FLUORESCENCE PROPERTIES OF THE ALLENIC CAROTENOID FUCOXANTHIN - ANALYSIS OF THE EFFECT OF KETO CARBONYL GROUP BY USING A MODEL-COMPOUND, ALL-TRANS-BETA-APO-8'-CAROTENAL

The fluorescence spectrum of a linear carotenoid, all-trans-beta-apo-8'-carotenal (beta-apo-8'-carotenal) at 20-degrees-C is reported for the first time. This pigment contains a keto carbonyl group and thus is regarded as a model compound of fucoxanthin, a main light-harvesting carotenoid in marine algae. When beta-apo-8'-carotenal was excited at 460 nm, fluorescence originating from plural states was simultaneously detected; one being located close to the 0-0 absorption band and the other, around 755 nm at its maximum. Locations of the fluorescence maxima were solvent dependent; the emission from the former varied its location in accordance with the locations of the absorption maximum, contrary to the emission from the latter whose location was only slightly affected by solvents. Relative intensities of both emissions were also solvent dependent; in a solvent with small polarizability (n-hexane), emission solely came from the former; on the other hand, in a solvent with high Polarizability (CS2), the emission from the latter was stronger. These observations suggest that the fluorescence of beta-apo-8'-carotenal originates both from the lowest (S1) and second (S2) singlet states. The solvent effect is explained by the interaction of solvent molecules with beta-apo-8'-carotenal molecules. Optical properties of beta-apo-8'-carotenal are discussed in relevance to the biological functioning of fucoxanthin in algal photosynthesis.