Fluorescence and time resolved photoacoustics of hypericin inserted in liposomes: dependence on pigment concentration and bilayer phase

Solvatochromic effects on the energy of visible absorbance and fluorescence transitions in hypericin have been studied in organic solutions and in phosphatidylcholine liposomes, suggesting that the molecule in this last case is preferentially located in the polar aprotic zone close to the lipid-water interface. Nonradiative and radiative decays of the pigment have also been studied, combining photocalorimetric and fluorescence techniques, as a function of the pigment concentration in the liposomal phase and of the gel-to-sol thermotropic transition of the phospholipidic bilayer. The results show that hypericin can sense the phase transition by exhibiting a stepwise increasing of the fluorescence quantum yield; concomitantly photoacoustic data indicate that in the gel phase, above a certain concentration of hypericin, clustering of the pigment promotes the formation of nonradiative long-lived species, whereas when the bilayer is in the sol phase ultrafast nonradiative pathways become the main deactivation channels upon increasing concentration. As a consequence, as shown by photoacoustic results, the photosensitized formation of singlet oxygen is prevented when the local concentration of hypericin in the lipidic phase is greater than about 30 mM.