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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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