タンニン及びフラボノイドによる自動酸化抑制機構(第4報) : タンニンとCu(II)イオンにより誘起されるリノール酸メチルの過酸化促進機構

Abstract

As reported previously, tannins inhibited efficiently the Cu (II)-catalyzed autoxidation of ascorbic acid and the peroxidation of methyl linoleate by the radical scavenging mechanism. However, the latter reaction was found to be accelerated remarkably in the presence of Cu (II) ion and tannin; that is, tannin was able to initiate the peroxidation of methyl linoleate in the presence of Cu (II) ion. The peroxidation of methyl linoleate induced under the coexistence of Cu (II) ion and tannin was kinetically investigated. The rate of oxidation, V, was represented as follows : V∝[methyl linoleate]1.0[Cu(II)]0.5[tannin]0.0 An acceleration mechanism by tannin was proposed to explain the above kinetics. It was found that tannin participates in both initiation and inhibition processes. The autoxidation was initiated by the superoxide radical, (O2)-·, which was produced by the reaction between reduced Cu (I) and dissolved oxygen, where the Cu (I) ion was produced through the reduction by tannin. This was proved by the fact that the removal of Cu (I) ions by the addition of cuproin in the reaction system completely diminished the peroxidation. Moreover, the participation of (O2)-· radical in the initiation reaction was confirmed by the use of ·OH and (O2)-· radical scavengers, respectively. Initiation efficiency of peroxidation by the Cu (II)-tannin couple was rather weak compared with that by the Cu (II)-ascorbic acid couple, and, therefore, the acceleration of peroxidation by tannin was observed only under conditions of remarkably high relative concentration of Cu (II) against tannin. A similar acceleration effect by tannin was found to occur by a metal ion such as Fe (III) whose redox potential is low.