The Acid-Promoted Reaction of the Green Tea Polyphenol Epigallocatechin Gallate with Nitrite Ions.

Exposure of 400 mM (-)-epigallocatechin-3-O-gallate (EGCG), the main polyphenolic constituent of green tea, to equimolar concns. of nitrite ions in 0.5 M HCl at 37° resulted in the formation of a distinct pattern of products that were trapped as phenazine derivs. by treatment with o-phenylenediamine. Repeated chromatog. fractionation eventually allowed isolation of four main species, which were identified by 2D NMR and MS anal. as 1b, derived from EGCG quinone 1a, the isomeric oximes 2b,c, arising from nitrosation of EGCG on the pyrogallol B-ring, and the dioxime 4b in which the A-ring was doubly substituted. At lower EGCG concns. (e.g., 25 mM) and at pH 3, reaction with equimolar amts. of nitrite gave 1b as the first formed species, whereas nitrosation products 2b,c and 4b became detectable only with excess nitrite. Similar reaction of chem. prepd. 1a with acidic nitrite led to the formation of 2b,c and 4b, suggesting that this quinone may be an intermediate in the nitrosation of EGCG. Exposure of green tea exts. to acidic nitrite ions resulted in the conversion of EGCG to 1a, detected as 1b. Overall these results substantiate literature reports suggesting that the protective effects of EGCG against nitrosation involve mainly an initial redox exchange process and hint at a hitherto unrecognized property of quinone 1a as a potential scavenger of nitrosating species.