Nanosecond-Scale Isomerization of the 4'-Carboxonium Cation Oxidatively Produced in Pyrimidine Units of DNA

The long-standing puzzle of the chemistry producing the Stubbe-Kozarich abasic site, which is the minor product in the oxidation of 2'-deoxycytidine units of DNA by Fe(II)-bleomycin, has been computationally solved in this study. Scrupulous DFT-based calculations that included extensive screening of the potential energy surface of model-solvated nucleotides and the elucidation of the chemical structure of the located nucleotide cations via natural bond orbital analysis demonstrated that the 2'-deoxycytidine unit bearing the 2'-deoxyribose ring 2e-oxidized at the 4'-position undergoes carboxonium ion- iminium ion (C═O⁺-C → C═N⁺) isomerization. This 1,2-elimination of the carbonyl group 4'-C═O from the carboxonium cation fragment is associated with minimal spatial reorganization of the molecule and appears to be an ultrafast reaction. The calculated barrier Δ G₀^# of 2.7 kcal mol^-1 for this isomerization is lower than that reported for the addition of water to oxocarbenium ions. Thus, this unusual nucleotide transformation is the key chemical reaction that yields the Stubbe-Kozarich product. Such a product cannot be formed for purine nucleotide units in DNA. The isomerization of 4'-dehydro-2'-deoxyribose-4'-carboxonium cations formed in these DNA units is slower because it destroys the purine aromaticity, and the cations are intercepted by water molecules before they isomerize.