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

Exposure of 400 μM (−)-epigallocatechin-3-O-gallate (EGCG), the main polyphenolic constituent of green tea, to equimolar concentrations of nitrite ions in 0.5 M HCl at 37 °C resulted in
the formation of a distinct pattern of products that were trapped as phenazine derivatives by
treatment with o-phenylenediamine. Repeated chromatographic fractionation eventually
allowed isolation of four main species, which were identified by 2D NMR and MS analysis 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 concentrations (e.g., 25 μM) and at pH 3, reaction with equimolar amounts 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 chemically prepared 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 extracts 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.