Valence anions of 9-methylguanine-1-methylcytosine complexes. Computational and photoelectron spectroscopy studies.

The photoelectron spectrum for the radical anion of 9-methylguanine-1-methylcytosine, MGMC(-), was recorded for the first time. To interpret the experimental results, B3LYP/6-31++G(d,p) level computational studies were carried out for the neutral and anionic complexes of MGMC/MGMC(-) stabilized by three hydrogen bonds and comprising canonical or low-energy tautomeric forms of the methylated nucleobases. The visualization of singly occupied molecular orbitals for the MGMC(-) anions indicates that they are valence-bound species since the excess electron is localized on a pi* orbital of cytosine. All but one of the studied anionic complexes are adiabatically stable at the applied B3LYP level of theory. The intensity maximum of the broad band in the photoelectron spectrum was measured at 2.1 eV. This value is very well reproduced by the calculated vertical detachment energy of the calculated global minimum geometry of the MGMC(-) anion. This structure is the Watson-Crick base pair anion with proton transferred from the N1 atom of guanine to the N3 site of cytosine. The calculated adiabatic electron affinities span a range of 0.39-0.60 eV. The consequences of electron attachment to the AT or GC base pairs incorporated within DNA are briefly discussed in the context of DNA damage by low-energy electrons.