An ESR study of the silica-induced autocondensation of polyflavonoid tannins

Abstract

Radical mechanisms of silica-induced polyflavonoid tannin autocondensation and hardening through alkaline pyran ring cleavage are shown to be important, especially at different levels according to the reaction conditions and type of tannin. For prodelphinidin tannins, radical mechanisms are shown to be more important than are ionic ones even under mild alkaline reaction conditions. For procyanidin tannins, radical mechanisms appear to be of little or no importance under mild alkaline reaction conditions, but acquire more importance at more alkaline pHs, a result confirmed also with a (+)-catechin monomer as a model compound. Under the latter conditions, the radical mechanism of pyran ring cleavage inverts the relative importance of the interflavonoid bond cleavage and pyran ring opening for procyanidins. Relative intensities of phenoxide radical and radical anion ESR signals indicate that pyran ring opening and autocondensation are rapid under alkaline conditions. © 1996 John Wiley & Sons, Inc.