Oxidative spectroelectrochemistry of two representative coumarins

Electrochimica Acta 56 (2011) 2919–2925

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Oxidative spectroelectrochemistry of two representative coumarins Xin-Ran Hu a , Jian-Bo He a,b,∗ , Yan Wang a , Yan-Wu Zhu a , Jing-Jing Tian b a Anhui Key Lab of Controllable Chemical Reaction and Material Chemical Engineering, School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China b Center for Analysis and Measurement, Hefei University of Technology, Hefei 230009, China

a r t i c l e

i n f o

Article history: Received 28 October 2010 Received in revised form 25 December 2010 Accepted 28 December 2010 Available online 8 January 2011 Keywords: Spectroelectrochemistry Oxidation Umbelliferone Benzotertonic acid Coumarin

a b s t r a c t Electrochemical oxidation of two isomeric coumarins, umbelliferone (UF, 7-hydroxycoumarin) and benzotertonic acid (BA, 4-hydroxycoumarin), were comparatively studied in aqueous buffer solutions by cyclic voltammetry, in situ long-path-length thin-layer UV–vis spectroelectrochemistry and ex situ ATRFTIR spectrometry. Both the coumarins undergo the completely irreversible oxidation but following totally different oxidation mechanisms. The 7-OH but not the 4-OH group can contribute to antioxidative activity of coumarin via an electron transfer mechanism. Electro-oxidation of UF occurs at the 7-OH position and produces an insulating polymer film at the electrode surface, which probably consists of a poly(ethylene oxide) backbone with coumarin side groups. The toxicity-related coumarin 3,4-epoxide is a possible intermediate in the UF oxidation. Electro-oxidation of BA occurs at the C3 C4 double bond, also yielding a non-conductive film at the electrode surface. In this process salicylaldehyde as the possible intermediate undergoes further oxidation to form the poly(aryl ether) film. The knowledge of the mechanisms of UF and BA oxidation should be helpful in understanding the roles and conversion of coumarins in their biological and chemical processes. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Coumarin (5,6-benzo-␣-pyrone) is among the best known oxygen heterocyclics with a ␦-lactone ring and has over 1300 derivatives found throughout the plant kingdom [1–3]. Numerous coumarin derivatives have been chemically synthesized, mainly motivated by their multiple biological activities including disease prevention, growth modulation and antioxidant properties [1,4]. Coumarin has shown anti-tumour activity in vivo, with the effect believed to be due to its metabolites, e.g., 7-hydroxycoumarin (umbelliferone, UF) [1]. UF is also considered to be the effective chemical structure in the treatment of high-protein edema [2]. Many 7-substituted coumarins are used as sunscreen agents and optical brighteners for textiles due to their UV-activity [5]. 4Hydroxycoumarin (benzotertonic acid, BA) is used as a precursor in the synthesis of pharmaceuticals especially for anticoagulants [6] and inhibitors against human NAD(P)H quinone oxidoreductase-1 [7]. On the other hand, the toxicity of coumarins as flavouring ingredients in foods has raised some concerns and food safety authorities have set a maximum limit of 2 mg kg−1 for foods and beverages in general, and a maximum level of 10 mg L−1 for alcoholic beverages [8]. Some reports have indicated that coumarin-induced

∗ Corresponding author at: School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China. Tel.: +86 551 2901462; fax: +86 551 2901450. E-mail address: [email protected] (J.-B. He). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.086

