Combination of Nitrogen Dioxide Radicals with 8-Oxo-7,8-dihydroguanine and Guanine Radicals in DNA:  Oxidation and Nitration End-Products

The oxidation and nitration reactions in DNA associated with the combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine (8-oxoGua) and guanine radicals were explored by kinetic laser spectroscopy and mass spectrometry methods. The oxidation/nitration processes were triggered by photoexcitation of 2-aminopurine (2AP) residues site-specifically positioned in the 2‘-deoxyribooligonucleotide 5‘-d(CC[2AP]TC[X]CTACC) sequences (X = 8-oxoGua or G), by intense 308 nm excimer laser pulses. The photoionization products, 2AP radicals, rapidly oxidize either 8-oxoGua or G residues positioned within the same oligonucleotide but separated by a TC dinucleotide step on the 3‘-side of 2AP. The two-photon ionization of the 2AP residue also generates hydrated electrons that are trapped by nitrate anions thus forming nitrogen dioxide radicals. The combination of nitrogen dioxide radicals with the 8-oxoGua and G radicals occurs with similar rate constants (∼4.3 × 10⁸ M^-1 s^-1) in both single- and double-stranded DNA. In the case of 8-oxoGua, the major end-products of this bimolecular radical−radical addition are spiroiminodihydantoin lesions, the products of 8-oxoGua oxidation. Oxygen-18 isotope labeling experiments reveal that the O-atom in the spiroiminodihydantoin lesion originates from water molecules, not from nitrogen dioxide radicals. In contrast, combination of nitrogen dioxide and guanine neutral radicals generated under the same conditions results in the formation of the nitro products, 5-guanidino-4-nitroimidazole and 8-nitroguanine adducts. The mechanistic aspects of the oxidation/nitration processes and their biological implications are discussed.