Photooxidation of lysozyme or serum albumin bound to meso-tetra-arylporphyrins

The photoprocesses of meso-tetra(4-sulfonatophenyl)porphyrin and meso-tetra(4-hydroxyphenyl)porphyrin were studied in the presence of lysozyme. Light-induced damage of the dyes and proteins were measured in aqueous solution at pH 4 and 8 under the exclusion of oxygen. The results were compared with those of bovine serum albumin, where virtually no photooxidation takes place. Non-covalent binding to the proteins is strongest for the diprotonated porphyrins. The porphyrin triplet state is longer lived due to shielding by binding to a protein but such binding essentially inhibits further reactions. Electron transfer to the excited porphyrin singlet state is indicated by a lower triplet yield for loading to lysozyme, in contrast to serum albumin. The quantum yields of protein damage are comparable and rather low, they depend on the protonation state and are largest (1.4 × 10−3) for the meso-tetra(4-sulfonatophenyl)porphyrin/lysozyme system. The effects of protein concentration and protonation state on the quantum yields of protein and dye damage were studied in detail.