Formation of a diimino-imidazole nucleoside from 2′-deoxyguanosine by singlet oxygen generated by methylene blue photooxidation Original Research Article

Singlet oxygen (1O2) is capable of inducing genotoxic, carcinogenic and mutagenic effects. It has previously been reported that the reaction of 1O2 with 2′-deoxyguanosine, which is a major target of 1O2 among the DNA constituents, leads to formation of various oxidized products including 8-oxo-7,8-dihydro-2′-deoxyguanosine and spiroiminodihydantoin, amino-imidazolone and diamino-oxazolone nucleosides. In addition to these products, we report that a novel diimino-imidazole nucleoside, 2,5-diimino-4-[(2-deoxy-β-d-erythro-pentofuranosyl)amino]-2H,5H-imidazole (dD), is formed by reaction of 2′-deoxyguanosine with 1O2 generated by irradiation with visible light in the presence of methylene blue under aerobic conditions. Its identification is based on identical chromatographic and spectroscopic data with an authentic compound, which we recently isolated and characterised from the reaction mixture of 2′-deoxyguanosine with reagent HOCl and a myeloperoxidase–H2O2–Cl− system. The yield of dD was increased by D2O and decreased by azide. dD was not generated from 8-oxo-7,8-dihydro-2′-deoxyguanosine. These results indicate that dD is generated by 1O2 directly from 2′-deoxyguanosine, but not via 8-oxo-7,8-dihydro-2′-deoxyguanosine. dD may play a role in the genotoxicity of singlet oxygen in cells.