DNA cleavage by UVA irradiation of NADH with dioxygen via radical chain processes

Efficient DNA cleaving-activity is observed by UVA irradiation of an O ₂-saturated aqueous solution of NADH (β-nicotinamide adenine dinucleotide, reduced form). No DNA cleavage has been observed without NADH under otherwise the same experimental conditions. In the presence of NADH, energy transfer from the triplet excited state of NADH (³NADH*) to O₂ occurs to produce singlet oxygen (¹O₂) that is detected by the phosphorescence emission at 1270 nm. No quenching of ¹O₂ by NADH was observed as indicated by no change in the intensity of phosphorescence emission of ¹O₂ at 1270 nm in the presence of various concentrations of NADH. In addition to the energy transfer, photoinduced electron transfer from ³NADH* to O ₂ occurs to produce NADH⁺ and O₂^-, both of which was observed by ESR. The quantum yield of the photochemical oxidation of NADH with O₂ increases linearly with increasing concentration of NADH but decreases with increasing the light intensity absorbed by NADH. Such unusual dependence of the quantum yield on concentration of NADH and the light intensity absorbed by NADH indicates that the photochemical oxidation of NADH with O₂ proceeds via radical chain processes. The O₂^- produced in the photoinduced electron transfer is in the protonation equilibrium with HO₂^-, which acts as a chain carrier for the radical chain oxidation of NADH with O₂ to produce NAD⁺ and H₂O₂, leading to the DNA cleavage.