Isolation and identification of singlet oxygen oxidation products of beta-carotene

Singlet oxygen is a highly reactive form of oxygen produced by many toxic photosensitizers. beta-Carotene quenches singlet oxygen catalytically through a very efficient physical reaction. However, concomitant chemical reactions during photosensitized oxidations consume beta-carotene. To investigate the hypothesis that chemical reactions with singlet oxygen consume beta-carotene, we characterized products of the photosensitized oxidation of beta-carotene. beta-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 degrees C. Reaction products were analyzed by reverse-phase HPLC, UV-vis spectrophotometry, and mass spectrometry. beta-Carotene oxidation products were identified as beta-ionone, beta-apo-14'-carotenal, beta-apo-10'-carotenal, beta-apo-8'-carotenal, and beta-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 beta-carotene autoxidation, since suppression of autoxidation by equimolar alpha-tocopherol did not diminish product formation. beta-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 beta-carotene may serve as markers for singlet oxygen quenching in biological systems.