How strong can the bend be on a DNA helix from cisplatin? DFT and MP2 quantum chemical calculations of cisplatin-bridged DNA purine bases

The B3LYP/6-31G(d) level of theory was used for the optimization of [Pt(NH3)(4)](2+), [Pt(NH3)(3)(H2O)](2+), cis-[Pt(NH3)(2)(H2O)(2)](2+), and related platinum complexes. In addition, water or ammonium ligands were replaced by DNA purine bases so that finally cis-diammineplatinum with two bases (Pt-bridged complexes) is obtained. Single point calculations using the MP2/6-31+G(d) method were performed on the obtained reference geometries and were utilized for estimating bond dissociation energies (BDEs) and stabilization energies, and for electron density analyses. After reoptimization, IR spectra were determined from HF second derivatives. It was found that replacement of both water and ammonium by the DNA base is an exothermic process (20-50 kcal/mol depending on the ligands present in the complex). Asymmetric structures with one interbase H-bond were obtained for cis-diammine-(N-7,N-7'-diadenine)-platinum and mixed cis-diammine-(N-7-adenine)-(N-7-guanine)-platinum complexes. In the case of the diguanine Pt-bridge, a symmetrical complex with two ammonium ...O-6 H-bonds was found. The higher stabilization energy of the di-guanine complex is linked to a larger component of the Coulombic interaction. However, the BDE of Pt-N-7(G) is smaller in this complex than the BIDE of Pt-N-7(G) from the mixed Pt-AG complex. Also, steric repulsion of the ligands is about 10 kcal/mol smaller for the asymmetrical Pt-AA and Pt-AG bridges. The influence of the trans effect on DBE can be clearly seen. Adenine exhibits the largest trans effect, followed by guanine, ammonium, and water. The strength of the H-bond can be determined from the IR spectra. The strongest H-bond is the interbase H-bridge between adenine and guanine in the mixed Pt-AG complex; otherwise, the H-bonds of adenine complexes are weaker than in guanine complexes. BIDE can be traced in the guanine-containing complexes. The nature of the covalent bonding is analyzed in terms of partial charges and MO. A general explanation of the lower affinity of transition metals to oxygen than nitrogen can be partially seen in the less favorable geometrical orientation of lone electron pairs of oxygen.