A possible role for singlet oxygen in the initiation of fatty acid autoxidation |
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Authors: | H Ralph Rawls P J Van Santen |
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Affiliation: | (1) Unilever Research Laboratory, Vlaardingen, The Netherlands;(2) Present address: Gulf South Research Institute, P.O. Box 26500, 70126 New Orleans, La. |
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Abstract: | A mechanism for the initiation of autoxidation in fatty acids is proposed which involves singlet state oxygen, formed through
a photosensitization reaction, as the reactive intermediate. Both singlet oxygen generated in a radio-frequency gasdischarge,
and photosensitization by natural pigments, were shown to catalyze the oxidation of methyl linoleate. The involvement of singlet
oxygen was shown by the identification of nonconjugated hydroperoxides as products common to both photooxidation and singlet
O2 oxidation. Nonconjugated hydroperoxides could not be detected among the free radical autoxidation products. Further proof
for the above mechanism was gained by showing that compounds known to react strongly with singlet oxygen, inhibited the photooxidation.
With the exception of chlorophyll, all sensitizers could be completely inhibited. Although singlet oxygen formation can account
for approximately 80% of the observed chlorophyll photooxidation, at least one other mechanism must be involved. It is postulated
that proton abstraction by the photoactivated carbonyl group of chorophyll could account for the remaining 20% of the observed
photooxidation. The conclusion is drawn that oxygen, excited to its singlet state by a photosensitization process, plays the
important role of forming the original hydroperoxides whose presence is necessary before the normal free radical autoxidation
process can begin. |
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