DISTRIBUTION OF PORPHYRINS IN THE VARIOUS COMPARTMENTS OF UNILAMELLAR LIPOSOMES OF DIPALMITOYL‐PHOSPHATIDYLCHOLINE AS PROBED BY FLUORESCENCE SPECTROSCOPY

Abstract— The spectroscopic properties of hematoporphyrin, hematoporphyrin‐dimethyl ester, uroporphyrin and uroporphyrinoctamethyl ester, incorporated into unilamellar liposomes of dipalmitoylphos‐phatidylcholine, have been studied with the aim to assess the distribution of porphyrins within the various liposomal compartments. The results obtained indicate that the highly hydrosoluble uroporphyrin is partitioned in the endoliposomal aqueous pool while its octamethylester is homogeneously distributed in the inner lipid monolayer. Hematoporphyrin and its dimethylester show an heterogeneous distribution within the phospholipid bilayer. At T = 25°C these porphyrins are preferentially located in the outer phospholipid monolayer. Detailed studies on hematoporphyrin indicate that the distribution between the inner and outer phospholipid monolayer is a function of temperature and liposome dimensions. In particular, the increase of temperature above the critical temperature for the liquid‐gel phase transition of the liposomes causes a partial shift of the porphyrin molecules toward the inner phospholipid monolayer. Moreover, the increase of liposome dimensions leads to a greater accessibility of porphyrin to the external medium.