toxicity is dependent on the formation of coumarin 3,4-epoxide under enzymatic catalysis [9–11]. It is noticeable that coumarin and its derivatives participate in redox regulation in some biological systems [4,7,9–14]. Electrochemical studies have been carried out in different coumarin-containing systems, mostly for the purpose of preparing novel functionalized compounds [3,15–22]. A partly halogenated coumarin phthalonitrile and corresponding metal-free, cobalt and zinc phthalocyanines were synthesized, and the redox processes of the complexes were studied by voltammetry and in situ spectroelectrochemistry [3]. A coumarin dye with a side ring was prepared and characterized with respect to photophysical and electrochemical properties for its application in dye-sensitized solar cells [15]. A series of Co(II), Ni(II), and Cu(II) complexes with Schiff bases of formyl coumarin derivatives were synthesized and subjected to electrochemical and in vitro antimicrobial studies [16]. Anodic oxidation of UF derivatives was performed in anhyd acetonitrile-lithium perchlorate for the synthesis of oxazolocoumarin derivatives [17]. Two novel photoreversible poly(ferrocenylsilanes) with coumarin side groups were synthesized and their photochemical reactivity and electrochemical behavior were investigated [18]. Copolymerization of polytriphenylamine with coumarin was carried out to improve the oxidation potential and LiFePO4 battery overcharge tolerance [19]. The natural compound scopoletin (7-hydroxy-6-methoxy-coumarin) was electropolymerized onto the electrode surfaces as an immobilization matrix of nucleic acids or proteins [20]. BA was used for

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Scheme 1. Chemical structures of umbelliferone (UF) and benzotertonic acid (BA).

electro-organic synthesis of coumestan derivatives [21,22]. Electrochemiluminescence of coumarin derivatives induced by hot electrons was studied at thin insulating film-coated aluminium electrodes in aqueous electrolyte solution [23]. The voltammetric characterization of several structurally similar coumarins has been reported for the potential use in developing methods for liquid chromatography with electrochemical detection of coumarins [24]. Recently, a spectroelectrochemical and chemical study on oxidation of 7,8-dihydroxy-4-methylcoumarin and some related compounds was conducted in aprotic medium [25]. Therefore, the understanding of the redox mechanisms of coumarins is important since their roles and applications in many aspects of biology and chemistry are based on the redox or electron transfer processes. Due to the lack of literature on the electrochemical oxidation mechanisms of coumarins, we carried out a spectroelectrochemical investigation on the oxidation of UF and BA, which are two representative and widely spread coumarins with one hydroxyl group at the benzene ring and the pyrone ring, respectively (Scheme 1). A long-optical-path thin-layer electrochemical cell (LTE-cell) was used for the in situ UV–vis measurements, allowing direct observation of the spectral change occurring in the thin-layer solution near the electrode surface. The obtained information and the resulting conclusions should be useful in understanding the roles and conversion of coumarins in their biological and chemical processes.

body was a polystyrene hollow tube with inner diameter of 2.2 mm, which was tightly impacted with a copper rod leaving a cavity of 2 mm depth at one end of the tube. The quadrate electrode body was a polystyrene plate with a cavity of 8.0 mm × 9.6 mm, as described previously [29]. The solid carbon paste was made from dry graphite powder and paraffin wax in a ratio of 5:2 (w/w). The wax was heated in an evaporating dish until molten, and then was mixed with the graphite powder to obtain a well blended paste. This paste was pressed firmly into the cavities of the two kinds of electrode bodies, and then the paste surface was polished successively with 800–4000 grit emery papers. The resulting sCPE was washed ultrasonically in doubly distilled water for 5 s to remove the stuck particles, and then stored in the BRSs prior to use. 2.3. Apparatus and procedures Electrochemical measurements including cyclic voltammetry and spectroelectrochemistry were carried out on a CHI660C electrochemical analyzer (Chenhua, Shanghai, China). The working electrode was the sCPE, used along with a platinum coil counter electrode and a KCl-saturated Ag/AgCl reference electrode (0.195 V vs. SHE, self-made). A conventional single-compartment cell was used for the voltammetric measurements. The time-dependent thin-layer UV–vis spectra were recorded in situ on an UV-2500 spectrophotometer (Shimadzu, Japan), to monitor the redox products of UF and BA at different pHs. The

