LIGAND EFFECTS IN PLATINUM BINDING TO DNA - A COMPARISON OF DNA-BINDING PROPERTIES FOR CIS AND TRANS-[PTCL2(AMINE)2] (AMINE = NH3, PYRIDINE)

The DNA binding properties of cis- and trans-[PtCl2(pyridine)2] have been examined and compared with their NH3 analogs, cis- and trans-DDP. The presence of a planar ligand reduces the rates of DNA binding but does not greatly affect the overall conformation of CT DNA, as measured by circular dichroism spectroscopy. The sequence specificity of trans- [PtCl2(py)2] includes alternating purine-pyrimidine sequences. The sequence specificity is further different between the two pyridine isomers, and the steric effects of two cis-pyridine groups are demonstrated by the appearance of relatively few binding sites in the 49-bp duplex. The effects of the pyridine ligand are further manifested by a greatly enhanced DNA-DNA interstrand cross-linking efficiency for the trans isomer, with a cross-link per adduct frequency of between 0.14 and 0.23, depending on the r(b) of the sample. The unwinding of closed circular pUC19 DNA by trans-[PtCl2(pyridine)2] is also more efficient than that by either DDP isomer, with an unwinding angle calculated at phi = 17-degrees (compare cis-DDP with phi = 13-degrees and trans-DDP with phi = 9-10-degrees). In contrast, little unwinding is induced by cis-[PtCl2(pyridine)2], with phi = 4-degrees. These results in particular invert the standard cis/trans structure-activity relationships observed previously for [PtCl2(NH3)2]. The results are discussed with respect to the previously demonstrated effect of activation of the trans-platinum geometry using sterically hindered ligands.