Electron Spin Resonance Spectroscopic Studies on the Radical

Jan 5, 2012 - The antioxidant activities and free radical scavenging capacities of (+)-catechin, pyrogallol, and red wine are investigated using the ...
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Electron Spin Resonance Spectroscopic Studies on the Radical Scavenging Capacities of Catechin and Pyrogallol Shaolin Mu* Department of Chemistry, Yangzhou University, Yangzhou 225002, Jiangsu Province, China

Chong Chen Laboratory Center of Yangzhou University, Yangzhou 225002, Jiangsu Province, China ABSTRACT: The antioxidant activities and free radical scavenging capacities of (+)-catechin, pyrogallol, and red wine are investigated using the electrochemical method and electron spin resonance (ESR) technique. Poly(aniline-co-o-aminophenol) (PAoA) is used as a source of the free radical. On the basis of the electrochemical measurements, among the samples used, the antioxidant activity is the strongest for pyrogallol and is weakest for red wine. However, the ESR measurements reveal the fact that (+)-catechin can effectively scavenge the free radical of PAoA; however, pyrogallol not only cannot scavenge the free radical but also makes the ESR signal intensity of PAoA pronouncedly increase. This result is just contrary to the previous reports based on the determination of the scavenging free radical of DPPH using the UV−visible spectral method, in which the maximum absorbance is related only to the reducing power of an antioxidant. This difference is due to the formation of the new free radical during the oxidation process of pyrogallol. It is found that the red wine used can scavenge the free radical, but its scavenging capacity is lower than that of (+)-catechin.

1. INTRODUCTION Catechins (flavan-3-ols) are polyphenolic compounds and belong to the group of flavonoids, which are present in plants and in foods of plant origin, including beverages such as tea and wine. A relatively high level of catechins in human daily diet is related to the reduction of common chronic diseases and the promotion of health.1 In addition, catechins show antibacterial, antiviral, and/or antibiotic activities.2 These biological activities of catechins are often ascribed to their antioxidant properties due to the high number of OH groups in their structures.3 Therefore, the study on the oxidation mechanism, antioxidant activities, and the free radical scavenging capacities of catechins has received recently a great deal of attention.4−9 In addition, the determination of thermodynamic parameters10,11 of catechins plays an important role in explaining antioxidative activities and in discussing biological and biochemical phenomenon. The flavonoid (+)-catechin has three rings, A, B, and C, with the resorcinol group in ring A, the catechol group in ring B, and a hydroxyl group at position 3 in ring C; see Scheme 1. It was demonstrated that the catechol B-ring (EOOH = 0.135 V vs SCE) is more easily oxidizable than the resorcinol A-ring (EOOH = 0.285 V vs SCE) based on their oxidation potentials.5,10 This result is expected because phenolic compounds with o- or p-diphenol groups are typically found to have lower oxidation potentials than compounds with mdiphenols or isolated phenols. Therefore, the redox potentials of a polyphenol could be a key factor that governs its © 2012 American Chemical Society

Scheme 1. (+)-Catechin Oxidation

antioxidant activity, which therefore is closely related to its radical scavenging capacity that was generally assessed on the basis of its capacity to scavenge 2,2-diphenyl-1-picrylhadrazyl (DPPH) free radicals.8,9,12,13 This experiment was carried out in a mixture of a testing sample and DPPH. After the redox reaction between them, the mixture was measured using an UV−visible spectrophotometer because DPPH was an intense color with a maximum absorbance at 516 nm. The previous reports demonstrated that the scavenging capacity of antioxidant increases with decreasing its oxidation potential, such as (+)-catechin and pyrogallol.8 In spite of the successful results on the oxidation mechanism and radical scavenging capacities of antioxidants that have been obtained,4−10 the study on the determination of radical scavenging capacities of them is still open because the early determination was not carried out using the electron spin Received: November 17, 2011 Revised: January 4, 2012 Published: January 5, 2012 3065

