PHOTOPHYSICAL PROPERTIES OF PORPHYRIN PLANAR AGGREGATES IN LIPOSOMES

This paper describes studies of some photophysical properties of non-c ovalent 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. T he relative weight of this species in different media, as compared to porphyrin monomers and stacked aggregates, has been deduced by fluores cence decay studies. In contrast with what is observed for stacked agg regates, 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 t imes are generally higher than those of the monomers, and depend on th e medium in which the planar aggregates are formed. The photooxidation properties of porphyrin planar aggregates, as revealed by oxygen cons umption and histidine photodegradation upon irradiation at 365 nm, wer e compared to those of the monomers. The extent of the photooxidation process is nearly 20-30% higher in planar aggregates than in the monom ers. In contrast, it is well known that cofacial aggregates are photoc hemically inert and only monomeric species of porphyrin are efficient photosensitizers. The biological relevance of these findings is discus sed.