A Survey of the Ionization Energies of the DNA Nitrogenous Bases via DFT-Based Calculations of their Potential Energy Surfaces

Abstract

The knowledge of the ionization energies (IE) of the four nitrogenous bases of DNA is crucial in understanding the DNA damage processes. So far, both theoretical and experimental results show that guanine has the least IE [J. Phys. Chem. Chem. Phys., 2014,15, 13833-13837]. But even though the measured IEs seemed to be consistent with the calculated ones, they are not in good agreement. The disagreements might be due to the fact that the IE depends on the location of the leaving electron and its electronic environment. Besides, the calculations of the ionization energies of a molecular ion need the consideration of all directions that are indispensable to define the geometry of the molecule. Another factor which was often obscure when investigating these IEs is the effect of the geometry transition from the relaxed neutral to the relaxed cation.In this study, we considered a relaxed neutral nitrogenous base, which is also assumed to be the geometry of the cation right after its formation. Each single atom of the neutral base (as well as each single atom of the cation) is considered as a site of the departing electron and the corresponding potential curves are calculated in all three directions (x, y, z) using the density functional theory and assuming that all other atoms are frozen during the ionization. Thus, this method of calculations of IE is an all-direction survey of the IEs of the DNA nitrogenous bases. It is different from the standard ones that lead to either vertical IE or adiabatic IE. It gives access to the potential energy surfaces and IEs that are in good agreement with some measurements. The results are reported and discussed in this presentation.