New insights in the DNA-[Cr(phen)2(dppz)]3+ binding and photocleavage properties by the complex with an intercalating ligand.

Due to the key role of DNA in cell life and pathological processes, the design of specific chemical nucleases, DNA probes and alkylating agents is an important research area for the development of new therapeutic agents and tools in Biochemistry. Hence, the interaction of small molecules with DNA has attracted in particular a great deal of attention. The aim of this study was to investigate the ability of [Cr(phen)(2)(dppz)](3+) to associate with DNA and to characterize it as photocleavage reagent for Photodynamic Therapy (PDT). Chromium(III) complex [Cr(phen)(2)(dppz)](3+), (dppz = dipyridophenazine, phen = 1,10-phenanthroline), where dppz is a planar bidentate ligand with an extended pi system, has been found to bind strongly to double strand oligonucleotides (ds-oligo) and plasmid DNA with intrinsic DNA binding constants, K(b,) of (3.9+/-0.3)x10(5) M(-1) and (1.1+/-0.1)x10(5) M(-1), respectively. The binding properties to DNA were investigated by UV-visible (UV-Vis) absorption spectroscopy and electrophoretic studies. UV-Vis absorption data provide clearly that the chromium(III) complex interacts with DNA intercalatively. Competitive binding experiments show that the enhancement in the emission intensity of ethidium bromide (EthBr) in the presence of DNA was quenched by [Cr(phen)(2)(dppz)](3+), indicating that the Cr(III) complex displaces EthBr from its binding site in plasmid DNA. Moreover, [Cr(phen)(2)(dppz)](3+), non-covalently bound to DNA, promotes the photocleavage of plasmid DNA under 457 nm irradiation. We also found that the irradiated Cr(III)-plasmid DNA association is able to impair the transforming capacity of bacteria. These results provide evidence confirming the responsible and essential role of the excited state of [Cr(phen)(2)(dppz)](3+) for damaging the DNA structure. The combination of DNA, [Cr(phen)(2)(dppz)](3+) and light, is necessary to induce damage. In addition, assays of the photosensitization of transformed bacterial suspensions suggest that Escherichia coli may be photoinactivated by irradiation in the presence of [Cr(phen)(2)(dppz)](3+). In sum, our results allow us to postulate the [Cr(phen)(2)(dppz)](3+) complex as a very attractive candidate for DNA photocleavage with potential applications in Photodynamic Therapy (PDT).