Tandem mass spectrometry studies of green tea catechins. Identification ofthree minor components in the polyphenolic extract of green tea

Liquid chromatography/electrospray ionization mass and tandem mass spectrom etry (MS/MS) techniques were used to identify two minor components and one new compound in the polyphenolic extract of green tea (Camellia sinensis), Identification and structure assignments were based on previously reported sub-structural features in the MS/MS product, precursor and neutral loss sc ans of reference samples, The structures of two minor components, related t o the known green tea components epicatechin gallate (ECC, 5) and epigalloc atechin gallate (EGCG, 6), are formed by methylation at the 3 "-O-position of the gallic acid moiety, The new compound contained a gallic acid ester g roup, but had only one phenolic group in either the A- or B-ring, relative to the structure of 5, High-resolution mass measurements supported the empi rical formula assigned to the new compounds. An important fragmentation for defining the position of methylation of the ester function involves ioniza tion of the phenolic group at the 4 "-position of the gallic acid, followed by elimination of the ester function as a neutral with concomitant formati on of the m/z 169 ion. If the 4 "-position is blocked by methylation, the f ormation of m/z 169 incorporating the gallic acid group would be blocked. T hus, the presence of an ion representing the eater group indicates a free 4 "-phenol and the absence of this ion would signify the 4 "-position as a s ite of methylation. The operation of this mechanism should be general and u seful in assigning the site of methylation of any polyphenolic ester group in natural products. A similar conclusion can be drawn concerning alkylatio n or esterification of the 4'-position of the catechins, i.e. blocking the 4'-phenol would prevent formation of the m/z 125 ion common to all of the c atechin compounds thus far examined, Therefore, mass spectral data are of c onsiderable importance in deducing the sites of alkylation or esterificatio n in the structures of the components of green tea. Copyright (C) 2000 John Wiley & Sons, Ltd.