Ab initio calculations of structure and ionization energies of the five guanine tautomers

Ionization of guanine might play a key role in radiative and oxidative damage of DNA following the ionization of latter. Recent VUV photoionization experiment reports accurate ionization energies (IEs), however, the interpretation of the spectrum is challenging due to variety of tautomers present in the experimental beam. In the present paper we theoretically investigated ionization processes of the five most stable guanine tautomers. The thermodynamic distribution at the temperature of the experiment has been evaluated, indicating the relative populations in the experimental beam. The equilibrium geometries of the neutral and cation guanine tautomers have been calculated and compared. Structural changes upon ionization turned out to be similar for all the tautomers. Calculated vertical and adiabatic IEs show close agreement with measured experimental data. Molecular orbitals (MOs) of the tautomers have also been investigated. MO analysis shows the order and character of the lowest ionized states, demonstrating the importance of correlation in the right description of the electronic structure.