AB-INITIO MOLECULAR-ORBITAL CALCULATIONS ON DNA RADICAL IONS .3. IONIZATION-POTENTIALS AND IONIZATION SITES IN COMPONENTS OF THE DNA SUGAR-PHOSPHATE BACKBONE

Ab initio molecular orbital calculations of various fragments of the D NA backbone have been performed to aid our understanding of charge loc alization and transfer following DNA irradiation. Koopmans ionization potentials of H3PO4, H2PO4- and several hydrated forms of the H2PO4-Na + complex were determined at the ROHF/3-21G level. Counterion interact ion and progressive hydration result in a substantial increase in the phosphate ionization potential, hence gradually disfavoring the phosph ate group as a potential localization site for the hole. The binding e nergy of the waters of hydration progressively decreases as saturation is approached. In the triply hydrated system, the average water-sodiu m binding energy is 31.2 kcal/mol H2O while the average water-phosphat e oxygen hydrogen bond energy is 9.4 kcal/mol H2O. The Koopmans ioniza tion potential of 2'-deoxyribose was determined, leading to the follow ing trend in IP: base < deoxyribose (D) < phosphate (P). Calculations performed on DP, DPD, and PDP reveal that the joining of D to DP resul ts in a decrease in IP of D-P while joining of P to D-P raises its IP. Subsequent hydration of DP and DPD via a sodium counterion results in a slight decrease in Koopmans IP. Unpaired spin densities determined in D, DP, and DPD localize the hole at the sugar ring oxygen, while ho le localization at the anionic phosphate oxygen is found in PDP. In de oxycytidine 3'-monophosphate, the spin is localized to the cytosine ba se pi electron system. Koopmans ionization potentials determined for D P and PDP in Z-DNA are ca. 0.2 eV lower than their B-DNA analogues.