SPECTROSCOPIC STUDIES OF A PHOTORECEPTOR MODEL SYSTEM

In photoresponsive ciliates, like Blepharisma japonicum and Stentor coerueus, the photoreceptor pigments responsible for photomotile reactions are hypesicin-type chromophores packed in highly osmiophylic subpeliicular granules. The photophysical properties and the photochemical reactivity of these clustered pigments are not known and can be quite peculiar. Liposomes loaded with hypericin can constitute a simple model system, appropriate for understanding the primary light-induced molecular events tniggerng the sensory chain in these microorganisms. Optical absorption, steady-state and time-resolved fluorescence, pulsed photoacustic calorimetry have been used to measure spectral distributions, fluorescence lifetimes, radiative and radiationless transitions quantum yields of hypericin, when assembled into liposomes made up of different phospholipids. With respect to hypericin ethanol solutions, both absorption and fluorescence maxima are 5-8 nm red shifted when the pigment is inserted into the lipidic microenvironment, regardless of the number N of hypericin molecules per liposome, whereas the reative fluorescence quantum yields and lifetimes decrease increasing N over 20. Measurements of radiationless transitions quantum yields in the different experimental conditions allow to calculate the energy balance for each model system.