ABINITIO MOLECULAR-ORBITAL CALCULATIONS ON DNA-BASE PAIR RADICAL IONS - EFFECT OF BASE-PAIRING ON PROTON-TRANSFER ENERGIES, ELECTRON-AFFINITIES, AND IONIZATION-POTENTIALS

Ab initio molecular orbital calculations have been performed in this study to estimate proton-transfer energies in DNA complementary base pair radical ions and the effect of base pairing on ionization potentials and electron affinities. The calculated (3-21G) adiabatic proton-transfer energy profile for the neutral GC is found to have a single potential minimum, i.e., no stable proton-transfer state, whereas the GC.- shows a second potential minimum which favors proton transfer (DELTAE = -5 kcal). High level calculations (6-31+G(d)) of various uncommon protonation states of the DNA bases and DNA base radical ions were performed to estimate the energy for proton transfer in .T+A, .C+G, TA.-, and CG.-. All transfers are energetically unfavorable, but proton transfer in the AT cation radical and anion radical is only slightly endothermic. Base pairing is not found to significantly affect the ionization potential of A or T in the AT base pair. However, base pairing lowers guanine's ionization potential by 0.54 eV while raising cytosine's ionization potential by 0.58 eV. Base pairing the order of the ionization potentials and electron affinities of thymine and cytosine which makes cytosine the most affinic DNA base and the least likely to be ionized. The order of ionization potentials in base pairs calculated at the 3-21G level is C > T much greater than A > G. Further investigation was performed on stacked four base (AT/GC) configurations. A 3-21G calculation of the anion radical of the stacked system with the neutral base pair geometries shows the electron localizes on thymine. However, on relaxation of the nuclear framework of the AT/GC system, the electron is found to preferentially localize on cytosine. Calculations on the effect of ion formation on hydrogen bond strengths in base pairs suggest that the hydrogen bonds in the GC anion and cation are greatly strengthened over the neutral GC parent, whereas only the hydrogen bonds in the cation are strengthened in AT over the neutral parent.