Singlet oxygen generation by porphyrins and the kinetics of 9,10-dimethylanthracene photosensitization in liposomes

Abstract

Two new sensitizers are introduced for a potential use in photodynamic therapy: Zn(2+)- and MG(2+)-tetrabenzoporphyrin (ZnTBP and MgTBP). A comparative study of the quantum yields of singlet oxygen generation (phi delta) of hematoporphyrin derivative (HpD), Photofrin II (PF-II), Zn(2+)-phthalocyanine tetrahydroxyl [ZnPC(OH)4] and the newly introduced sensitizers ZnTBP and MgTBP in liposomes, as well as the kinetics of a photochemical reaction sensitized by them, was made by employing the fluorescent membrane probe 9,10-dimethylanthracene (DMA). We followed the photosensitization of DMA in real time by monitoring its fluorescence decrease at 457 nm and found that DMA's photosensitization is oxygen mediated. The kinetic traces of the photosensitization reactions were fitted to an analytical function, and the phi delta values were evaluated. At 10 microM sensitizer in an aqueous suspension of 2 mg/mL egg phosphatidylcholine (EPC), HpD was found to have the largest value of phi delta (0.215), followed by PF-II (0.191), ZnTBP (0.023), MgTBP (0.019) and ZnPC(OH)4 (0.005). As a test of the method, phi delta for methylene blue in ethanol was measured and found to be 0.45 as compared to 0.52 reported in the literature. Due to difference in the sensitizers' absorbances at the laser's wavelength, the reaction photosensitized by ZnTBP was the fastest with a time constant of 6.7 min, followed by MgTBP (8.7), PF-II (11.9), HpD (17.1) and ZnPC(OH)4 (31.2), all at equal sensitizers' concentrations and laser intensities. The binding constants of the sensitizers to EPC liposomes are also reported.