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.