2. Experimental 2.1. Chemicals and solutions UF and BA (98% purity each) were purchased from Shanghai Nuotai Chem Company and was used as received. Spectrograde graphite powder (320 mesh) and spectrograde paraffin wax (solidification point 62–65 ◦ C) were purchased from Shanghai Chemical Works for preparing the sCPE. Doubly distilled water was prepared in an all-glass distillatory apparatus for solution preparation. All other chemicals were of analytical grade from China-Reagent group. High pure N2 was used for solution deaeration. The supporting electrolytes were 0.2 mol dm−3 Britton–Robinson buffered solutions (BRS) with various pH values plus 0.5 mol dm−3 KCl. The stock solutions of UF and BA both with a concentration of 1.0 mmol dm−3 were prepared with ethanol as solvent and stored at 4 ◦ C in a refrigerator. Before use, the stock solution was diluted to various desired concentrations with the buffer supporting electrolytes. 2.2. Electrode preparation The solid carbon paste electrode (sCPE) was selected in this work with solid paraffin wax as a binder, because of its advantages of low noise and background current, improved reproducibility, robust in operation and better stabilization against organic solvents than oily liquids [26–28]. A disk sCPE with a smaller geometrical area of 3.8 mm2 was prepared for the voltammetric measurements, while a quadrate sCPE with a larger area of 77 mm2 was fabricated for the in situ spectroelectrochemical experiments. The disk electrode

Fig. 1. CVs of UF (A) and BA (B) both of 0.10 mmol dm−3 . Buffer pH (1 → 6): 1.8, 3.2, 5.8, 7.4, 8.0, 9.2; scan rate: 100 mV s−1 . Inset: the dependences of the peak potential on pH.

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Fig. 2. In situ thin layer UV–vis spectra during the constant potential oxidation of UF of 0.10 mmol dm−3 . (A) pH = 3.2, E = 0.95 V; (B) pH = 7.4, E = 0.72 V; (C) pH = 8.0, E = 0.72 V; (D) pH = 9.2, E = 0.70 V. Spectral tracing was repeated every 60 s after the potential was applied. The first line in every panel was recorded prior to electrolysis.

LTE-cell was home-made using a commercial available quartz photometric cell as the cell body [29], with a long-light-path of 10 mm and a thin-layer thickness of 0.2 mm. The incident light beam parallel to the electrode surface goes through the thin-layer electrolyte solution of 0.2 mm thickness on the surface of the working electrode. The time constant of the cell is less than 1 ms in 0.5 mol dm−3 KCl solution, which was characterized by chronoamperometric experiments. The sCPE surfaces after subjected to oxidation of the two coumarins in pH 7.4 BRS were characterized with attenuated total reflectance (ATR)-FTIR spectra, using a FTIR spectrophotometer Nicolet 6700 (Thermo Nicolet Corp., Madison, WI) equipped with a detector of deuterated triglycine sulphate (DTGS). The samples were located in contact with ATR element (ZnSe crystal) at room ambient temperature. FTIR spectra were collected with 32 scans and at a resolution of 4 cm−1 . All spectra were rationed against a background of air spectrum. After every scan, a new reference air background spectrum was taken. Before experiment, the electrochemical cell was washed with doubly distilled water and ethanol successively for 1 min under ultrasonication. All experiments were conducted at room temperature (22 ± 1 ◦ C). The electrolyte solution was deaerated, by passing nitrogen for about 15 min to remove dissolved oxygen, before each run. After each run, the working electrode was cleaned by repetitive cyclic scans between −0.4 and 1.6 V in 1.0 mol dm−3 KCl water–ethanol solution, until only the background current remained. 3. Results and discussion 3.1. Cyclic voltammetry Fig. 1 presents the cyclic voltammograms (CVs) of UF and BA in different pH BRSs. Only one anodic peak was observed for each

