Dietary polyphenols like epigallocatechin-3-gallate (EGCG)—which represents the most abundant flavan-3-ol in green tea—are subject of several studies regarding their bioactivity and health-related properties. On many occasions, cell culture or in vitro experiments form the basis of published data. Although the stability of these compounds is observed to be low, many reported effects are directly related to the parent compounds whereas the impact of EGCG degradation and autoxidation products is not yet understood and merely studied. EGCG autoxidation products like its dimers theasinensin A and D, “P2” and oolongtheanin are yet to be characterized in the same extent as their parental polyphenol. However, to investigate the bioactivity of autoxidation products—which would minimize the discrepancy between in vitro and in vivo data—isolation and structure elucidation techniques are urgently needed. In this study, a new protocol to acquire the dimers theasinensin A and D as well as oolongtheanin is depicted, including a variety of spectroscopic and quadrupole time-of-flight high-resolution mass spectrometric (qTOF-HRMS) data to characterize and assign these isolates. Through nuclear magnetic resonance (NMR) spectroscopy, polarimetry, and especially circular dichroism (CD) spectroscopy after enzymatic hydrolysis the complementary atropisomeric stereochemistry of the isolated theasinensins is illuminated and elucidated. Lastly, a direct comparison between the isolated EGCG autoxidation products and the monomer itself is carried out regarding their antioxidant properties featuring Trolox equivalent antioxidant capacity (TEAC) values. These findings help to characterize these products regarding their cellular effects and—which is of special interest in the flavonoid group—their redox properties.

Infusions of the tea plant Camellia sinensis L. have been intensively studied regarding their constituents and especially their bioactivity. While the compound classes of catechins and xanthines are strongly associated with tea, tea consumption as a source for oxidation products based on the flavan-3-ol epigallocatechin gallate (EGCG) is rather unknown. The formation of theasinensins, which represent flavan-3-ol dimers, was already observed under cell culture conditions and also occurs during tea fermentation and brewing. However, oxidation product contents in tea infusions and their bioactive effects have not yet been sufficiently evaluated. In this study, flavan-3-ols, xanthines, and EGCG oxidation products were quantified in 26 tea samples of different fermentation degrees via validated high-performance liquid chromatography coupled with tandem mass spectrometry. Lastly, the cytotoxicity of these oxidation products on HepG2 hepatocarcinoma cells was examined. As a result, theasinensin A was found as an abundant oxidation product in fermented teas with higher cytotoxic potential in comparison to its parental EGCG.

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