AN ESR STUDY OF THE SILICA-INDUCED AUTOCONDENSATION OF POLYFLAVONOID TANNINS

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