DNA cleavage via superoxide anion formed in photoinduced electron transfer from NADH to γ-cyclodextrin-bicapped C60 in an oxygen-saturated aqueous solution

gamma-Cyclodextrin-bicapped C-60 (C-60/gamma-CyD) shows an efficient DNA cleaving-activity in the presence of NADH (beta-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 (O-1(2)) has been detected by the ESR spin-trapping of the C-60/gamma-CyD-O-2 system. This indicates that neither triplet excited state of C60/gamma-CyD (C-3(60)*/gamma-CyD) nor O-1(2) produced via an energy transfer from C-3(60)*/gamma-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 C-3(60)*/gamma-CyD occurs to yield two equivalents of the radical anion (C-60(.-)/gamma-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(.-)/gamma-CyD with use of the laser flash photolysis for the C-60/gamma-CyD-NADH system. In the presence Of O-2, C-60(.-)/gamma-CyD disappears via the electron transfer to O-2 and an electron transfer from NADH to O-1(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 O-36(2).- and O-32(2). This indicates that the electron transfer from C-60(.-)/gamma-CyD to O-2 proceeds via an outer-sphere pathway. The O-2(.-) thus produced gives H2O2, ultimately yielding hydroxyl radical, which is shown to be an actual DNA-cleaving reagent.