Binding of Pt(NH₃)₃²⁺ to Nucleic Acid Bases

The ab initio SCF energies for Pt(NH₃)₃²⁺ binding to guanine, adenine, cytosine, and thymine are calculated. A relativistic effective potential is used to represent the core electrons of Pt, and compact effective potentials are also used to replace the core electrons in carbon, nitrogen, and oxygen to simplify the calculation of these large molecules. In order to analyze the bonding, SCF calculations were also done for H₂O, NH₃, imidazole, pyrimidine, 2- and 4-pyrimidone, and several deprotonated anions. The binding is calculated to have a large electrostatic component, but there is a significant contribution from polarization of the base. The valence-all-electron binding energies can be reproduced by an SCF energy for a system where the Pt(NH₃)₃²⁺ complex is replaced by an effective point charge, Z_eff. The binding order for all the sites on the nucleic acid bases was calculated by this means after checking the accuracy with all-valence-electron calculations on binding to the N7 and 06 sites on guanine. Force constants were calculated for one-dimensional energy curves between Pt(NH₃)₃²⁺ and selected bases. Valence-all-electron SCF calculations were used to show that chelate bonding of the N7 and 06 sites of guanine to either Pt(NH₃)₂²⁺ or Pt(NH₃)₃²⁺ is unlikely to compete with intrastrand binding of the N7 sites on neighboring guanines.