Analysis of Redox Properties of Polyphenols with Resorcinol Moiety
Authors: Kanno Sorai , Kitagawa Yuito , Shioji Mayo , Matsumoto Kenji , Kuwabara Tomohiko , Mizuguchi Hitoshi , Tsujino Yoshio , Osakai Toshiyuki , Hotta Hiroki 
Published: December 2024

Introduction
Plant polyphenols have shown great promise for a prevention and treatment of national diseases such as cancer and diabetes. Polyphenols are organic substances with multiple phenolic hydroxyl groups in the same molecule, and are components of bitterness and pigmentation present in all higher plants. More than 8,000 natural polyphenols are known, and their intake is known to have health-promoting effects for humans. This is due to the antioxidant properties of polyphenols. Because they are antioxidants found in plants, they are readily available through the diet. Many polyphenols are known to have a dihydroxybenzene skeleton, and we recently reported that the electron transfer between polyphenols and ABTS radical is different for ortho-, meta-, and para-dihydroxybenzene, depending on their reaction mechanisms1. Polyphenols with a para-dihydroxybenzene moiety are known to undergo a stable two-electron transfer to form the quinone, while those with ortho-dihydroxybenzene are known to polymerize upon oxidation. Polyphenols with meta-dihydroxybenzene moiety are reported to undergo irreversible oxidation, which appears to be an addition reaction with radicals1. Among them, meta-substituted compounds have relatively high redox potential and seemingly low antioxidant capacity, but some of them, such as resveratrol and kaempferol, show very high activity. In this study, several polyphenols with meta-substituted dihydroxybenzenes were analyzed for their structural changes upon electrolytic oxidation.
Experiments
40 mM Tris-HCl, 50% (v/v) methanol solution containing 0.5 mM polyphenol was used as a sample solution. The samples were analyzed by HPLC after oxidation treatment in a flow total electrolysis cell. A photodiode array and electrochemical detector (ECD) were used as detectors. Cyclic voltammetry (CV) measurements were performed beforehand to confirm the potential dependence of each substance on oxidation.
Results and Discussion
Resorcinol was whole electrolyzed in a flow-through whole-electrolysis cell and the products were measured by HPLC-DAD (diode array detector)-ECD. Several oxidation products were detected, and the detection potential dependence of the ECD detection of the most abundant oxidation product is shown in Figure 1a. The horizontal axis shows the applied potential, and the vertical axis is the ECD peak area divided by the UV absorption peak area. The concentration difference was corrected by dividing by UV absorption. The obtained corrected values were normalized by the maximum value. The obtained oxidation products were found to be oxidized above 0.5 V. Despite all of the sample were oxidized, the products were again shown to have reducing power, indicating that it got re-reduction ability through subsequent reactions. The potential dependence of resorcinol obtained from the same analysis shows that the oxidation current of resorcinol is observed above 0.7 V and becomes constant around 1.0 V, indicating that the oxidation product is a compound with a stronger reducing power of about 200 mV. An experiment similar to that in Fig. 1a was performed for catechin. The results are shown in Figure 1b. It is interesting that it was reoxidized despite being an oxidation product due to total electrolysis, but unlike the results for resorcinol, it showed the same potential dependence tendency as catechin. When detected very shortly after electrolytic oxidation, short-lived intermediates formed that oxidized at potentials lower than 0.4 V2. The difference between resorcinol and catechin may be due to the fact that catechins with catechol moiety have inherently lower oxidation potentials. These results suggest that resorcinol may generate catechol moieties through subsequent chemical reactions associated with oxidation. Structural analysis of other substances is currently underway.
*1. K. Matsumoto, M. Taniarashi, Y. Tsutaho, A. Yamada, A. Yosho, T. Osakai, H. Hotta
Redox reactions between ABTS•+ and dihydroxybenzenes as studied by cyclic voltammetry
Anal. Sci., 38 (2022), pp. 227-230,
*2. T. Kuwabara, R. Hashimoto, K. Matsumoto, H. Hotta, M. Iiyama, T. Takayanagi, H. Mizuguchi
Detection behavior of phenolic compounds in a dual-electrode system assembled from track-etched membrane electrodes
J.Electroanal.Chem., 928 (2023), Article 117039,

Figure 1