4.7 Article

Oligomerization Mechanisms of Tea Catechins Involved in the Production of Black Tea Thearubigins

Journal

JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY
Volume 71, Issue 41, Pages 15319-15330

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.jafc.3c05007

Keywords

black tea; thearubigins; catechin; dehydrotheasinensin; quinone; oxidation

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Thearubigins (TRs) are chemically ill-defined black tea pigments composed of numerous catechin oxidation products. The oligomerization mechanisms of TRs are poorly understood, but it was found that the A-ring methine carbons of catechins participate in the oligomerization process. Inter-molecular oxidative couplings between the B-rings and A-rings of pyrogallol-type catechins are involved in the oligomerization, which is important in the production of TRs.
Thearubigins (TRs) are chemically ill-defined black tea pigments composed of numerous catechin oxidation products. TRs contain oligomeric components; however, the oligomerization mechanisms are poorly understood. The comparison of the C-13 nuclear magnetic resonance (NMR) spectra of TRs with different molecular sizes suggested the participation of A-ring methine carbons in the oligomerization. Crushing fresh tea leaves with phloroglucinol, a mimic of the catechin A-rings, yielded the phloroglucinol adducts of the B-ring quinones of pyrogallol-type catechins and dehydrotheasinensins, indicating that intermolecular oxidative couplings between pyrogallol-type B-rings and A-rings are involved in the oligomerization. This is supported by the comparison of the C-13 NMR spectra of the oligomers generated from the dehydrotheasinensins and epicatechin. Furthermore, the presence of the quinones or related structures in the catechin oligomers is shown by condensation with 1,2-phenylenediamine. The pyrogallol-type catechins account for approximately 70% of tea catechins; therefore, the B-A ring couplings of the pyrogallol-type catechins are important in the catechin oligomerization involved in TR production.

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