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Figure 1. (A) Cyclic voltammograms of (1) 1.0 mM pyrogallol, (2) 1.0 mM (+)-catechin, and (3) 50% (v/v) red wine in 0.20 M phosphate, pH 5.0, at a scan rate of 10 mV s−1. (B) The change in the open circuit potential of poly(aniline-co-o-aminophenol) in different solutions: (1) 10 mM pyrogallol and 0.20 M phosphate, (2) 1.0 mM (+)-catechin and 0.20 M phosphate, pH 5.0, (3) red wine of 12% (v/v) alcohol with pH 2.66. A reference electrode of Ag/AgCl with a saturated KCl solution.

that, a Pt wire with PAoA was washed with distilled water and then cycled between −0.20 and 0.80 V (vs SCE), in a 0.20 M H2SO4 solution. The aim for this is to remove reactants in PAoA film. By ending cycling, the potential remained at 0.80 V, indicating that PAoA was at the oxidized form. Finally, this electrode was used for the ESR measurements in an ESR cell.27 Before the PAoA electrode was inserted into an ESR cell, the solution in the ESR cell was deoxygenated by passing through pure nitrogen for 5 min, and then the ESR signal of the PAoA electrode was measured without a continuous flow of nitrogen. A Bruker A300 spectrometer was used for the ESR measurements. The microwave power was set at 20 mW. The modulation amplitude was set at 1.0 G. The ESR spectra were recorded every 90 s. The SEM image of the polymer was measured on a field emission scanning electron microscope (SEM) S-4800 II FE-SEM instrument.

resonance (ESR) technique. It is well-known that the free radicals can be generated during the oxidation processes of polyphenols and aromatic phenols, which would affect the assessment of the radical scavenging capacities based on the determination of UV−visible spectra. Polyaniline is a stable conducting polymer with free radicals;14,15 its ESR measurements provided a wealth of information on its conduction mechanism, the potential dependence of interconversion between polaron and bipolaron, and the origin of the free radical.16−24 Polyaniline has been used as free radical scavengers of DPPH based on the determination of UV−visible spectra.25 An appropriate experimental condition for determining the radical scavenging capacity of (+)-catechin would be performed in the near neutral aqueous solutions. The ESR measurements of poly(aniline-co-oaminophenol) (PAoA) in aqueous solutions confirmed that it is suitable for determining radical scavenging capacities of antioxidants.26 In this work, we used ESR technique and PAoA to approach the free radical scavenging capacities of (+)-catechin, pyrogallol, and red wine because the pyrogallol ring is contained in (−)-epigallocatechin and catechins are contained in red wines. Among the three samples used, the ESR spectra of PAoA show a surprising result that pyrogallol with the strongest antioxidant activity not only does not exhibit free radical scavenging ability but also makes the ESR signal intensity of PAoA pronouncedly increase; however, (+)-catechin and red wine show free radical scavenging capacities. The cause for this experimental phenomenon is discussed in more detail in this work.

3. RESULTS AND DISCUSSION 3.1. Assessment of Antioxidant Activities of (+)-Catechin, Pyrogallol, and Red Wine Based on the Electrochemical Determination. Figure 1 shows the cyclic voltammograms of the first cycle for pyrogallol of 1.0 mM (curve 1), (+)-catechin of 1.0 mM (curve 2), and 50% (v/v) red wine (curve 3) in 0.20 M phosphate buffer with pH 5.0. An oxidation peak at 0.34 V occurs on curve 1, which is caused by the oxidation of hydroxyl groups in pyrogallol; however, on the reverse scan, only a small reduction peak occurs at 0.14 V; in addition, the oxidation peak potential shifts toward more positive potentials accompanied with a rapid decrease in the peak current with an increasing number of cycles (omitted here). The above experimental phenomenon indicates that pyrogallol oxidation is irreversible. A golden film was observed on the platinum electrode during the oxidation process of pyrogallol, which resulted in a decrease in the electrolytic rate. An oxidation peak at 0.42 V occurs on curve 2, which is attributed to the oxidation of the OH groups of ring B in (+)-catechin.5,10 Curve 3 shows a broad wave centered at 0.48 V, which is caused by the oxidation of catechins and other polyphenols in the red wine. On the basis of the oxidation peak potentials and their peak currents in Figure 1A, we come to a conclusion that pyrogallol is more easily oxidizable than (+)-catechin, which is in good agreement with the previous report,8 and pyrogallol has the highest antioxidant activity and the red wine has the lowest one among the three samples used.

