Ru(II) carbazole thiosemicarbazone complexes with four membered chelate ring: Synthesis, molecular structures and evaluation of biological activities

Abstract

Formation of ruthenium(II) complexes of the type [RuH(CO)(PPh3)2(L)] (where L=N-Substituted 9-ethyl carbazole thiosemicarbazone ligands) has been described from the reactions of [RuHCl(CO)(PPh3)3] and substituted carbazole thiosemicarbazones in 1:1 equivalent respectively. The composition of the complexes was established by elemental analysis, IR, NMR (1H ,13C and 31P) and UV–visible spectral methods. The solid state molecular structure of the ligands (L1−L3) and one of the complexes have been analysed by single-crystal X-ray studies, and found that the ruthenium(II) complexes possess a pseudo-octahedral geometry. The thiosemicarbazone ligand is coordinated to ruthenium as a monoanionic bidendate N,S-donor forming a four-membered chelate ring with a bite angle of 64.47(5)°. The stability of the complexes in aqueous medium was confirmed by UV–visible and ESI-Mass spectral studies. The DNA binding interactions of the complexes with Calf thymus DNA have been investigated by absorption, emission, elctrochemical, circular dichromism and viscosity measurements revealed that the complexes could interact with DNA via intercalation. Further, their protein binding ability was monitored by the quenching of tryptophan emission using bovine serum albumin (BSA) as a model protein. The alterations in the secondary structure of BSA by the complexes were confirmed with synchronous and three-dimensional fluorescence spectral studies. The ability of complexes to cleave BSA varies from 3>2>1 in the presence of activator like H2O2, as revealed from SDS-PAGE is consistent with their strong hydrophobic interaction with the protein. Free-radical scavenging ability of all the complexes were also carried out against a panel of radicals such as DPPH, NO, OH, O2– and reducing power assay under in vitro experimental conditions. The potential of complexes to act as anticancer agents is thoroughly examined on human cervical cancer cell line HeLa, Osteosarcoma cell line MG-63 and a normal mouse embryonic fibroblasts cell line NIH-3T3 and screening shows the HeLa cell line exhibits maximum cytotoxicity. The correlation of cytotoxicity of these complexes to their hydrophobicity shows that an appropriate value of the hydrophobicity is essential for high antiproliferative activity. Further, the morphological changes and apoptosis have been evaluated by AO-EB staining techniques and flow cytometry analysis against HeLa cell line.