Time-resolved pH-dependent fluorescence of hydrophilic porphyrins in solution and in cultivated cells

Fluorescence decay kinetics and time-gated (nanosecond) emission spectra of the hydrophilic photosensitizers meso-tetra(4-sulfonatophenyl)porphyrin (TPPS4) and uroporphyrin III (UP III) are reported. These substances are characterized by low aggregation, preferential accumulation within lysosomes and a pH-dependent composition of unprotonated and protonated species. A comparison of TPPS4 and UP III in buffer solutions and in confluently growing RR 1022 epithelial cells showed that the intracellular pH value of the environment of both photosensitizers was about 4.7. A slight decrease by 0.10-0.15 pH units occurred after light exposure which (in the case of TPPS4) was concomitant with a lethal damage of the cells. A photoproduct at 640 nm with a characteristic fluorescence lifetime of 4.3 +/- 0.8 ns was detected for UP III in buffer solutions at pH values above 5. The absence of this photoproduct in epithelial cells again indicated that UP III was located within lysosomes.