Analysis of Redox Reaction of Caffeic Acid Under UV Irradiation By Electrochemical Measurements
Authors: Kitagawa Yuito , Kanno Sorai , Matsumoto Kenji , Tsujino Yoshio , Osakai Toshiyuki , Hotta Hiroki 
Published: December 2024

Introduction
Since the discovery of the French Paradox, focus on polyphenols as substances with health benefits has continued to increase. Caffeic acid (Caf, 3,4-dihydroxycinnamic acid), the subject of this study, has also focused on as a polyphenol with antioxidant activity to prevent diseases caused by reactive oxygen species. Caf is one of the major antioxidants and is present in a variety of plants, making it easily available. We have extensively studied the electrochemical properties of polyphenols including caffeic acid[1-4]. We have found that cyclic voltammograms (CV) of Caf exhibit a unique behavior in which the oxidation current increases and the reduction current decreases when the electrode is irradiated with UV light. It is known that the trans-alkene moiety of the side chain of Caf is isomerized to the cis form upon UV irradiation[5], and it is also known that the cis form is converted to esculetin[6]. Based on these findings, we investigated the mechanism of the oxidation reaction of Caf under UV irradiation.

Experiments

A water/methanol (MeOH) (50/50, v/v) mixed solution containing 0.25 mM trans-Caf and 0.1 M citrate buffer (pH 6.0) was used as a sample solution. Cyclic voltammograms (CV) were obtained while irradiating the electrode surface with UV light. UV light from an Hg lamp was irradiated to the sample solution with the wavelength controlled by a bandpass filter. The UV-irradiated solution contained the trans- and cis-Caf, which were separated by HPLC and detected by UV absorption and electrochemical detector (ECD). A water/MeOH (85/15, v/v) mixed solution containing 0.085% (v/v) formic acid was used as an eluent for HPLC measurements. A flow whole-electrolytic cell was used as an ECD.

Results and Discussion

When the UV-irradiated solution was analyzed by HPLC, trans-Caf, cis-Caf, and esculetin were detected. The CVs during UV irradiation showed that the wavelength that most contributes to the isomerization of Caf was at 330 nm, which was the absorption peak derived from the trans-alkene chain of the side chain of trans-Caf. In contrast, at 260 nm where there was almost no absorption of trans-Caf, the oxidation current corresponding to cis form was very small. These results show that the isomerization of Caf was dependent on the absorption spectrum of trans-Caf. The results of coulometry using the flow whole-electrolytic cell showed that the number of electrons through the oxidation, n of trans-Caf, cis-Caf, and esculetin were 2.0, 4.4, and 4.4, respectively. These n-values indicated that the oxidation of cis-Caf and esculetin were not stop at quinone form (n = 2), but were further oxidized. Furthermore, the results of multi-scan CV using an HPLC-electrolytic cell (Figure 1) indicated that in the first cycle (black line), cis-Caf underwent a two-step oxidation reaction, the first step being irreversible and the second step reversible. The peak potential of the first step oxidation was more negative than that of trans-Caf, and that of the second step of cis-Caf was more negative than that of esculetin. In other words, it was found that an easily oxidizable substance with a lower oxidation-reduction potential than the trans-Caf was produced. From these results, cis-Caf produced by UV irradiation is suggest to have higher antioxidant activity than trans-Caf.
References:
[1] H. Hotta, H. Sakamoto, S. Nagano, T. Osakai, Y. Tsujino, Biochim. Biophys. Acta
2001, 1526, 159.
[2] H. Hotta, M. Ueda, S. Nagano, Y. Tsujino, J. Koyama, T. Osakai, Anal. Biochem.
2002, 303, 66.
[3] H. Hotta, S. Nagano, M. Ueda, Y. Tsujino, J. Koyama, T. Osakai, Biochim. Biophys. Acta 2002, 1572, 123.
[4] K. Matsumoto, M. Taniarashi, Y. Tsutaho, A. Yamada, A. Yosho, T. Osakai, H. Hotta, Anal. Sci. 2022, 38, 227.
[5] Kahnt, Phytochemistry, 1967, 6, 755.
[6] F. Parrino, A. Di Paola, V. Loddo, I. Pibiri, M. Bellardita, L. Palmisano Appl. Catal. B, 2016, 182, 347.

Figure 1