Synthesis and X-ray structure of a new zinc(ii) coordination polymer: interaction with DNA and double stranded RNA and elucidation of the molecular aspects of the binding to bovine serum albumin

Abstract

A novel ligand bridged zinc(II) coordination polymer, [Zn(4-Me-5-CHOIm)₂(HCOO)](ClO₄) has been synthesized and characterized by single crystal X-ray diffraction studies. The structural analysis revealed that the complex exhibits two types of 2D network by weak and strong hydrogen bonds. Interestingly, an extended supramolecular layer network has been observed in the solid state structure through weak face-to-face π-stacking interactions. The interaction of the complex with calf thymus DNA (CT-DNA) and double stranded ribonucleic acid, polyinosinic·polycytidylic acid (poly(I)·poly(C)) has been explored by UV-Vis absorption, fluorescence displacement, circular dichroism (CD) spectral and isothermal titration calorimetric (ITC) methods. The spectral titration method revealed that the compound could interact with CT-DNA and RNA through partial intercalation with a binding affinity of an order of 10⁵. ITC experiments indicate an exothermic reaction in both cases but RNA interaction is favored by positive entropy. On the other hand, the complex could quench the intrinsic fluorescence of BSA in a static quenching process with a quenching constant of the order of 10⁴. A specific binding distance of 2.75 nm between Trp 212 (donor) and the complex (acceptor) was obtained from Forster resonance energy transfer studies (FRET). Competitive binding using site markers, warfarin and ibuprofen, having definite binding sites demonstrated that the complex binds to site I (subdomain IIA) on BSA. Circular dichroism (CD), synchronous fluorescence, and three-dimensional fluorescence spectroscopy experiments reveal that binding with the complex alters the BSA secondary structure by a reduction of the α-helical organization and increases the coiled structure and the ITC experiment indicates that the binding was enthalpy driven with a slight negative entropy effect.