Ionization Energies of the Nucleotides

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The vertical ionization energies of the four nucleotides have been computed. Geometries have been chosen to mimic orientations as they appear in B-DNA. The negative charge on the phosphate was neutralized by protonation, and also by the inclusion of counterions. Calculations have been performed with electron propagator methods (P3), Møller-Plesset second-order perturbation theory, and density functional theory to determine the nature of the orbitais associated with the highest lying ionization energies. Calculations at the MP2/6-311G(d,p)//P3/6-311G(d,p) level of theory yield vertical ionization energies for 5′-dTMP 9.05 eV, for 5′-dCMP 8.40 eV, for 5′-dAMP 8.16 eV and for 5′-dGMP 7.96 eV. In all cases the highest occupied molecular orbital resides on the base moieties.