Singlet oxygen linoleate

Molecular oxygen, as distinct from reactions involving radicals or singlet oxygen, is directly inserted into free fatty acids by lipoxygenase (LOX) enzymes. Lipoxygenases, both regio- and stereospecific enzymes, react on the 1,4-pentadienyl moieties such as those of linoleic and a-linolenic acids. 

Terao, J., Yamauchi, R., Murakauri, H., Matsushita, S. 1980. Inhibitory effects of tocopherols and (3-carotene on singlet oxygen-initiated photooxidation of methyl linoleate and soybean oil. J. Food Proc. Preserv. 4, 79-93. 

Koga S, Nakano M, Uehara K (1991) Mechanism for the generation of superoxide anion and singlet oxygen during heme compound-catalyzed linoleic acid hydroperoxide decomposition. J Biolumin Chemilumin 289 223-229... 

The singlet oxygen reacts directly with the double bond by addition, and shifts the double bond one carbon away. The singlet oxygen attack on linoleate produces four hydroperoxides as shown in Figure 2-20. Photooxidation has no induction period, but the reaction can be quenched by carotenoids... 

Figure 2-20 Photooxidation. Singlet-oxygen attack on oleate produces two hydroperoxides linoleate yields four hydroperoxides...

Ogoshi and coworkers reported regio- and stereospecific oxidation of linoleic acid by singlet oxygen, which was generated by CDx-sandwiched porphyrin sensitization [125] (Scheme 21). 

Photooxidation is much faster than autoxidation the reaction of linoleate with singlet oxygen is approximately 1500 times faster than that with triplet oxygen (47). There is less difference in the rate of photooxidation between monoenes and polyenes than is seen in autoxidation. The relative rates for oleate, linoleate, linolenate, and arachidonate are 1.0, 1.7, 2.6, and 3.1 (48, 49). This contrasts with the 40-fold increase in rate of autoxidation between oleate and linoleate. 

Ketones Aliphatic ketones formed by autoxidation of lipids also contribute to the flavor of oils and food products. For example, Guth and Grosch (13) identified l-octen-3-one as one of the odor-active compounds in reverted soybean oil. This compound was described as metallic and mushroom-like. The reaction pathway for the formation of l-octen-3-one from the linoleate-10-hydroperoxide via the p-scission route is illustrated in Figure 2. 10-Hydroperoxide of linoleate is not the usual hydroperoxide formed by autoxidation of linoleate however, it is one of the major hydroperoxides formed by the photosensitized oxidation (singlet oxygen reaction) of linoleate (14). 

The /S-CD sandwiched porphyrin 81 was able to provide a regioselective and stereoselective oxidation by singlet oxygen. Hydroperoxidation of linoleic acid was obtained selectively at the double bond with 82%... 

TABLE 28 Hydroperoxidation of Linoleic Acid by Singlet Oxygen Generated by Photosensitization ... 

Figure 3.6 Oxidative breakdown of hydroperoxy cyclic peroxides of linoleate leading to pentane and hexanal. Reproduced from Min, D. B., Lee, S. H. and Lee, E. C., Singlet oxygen oxidation of vegetable oils, in Flavor Chemistry of Lipid Foods (eds D. B. Min and J. H.

Chlorophyll, methylene blue, protoporphyrins and erythrosine react with unsaturated fatty esters by type II photosensitized oxidation in which singlet oxygen produces hydroperoxides by an entirely different mechanism from free radical autoxidation. In marked contrast to autoxidation, the distribution of hydroperoxides produced from oleate, linoleate and linolenate in the presence of singlet oxygen is very different, and is discussed in Section B below. 

Figure 3.1. Oxidation of oleate and linoleate by singlet oxygen.

Linoleate treated with singlet oxygen produces a mixture of four hydroperoxide isomers ... 

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