Comparison of sensitizers in the photooxidation of unsaturated fatty acids and their methyl esters

During the photosensitized oxidation of methyl oleate and methyl linoleate the phaeophytins a and b were more effective than the chlorophylls a and b in producing hydroperoxides. This difference can be ascribed to the superior stability of the phaeophytins. Analysis of the monohydroperoxide isomers confirmed that these plant pigments act as sensitizers that produce singlet oxygen. Riboflavin in methanol and curcumin in benzene were also identified as type II sensitizers. During a riboflavin sensitized photooxidation oleic acid emulsified in water underwent some autoxidation in addition to oxidation by singlet oxygen. Chloranil produced singlet oxygen but was also able to initiate some autoxidation of methyl oleate and linolenic acid dissolved in benzene. HPLC analysis of the monohydroperoxide isomers after hydrogenation and conversion of the hydroxystearic acids to phenacyl esters was used as a sensitive method to distinguish between autoxidation and oxidation by singlet oxygen.