Active Oxygen Species Generated from Photoexcited Fullerene (C60) as Potential Medicines: O2-• versus 1O2

To characterize fullerenes (C60 and C70) as photosensitizers in biological systems, the generation of active oxygen species, through energy transfer (singlet oxygen 1O2) and electron transfer (reduced active oxygen radicals such as superoxide anion radical O2-• and hydroxyl radical •OH), was studied by a combination of methods, including biochemical (DNA-cleavage assay in the presence of various scavengers of active oxygen species), physicochemical (EPR radical trapping and near-infrared spectrometry), and chemical methods (nitro blue tetrazolium (NBT) method). Whereas 1O2 was generated effectively by photoexcited C60 in nonpolar solvents such as benzene and benzonitrile, we found that O2-• and •OH were produced instead of 1O2 in polar solvents such as water, especially in the presence of a physiological concentration of reductants including NADH. The above results, together with those of a DNA cleavage assay in the presence of various scavengers of specific active oxygen species, indicate that the active oxygen species primarily responsible for photoinduced DNA cleavage by C60 under physiological conditions are reduced species such as O2-• and •OH.