Artificial models of biological photoreceptors: effect of quenchers on the fluorescence properties of hypericin embedded in liposomes

Liposomes loaded with hypericin at different local concentrations were used as artificial models of biological light sensors. The steady state and time-resolved fluorescence of the chromophore were investigated in the presence of fluorescence quenchers which penetrate differently into and diffuse differently through lipid bilayers: iodide (I-), 9,10-anthraquinone-2,6-disulphonate (AQDS2-) and 9,10-anthraquinone-2-sulphonate (ACMS-). The results of the experiments with I- indicate that two of the three fluorescent species detected in our system, the long-lived and intermediate-lived species, are relatively close to the lipid-water interface. Anthraquinone compounds, which partially penetrate into the liposome, not only significantly quench the chromophore fluorescence emission, but also, apparently, affect the distribution of hypericin molecules in the vesicle, shifting the long-lived fluorescent molecules towards chemical-physical configurations characterized by shorter fluorescence lifetimes.