107(P36) カテキン類のDPPHラジカルに対する抗酸化機構(ポスター発表の部)

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Stoichiometric factors of catechins on DPPH radicals. We have reported the radical scavenging activity of catechins. The present study is an attempt to extend the understanding of the scavenging activity of the catechins. Stoichiometric factors of catechins and their related compounds were first examined with an excess of DPPH radicals at the initial stage of reaction. The estimated Stoichiometric factor of 2 was observed with 4-MC, +C, and (-)-epicatechin, while (-)-epigallocatechin and ethyl gallate were 5, (-)-epicatechin gallate with was 7 and (-)-epigallocatechin gallate was 10. The 2 adjacent hydroxyl groups of 4-MC and the catechol structure in the Bring of +C and epicatechin may determine their stoichiometric factors. However, after the rapid reactions of 4-MC and +C with DPPH radicals in the early stage, slow reactions were observed to proceed. The catechins having an ortho-trihydroxyl group showed the factors greater than the number of hydroxyl groups they possess, and hence the reactions with DPPH radicals were considered to be more complex. Further, we tried to examine the mechanisms of radical scavenging reactions of 4-MC and +C on the DPPH radicals for the purpose of discovering a clue to the elucidation of the complex radical scavenging reaction of catechins. Radical scavenging mechanism of 4-MC Isolation and identification of reaction products formed from 4-MC after the reaction of the compound with an equal molar of the DPPH radicals in EtOH were performed. Two reaction products were obtained; One of which was relatively stable while the other was instable. FAB-MS and NMR analyses showed that the stable product is 4-methyl-5-ethoxy-ortho-benzoquinone(3). The instable product formed as a sole reaction product when the reaction was carried out in CH_3CN. 4-MC and compound 3 were produced respectively from the instable compound by reduction with ascorbic acid and by the addition of EtOH. NMR experiments using the reaction mixture in combination with the above result clearly indicated that the instable product is 4-methyl-ortho-benzoquinone (4). The time course analysis of the formation of compounds 3 and 4 was performed by HPLC. Formation of the reaction intermediate (4) began to take place along with the rapid decrease of 4-MC immediately after initiating the reaction. Subsequent reaction in which the formation of 3 and the regeneration of 4-MC took place was observed along with the decrease of 4. On the basis of the result, radical scavenging mechanism of 4-MC on the DPPH radicals in EtOH were proposed as shown in Fig. 5. Radical scavenging mechanism of +C +C was reacted with an equal molar of DPPH radicals in EtOH under dark conditions. The reaction product 5 could not be isolated because of its instability. The structure of 5 was estimated from the structure of 6 which was produced from 5 by reduction of ascorbic acid. The ortho-bezoquinone form of +C (7) was identified as the first product of the scavenging reaction by the same experiments as used in the identification of 4. On the other hand, when the reaction was conducted under irradiation of fluorescent or day light, compound 8 was formed through 7, while 5 was not formed. +C and 4-MC were found to have firstly a rapid scavenging activity followed by a slow scavenging ability. On the basis of the results obtained in this study the rapid scavenging activity was thought to be maintained until the catechol moiety in +C and 4-MC was converted to ortho-benzoquinone form. This reaction consumed 2 molecules of DPPH radicals which agreed with the stoichiometric factors of +C and 4-MC. Although the DPPH radical may be scavenged rapidly by 4-MC and +C which were regenerated in the process of formation of 3 and 5 from compounds 4 and 6 respectively, the apparent slow scavenging activity observed may be due to the rate-limiting reactions of 4 and 6 with EtOH.
1998-08-31

107(P36) カテキン類のDPPHラジカルに対する抗酸化機構(ポスター発表の部)

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