4. On the other hand, this wave current is relatively low in comparison with the peak currents attributed to the oxidation of morin. Zieja, GadowskaTrzos, and Stojek (2001) reported that this wave could also be
10.0 0.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Potential (V)
-10.0
Fig. 1. Voltammograms of morin electrooxidation at Pt electrode; 1 – cyclic voltammogram, 2 – differential pulse voltammogram, 3 – cyclic voltammogram recorded in the supporting electrolyte; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile, v = 0.1 V s1.
caused by the oxidation of impurities such as water and other organic substances. From the dependence shown in Fig. 1 (curves 1 and 2) it follows that morin is irreversibly oxidised in at least two steps at potentials lower than those of electrolyte decomposition. The half-wave potential (E1/2) of the first step of hydroxymorin oxidation, as determined by cyclic voltammetry, is 0.69 V. This corresponds to the peak potential from the differential pulse voltammetry measurements; the half-wave potential (E1/2) of the second step is equal to 1.07 V.
3.2. Influence of scan rate and of morin concentration electrooxidation The effect of polarisation rate on the electrooxidation of morin was investigated by cyclic voltammetry with scan rates of 0.01– 0.50 V s1 (Fig. 2). The peak potential and current were determined for the first step of the electrode reaction of morin oxidation, whereas only the peak potential was measured for the second step. In scanning negative potentials, only fluctuations in the current are perceptible, and the accurate localisation of potential peaks is not evident. Two approaches widely used to study the reversibility of electrochemical reactions and to determine whether a reaction rate is controlled by adsorption or diffusion are the analyses of ip on v1/2 and of ln ip on ln v curves. Fig. 3 shows these plots for the first oxidation peak of morin in acetonitrile. For reversible or irre400.0 I
350.0
3
300.0 Current (µA)
2.2. Measurement methods
3
50.0
2. Materials and methods
(1) Acetonitrile (CH3CN) pure p.a. from POCh Gliwice, Poland, (2) Tetrabutylammonium perchlorate (C4H9)4NClO4) from Fluka was used as a supporting electrolyte.
20.0
Current (µA)
40.0 II
100.0
Pure morin hydrate (20 ,3,40 ,5,7-pentahydroxyflavone) was obtained from a commercial source (Sigma–Aldrich) and used as received. The chemicals used for the preparation of the flavonoid solutions were as follows:
50.0
1
250.0 Current (µA)
Nematollahi & Malakzadeh, 2003; Wang, Xu, Zhao, & Hu, 2007; Zhou, Kikandi, & Sadik, 2007) but also for understanding the mechanisms of their oxidation or reduction processes (Janeiro, Corduneanu, & Brett, 2005; Timbola, Souza, Giacomelli, & Spinelli, 2006; Timbola, Souza, Soldi, Pizzolatti, & Spinelli, 2007). One of the parameters suitable for providing information about the antioxidant capacity of flavonoids is the half-wave potential (E1/2). Thus, a flavonoid species that shows a low E1/2 value can be regarded as a good free radical scavenger.
II
250.0
2
200.0
1
150.0 100.0
4
50.0 0.0 -50.0
0.0
0.3
0.6
0.9
1.2
1.5
1.8
Potential (V)
Fig. 2. Cyclic voltammograms of morin oxidation at Pt electrode; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at various scan rates; curve 1 – v = 0.01 V s1, 2 – v = 0.05 V s1, 3 – v = 0.1 V s1, 4 – v = 0.01 V s1 of the supporting electrolyte (0.1 mol L1 (C4H9)4NClO4 in acetonitrile).
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0.84
A Ep (V) vs Fc+/Fc
0.82
0.80
I peak
0.78
0.76 0.0
0.2
0.4
0.6 v (V s-1)
0.8
1.0
0.84
Ep = {0.0145[ln v (V s-1)]} V + 0.8246 V R² = 0.995
Ep (V) vs. Fc+/Fc
0.83
B
0.81 0.80
I peak
0.78 0.77 -4.0
-2.0 ln v (V s-1)
-1.0
0.0
7.0
Fig. 3. (A) The dependence of the anodic peak current (ip) on the square root of potential scan rate (v); (B) the dependence of anodic peak current on the potential scan rate in double logarithm coordinates for the oxidation of morin at Pt electrode; c = 2.0 103 mol L1 in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile.
