Photophysical Properties of Porphyrin Planar Aggregates in Liposomes

This paper describes studies of some photophysical properties of non-covalent planar aggregates of hematoporphyrin and protoporphyrin. This porphyrin species has been recently discovered and can be generated in lipid bilayers such as liposomes and inner mitochondrial membranes. The relative weight of this species in different media, as compared to porphyrin monomers and stacked aggregates, has been deduced by fluorescence decay studies. In contrast with what is observed for stacked aggregates, promotion of planar suprastructures can occur both in aqueous and lipid environments. The spectroscopic properties are very similar to those of the corresponding monomers, in particular as regards the shape of the absorption and emission spectra. The fluorescence decay times are generally higher than those of the monomers, and depend on the medium in which the planar aggregates are formed. The photooxidation properties of porphyrin planar aggregates, as revealed by oxygen consumption and histidine photodegradation upon irradiation at 365 nm, were compared to those of the monomers. The extent of the photooxidation process is nearly 20-30% higher in planar aggregates than in the monomers. In contrast, it is well known that cofacial aggregates are photochemically inert and only monomeric species of porphyrin are efficient photosensitizers. The biological relevance of these findings is discussed.