FLUORESCENCE AND TRIPLET-TRIPLET ABSORPTION QUENCHING OF HYPERICIN IN LIPOSOMES OF DIPALMITOYLPHOSPHATIDYLCHOLINE - APPLICATION TO THE STUDY OF THE INTERVESICULAR DIFFUSION OF HYPERICIN

Liposomes of dipalmitoylphosphatidylcholine (DPPC) have been employed as model systems for the study of the singlet oxygen formation sensitized by hypericin in membranes. In order to better understand the molecular mechanism of action of hypericin, the present communication provides some quantitative information on the self quenching of the excited singulet and triplet states of this hydrophobic dye in vesicles. From the results obtained in measurements of the quenching of the fluorescence and the triplet-triplet absorption two mechanisms were proposed : - a dynamic quenching mechanism with the bimolecular rate constants : (1)k(A) = 2.10(9) M(-1).s(-1) for the singlet excited state (3)k(A) = 6.5.10(4) M(-1).s(-1) for the triplet excited state - a static quenching mechanism where a long range coulombic interaction is believed to occur. The liposomes beeing potential drug delivery vehicles, the self quenching of the fluorescence has been used as a tool to study the intervesicular diffusion of hypericin. From our experiments it was deduced the rate constants of the exchange of this molecule between two liposomes : k(ech) = 3.3.10(-4)s(-1) at 20 degrees C and k(ech) = 9.2.10(-4) s(-1) at 50 degrees C.