C 6.0
ln ip (µA)
versible systems without kinetic complications, ip varies linearly with v1/2, intercepting the origin of the coordinates. However, the dependence of ip on v1/2 is not linear and presents a value different from zero for the linear coefficient if the electrode process is preceded or followed by a homogeneous chemical reaction. Within the polarisation rate range from 0.01 to 0.5 V s1, the electrooxidation peak current depends linearly on the square root of polarisation rate and is described by the following equation:
-3.0
5.0
ln ip = {34.32 (Ep-E0) (V)} µA - 9.02 µA R² = 0.9955 4.0 0
1
0.02
ip ¼ 793:89½v ðV s1 Þ2 lA þ 43:069 lA;R2 ¼ 0:9983
0.04
0.06
(Ep-E0) (V)
This linear fit does not precisely intercept the origin of the coordinates (3 A). This dependence suggests that the electrode process of peak I is diffusion-controlled but can also be preceded by a chemical reaction. On the other hand, the dependence of ln ip on ln v is linear and is described by the following equation:
Fig. 4. (A) The dependence of peak potential (Ep) on the potential scan rate (v) for the oxidation of morin in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at Pt electrode; (B) the dependence of peak potential on the ln v for the oxidation of morin in 0.1 mol L1 (C4H9)4NClO4 in acetonitrile at Pt electrode; (C) variation of ln ip versus (Ep E0) for the oxidation of morin in acetonitrile at Pt electrode.
ln ip ¼ 0:4129ln v ðV s1 ÞlA þ 6:6772 lA;R2 ¼ 0:9955
electrochemical reaction at the electrode surface. The value of the electron transfer coefficient for the reaction can be obtained from the following Eq. (1) (Bard & Faulkner, 2001; Harrison & Khan, 1970; Nicholson & Shain, 1964):
(Fig. 3B). The slope of this fit is 0.412, which indicates that the process is only controlled by diffusion. A slope close to 0.5 is expected for diffusion-controlled electrode processes, and a slope close to 1.0 is expected for adsorption-controlled processes (Bard & Faulkner, 2001; Brett & Brett, 1993; Kissinger & Heineman, 1996; Timbola et al., 2007). Fig. 4 A depicts the dependence of EpI on the scan rate for the electrooxidation of morin in acetonitrile under the same conditions as those shown in Fig. 2. For a reversible electrochemical reaction, the peak potential is independent of v. Therefore, we can conclude that the heterogeneous electronic transfer at peak I is irreversible or that there is a homogeneous chemical reaction following each
Ep ¼
RT ln v þ const 2bnb F
ð1Þ
where Ep – peak potential (V), R – universal gas constant (8.314 J K1 mol1), F – Faraday constant (96,487 C mol1), T – Kelvin temperature (298 K), bnb – anodic transfer coefficient, v – scan rate (V s1). This equation is valid for a totally irreversible diffusion-controlled process. Using the dependence of the anodic peak potential on the neperian logarithm of the potential sweep rate (Fig. 4B), a
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peak currents on the square root of the potential sweep rates (Fig. 3A) and the Randles–Sevcik Eq. (5) (Bard & Faulkner, 2001): 1
1
1
ip ¼ ð2:99 105 Þðbnb Þ2 C D2 v 2
ð6Þ 2
1
where D is the diffusion coefficient (cm s ). The diffusion coefficient for morin was calculated to be (5.45 ± 0.05) 106 cm2 s1. The diffusion coefficient calculated from Hayduk and Laudie’s Eq. (6) is 5.79 106 cm2 s1 (Schranke, Murphy, Doucette, & Hintze, 1999):
D¼
Fig. 5. (A) Cyclic voltammograms of morin oxidation at Pt electrode for various concentration; curve 1 – c = 76.4 ppm, 2 – c = 151.2 ppm, 3 – c = 310.8 ppm, 4 – c = 627.2 ppm, 5 – recorded in the supporting electrolyte (0.1 mol L1 (C4H9)4NClO4 in acetonitrile); (B) dependence of the anodic peak current on morin concentration, v = 0.01 V s1.