compound without the corresponding cathodic peak, indicating the irreversible oxidation of both the coumarins. The anodic peak of UF occurred at less positive potentials than that of BA, especially in neutral and alkaline media. For example, the oxidation potentials of UF and BA at the physiological pH (7.4) are 0.73 and 1.09 V (vs. Ag/AgCl/saturated KCl), respectively. Therefore, the 7-hydroxyl group can contribute to antioxidative activity of coumarin more than the 4-hydroxyl group. The oxidation peak potential (Epa ) of UF was shifted to more negative values with increasing pH for pH ≤ 7.4 (Fig. 1A), following a linear relationship Epa /V = 1.189 − 0.0630 pH (R = −0.9985). The equation slope of near −59 mV per pH unit support a mechanism involving the same number of electrons and protons. At the pH values greater than 7.4, the peak potential did not depend on the pH of medium, suggesting the absence of proton transfer in these pH conditions. The pKa value of the 7-OH of UF has been reported to be 7.75 [30]. Accordingly, the oxidation of UF should take place at the hydroxyl site, through one-electron one-proton transfer for the undissociated molecules but only one-electron transfer for the deprotonated phenolate anions. The oxidation peak current increased with increasing pH in the range below pKa (Fig. 1A), suggesting that the directly oxidizable form could be the deprotonated UF. As for BA, the oxidation peak current decreased with increasing pH, while the peak potential showed little change in the whole pH range tested (Fig. 1B), suggesting that no protons were involved in the charge transfer step. Therefore, the oxidation of BA should not occur at the 4-OH group. Nevertheless, the ionization degree of the 4-OH group (pKa = 5.1 [31]) may be the most probable cause for the decrease of the peak current with increasing pH, since this group is the most pH-sensitive part in BA molecule. The directly oxidizable form should be the undissociated BA, the amount of which can be compensated by fast shift of the dissociation equilibrium preceding the

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Fig. 4. (a) ATR-FTIR spectra of the polymerized UF film on a sCPE substrate. The film was electrodeposited in pH 7.4 BRS at a constant potential of 0.72 V for 10 min. (b) FTIR of UF powder in KBr pellet is also shown with band intensities reduced by 60 times for comparison.

Fig. 3. Decomposition of the first spectrum (A) and the last spectrum (B) in Fig. 2B to two absorption bands assigned to the protonated and deprotonated UF. The spectral baselines were subtracted prior to decomposition.

electron transfer, especially at the pH values greater than the pKa . 3.2. Spectroelectrochemistry of UF In situ UV–vis spectra were recorded during the controlled potential oxidation of UF in different pH BRSs (Fig. 2). The potentials for the oxidation were set at 0.95, 0.72, 0.72 and 0.70 V for pH 3.2, 7.4, 8.0 and 9.2, respectively. At the pH 3.2, UF shows two absorption bands at 216 nm and 324 nm (Fig. 2A, the first spectrum), the former corresponding to the benzene ring and the latter to the pyrone ring. The 324 nm band decreased in intensity with increasing pH until disappeared at pH 9.2, while two new bands grew at 230 and 368 nm (compare the first spectra in Fig. 2B–D). The 324 nm and the 368 nm bands have been assigned to the enol form (protonated form) [32,33] and the deprotonated anion [30,32,34] of UF, respectively. Therefore, the relative intensities of the two bands correlate with the relative amount of the protonated and deprotonated forms of UF. The electro-oxidation of UF led all these absorption bands to diminish in intensity proportionally, whether in acidic or alkaline media (Fig. 2), without appearance of any isosbestic point. This finding reflects a pure decrease in the concentration of the reactant, without any soluble light-absorbing product being formed. For further confirmation, we estimated the pKa value of UF from the relative intensities of the two bands at 324 and 368 nm recorded before and after electro-oxidation (Fig. 3). The

