Water-soluble dinuclear zinc(II) complexes bridged by auxiliary para-substituted benzoate ligands: Synthesis, structure, spectroscopic characterization and protein binding affinity

Abstract

This work describes the synthesis, structure, spectroscopy and evaluation of protein binding affinity with bovine serum albumin (BSA) of a new series of water-soluble dizinc(II) complexes of molecular formula Na₂[Zn₂(cpdp)(μ-p-O₂CC₆H₄(Cl))](2Br) (1), [Zn₂(cpdp)(μ-p-O₂CC₆H₄(CH₃))]·5H₂O (2) and [Zn₂(cpdp)(μ-p-O₂CC₆H₄(NO₂))] (3) (H₃cpdp = N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol; p-C₆H₄(Cl)(CO₂H) = para-chlorobenzoic acid; p-C₆H₄(CH₃)(CO₂H) = para-methylbenzoic acid; p-C₆H₄(NO₂)(CO₂H) = para-nitrobenzoic acid). All three complexes were characterized by multiple analytical techniques, including single crystal X-ray crystallography. Crystal structure analysis disclosed that the zinc centers in 1–3 are assembled via one endogenous bridging alkoxide group of cpdp³⁻ ligand, and one exogenous bridging para-chlorobenzoate/para-methylbenzoate/para-nitrobenzoate, with a self-aggregated closely packed arrangement having an average internuclear Zn∙∙∙Zn separation of 3.530 Å. UV–Vis absorption and fluorescence emission spectroscopy, and DFT computation were employed to investigate their binding interactions with BSA protein. Synchronous fluorescence spectra clearly suggest that 1–3 bind to the active sites of BSA, showing that the binding response is more or less equally prominent towards both tryptophan and tyrosine. Fukui functions at the zinc sites, including HOMOs and LUMOs in 1–3 were calculated by DFT computation to predict the probable zinc centers anticipated in the binding phenomena.