Singlet oxygen oleate

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

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. 

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.

Soybean oil containing a mixture of oleate, linoleate and linolenate triacylglycerols can produce 14 positional hydroperoxides by autoxidation and photosensitized oxidation (Chapters 2 and 3). Soybean oil and other vegetable oils, such as safflower oil and com oil, that do not contain linolenate, produce an unexpectedly high concentration of the 12-hydroperoxide isomer at peroxide values below 50. The 12-hydroperoxide isomer appears to be derived from photosensitized oxidation because its concentration is decreased in the presence of singlet oxygen quenchers such as )5-carotene and a-tocopherol (Chapter 3). 

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