Ionization-induced proton transfer in model DNA base pairs: A theoretical study of the radical ions of the 7-azaindole dimer

Proton-transfer reactions of the radical onion and cation of the 7-Azoindole (7AI) dimer were investigated by means of density functional theory (DFT). The calculated results for the dimer onion and cation were very similar. Three equilibrium structures, which correspond to the non-proton-transferred (normal), the single-proton-transferred (SPT) and the double-proton-transferred (tautomeric) forms, were found. The transition states for proton-transfer reactions were also located. The calculations showed that the first proton-transfer reaction (normal-->SPT) is exothermic and almost barrier-free; therefore, it should occur spontaneously in the period of a vibration. In contrast, the second proton-transfer reaction (SPT-->tautomer) was found to be for less-probable in terms of reaction energy and barrier. Hence, it was concluded that both (7AI)(2)(.-) and (7AI)(2)(.+) exist in the SPT form. The conclusion was further confirmed by the calculated electron vertical detachment energy (VDE) of the SPT form of (7AI)(2)(.-), 1.33 eV, which is very close to the experimental measurement of 1.35 eV. The calculated VDEs of the normal and tautomeric (7AI)(2)(.-) forms were too small compared to the experimental value. The proton transfer process was found to be multidimensional in nature involving not only proton motion but also intermolecular rocking motion. In addition, IR spectra were calculated and reported. The spectra of the three structures showed very different features and, therefore, can be considered as fingerprints for future experimental identifications. The implications of these results to biology and spectroscopy are also briefly discussed.