A novel photochemical approach is described for synthesizing site-specific 8-nitro-2′deoxyguanosine (8-nitro-dG) adducts DNA. The method is based on the bimolecular reaction of a neutral, deprotonated guanine radical [G(-H)·] in DNA and nitrogen dioxide (·NO2) radicals. This approach is illustrated using the single-stranded oligodeoxyribonucleotide 5′-d(CCATCGCTACC) dissolved in an aqueous solution of nitrite and bicarbonate anions at pH 7.5. The photochemical synthesis was triggered by the selective photodissociation of persulfate anions to yield SO4·- radical anions by either 308 nm XeCl excimer laser pulses or by a continuous irradiation with 290-340 nm light from a 1000 W Xe lamp. The sulfate radicals formed generate the CO3·- and ·NO2 radicals by one-electron oxidation of the bicarbonate and nitrite anions. In turn, the CO3·- radicals site-selectively generate G(-H)· radicals in DNA that combine with ·NO2 to form 8-nitro-dG lesions in the oligonucleotide. The nitrated oligonucleotides were purified by reversed-phase HPLC techniques and are stable at 4 °C for at least 4 days, but depurinate at ambient temperatures of 23 °C at pH 7 with a half-life of ∼20 h. The nature of the reaction and decomposition products were studied by a combination of ESI and MALDITOF mass spectrometric techniques.
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