COMPARATIVE KINETICS OF INDUCED RADICAL AUTOCONDENSATION OF POLYFLAVONOID TANNINS .1. MODIFIED AND NONMODIFIED TANNINS

摘要

Comparative kinetics of the radical autocondensation induced by SiO2 of a polyflavonoid tannin, namely quebracho tannin a mostly profisetinidin/prorobinetinidin tannin, in its natural extract state, sulfited, carbohydrate free, and in its adhesive intermediate form were carried out by electron spin resonance (ESR). The results obtained not only confirmed the existence of strong radical mechanisms of tannin autocondensation in the presence of dissolved SiO2, but also pointed out new effects of interest in such a reaction. The SiO2 induced autocondensation proceeds at a faster rate, and radical surge and concentration decay appear to be more marked and more rapid the more colloidal the tannin solution is. This appears to indicate that a colloid-induced intramicellar radical mechanism of the reaction is at work. As a consequence the intensity and rate of the radicals surge and decay decreases passing from more colloidal to less colloidal tannin solutions. Thus, the decay rate decreases passing from natural tannin to adhesive intermediate, to carbohydrate-free tannin, to almost disappear for sulfited tannin. The colloidal state of the solution appears to depend mostly, but not only, on the presence of the polymeric carbohydrates in the extract. (C) 1996 John Wiley & Sons, Inc. [References: 14]