Photosensitizing properties of chlorins in solution and in membrane-mimicking systems

The photosensitizing properties of three chlorins, meso-tetra(3-hydroxyphenyl) chlorin (m-THPC), chlorin e6 (Ce6) and meso-tetraphenylchlorin substituted by two adjacent sulfonated groups (TPCS2a) are compared in solution and when incorporated in dioleoyl-sn-phosphatidylcholine (DOPC) liposomes. In solution, the three chlorins possess a similar efficacy to generate singlet oxygen (quantum yield similar to 0.65). The formation of conjugated dienes was used to determine their ability to induce the peroxidation of methyl linoleate as a target of singlet oxygen. In ethanol solution, the apparent quantum yield for this process is the same for the three chlorins and its value agrees with that expected from the known rates for the decay of singlet oxygen and its reaction with methyl linoleate. When incorporated in liposomes, the order of efficacy is m-THPC > TPCS2a > Ce6. This order is tentatively assigned to the relative embedment of the photosensitizer within the lipidic bilayer, TPCS2a and Ce6 being anchored by their negative chains nearer to the water-lipid interface. The photoinduced permeation of the lipidic bilayer by these chlorins was investigated by measuring the release of carboxy fluorescein entrapped into DOPC liposomes. The charged chlorins, in particular TPCS2a, are the most efficient, a result discussed in relation with the technology of photochemical internalization, PCI.