Interaction with calf-thymus DNA and photoinduced cleavage of pBR322 by rhodium(III) and iridium(III) complexes containing crown thioether ligands

Abstract

In this report, we present our investigation on the photoinduced cleavage of plasmid pBR322 and the binding interactions with calf-thymus (CT) DNA by a series of thioether metal complexes. The complexes of interest are rhodium and iridium complexes containing thiacrown ligands 1,4,7-trithiacyclononane (9S3) and 1-oxa-4,7-dithiacyclononane (9S2O), and the complexes are abbreviated as [Rh(9S3)Cl3], [Rh(9S2O)Cl3], and [Ir(9S3)Cl3]. In the nicking assay, pBR322 was treated with each complex and irradiated at 254 and 350 nm, respectively, in concentration- and time-dependent studies. The nicking assay revealed that, under exposure to 254-nm radiation, [Rh(9S3)Cl3] and [Rh(9S2O)Cl3] cleaved pBR322 efficiently forming a nicked form, while [Ir(9S3)Cl3] was least efficient. For the 350-nm irradiation, a similar trend was observed, with [Rh(9S3)Cl3] being the most efficient one, however with a lower efficiency than at 254 nm. An ethidium bromide displacement assay was also carried out to evaluate the binding interaction of each compound with CT-DNA by titrating the pre-equilibrated complex of CT-DNA and EB with the investigated complexes. The efficient concentration to achieve a 50% loss in fluorescent emission was found to be 24 μM for [Rh(9S3)Cl3] and 35 μM for [Rh(9S2O)Cl3], while [Ir(9S3)Cl3] was an ineffective DNA binder.