The role of singlet oxygen and hydroxyl radical in the photobleaching of meso-substituted cationic pyridyl porphyrins in the presence of folic acid

Using a photosensitizer (PS), light, and oxygen, photodynamic therapy creates cytotoxic reactive oxygen species, such as singlet oxygen ( 1 O 2 ), that kill cancer cells. Many cancer cell lines have up to 300 times more folic acid receptors than healthy cells. Therefore, folic acid is often used to improve selectivity of PSs. Photobleaching poses a disadvantage for PSs. In this paper, we have studied the photoinduced changes of meso-substituted cationic pyridyl porphyrins in the presence of folic acid using fluorescence and absorption spectroscopy. In this work, it was demonstrated that L-histidine, which is a 1 O 2 quencher, and D-mannitol, which is a hydroxyl radical quencher, can reduce photobleaching of cationic porphyrins and their interaction products with FA. This implies both singlet oxygen and hydroxyl radicals are involved in photobleaching. Additionally, our study revealed certain important features of the photobleaching of cationic porphyrins in the presence of folic acid.