overlapping band contour was decomposed into two independent components assigned to the protonated and deprotonated forms, respectively, by means of a computer program that was self-designed based on the spectra of the two pure forms of UF. Two nearly equal pKa values, 7.88 and 7.85, were obtained from the decomposition of the first and the last spectra in Fig. 2B, respectively, very close to the pKa values reported in the literature (pKa = 7.75 [30] and pKa ≈ 8 [35]). The result excludes the formation of soluble light-absorbing products, and demonstrates that the ionization equilibrium of the 7-OH was maintained during its oxidation process. It is thus concluded that the oxidation product of UF was deposited on the electrode surface. The resulting coating should be non-conductive, obstructing the exhaustive electrolysis of UF from the thin-layer solution, since the absorption bands of UF did not vanish with increasing electrolysis time. The electrode passivation by the oxidation product was also evidenced by a multi-cyclic voltammetry experiment, in which the UF oxidation peak disappeared completely beginning with the second cycle (1 mmol dm−3 UF, scan rate 20 mV s−1 ). Similar result has been reported by Gajovic-Eichelmann et al. who first prepared a non-conductive but highly hydrophilic polymer film at the surface of carbon or noble metal electrodes, by electropolymerization of the natural compound scopoletin (7-hydroxy-6-methoxy-coumarin) [20]. The ATR-FTIR of the sCPE surface after subjected to UF oxidation at 0.72 V in pH 7.4 BRS is shown in Fig. 4(a), along with the FTIR of the UF power (Fig. 4(b)) for comparison. The paraffin in the sCPE substrate gave its as (CH2 ) band at 2920 cm−1 , s (CH2 ) at 2845 cm−1 , ı(CH2 ) at 1462 cm−1 , ıs (CH3 ) at 1373 cm−1 and  r [(CH2 )n ] (n ≥ 4) at 717 cm−1 (the former two not shown in Fig. 4(a)). The bands in the range of 1600–1454 cm−1 are due to the aromatic C C ring stretching vibrations. The shoulder band at 1110 cm−1 (C–O–C stretching) and the small band at 1335 nm−1 (COO stretching) in the spectrum (a) suggest that the lactone group remains in the oxidation product of UF. The strong C–O stretching of the phenolic hydroxyl of UF (shown at 1234 cm−1 in spectrum (b)) has vanished from the spectrum (a), due to the oxidation of the 7-OH group. More than this, the spectrum (a) also does not support other forms of oxygen directly attached to a benzene ring, such as quinine, aryl ether and peroxide. Instead, an alkyl ether structure is suggested by the new absorption band around 1055 cm−1 (C–O–C stretching). A possible oxidation mechanism is therefore proposed for the electro-oxidation of UF as in Scheme 2. The initial proton and electron transfer steps produce a reactive phenoxy radical, followed by a dimerization reaction with another UF anion to form

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Scheme 2. An electro-oxidative polymerization mechanism proposed for UF.

an alkyl-aryl ether. The latter undergoes successive oxidation and polymerization leading to chain prolongation until the growing oligomer completely covers the electrode surface. The resulting final polymer is a poly(alkyl ether) with a poly(ethylene oxide) backbone and coumarin side groups. In this process an epoxidation of UF occurs across the 3,4-C C bond, by receiving an oxygen atom from the phenoxy radical. Coumarin 3,4-epoxide has been identified as an intermediate of short half-life in some enzymatic catalytic systems, and was demonstrated to be responsible for coumarininduced toxicity [9–11]. The hydroxyl free radicals generated in the present process may combine to form H2 O2 , as determined during the oxidation of coumarins by human cytochrome P450 2A6 [13]. The poly(ethylene oxide) backbone constitutes the hydrophilic part of the film through the hydrogen bond between the oxygen atom and water molecule, therefore the excellent hydrophilic property of the poly(scopoletin) film (contact angle 10 ± 1◦ ) [20,36] can be well understood.

Fig. 5. In situ thin layer UV–vis spectra during the constant potential oxidation of 4hydroxycoumarin of 0.10 mmol dm−3 . (A) pH = 1.8, E = 1.15 V; (B) pH = 7.4, E = 1.10 V. Spectral tracing was repeated every 30 s after the potential was applied. The first line in every panel was recorded prior to electrolysis.