2. EXPERIMENTAL SECTION Aniline was distilled under reduced pressure before use. (+)-Catechin hydrate (≥98%) was purchased from SigmaAldrich. Other chemicals were of analytical reagent grade. Doubly distilled water was used to prepare all solutions. Red wine with 12% (v/v) alcohol (Dynasty OAK 160) was produced in 2008; its pH is 2.66. The pH values of the solutions were determined with a PXD-12 pH meter. All electrochemical experiments were performed on a CHI 407 electrochemical workstation. PAoA was synthesized using cyclic voltammetry between −0.20 and 0.80 V (vs SCE), in a mixture containing 0.20 M aniline, 10 mM o-aminophenol, and 0.60 M H2SO4. A platinum wire (0.5 mm diameter) was used as a working electrode. After 3066

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Figure 2. ESR spectra of poly(aniline-co-o-aminophenol), (A) in 0.20 M phosphate, (B) in a solution containing 1.0 mM (+)-catechin and 0.20 M phosphate, pH 5.0. An ESR spectrum was recorded every 90 s: (1) first record through (5) fifth record.

Figure 3. ESR spectra of poly(aniline-co-o-aminophenol) in 0.20 M phosphate buffer with different pyrogallol concentrations of pH 5.0: (A) 10 mM pyrogallol and (B) 0.5 mM pyrogallol, (1) without pyrogallol and (2−6) containing pyrogallol. An ESR spectrum was recorded every 90 s.

the basis of potentials measured at 600 s, pyrogallol has the highest antioxidant activity and the red wine has the lowest one among the three testing samples used. Therefore, the result shown in Figure 1B is in good agreement with the result shown in Figure 1A. 3.2. Assessment of Radical Scavenging Capacities of (+)-Catechin and Pyrogallol Using ESR Technique. The unpaired spin density of polyaniline or PAoA at the oxidized form is generally greater than that of reduced polyaniline or reduced PAoA. This means that the ESR signal intensity of PAoA is stronger than that of the reduced PAoA at the same amount. Figure 2A shows the ESR spectrum of PAoA in 0.20 M phosphate buffer of pH 5.0; its ESR signal intensity measured with the peak height is 7.84 × 106 (arbitrary unity). After that the ESR spectra of this electrode were recorded in a solution containing 1.0 mM (+)-catechin and 0.20 M phosphate with pH 5.0, which are shown in Figure 2B. Curve 1 in Figure 2B is the ESR spectrum of the first record. Its ESR signal intensity is 5.28 × 105, which is only 6.7% of PAoA shown in Figure 2A. As the reaction of PAoA and (+)-catechin proceeds, the ESR signal intensity in Figure 2B increases slowly with time. The ESR signal intensity of curve 5 in Figure 2B is still less than 10% of PAoA shown in Figure 2A. The results from Figure 2 demonstrate that (+)-catechin has a very strong free radical scavenging capacity. This is because its oxidation potential is lower than that of PAoA; hence, it has a powerfully reducing power. As can be seen in Figure 2B, the ESR signal intensity increases slowly with time as the reaction proceeds; one of the reasons for this would be attributed to the decrease in the local

To confirm their antioxidant activities, the open circuit potential of the PAoA electrode was measured in an ESR cell with a testing solution and a reference electrode of Ag/AgCl with a saturated KCl solution. Before that, the equilibrium potential of the PAoA electrode was first measured in a 0.20 M phosphate buffers of pH 5.0 and 2.66, respectively. The latter is for the determination of the red wine because its pH is 2.66. The results are shown in Figure 1B, in which the potentials denoted with PBS at zero on the time axis present the equilibrium ones of the PAoA electrodes. Curve 1 in Figure 1B shows the change in the potential of the PAoA electrode with time, in a solution containing 10 mM pyrogallol and 0.20 M phosphate with pH 5.0. It is clear that the potential decreases rapidly from 0.37 V of the equilibrium potential to 0.129 V at 10 s after moving the PAoA electrode from 0.20 M phosphate buffer solution into the solution containing pyrogallol and then decreases slowly with time to 0.099 V at 600 s. After that the PAoA electrode was immediately washed with distilled water and then moved into the phosphate buffer solution of pH 5.0 without pyrogallol to determine its potential change with time again; its potential increases with time, which is also shown in Figure 1B denoted with PBS. Curves 2 and 3 in Figure 1B show the change in the potential of PAoA electrode with time, in a solution containing 1.0 mM (+)-catechin and 0.20 M phosphate with pH 5.0 and the red wine with 12% (v/v) alcohol of pH 2.66, respectively. Their potentials decrease from 0.37 (curve 2) and 0.40 V (curve 3) to 0.238 and 0.276 V at 600 s, respectively. The results in Figure 1B indicate that PAoA was reduced by pyrogallol, (+)-catechin, and the red wine. On 3067

