Characterization and DNA interaction of the Pt(II)(pq)(bdt) complex: A theoretical and experimental research

A mononuclear complex Pt(pq)(bdt) (1) (where pq = 2-(2′pyridyl)quinoxaline and bdt = benzene-1,2-dithiolate) has been prepared and characterized by NMR spectroscopy, ES mass spectroscopy and elemental analysis. Furthermore, its molecular and electronic structure has been fully elucidated by means of the density functional theory (DFT) and time-dependent density functional theory (TDDFT). The former reveals an extensive distortion of the planarity of complex 1, while the latter an intense mixed metal ligand to ligand charge transfer (MM′LLCT) transition in the visible region of the spectrum. Interactions of complex 1, the free ligands pq and bdt with double stranded calf thymus DNA before and after illumination were studied by UV-spectrophotometric (melting curves) and circular dichroism (CD) measurements, indicating that complex 1 is able to form adducts with DNA and to distort the double helix by changing the base stacking. Under our experimental conditions, it is unclear that complex 1 can photocleave DNA. Viscosity changes of calf thymus DNA (CT-DNA) in the presence of an incremental amount of complex 1 demonstrate that in very low ratios, [1]/[DNA] ⩽ 0.02, this complex binds intercalatively to the DNA, while in higher ratios a partial or a non-classical interacalation should occur due to the tetrahedral distortion of the molecule and the existence of the dithiolato-ligand.