COMPARATIVE KINETICS OF THE INDUCED RADICAL AUTOCONDENSATION OF POLYFLAVONOID TANNINS .2. FLAVONOID UNITS EFFECTS

Comparative kinetics of the radical autocondensation induced by SiO2 o n a series of polyflavonoid tannins, namely, pine, pecan, mimosa, queb racho, gambler, sumach, and on the catechin monomer as a model compoun d were carried out by electron spin resonance. The induced radical aut ocondensation appeared to be independently catalyzed by the known base mechanism, as well as SiO2 and Lewis acid attack directly at the hete rocycle oxygen. The reaction occurs in two definite steps: the first, the radical-anion formation, the second, the condensation proper with other flavonoid units of the reactive sites formed. The rate determini ng step depends on both the main flavonoid unit structure of each tann in and particularly on the level of colloidal state of the tannin solu tion and the number-average degree of polymerization (DPn), with the l atter two parameters being the main determining ones for the second re action step and the first two for the first reaction step. It is, howe ver, the combination of the three parameters that determines the total observable effect for each of the flavonoid tannins. The SiO2 attack. at the heterocycle ether oxygen is of such an intensity that the A-ri ngs' phenoxide radicals, which drive the reaction, surge very rapidly to such a higher proportion than the B-rings phenoxide radicals that t he B-rings also start to surge later by shifting to the left of the B- . reversible arrow A(.) equilibrium. There are also indications that i onic mechanisms might be more important for the second step of the rea ction. Different radical-anion species and the relative movements of t he relevant equilibria involved can be clearly identified from the spe ctra peaks. The initial, maximum intensity of the peaks has been shown to be the parameter defining the first step of the reaction, while th e radical decay rate has been shown to refer to the second step of the reaction. Hydrolyzable tannins have been shown not to undergo neither any silica-induced radical surge nor autocondensation as predictable from their structures. (C) 1997 John Wiley & Sons, Inc.