Calculation of the ionization potentials of the DNA bases in aqueous medium

Projected MP2/6-31++G(d,p) ionization potential calculations have recently been performed on the DNA bases in the gas phase and in aqueous solution (J. Phys. Chem. A, 2004, 6373). The goal of the present work is to explore methods to perform these same calculations with density functional theory. New results for the vertical ionization potentials for the DNA bases at the B3LYP/6-31++G(d,p) level are close to the PMP2 results and also close to the experimental results. Vertical ionization potentials for the DNA bases in aqueous medium at the PCM/B3LYP/6-31++G(d,p) level are thymine 5.41, (5.36), cytosine 5.32, (5.24), adenine 5.05, (5.08), and guanine 4.71, (4.77) eV. The numbers in parentheses are the previous PCM/PMP2 results. Again, the DFT results are comparable to the PMP2 results. Results are also presented for the vertical ionization potential of 5-MeC in the gas phase and also in aqueous solution. This results in a cytosine base that has an IP more like a purine and may therefore have to be considered to be in competition with guanine as a hole trap.