AB-INITIO MOLECULAR-ORBITAL CALCULATIONS OF RADICALS FORMED BY H-CENTER-DOT AND CENTER-DOT-OH ADDITION TO THE DNA BASES - ELECTRON-AFFINITIES AND IONIZATION-POTENTIALS

Ab initio molecular orbital calculations of the ionization potentials (IP's) and electron affinities (EA's) of DNA base radicals formed by addition of H-. or OH. to DNA bases are presented in this work. IP's and EA's are obtained by calculating the energies of the LUMO and HOMO (Koopmans' values) of the cationic and anionic states, respectively. Scaling of the theoretical values to experimentally known ionization potentials and electron affinities of other small radical compounds leads to predicted trends in IP's and EA's of the DNA base adducts. These trends are shown to be in good agreement with experimental redox trends and aid our understanding of possible electron transfer processes. The present results indicate the OH. and H-. adducts of the pyrimidines at C6 are most oxidizing, while the H-. adduct of cytosine at N3 is most reducing. The calculated trend in electron affinities in conjunction with experimental observations results in the prediction that only a fraction of the base adducts will be reduced via electron transfer processes from thiols. All five sugar radicals have nearly equal electron affinities; however, their ionization potentials substantially differ. Delocalization effects which result in less electron repulsion in the anionic state are shown to account for differences in EA and IP scales. Conformational distortion of the purine bases upon adduct formation at the C4 and C5 sites is shown to be significant.