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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. October 2005, Volume 4, Number 10, 711-720

Antioxidant Activity of Diphenylamine-Related Compounds as Scavengers of Carbon Radicals
Seiichiro Fujisawa, Mariko Ishihara, and Yoshinori Kadoma
Internet Electron. J. Mol. Des. 2005, 4, 711-720

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Abstract:
Diphenylamine-related compounds, which are singlet oxygen scavengers, have been utilized as antioxidants. Quantitative in vitro studies of these amine antioxidants and their inhibition of autooxidation have been performed under aerobic conditions, but the relevance of these studies to biological systems is unclear. Since biological systems have low oxygen tensions, the effectiveness of amine antioxidants may differ markedly between nonbiological and biological conditions. Kinetic studies of radical scavenging by phenylamines are needed to clarify this point. Also, the ability of these antioxidants to scavenge carbon-centered radicals remains unknown. In the present study, we evaluated the efficiency of radical scavenging by the diphenylamine-related compounds. Polymerization of methyl methacrylate initiated by AIBN in the presence or absence of amine antioxidants was studied by DSC at 70°C. Time-exotherm curves were used to determine time-conversion curves, and IP, Rpcon and Rpinh were determined. A linear relationship between the length of the IP and the concentration of antioxidants was obtained. Also, values of Rpinh/Rpcon decreased linearly as the concentration increased. The n values for DPPD, HDPA and MEAN were approximately 2, 2 and 1, respectively. Rpinh/Rpcon values declined in the order MEAN > DPPD > HDPA. In particular, HDPA markedly decreased the Rpinh/Rpcon value, i.e. the KCL. kinh/kp was calculated from IP and Rpinh/Rpcon, and the value declined in the order MEAN (11.6) > HDPA (6.8) > DPPD (4.2). HDPA, DPPD and MEAN are efficient scavengers of carbon-centered radicals derived from AIBN. In particular, HDPA strongly suppresses growing MMA radicals, probably because of formation of semiquinone radicals and, consequently, phenyl-1,4-benzoquinonemonoimine. The kinh for MEAN was about twice that for DPPD or HDPA. Previously reported kinh values for DPPD in reactions with linoleic acid peroxy radicals and poly (styryloperoxy) radicals are much higher than that determined in the present study. Although the methodologies of these studies were rather different, the discrepancy is probably due to differences in oxygen tension. Biomimetic evaluation of antioxidants should be performed under low oxygen tension.

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