Mechanisms of oxidation of guanine in DNA by carbonate radical anion, a decomposition product of nitrosoperoxycarbonate

Peroxynitrite is produced during inflammation and combines rapidly with carbon dioxide to yield the unstable nitrosoperoxycarbonate, which decomposes (in part) to CO₃^̇- and ^̇NO₂ radicals. The CO₃^̇- radicals oxidize guanine bases in DNA through a one-electron transfer reaction process that ultimately results in the formation of stable guanine oxidation products. Here we have explored these mechanisms, starting with a spectroscopic study of the kinetics of electron transfer from 20-22mer double-stranded oligonucleotides to CO₃ ^̇- radicals, together with the effects of base sequence on the formation of the end-products in runs of one, two, or three contiguous guanines. The distributions of these alkali-labile lesions were determined by gel electrophoresis methods. The cascade of events was initiated through the use of 308 nm XeCl excimer laser pulses to generate CO₃^̇- radicals by an established method based on the photodissociation of persulfate to sulfate radicals and the oxidation of bicarbonate. Although the Saito model (Saito et al., J. Am. Chem. Soc. 1995, 117, 6406-6407) predicts relative ease of one-electron oxidations in DNA, following the trend 5′- ⋯GGG⋯ > 5′⋯GG⋯ > 5′-G′, we found that the rate constants for CO₃^̇-mediated oxidation of guanines in these sequence contexts (k₅) showed only small variation within a narrow range [(1.5-3.0) × 10⁷M ^-1S^-1]. In contrast, the distributions of the end-products are dependent on the base sequence context and are higher at the 5′-G in 5′-⋯GG⋯ sequences and at the first two 5′-guanines in the 5′-⋯GGG⋯ sequences. These effects are attributed to a combination of initial hole distributions among the contiguous guanines and the subsequent differences in chemical reaction yields at each guanine. The lack of dependence of k₅ on sequence context indicates that the one-electron oxidation of guanine in DNA by CO₃^̇- radicals occurs by an inner-sphere mechanism.