Proton Transfer Induced SOMO-to-HOMO Level Switching in One-Electron Oxidized A-T and G-C Base Pairs: A Density Functional Theory Study
American Chemical Society (ACS) -- The Journal of Physical Chemistry B
DOI 10.1021/jp5028004
Abstract(s)

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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.