Studies on Inhibition Mechanism of Autoxidation by Tannins and Flavonoids. I. Inhibition Mechanism of Tannins on Cu (II)-Catalyzed Autoxidation of Ascorbic Aicd

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Abstract
Inhibitory effects of tannins and related compounds on Cu(II)-catalyzed autoxidation were studied by kinetic and ESR measurements. The order of inhibition was as follows: ellagic acid > quercetin > geraniin > tannic acid > gallic acid > (-)-epicatechin gallate > control = pyrogallol = (-)-epicatechin. Formation constant of Cu(II) complex with tannic acid JP was found to be larger than that with geranin by the method of Scatchard plot. Therefore, the effectiveness of geraniin over tannic acid JP is attributable to the radical scavenging ability rather than the Cu(II)-blocking ability. Polyphenols of smaller molecules, such as gallic acid and pyrogallol, etc., showed a minor inhibitory effect. The radical scavenging mechanism was supported by the electron spin resonance (ESR) measurements of these inhibitor radicals. The inhibitor with a large inhibitory effect exhibited a stable ESR signal, while polyphenols of smaller molecules exhibited only unstable or no ESR signals. Quercetin, a flavonoid, also showed a stable ESR signal and had a large inhibitory effect. Ellagic acid, in spite of its smaller molecular weight, showed the strongest inhibitory effect and exhibited a large and stable ESR signal.

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