thearubigin and flavan-3-ol

Thearubigins are the most abundant pigments found in black tea, comprising polyphenolic oxidation products, whose composition and chemical nature have remained unresolved until recently. In the course of studying the mechanism of thearubigin formation from green tea flavan-3-ols, a model system, based on electrochemical oxidation of one of the main tea flavan-3-ol substrates, epigallocatechin gallate (EGCG), was employed. Reaction intermediates and products were subsequently analysed using mass spectrometry techniques, allowing for the identification of key intermediates and products. The results provided, for the first time, spectroscopic evidence for the structures of primary oxidation products, and led to the conclusion that oxidation is mainly taking place on the B-ring and the galloyl group, where the oxidized components undergo oxidative coupling for the formation of theaflavins, theasinensins and polyhydroxylated flavan-3-ols, all precursors for thearubigin formation. Furthermore, density functional theory (DFT) calculations were carried out to support key findings.

Topics: Catechin; Flavonoids; Mass Spectrometry; Oxidation-Reduction; Polyphenols; Tea

Flavanol depleted whole fresh green tea leaf powder, as reported in the literature, was used as matrix for a systematic study of the endogenous oxidative enzymatic conversion of selected flavanol combinations to theaflavins and thearubigins. The activity of the two crucial enzymes polyphenol oxidase (PPO) and peroxidase (POD) was controlled individually through addition of H2O2 and/or O2. Using the endogenous peroxidase only it was shown that (-)-epicatechin alone did not react with POD. According to these results it is possible that theaflavin formation occurs via reaction of a flavanol quinone with a nonquinone flavanol. It was confirmed that only a dihydroxy-B-ring flavanol with a trihydroxy-B-ring flavanol gave a theaflavin upon enzymatic oxidation. Use of horseradish peroxidase in the presence of a flavanol depleted tea leaf matrix led to significantly higher kinetics on theaflavin 3-gallate degradation compared to the absence of leaf matrix, suggesting a catalytic effect of the leaf matrix not reported before.

Topics: Biflavonoids; Camellia sinensis; Catechin; Catechol Oxidase; Fermentation; Flavonoids; Food Handling; Models, Biological; Oxidation-Reduction; Peroxidase; Plant Leaves; Polyphenols; Tea