Studies on Inhibition Mechanism of Autoxidation by Tannins and Flavonoids. III. : Inhibition Mechanism of Tannins Isolated from Medicinal Plants and Related Compounds on Autoxidation of Methyl Linoleate

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Abstract
Inhibitory effects of about twenty five tannins and their related compounds on autoxidation of methyl linoleate were studied by kinetic, static and in situ electron spin resonance (ESR) measurements. Almost all tannins examined showed nearly as large inhibitory effects as that of .alpha.-tocopherol. Polyphenols with low molecular weight, such as gallic acid, pyrogallol etc., showed a minor inhibitory effect. The rate of inhibition by tannins was represented as follows: Vinh = -d[O2]/dt = k[RH]1.2[AIBN]0.6[Tannin]-0.4. This kinetics was reasonably explained by a proposed radical scavenging mechanism by tannins, where the tannins acted as scavenger of chain carrying peroxy radicals. The inhibitory activities of tannins were dependent on both the type of phenolic groups and their numbers in the molecules. The scavenging activity of the phenolic group was in the order of hexahydroxydiphenoyl (HHDP) group > galloyl group. That is, the tannins with many HHDP groups in the molecule exhibited larger inhibitory effects compared with tannins with galloyl groups only. In situ ESR detection of tannin radicals under the inhibitory conditions was carried out, and transient ESR signals of tannin radicals were observed in good resolution. The signals coincided well with those obtained by air-oxidation of tannins in aqueous alkaline DMSO solution carried out in separate ESR measurements. Thus, the radical scavenging mechanism by tannins was confirmed to be operative in our inhibited peroxidation systems.