Synthesis, characterisation, DNA binding interactions, and biological activity of nickel Schiff base complexes

A series of fourteen new nickel Schiff base complexes was synthesised by a two-step procedure. Initially 2,4,6-trihydroxybenzaldehyde was reacted with 1-(2-chloroethyl)piperidine hydrochloride, 4-(2-chloroethyl)morpholine hydrochloride or 1-(3-chloropropyl)piperidine hydrochloride in the presence of K2CO3 to afford three organic precursor compounds featuring different pendant groups. These compounds were then successfully reacted with different diamines in the presence of Ni(II) to form a series of nickel Schiff base complexes featuring four pendant groups. All new organic compounds and nickel complexes were characterised using 1D and 2D nuclear magnetic resonance (NMR) spectroscopic methods, elemental microanalysis and electrospray ionisation mass spectrometry (ESI-MS). The solid-state structures of four nickel complexes were determined by single crystal X-ray crystallography and revealed that the coordination geometry around the nickel ion was square planar in each case. The ability of the nickel complexes to bind to the double stranded DNA molecule D2, the tetramolecular G-quadruplex Q4, the unimolecular G-quadruplex Q1 in its parallel, anti-parallel and hybrid topologies, the parallel unimolecular G-quadruplex c-KIT1, and the fluorescently labelled unimolecular G-quadruplex F21T, was investigated using ESI-MS and circular dichroism (CD) spectroscopy, Fluorescence Indicator Displacement (FID) assays, Fluorescence Resonance Energy Transfer (FRET) melting assays and molecular docking