ISOLATION AND IDENTIFICATION OF SINGLET OXYGEN OXIDATION-PRODUCTS OF BETA-CAROTENE

Singlet oxygen is a highly reactive form of oxygen produced by many to xic photosensitizers. beta-Carotene quenches singlet oxygen catalytica lly through a very efficient physical reaction. However, concomitant c hemical reactions during photosensitized oxidations consume beta-carot ene. To investigate the hypothesis that chemical reactions with single t oxygen consume beta-carotene, we characterized products of the photo sensitized oxidation of beta-carotene. beta-Carotene and the photosens itizer rose bengal were dissolved in toluene/methanol (85:15 v/v), whi ch was bubbled with 02 and illuminated with a quartz-halogen lamp for 30 min at 5-degrees-C. Reaction products were analyzed by reverse-phas e HPLC, UV-vis spectrophotometry, and mass spectrometry. Beta-Carotene oxidation products were identified as beta-ionone, beta-apo-14'-carot enal, beta-apo-10'-carotenal, beta-apo-8'-carotenal, and beta-carotene 5,8-endoperoxide. Formation of these products was dependent on the pr esence of the photosensitizer. The products apparently were formed fro m the action of singlet oxygen rather than by photochemically-initiate d beta-carotene autoxidation, since suppression of autoxidation by equ imolar a-tocopherol did not diminish product formation. Beta-Carotene autoxidation initiated by 2,2'-azobis(2,4-dimethylvaleronitrile), whic h 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 si nglet oxygen quenching in biological systems.