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

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.