Fig. 6. (a) ATR-FTIR spectra of the polymerized BA film on a sCPE substrate. The film was electrodeposited in pH 7.4 BRS at a constant potential of 1.10 V for 10 min. (b) FTIR of BA powder in KBr pellet is also shown with band intensities reduced by 64 times for comparison.

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Scheme 3. An electro-oxidation mechanism proposed for BA.

3.3. Spectroelectrochemistry of BA Fig. 5 presents the in situ UV–vis spectra recorded during the BA oxidation. The potentials for the oxidation were set at 1.15 V for pH 1.8 and 3.2, and at 1.10 V for pH 5.8, 7.4, 8.0 and 9.2, respectively. At the pH values of 1.8 and 3.2, BA displayed its characteristic absorption bands around 267, 278 and 300 nm with a shoulder at ca. 312 nm (Fig. 5A); whereas at pH values of 5.8, 7.4, 8.0 and 9.4, it exhibited one main band around 285 nm along with a shoulder at ca. 296 nm (Fig. 5B). This pH-dependent difference may be attributed to the ionization degree of the 4-OH group of BA, since its pKa = 5.1 [31]. After an oxidation potential was applied, all the above bands were decreased in intensity, but did not disappear with increasing electrolysis time, suggesting the passivation of the electrode surface. On the other hand, three isosbestic points are shown at 230, 253 and 320 nm for pH 1.8, and 242, 262 and 320 nm for pH 7.4, indicating the formation of some soluble products. Accordingly, the BA should have been ring-opened, upon its oxidation occurring at the lactone ring, liberating soluble small molecules without a large conjugated structure. Apart from the C2 –O1 bond of the lactone that may be opened by hydrolysis, the C3 C4 double bond can split open, as is the case in the chemical oxidation of BA in aqueous alkaline medium by permanganate [37], or by diperiodatonickelate(IV) with [38] and without [39] ruthenium(III) as catalyzer. In these reports salicylic acid and oxalic acid were identified as the end products of BA oxidation. Under the present electro-oxidation conditions, salicylic acid can be further oxidized, but showed the spectral changes different from in Fig. 5. An additional strong absorption band was observed around 400 nm upon the electro-oxidation of salicylic acid at pH 7.4 (but not at pH 1.8), indicating a soluble product with a more extensively conjugated structure than the parent salicylic acid (data not shown). Interestingly, the electro-oxidation of salicylaldehyde exhibited the same spectral change trend (as reported in the literature [40]) as that shown in Fig. 5, supporting that salicylaldehyde is a possible intermediate from the electro-oxidation of BA. The ATR-FTIR of the sCPE surface after subjected to the BA oxidation at 1.10 V in pH 7.4 BRS is shown in Fig. 6(a), along with the FTIR of the BA power (Fig. 6(b)) for comparison. The paraffin in the sCPE substrate gave its characteristic bands at 2920, 2845, 1462, 1375 and 720 cm−1 , all nearly coinciding with the corresponding bands in Fig. 4(a). Two strong absorptions around 1153 and 1210 cm−1 can be assigned to the symmetric and asymmetric stretching of aryl ether group [41–43], indicating that the deposited product was linked by aryl ether bonds. The bands at 1309 and 1101 nm−1 disap-

