Meso-substituted cationic porphyrins of biological interest. Photophysicaland physicochemical properties in solution and bound to liposomes

A series of cationic porphyrins with 1-4 positive charges are studied: mono (N-methyl-4-pyridyl)triphenylporphine chloride [Mono], cis (N-methyl-4-pyr idyl)diphenylporphine chloride [Cis], tri(N-methyl-4-pyridyl)monophenylporp hine chloride [Tri] and tetra(N-methyl-4-pyridyl)porphine chloride [Tetra], Their photophysical properties are measured in small unilamellar vesicles and compared with those in homogeneous solution. Liposomes of L-alpha-dimyr istoyl-phosphatidylcholine (100 nn diameter) and L-alpha-dipalmitoyl-phosph atidylcholine (50 nm diameter) in phosphate-buffered saline (pH = 7.4) or D 2O 0.15 M NaCl were used. The effect of the medium microheterogeinity is di scussed. The triplet quantum yields in liposomes for all the porphyrins are about 0.7, similar to the value obtained for Tetra in aqueous media. The s inglet molecular oxygen quantum yields for the hydrophilic compounds Tri an d Tetra are greater than those of the hydrophobic ones, Mono and Cis, Also, association constants (K-L) of the dyes to liposomes and their localizatio n within the membranes are determined from fluorescence and fluorescence po larization measurements, respectively, K-L values are in the range of 10(4) -10(5) M-1 for all the compounds, indicating that hydrophobic and coulombic interactions between porphyrins and liposomes are responsible for the dye association. Fluorescence polarization experiments indicate that Mono and C is can penetrate into the lipidic phase, and that Tri and Tetra are located near the polar heads of the lipidic molecules.