Epimerization of tea catechins and O-methylated derivatives of (-)-epigallocatechin-3-O-gallate:: Relationship between epimerization and chemical structure

Epimerization at C-2 of O-methylated catechin derivatives and four major tea catechins were investigated. The epimeric isomers of (-)-epicatechin (I), (-)-epicatechin-3-O-gallate (II), (-)-epigallocatechin (III), (-)-epigallocatechin-3-O-gallate (IV), and (-)-epigallocatechin-3-O-(3-O-methyl)-gallatetype (V) in green tea extracts increased time-dependently at 90 degreesC. The epimerization rates of authentic tea catechins in distilled water are much lower than those in tea infusion or in pH 6.0 buffer solution. The addition of tea infusion to the authentic catechin solution accelerated the epimerization, and the addition of ethylenediaminetetraacetic acid, disodium salt (/) decreased the epimerization in the pH 6.0 buffer solution. Therefore, the metal ions in tea infusion may affect the gallate (VI)l in pH 6.0 buffer solution after heating at 90 / for 30 min were 42.4%, 37.0%, 41.7%, and 30.4%, respectively. These values were higher than those of I of IV and VI were hardly epimerized. These results suggest that the hydroxyl moiety on the B ring of catechins plays an important role in the epimerization in the order 3',4',5'-triol type > 3',4'-diol type much greater than 3',5'-diol type.