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In the present study, we show that for one-electron oxidized A-T or G-C base pairs the singly occupied molecular orbital (SOMO) is located on A or G and is lower in energy than the doubly occupied highest-occupied molecular orbital (HOMO) localized to the pyrimidines, T or C. This directs second ionizations to the pyrimidine bases resulting in triplet state diradical dications, (A(•+)-T(•+)) and (G(•+)-C(•+)). On interbase proton transfer, the SOMO and HOMO levels switch and the second oxidation is redirected to G and A. For G-C, the doubly oxidized singlet G(-H)(+)-C(H(+)) is more stable than its triplet (G(•+)-C(•+)); however, for A-T, the triplet (A(•+)-T(•+)) lies lowest in energy. The study demonstrates that double ionization of the A-T base pair results in a triplet dication diradical, which is more stable than the proton-transferred triplet or singlet species; whereas, double ionization of the G-C base pair, the proton transferred doubly oxidized singlet, G(-H)(+)-C(H(+)), is more stable and has both oxidations on guanine. In DNA, with both A-T and G-C, multiple oxidations would transfer to the guanine base alone.

Proton Transfer Induced SOMO-to-HOMO Level Switching in One-Electron Oxidized A-T and G-C Base Pairs: A Density Functional Theory Study