An unusual (H₂O)₂₀ discrete water cluster in the supramolecular host of a charge transfer platinum(II) complex: cytotoxicity and DNA cleavage activities

Abstract

The chemical reaction of Pt^II(L¹)Cl₂ [L¹=N-4-tolylpyridine-2-aldimine] with a bidentate N,S-donor atom ligand, 2-methylthioaniline, (HL²) in alkaline methanolic medium yielded a mixed ligand donor-acceptor complex, [Pt^II(L¹)(L²)]Cl, [1]Cl. The complex has been characterized by different spectroscopic and electrochemical techniques. The complex showed intense interligand charge transfer (ILCT) transition in the long wavelength region of UV-vis spectrum (>600 nm). The single-crystal X-ray structure of complex, [1]Cl·3.3H₂O is reported. The cationic complex upon crystallization from aqueous methanol solvent produces an assembly of discrete, three dimensional (H₂O)₂₀ guest moiety within the reference Pt-complex host lattice. The water assembly showed a unique type of aggregation of a distorted cube encapsulated by hydrogen bonded network of a twelve-water ring. The complex displayed one reversible cathodic response at -0.75 V and two irreversible anodic responses at 0.42 and 0.79 V versus Ag/AgCl reference electrode. The redox processes are characterized by EPR and spectroelectrochemistry. Density-functional theory calculations were employed to confirm the structural features and to support the spectral and redox properties of the complex. The square-planar complex has been found to intercalate DNA. Fluorescence spectroscopy, circular dichroism, cyclic voltammetry, viscosity measurements, together with DNA melting studies have been employed to characterize the binding of [1]Cl with calf thymus DNA. Agarose gel electrophoresis indicates that the complex cleaves supercoiled (SC) pUC19 plasmid DNA to its nicked circular (NC) form via singlet oxygen. As determined by a MTT assay, [1]Cl exhibits significant cytotoxicity with IC₅₀ value 58µM.