DNA Cleavage via Superoxide Anion Formed in Photoinduced Electron Transfer from NADH to γ-Cyclodextrin-Bicapped C₆₀ in an Oxygen-Saturated Aqueous Solution

γ-Cyclodextrin-bicapped C 60 (C 60 /γ-CyD) shows an efficient DNA cleaving-activity in the presence of NADH (β-nicotinamide adenine dinucleotide, reduced form) in an O 2 -saturated aqueous solution under visible-light irradiation. No DNA cleavage has been observed without NADH under experimental conditions that are otherwise the same, although singlet oxygen ( 1 O 2 ) has been detected by the ESR spin-trapping of the C 60 /γ-CyD-O 2 system. This indicates that neither triplet excited state of C 60 /γ-CyD ( 3 C 60 */γ-CyD) nor 1 O 2 produced via an energy transfer from 3 C 60 */γ-CyD to O 2 is an actual reactive species, which is responsible for the DNA damage under the present experimental conditions. In the presence of NADH, photoinduced electron transfer from NADH to 3 C 60 */γ-CyD occurs to yield two equivalents of the radical anion (C 60 •- /γ-CyD), which exhibits its characteristic NIR band at 1080 nm. The dynamics of the photoinduced electron transfer have been examined by monitoring the decay of triplet−triplet absorption band at 740 nm and concomitant rise of the NIR absorption band at 1080 nm due to C 60 •- /γ-CyD with use of the laser flash photolysis for the C 60 /γ-CyD-NADH system. In the presence of O 2, C 60 •- /γ-CyD disappears via the electron transfer to O 2 and an electron transfer from NADH to 1 O 2 to produced O 2 •- . The formation of O 2 •- has been confirmed by the spin trap with DEPMPO (5-diethoxyphosphoryl-5-methyl-1-pyrroline- N -oxide), which is an efficient O 2 •- -trapping agent. The reorganization energy for the reduction of O 2 to O 2 •- is evaluated as 43.4 kcal mol -1, which agrees with the literature value determined directly for the self-exchange between 36 O 2 •- and 32 O 2 . This indicates that the electron transfer from C 60 •- /γ-CyD to O 2 proceeds via an outer-sphere pathway. The O 2 •- thus produced gives H 2 O 2, ultimately yielding hydroxyl radical, which is shown to be an actual DNA-cleaving reagent.