Active Oxygen Species Generated from Photoexcited Fullerene (C60) as Potential Medicines: O2-•versus1O2

Abstract

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.