peared, implying the absence of the lactone group in the deposited product at the electrode surface. An electro-oxidation mechanism is therefore proposed for BA as shown in Scheme 3. Upon oxidation at the C3 –C4 bond, a molecule of water is added across the bond, making the bond break. The BA thus is decomposed into salicylaldehyde phenoxy radical (SAR) and glycolic acid (GA). The SARs then undergo successive oxidation and polymerization producing a polyether film at the electrode surface. One possible pathway is that the SAR is oxidized to phenol and formate [44], followed by the oxidative polymerization of phenol via aryl ether linkages [45]. The GA may be further oxidized to oxalic acid and finally to H2 O and CO2. 4. Conclusions The comparative study of two isomeric coumarins, UF and BA, provides a more profound insight into the electrochemical oxidation mechanisms in the aqueous buffer solutions with different pHs. Both the coumarins undergo the completely irreversible oxidation but following totally different oxidation mechanisms. Electro-oxidation of UF occurs at the 7-OH, leading to an insulating polymer film completely blocking the electrode surface, without liberating any soluble species from the reactant. The resulting polymer is probably a poly(alkyl ether) with a poly(ethylene oxide) backbone and coumarin side groups. The toxicity-related coumarin 3,4-epoxide is a possible intermediate during the formation of the poly(alkyl ether). The ionization equilibrium of the 7-OH was maintained during the oxidation process. Electro-oxidation of BA occurs at the C3 C4 double bond, instead of the 4-OH, also yielding a non-conductive but poly(aryl ether) film at the electrode surface along with the liberation of soluble products. In this process salicylaldehyde as the main intermediate undergoes further oxidation to form the polyether. The very high oxidation potential of BA excludes the possibility that the 4-OH might contribute to the antioxidant activity of coumarins via an electron transfer mechanism. The obtained information and the resulting conclusions should be useful in understanding the roles and conversion of coumarins in their biological and chemical processes. Acknowledgments The authors gratefully acknowledge the financial support from the National Nature Science Foundation of China (Nos. 20776033, 20972038).

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References [1] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

A. Lacy, R. O’Kennedy, Curr. Pharm. Des. 10 (2004) 3797. N. Farinola, N. Piller, Lymphat. Res. Biol. 3 (2005) 81. A. Alemdar, A.R. Özkaya, M. Bulut, Polyhedron 28 (2009) 3788. J.Z. Pedersen, C. Oliveira, S. Incerpi, V. Kumar, A.M. Fiore, P.D. Vito, A.K. Prasad, S.V. Malhotra, V.S. Parmar, L. Saso, J. Pharm. Pharmacol. 59 (2007) 1721. J.P. Shubha, Puttaswamy, Prog. React. Kinet. Mech. 33 (2008) 313. M. Pisklak, D. Maciejewska, F. Herold, I. Wawer, J. Mol. Struct. 649 (2003) 169. K.A. Nolan, J.R. Doncaster, M.S. Dunstan, K.A. Scott, A.D. Frenkel, D. Siegel, D. Ross, J. Barnes, C. Levy, D. Leys, R.C. Whitehead, I.J. Stratford, R.A. Bryce, J. Med. Chem. 52 (2009) 7142. C. Sproll, W. Ruge, C. Andlauer, R. Godelmann, D.W. Lachenmeier, Food Chem. 109 (2008) 462. S.L. Born, D. Caudill, B.J. Smith, L.D. Lehman-McKeeman, Toxicol. Sci. 58 (2000) 23. S.L. Born, D. Caudill, K.L. Fliter, M.P. Purdon, Drug Metab. Dispos. 30 (2002) 483. J.D. Vassallo, S.M. Hicks, S.L. Born, G.P. Daston, Toxicol. Sci. 82 (2004) 26. H.-C. Lin, S.-H. Tsai, C.-S. Chen, Y.-C. Chang, C.-M. Lee, Z.-Y. Lai, C.-M. Lin, Biochem. Pharmacol. 75 (2008) 1416. C.-H. Yun, K.-H. Kim, M.W. Calcutt, F.P. Guengerich, J. Biol. Chem. 280 (2005) 12279. R.W. Miller, J.-C. Sirois, H. Morita, Plant Physiol. 55 (1975) 35. Z.-S. Wang, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, S. Suga, H. Arakawa, H. Suglhara, J. Phys. Chem. B 109 (2005) 3907. A.D. Kulkarni, G.B. Bagihalli, S.A. Patil, P.S. Badami, J. Coord. Chem. 62 (2009) 3060. E. Abdelghani, A. Abd El-Aal, W. Shehab, M. El-Mobayed, Synthesis-Stuttgart (2003) 1373. B. Ren, D. Zhao, S. Liu, X. Liu, Z. Tong, Macromolecules 40 (2007) 4501. X.M. Feng, J.Y. Zheng, J.J. Zhang, R.F. Li, Z.J. Li, Electrochim. Acta 54 (2009) 4036. N. Gajovic-Eichelmann, E. Ehrentreich-Forster, F.F. Bier, Biosens. Bioelectron. 19 (2003) 417.