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Figure 4. (A) ESR spectrum of poly(pyrogallol) and (B) image of poly(pyrogallol) film.

Figure 5. ESR spectra of poly(aniline-co-o-aminophenol): (A) the red wine with 12% alcohol, (B) the red wine in 0.20 M citrate buffer containing 6% (v/v) alcohol; (1) 0.20 M phosphate buffer for part A, 0.20 M citrate buffer for part B, and (2−6) containing red wine, pH 2.66. An ESR spectrum was recorded every 90 s.

shown in Figure 3B. Curve 1 in Figure 3B is the ESR spectrum of PAoA in 0.20 phosphate buffer of pH 5.0. Curves 2−6 are the ESR spectra of PAoA in the pyrogallol solution. The ratio of the signal intensity of curve 2 to the signal intensity of curve 1 is 2.5, which is smaller than that in Figure 3A because a lower pyrogallol concentration was used for the experiment, as shown in Figure 3B. Now, the question is why pyrogallol not only cannot scavenge the free radical but also makes the ESR signal intensity of PAoA pronouncedly increase. One of the reasons is a decrease in pH on the PAoA surface due to pyrogallol oxidation, which causes only a slight increase in the ESR signal intensity (Figure.2B) during the measuring ESR process. Therefore, such a pronounced increase of the ESR signal intensity in Figure 3 would be caused by the new free radical formation in the reaction process. 3.3. ESR Spectrum and Image of the Product of Pyrogallol Oxidation. We found a golden film on the platinum electrode during the oxidation process of pyrogallol as mentioned previously, which should be a polymer, i.e., poly(pyrogallol). The polymerization of pyrogallol is suggested as follows:

pH on the PAoA surface, which results in the increase in the ESR signal intensity of PAoA. Even though the phosphate buffer was used here, the rapid oxidation of (+)-catechin shown in Figure 1B liberates an amount of protons that is greater than that needed for PAoA reduction after the onset of the reaction between PAoA and (+)-catechin, and the capillary of the ESR cell is very thin, which limits proton diffusion in good time. As a result, the pH decreases on the PAoA surface after the onset of the reaction, which results in the faster increase in the ESR signal intensity at the beginning measurements, and then the ESR signal intensity tends to a steady state because the reaction rate becomes slow as the reaction proceeds. Curve 1 in Figure 3A is the ESR spectrum of PAoA in 0.20 M phosphate buffer of pH 5.0. Curves 2−6 are the ESR spectra of PAoA in a solution containing 10 mM pyrogallol and 0.20 M phosphate with pH 5.0. Clearly, the ESR signal of PAoA on curve 2 for the first determination in the pyrogallol solution is much higher than that on curve 1. The ratio of the signal intensity of curve 2 to the signal intensity of curve 1 is 5.1. Then the ESR signal intensity of PAoA in the pyrogallol solution increases continuously with time. This result is contrary to expectation because its oxidation peak potential is lower than that of (+)-catechin (Figure 1A) and its reducing power is much stronger than that of (+)-catechin (Figure 1B), meaning that pyrogallol should scavenge the free radical of PAoA, and its free radical scavenging activity should be stronger than that of (+)-catechin. To confirm the result shown in Figure 3A, the ESR measurements of PAoA were carried out in a same solution but containing 0.5 mM pyrogallol. The result is

Therefore, poly(pyrogallol) deposited on the platinum electrode was used to determine ESR spectrum. A surprising result is that an ESR signal is detected in Figure 4A. This is the primary reason for the increase in the ESR signal intensity of 3068

