SPECTROSCOPIC AND PHOTOACOUSTIC STUDIES OF HYPERICIN EMBEDDED IN LIPOSOMES AS A PHOTORECEPTOR MODEL

In photoresponsive ciliates, like Blepharisma japonicum and Stenter co eruleus, the photoreceptor pigments responsible for photomotile reacti ons are hypericin-type chromophores packed in highly osmiophilic subpe llicular granules. Liposomes loaded with hypericin can constitute a si mple model system, appropriate for understanding the primary light-ind uced molecular events triggering the sensory chain in these microorgan isms. Optical absorption, steady-state and time-resolved fluorescence and pulsed photoacoustic calorimetry have been used to measure spectra l distributions, fluorescence lifetimes, radiative and radiationless t ransition quantum yields of hypericin when assembled into egg L-alpha- phosphatidylcholine liposomes. With respect to hypericin ethanol solut ions, both absorption and fluorescence maxima are 5 nm red shifted whe n the pigment is inserted into the lipidic microenvironment, regardles s of the hypericin local concentration. Increasing by 100 times the hy pericin local concentration decreases the relative fluorescence quantu m yield by a factor of around 150 and the fraction of thermally releas ed energy, conversely, increases from 0.6 to 0.9. From the analysis of fluorescence lifetimes and their relative amplitudes it appears that a subnanosecond living component is predominant at the highest hyperic in local concentrations.