13:26 105
l1:4 v 0:589 o
ð7Þ
where l is the viscosity of the solvent (centipoises) and mo is the molar volume (cm3 g1 mole1). The effect of morin concentration on the electrode reaction of the substrate was investigated in the range from 76.4 ppm (0.255 mmol dm3) to 627.2 ppm (2.08 mmol dm3). Cyclic voltammetry and a dependence of oxidation peak current on the concentration is presented in Fig. 5. This dependence is linear in all the concentration range of morin. An increase in the concentration of morin causes significant increase in the peak current. The linear dependence up to morin concentration of (ip = {0.5492[c(ppm)]}lA–15.58724 lA, R2 = 0.9985)) can be applied in determination of morin concentration at Pt electrodes. The peak currents of the I and II steps of morin electrooxidation increased with increasing concentration of the substrate, halfwave potential (E1/2) of the morin electrooxidation slightly shifts towards more positive values. The half-wave potential of the I step (E1/2) is 0.72 ± 0.08 V, and that of the II is 1.27 V. The calculated anode transition coefficient (bnb) is 0.88 ± 0.05, and the heterogeneous rate constant of step I of morin electrooxidation at the half-wave potential (kbh) is (2.84 ± 0.05) 104 cm s1 (Galus, 1994). 3.3. Morin oxidation processes
value of 0.89 for the electron transfer coefficient (bnb) was obtained for peak I. To obtain information on the rate-determining step, the Tafel slope b was determined using the following equation for a totally irreversible diffusion-controlled process (Harrison & Khan, 1970; Yaghoubian et al., 2011):
Ep ¼ b=2ln v þ constant
ð2Þ
Based on Eq. (2), the slope of the dependence of Ep on ln v is b/2, where b represents the Tafel slope. The slope of Ep vs. ln v plot is o Ep/o (ln v) and was found to be 14.5 mV in this work (Fig. 4B); thus, b = 2 14.5 mV = 29 mV. From the dependence given in Eq. (3) (Bard & Faulkner, 2001):
ip ¼ 0:227 FAC exp½ðbnb f ÞðEp E0 Þ
ð3Þ
where: ip – peak current (A), f = F/RT, A – electrode area (cm2), C⁄ – bulk concentration of morin (mol cm3), E0 – formal potential of an electrode (V). This equation allowed to obtain a dependence:
ln ip ¼ ln a ðbnb f ÞðEp E0 Þ
ð4Þ 0
The dependence of ln ip on (Ep–E ) is linear (Fig. 4C) and is described by the following equation:
ln ip ¼ 34:32ðEp E0 ÞðVÞlA 9:02 lA;R2 ¼ 0:9955
The determined parameters are confirmed by quantum-chemical calculations. The distribution of electron charges in the investigated molecules is not uniform and determines the reactivity of particular positions (Fig. 6A) (Kolos, 1978). The energy of the highest filled orbital (EHOMO – ionisation potential) determines the ease of giving up electrons and indicates the site that is most susceptible to oxidation. The highest electron density in the morin molecule is observed in rings A and B, suggesting the ease of oxidation of the hydroxyl groups in these rings. The oxidation mechanism of hydroxymorin in subsequent electrode steps and its antioxidant activity are connected with the number of hydroxyl groups and their position in the three aromatic rings of this compound. Morin presents conjugation between rings A and B. It has different pharmacophores, including the moiety in ring B and the three hydroxyl groups in rings A and C. At the most positive potential (the first electrode stage), the hydroxyl group of ring B is oxidised and one electron and one proton are exchanged. The hydroxyl group at position 3 at ring C should be oxidised afterwards. The current of peak I is very high compared with the current of peak II, in agreement with the higher radical scavenging activity associated with the oxidation of the ring B moiety. Based on the electroanalytical investigations and literature data, one can deduce the mechanism of hydroxymorin oxidation (Fig. 6B) (Janeiro & Brett, 2004; Wang, Zhang, Tong, Xu, & Yang, 2011).
ð5Þ 4. Conclusions
and a slope of 34.32. The calculated anodic transfer coefficient (bnb) is equal to 0.88. The diffusion coefficient for morin was calculated on the basis of the slopes of the linear dependence of the anodic
Owing to its antioxidant capacity, morin is an important flavonoid. In our study, the electrochemical behaviour of morin at a Pt
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A
B
HO
B
O
HO
A
_ O
OH
C
B
O
HO
+
- e, - H
A OH
C
OH
O
O
B
O
A
C
O
OH
O
O OH
O
HO HO
or
C O
O
B
O
A
A
OH
O
B
O
HO
- e, - H
C
O
HO
OH
O
HO Tautomerization
.
+
OH OH
O
Fig. 6. (A) Electron density and probable sites in morin molecule susceptible electrooxidation; (B) proposed oxidation mechanism of morin.
electrode was investigated. The investigated flavonoid is irreversibly oxidised in at least two electrochemical steps. In the first electrochemical step, the hydroxyl group of ring B is oxidised and one electron and one proton are exchanged. In the second electrode step, the hydroxyl groups of ring C are oxidised at a more positive potential and successive electrons and protons are exchanged. Based on the experimental results, the electrochemical reaction mechanisms of morin at Pt were proposed. Corresponding electrochemical parameters were calculated, including the diffusion coefficient of morin, the transfer coefficient and the rate constant. It is helpful for us to understand the kinetics of morin. The data obtained are consistent with available knowledge and suggest that voltammetric studies on mechanically transferred solids may provide a convenient method for elucidating the electrochemical oxidation mechanisms of this type of compound in anhydrous media. References Bard, A. J., & Faulkner, L. R. (2001). Electrochemical methods, fundamentals and applications (2nd ed.). New York: John Wiley & Sons, pp. 236, 503, 709. Braca, A., Fico, G., Morelli, I., De Simone, F., Tomè, F., & De Tommasi, N. (2003). Antioxidant and free radical scavenging activity of flavonol glycosides from different Aconitum species. Journal of Ethnopharmacology, 86, 63–67. Brett, C. M. A., & Brett, A. M. O. (1993). Electrochemistry: Principles, methods, and applications. New York: Oxford University Press, p. 427.
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