Direct phosphorescent detection of primary event of photodynamic action

Highly phosphorescent photosensitizer Pd-tetra (o-methoxy-p-sulfo)phenyl porphyrin (Pd- MSPP) was used to follow primary events of photodynamic therapy (PDT). The sensitizer forms complexes with proteins in solutions and biosystems showing remarkable hypsochromic shifts of band and an increase of the quantum yield and life time of phosphorescence at the binding to proteins. It was found the life time of phosphorescence depends mostly on the energy of the lower triplet state of the sensitizer. Therefore photo-chemical quenching of the triplets by cell components is negligible. The quenching of the sensitizer triplets is observed in the presence of free oxygen in biosystems. The degree of quenching varies in dependence on the kind and state of the system and is governed by oxygen concentration and viscosity of the media. It was concluded that the singlet oxygen mechanism of photosensitization is operative only at the primary step of the process.