Synthetic and naturally occurring pigments, with photosensitizing activity in vivo and in vitro, have been studied both in homogeneous solutions and bound to supramolecular systems of biological relevance. Traditional spectroscopic techniques and time resolved photoacoustics (LIOAS) have been employed in combination, to characterize the deexcitation pathways of the photosensitizer under investigation: this approach allows to draw a more complete picture of the fate of the energy absorbed by the pigment. The influence of melanins on the deexcitation processes has been studied for cationic porphyrins and 8-methoxypsoralen; in both cases the presence of melanin enhances the rate of intersystem crossing from the singlet excited to the triplet state of the photosensitizers. In contrast to 8-methoxypsoralen, trimethylpsoralen in organic solutions, has been shown to photosensitize the production of singlet oxygen with relatively high efficiency: in this case the quantum yield of formation of singlet oxygen has been determined employing LIOAS, through energy balance and kinetic considerations. Systems under current investigation include hypericin and hypericin-like pigments inserted in phospholipidic liposomes, an environment resembling the natural localization of this kind of pigments in vivo. The self quenching of fluorescence in the bilayer, is accompanied by decreased triplet formation quantum yield and subsequent lowering of singlet oxygen formation, as detected by LIOAS. Photoinduced electron transfer from hypericin to acceptors, which has been proved to occur with high efficiency in organic solutions, has also been investigated in liposomes. LIOAS measurements have also revealed a fast photoinduced molecular expansion in hypericin-like pigments extracted from Blepharisma japonicum, whose origin is still under investigation.
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