Arachidonic acid and methyl arachidonate were oxygenated with singlet oxygen using methylene blue as a photosensitizer. The products of methyl arachidonate were separated into monohydroperoxides (MHP) and further oxygenated peroxides (FOP). Accumulation of the eight regioisomers of MHP occurred first, followed by formation of FOP. After hydrogenation, FOP were identified as isomeric dihydroxy icosanoate and epoxy hydroxy icosanoate. The structure of FOP may possibly be dihydroperoxides consisting of sixteen regioisomers. The quantitative ratios of MHP regioisomers produced from arachidonic acid were determined by gas chromatography-mass spectrometry as follows: 6-, 14-, <5-, 8-, 9-, 11-, 12-, <15-isomer.

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