Cholesterol, reaction + singlet

Figure 2.9 Chemical quenching of singlet oxygen, (a) The ene reaction — addition of singlet oxygen to an olefin with allylic hydrogen (b) the ene reaction of cholesterol (c) endoperoxide formation by singlet oxygen to imidazole residue as in histidine.

Suwa, K., Kimura, T., and Schaap, A.P. (1978) Reaction of singlet oxygen with cholesterol in liposomal membranes. Effect of membrane fluidity on the photo-oxidation of cholesterol, Photochem. Photobiol., 28 469 473. 

Until now it has usually been assumed that most singlet oxygen reactions could be attributed to reactions of A (11, 12, 13, 39, 54). As far as we are aware, the only published attempt to find experimental evidence for a difference in the reactivity of and A gave negative results (10). We now discuss some data on the sensitized photo-oxygenation of cholesterol, which we believe provides evidence for involvement of both and A. ... 

Oxidation of polyunsaturated fatty acids (PUFA) in lipoproteins may be mediated by reactive species such as radicals, transition metals, other electrophiles, and by enzymes. Once initiated, oxidation of lipids may proceed by a chain reaction, illustrated in Fig. 4 (R5). In step I, an oxidant captures an electron from a PUFA to produce a lipid radical. In step 2, after rearrangement, the conjugated diene radical reacts rapidly with singlet oxygen to produce a lipid peroxide radical, which is the kinetically preferred reaction (step 3) (B5). The chain can be terminated if the lipid radical reacts with an antioxidant to produce a stable peroxide (step 4). Otherwise, the peroxyl radical can react with another polyunsaturated fatty acid as shown in step 5 to perpetuate a chain reaction. The chain reaction requires production of lipid peroxides, giving it the name peroxidation. Fatty acids oxidized in the core are largely triglycerides and cholesterol esters, while toward the outer layer fatty acids in phospholipids are oxidized. 

Uemi, M., Ronsein, G.E., Miyamoto, S., Medeiros, M.H., Di Mascio, P. (2009) Generation of cholesterol carboxyal-dehyde by the reaction of singlet molecular oxygen [02 (IDelta(g))] as well as ozone with cholesterol. Chem. Res. Toxicol, 22(5), 815-884. 

It has been suggested but now seems rather improbable, that singlet oxygen may be involved in this phagocytosis chemiluminescence [118]. This conclusion is supported experimentally by means of the most reliable method available at present for the chemical detection of singlet oxygen its unambiguous reaction with cholesterol (p. 13). 

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