Oxidation of guanine by carbonate radicals derived from photolysis of carbonatotetramminecobalt(III) complexes and the pH dependence of intrastrand DNA cross-links mediated by guanine radical reactions

The carbonate radical anion CO₃^•- is a decomposition product of nitrosoperoxycarbonate derived from the combination of carbon dioxide and peroxynitrite, an important biological by product of the inflammatory response. The selective oxidation of guanine in DNA by CO₃^• radicals is known to yield spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh) products, and also a novel intrastrand cross-linked product: 5′-d(CCATCG*CT*ACC), featuring a linkage between guanine C8 (G*) and thymine N3 (T*) atoms in the oligonucleotide (Crean et al., Nucleic Acids Res. 2008, 36, 742-755). Involvement of the T-N3 (pK_a of N3-H is 9.67) suggests that the formation of 5′-d(CCATCG*CT*ACC) might be pH-dependent. This hypothesis was tested by generating CO₃C^•- radicals through the photodissociation of carbonatotetramminecobalt(III) complexes by steady-state UV irradiation, which allowed for studies of product yields in the pH 5.0-10.0 range. The yield of 5′-dCCATCG*CT*ACC) at pH 10.0 is ∼45 times greater than at pH 5.0; this is consistent with the proposed mechanism, which requires N3(H) thymine proton dissociation followed by nucleophilic addition to the C8 guanine radical.