On the Involvement of Singlet Oxygen in the Biosynthesis of Oxygenation Products of the Furocoumarin Imperatorine

Abstract

The role of singlet oxygen (1O2) in the photo-oxidation of the furocoumarin im peratorine was investigated in vitro. Irradiation with visible light and sensitization with methylene blue yielded the imperatorine oxidation product isogosferol and the corresponding ketone as main products. The involvement of 1O2 was dem onstrated by studying the rate of oxidation under conditions that affect the lifetime of 1O2. Com pared to a range of other furocoumarins, im peratorine appeared to be moderately active as a 1O2 generator. The extent of 1O2 production correlated with the skin sensitizing activity. Upon irradiation of im peratorine itself with UVA light (360 nm) no isogosferol formation is observed, probably as a consequence of its photochemical instability. Irradiation with visible light (λ > 400 nm) of a chlorophyll chromophore containing sensitizer in the presence of imperatorine, yielded isogosferol and the corresponding ketone product. This dem onstrates that in the formation of 1O2 oxidation products of imperatorine in plants naturally occuring sensitizers e.g. chlorophyll and visible light are involved, rather than 1O2 produced by im peratorine or other furocoumarins and UVA light. The protective effect on the chlorophyll sensitized im peratorine oxidation by the 1O2- and chlorophyll triplet quencher β-carotene was demonstrated in a lipophilic solvent.