Isolation and Identification of Singlet Oxygen Oxidation Products of β-Carotene

Singlet oxygen is a highly reactive form of oxygen produced by many toxic photosensitizers. β-Carotene quenches singlet oxygen catalytically through a very efficient physical reaction. However, concomitant chemical reactions during photosensitized oxidations consume β-carotene. To investigate the hypothesis that chemical reactions with singlet oxygen consume β-carotene, we characterized products of the photosensitized oxidation of β-carotene. β-Carotene and the photosensitizer rose bengal were dissolved in toluene/methanol (85:15 v/v), which was bubbled with O₂ and illuminated with a quartz–halogen lamp for 30 min at 5 °C. Reaction products were analyzed by reverse-phase HPLC, UV–vis spectrophotometry, and mass spectrometry. β-Carotene oxidation products were identified as β-ionone, β-apo-14′-carotenal, β-apo-10′-carotenal, β-apo-8′-carotenal, and β-carotene 5,8-endoperoxide. Formation of these products was dependent on the presence of the photosensitizer. The products apparently were formed from the action of singlet oxygen rather than by photochemically-initiated β-carotene autoxidation, since suppression of autoxidation by equimolar α-tocopherol did not diminish product formation. β-Carotene autoxidation initiated by 2,2′-azobis(2,4-dimethylvaleronitrile), which generates peroxyl radicals, yielded a different product distribution than that from photosensitized oxidation. Specific products formed by singlet oxygen oxidation of β-carotene may serve as markers for singlet oxygen quenching in biological systems.