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but also makes the ESR signal intensity of PAoA pronouncedly increase. This result is just contrary to previous reports based on the determination of the scavenging free radical of DPPH using UV−visible spectral technique, in which the maximum absorbance is related to the reducing power of an antioxidant. This difference is due to the formation of the new free radical during the oxidation process of pyrogallol. This means that the antioxidant activity of an antioxidant only depends on its oxidation potential; however, the free radical scavenging capacity of an antioxidant not only depends on its oxidation potential but also depends on its product that carries free radical or not. Therefore, the ESR technique is a good tool to assess the free radical scavenging capacity of an antioxidant, which is better than the UV−visible spectral method based on the determination of the maximum absorbance of DPPH. This work demonstrates that the red wine used can scavenge the free radical, but its scavenging capacity is lower than that of (+)-catechin.

PAoA in the pyrogallol solution because of the formation of poly(pyrogallol) carried with free radical. Figure 4B shows the image of poly(pyrogallol) film on the platinum disk, which is constructed of interwoven fibers with diameters of about 60− 200 nm with different lengths, indicating that after the formation of the polymer nuclei, the polymerization of pyrogallol took place on the polymer nuclei to form thick fibers rather than on the naked platinum surface. This may be due to the free radical in poly(pyrogallol), which decreases the activation energy for pyrogallol polymerization. The oxidation of pyrogallol generated the free radicals as discussed above. The question is, what about the oxidation of (+)-catechin? In this case, the (+)-catechin solution used for the experiment shown in Figure 1 was cycled between −0.20 and 0.80 V (vs Ag/AgCl) for 100 cycles. After that the working electrode was used for the ESR measurement, and no ESR signal was detected. In addition, the in situ electrochemical-ESR measurements for the above (+)-catechin solution were performed at 0.42, 0.60, and 0.80 V (vs Ag/AgCl), respectively; also no ESR signal is detected. This result indicates that the free radical scavenging capacity of (+)-catechin is only related to its oxidation potential, i.e., its reducing power. The ESR measurements for the electrochemical oxidation of (+)-catechin and pyrogallol demonstrate that the oxidation mechanism of (+)-catechin is different from that of pyrogallol. 3.4. Determination of the Radical Scavenging Capacity of the Red Wine. Curve 1 in Figure 5A is the ESR spectrum of PAoA in a 0.20 M phosphate buffer of pH 2.66; curves 2−6 show the ESR spectra of PAoA in the red wine with 12% (v/v) alcohol of pH 2.66. Figure 5A shows that the ESR signal intensity of curve 2 is slight lower than that of curve 1. This means that the red wine has a free radical scavenging capacity that is very low compared to that of (+)-catechin. Curves 2−6 in Figure 5A show an increase in the ESR signal intensity of PAoA with time, and the intensity on curve 6 exceeds that of curve 1. This is probably caused by a decrease in pH on the PAoA surface due to the oxidation of catechins and other polyphenols in the red wine and the low buffer capacity of the red wine. Clearly, the experimental result shown in curves 2−6 of Figure 5A is similar to that of Figure 2B, in which the ESR signal intensity of PAoA also increases slightly with time in a solution consisting of (+)-catechin and 0.20 M phosphate. In this case, the ESR measurements of PAoA were performed in a 6% (v/v) alcohol solution that consisted of the red wine and 0.20 M citrate with pH 2.66. The results are shown on curves 2−6 of Figure 5B. Curve 1 in Figure 5B is the ESR spectrum of PAoA in 0.20 M citrate buffer of pH 2.66. Clearly, the ESR signal intensity of curve 2 is much lower than that of curve 1 in Figure 5B. As compared with Figure 5A, the decrease in the ESR signal intensity of PAoA in Figure 5B becomes more pronounced even when only 6% (v/v) alcohol is contained in the determined solution. Therefore, the red wine used has a free radical scavenging ability. The difference between part A and B of Figure 5 is caused by the different buffer capacities, because the buffer capacity of the citrate solution is much greater than that of the red wine solution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES

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4. CONCLUSION In summary, the electrochemical measurements show that the antioxidant activity of pyrogallol is stronger than that of (+)-catechin. However, the ESR measurement demonstrates that (+)-catechin can effectively scavenge the free radical of PAoA but pyrogallol not only cannot scavenge the free radical 3069

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