The influence of human serum albumin on the photogeneration of singlet oxygen by meso-tetra(4-sulfonatophenyl)porphyrin. An infrared phosphorescence study.

meso-Tetra(4-sulfonatophenyl)porphyrin (TPPS4) is a water soluble photosensitizer, which is currently clinically tested as a PDT drug. In our contribution, we present IR spectral- and time-resolved phosphorescence data reflecting the influence of human serum albumin (HSA) on singlet oxygen photogeneration by TPPS4. IR emission of TPPS4 was studied in samples containing various concentrations of HSA in phosphate buffer. The observed changes in spectral and temporal behaviour of TPPS4 and singlet oxygen phosphorescence caused by the addition of HSA are equivalent to the effect of nitrogen purging of HSA-free solutions of TPPS4. The main feature induced by addition of HSA appears to be the occurrence of a long-lived (tens of microseconds) photosensitizer phosphorescence at 900 nm besides ordinary short-lived (approximately 2 micros) one at 820 nm. It is accompanied by presence of a long-lived component of singlet oxygen emission with lifetime roughly corresponding to that of the long photosensitizer phosphorescence component. Moreover, the quantum yield of singlet oxygen phosphorescence decreases with increasing HSA concentration, while total quantum yield of TPPS4 phosphorescence rises. These facts are explained by a shielding effect of HSA on bound molecules of TPPS4 against quenching by oxygen which is analogous to oxygen removal by nitrogen purging.