Electron Paramagnetic Resonance Studies of the Effectiveness of Myoglobin and Its Derivatives as Photosensitizers in Singlet Oxygen Generation

ABSTRACT The effectiveness of myoglobin and its derivatives as photosensitizers in singlet oxygen (1O2) generation was determined by electron paramagnetic resonance (EPR) spectroscopy using a spin trapping technique. A stable nitroxide radical adduct, 2,2,6,6,‐tetramethyl‐4‐piperidone‐N‐oxyl (TAN), was formed and detected by EPR. Native myoglobin and apomyoglobin exhibited no photosensitizing function, whereas hematin appeared to be a weak sensitizer. Protoporphyrin IX ring showed strong photosensitizing activity. A 1O2 quencher, N,N N′N′‐tetramethylethylene diamine (TMEDA) and the combination of TMEDA and a free radical scavenger, butylated hydroxyanisole (BHA) drastically reduced the EPR signal of TAN when protoporphyrin IX solution was illuminated indicating that 1O2 was produced using protoporphyrin IX ring as a sensitizer.