Mechanism of oolongtheanin formation via three intermediates

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Highlights

  • A novel intermediate was found in the formation of oolongtheanins from catechins.

  • A mechanism of oolongtheanin formation was then proposed.

  • This mechanism was confirmed by GC–MS and 1H NMR experiments.

Abstract

To clarify the mechanism of oolongtheanin formation, the oxidations of (−)-epigallocatechin and (−)-epigallocatechin gallate were investigated, and key intermediates were isolated. These intermediates were determined to be dehydrotheasinensins and pro-oolongtheanins, which we have reported previously, and one novel intermediate. Based on the chemical structures of these intermediates, a mechanism was proposed for oolongtheanin formation from catechins, and confirmed by NMR and GC–MS analysis.

Introduction

Catechins are polyphenols that are found in abundance in tea leaves and known to exhibit a range of various bioactivities, including antioxidant [1], [2], [3] and antitumor [2], [4] activities. Tea can be classified according to the differences in the processing methods, with examples including green tea (unfermented tea), oolong tea (semi-fermented tea), and black tea (fermented tea). In the manufacturing of oolong and black teas, the catechins present in the tea leaves are oxidized by polyphenol oxidase, resulting in the formation of dimers and polymers. Oolong tea has attracted particular attention because of its health benefits, such as its anti-obesity effects [5], [6], [7], [8], [9], [10], and these have been tentatively attributed to the oolongtheanins [11], [12], [13], which are pyrogallol-type catechin dimers found in oolong tea leaves [14], [15], [16], [17]. However, detailed studies have been limited thus far because of these components being part of a complex mixture, thereby rendering preparative scale isolation challenging.

Previously, we reported the transformation of (−)-epicallocatechin gallate ((−)-EGCg, 1) to oolongtheanin-3′-O-gallate (2) via two key intermediates, namely, dehydrotheasinensin A (3) and pro-oolongtheanin-3′-O-gallate (4) [11]. In the present study, we report that the treatment of (−)-epigallocatechin ((−)-EGC, 5) with CuCl2 led to the discovery of a novel intermediate that allowed us to propose the mechanism of oolongtheanin formation.

Section snippets

Results and discussion

In our previous study, we reported the three-step transformation of 1 to 2 upon treatment with CuCl2 [11]. Therefore, in this study, we conducted the transformation of EGC (5) using the reported method. The HPLC chromatogram of the resulting reaction solution showed a new peak corresponding to dehydrotheasinensin C (6, Intermediate I) [18], which contained the same key moiety as dehydrotheasinensin A (3) [19], one of the intermediates in the formation of 2 [12]. Thus, following the previous

Conclusions

We herein report our investigation on the formation of oolongtheanins through the oxidation of (−)-EGC and (−)-EGCg, and identification of the reaction intermediates. Indeed, we successfully determined the chemical structure of a novel intermediate, which allowed us to propose the mechanism of oolongtheanin formation from catechins. These results are of importance because of the beneficial health properties of such polyphenols, and investigation of their mechanism of formation and chemical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research (C) (Grant No. 18K05486) from the Japan Society for the Promotion of Science. The authors thank Dr. Shin-ishi Nakatsuka (Nagara Science Co.) for providing a sample of (−)-EGCg, and Dr. Kohei Nakamura (Gifu university) for GC-MS measurements.

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