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

Abstract

γ-Cyclodextrin-bicapped C60 (C60/γ-CyD) shows an efficient DNA cleaving-activity in the presence of NADH (β-nicotinamide adenine dinucleotide, reduced form) in an O2-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 (1O2) has been detected by the ESR spin-trapping of the C60/γ-CyD-O2 system. This indicates that neither triplet excited state of C60/γ-CyD (3C60*/γ-CyD) nor 1O2 produced via an energy transfer from 3C60*/γ-CyD to O2 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 3C60*/γ-CyD occurs to yield two equivalents of the radical anion (C60•-/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 C60•-/γ-CyD with use of the laser flash photolysis for the C60/γ-CyD-NADH system. In the presence of O2, C60•-/γ-CyD disappears via the electron transfer to O2 and an electron transfer from NADH to 1O2 to produced O2•-. The formation of O2•- has been confirmed by the spin trap with DEPMPO (5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide), which is an efficient O2•--trapping agent. The reorganization energy for the reduction of O2 to O2•- is evaluated as 43.4 kcal mol-1, which agrees with the literature value determined directly for the self-exchange between 36O2•- and 32O2. This indicates that the electron transfer from C60•-/γ-CyD to O2 proceeds via an outer-sphere pathway. The O2•- thus produced gives H2O2, ultimately yielding hydroxyl radical, which is shown to be an actual DNA-cleaving reagent.