2925

[21] S.M. Golabi, D. Nematollahi, J. Electroanal. Chem. 430 (1997) 141. [22] S.M. Golabi, D. Nematollahi, J. Electroanal. Chem. 420 (1997) 127. [23] M. Helin, Q. Jiang, H. Ketamo, M. Håkansson, A.-M. Spehar, S. Kulmala, T. AlaKleme, Electrochim. Acta 51 (2005) 725. [24] Q. Wu, H.D. Dewald, Electroanalysis 13 (2001) 45. [25] R. Petrucci, L. Saso, V. Kumar, A.K. Prasad, S.V. Malhotra, V.S. Parmar, G. Marrosu, Biochimie 92 (2010) 1123. ˇ [26] I. Svancara, A. Walcarius, K. Kalcher, K. Vytˇras, Cent. Eur. J. Chem. 7 (2009) 598. [27] J.-B. He, Y. Wang, N. Deng, Z.-G. Zha, X.-Q. Lin, Electrochim. Acta 52 (2007) 6665. [28] J.-B. He, F. Qi, Y. Wang, N. Deng, Sens. Actuators B: Chem. 145 (2010) 480. [29] J.-B. He, Y. Wang, N. Deng, X.-Q. Lin, Bioelectrochemistry 71 (2007) 157. [30] T. Moriya, Bull. Chem. Soc. Jpn. 56 (1983) 6. [31] R. Dondon, S. Fery-Forgues, J. Phys. Chem. B 105 (2001) 10715. [32] D. Jacquemin, E.A. Perpète, X. Assfeld, G. Scalmani, M.J. Frisch, C. Adamo, Chem. Phys. Lett. 438 (2007) 208. [33] K. Azuma, S. Suzuki, S. Uchiyama, T. Kajiro, T. Santa, K. Imai, Photochem. Photobiol. Sci. 2 (2003) 443. [34] T. Moriya, Bull. Chem. Soc. Jpn. 61 (1988) 1873. [35] D. Jacquemin, E.A. Perpète, I. Ciofini, C. Adamo, J. Phys. Chem. A 112 (2008) 794. [36] H. Cai, C. Shang, I.-M. Hsing, Anal. Chim. Acta 523 (2004) 61. [37] R.M. Mulla, S.T. Nandibewoor, Polyhedron 23 (2004) 2507. [38] R.S. Shettar, S.T. Nandibewoor, J. Mol. Catal. A: Chem. 234 (2005) 137. [39] R.S. Shettar, N.P. Shetti, S.T. Nandibewoor, E-J. Chem. 6 (2009) 601. [40] N. Matyasovszky, M. Tian, A.C. Chen, J. Phys. Chem. A 113 (2009) 9348. [41] M. Nechifor, React. Funct. Polym. 69 (2009) 27. [42] R.T.S.M. Lakshmi, J. Meier-Haack, K. Schlenstedt, H. Komber, V. Choudhary, I.K. Varma, React. Funct. Polym. 66 (2006) 634. [43] M. Obi, S.y. Morino, K. Ichimura, Chem. Mater. 11 (1999) 656. [44] G.P. Chen, L.L. Poulsen, D.M. Ziegler, Drug Metab. Dispos. 23 (1995) 1390. [45] M.A. Heras, S. Lupu, L. Pigani, C. Pirvu, R. Seeber, F. Terzi, C. Zanardi, Electrochim. Acta 50 (2005) 1685.