Dye sensitised photo-oxidation of the methyl and phenyl esters of oleic and linoleic acids

✍ Scribed by Jorge N. Chaon; George R. Jamieson; Roy S. Sinclair

• Publisher: Elsevier Science
• Year: 1987
• Tongue: English
• Weight: 877 KB
• Volume: 43
• Category: Article
• ISSN: 0009-3084
• DOI: 10.1016/0009-3084(87)90028-4

✦ Synopsis

The dye-sensitised photo-oxidation of phenyl oleate and the methyl and phenyl esters of llnoleic acid has been followed by UV-visibie spectroscopy, peroxide value determinations and a combination of chromatographic techniques. The methyl and phenyl esters of llnoleic acid showed identical photo-oxidation behaviour. The initially formed monohydroperoxides were hydrogenated and concentrated, and HPLC was used to separate and quantify the resulting isomeric hydroxystearates. The identity of the separated products was confirmed by specific synthesis and analysis of the 10-, 12-and 13-hydroxystearates. The behaviour of four different types of dye-sensitisers was studied (methylene blue (MB), erythrosin (Er), haematoporphyrin (Hp), and r~oflavin (RF)) and the isomeric product distributions interpreted in terms of a dual mechanism involving both singlet oxygen and a radical attack across the double bonds of the esters. RF showed the greatest radical contribution, and with MB and phenyl linoleate the radical contribution changed with the extent of peroxidation. The relative rates and quantum efficiencies of the photo-oxidation reaction using MB and Er were determined by analysis of the lamp emission profile and the absorption curves of the dyes and compared with some preliminary measurements of singlet oxygen yields (in the range 0.45-0.6) obtained from a laser flash irradiation study. The overall quantum yield of the reaction was shown to be quite small (~10 -=) but consistent with the relative rate of the decay in methanol of singlet oxygen, compared with its rate of reaction with phenyl oleate and llnoleate in the same solvent.