Synthesis, Structure, Spectroscopic Characterization, and Protein Binding Affinity of New Water-Soluble Hetero- and Homometallic Tetranuclear [CuII2ZnII2] and [CuII4] Clusters

Abstract

Two new water-soluble hetero- and homometallic tetranuclear clusters, Na4[Cu2Zn2(ccdp)2(μ-OH)2]·CH3OH·6H2O (1) and K3[Cu4(ccdp)2(μ-OH)(μ-OH2)]·14H2O (2), have been synthesized in methanol–water at room temperature by exploiting the flexibility, chelating ability, and bridging potential of a carboxylate-rich dinucleating ligand, N,N′-bis(2-carboxybenzomethyl)-N,N′-bis(carboxymethyl)-1,3 diaminopropan-2-ol (H5ccdp). Complex 1 is obtained through the self-assembly of two monoanionic [CuZn(ccdp)]−fragments, which are, in turn, exclusively bridged by two μ-OH–groups. Similarly, complex 2 is formed through the self-assembly of two monoanionic [Cu2(ccdp)]− species exclusively bridged by one μ-OH– and one μ-OH2 groups. Complexes 1 and 2 are fully characterized in the solid state as well as in solution using various analytical techniques including a single-crystal X-ray diffraction study. The X-ray crystal structure of 1 reveals that two CuII centers are in a distorted square-pyramidal geometry, whereas two ZnII centers are in a distorted trigonal-bipyramidal geometry. The solid-state structure of 2 contains two dinuclear [Cu2(ccdp)]− units having one CuII center in a distorted square-pyramidal geometry and another CuII center in a distorted trigonal-bipyramidal geometry within each dinuclear unit. In the powder state, the high-field EPR spectrum of complex 1 indicates that two CuII ions are not spin-coupled, whereas that of complex 2 exhibits at least one noninteracting CuII center coordinated to a nitrogen atom of the ligand. Both complexes are investigated for their binding affinity with the protein bovine serum albumin (BSA) in an aqueous medium at pH ∼7.2 using fluorescence spectroscopy. Synchronous fluorescence spectra clearly reveal that complexes 1 and 2 bind to the active sites in the protein, indicating that the effect is more pronounced toward tyrosine than tryptophan. Density functional theory calculations have been carried to find the Fukui functions at the metal sites in complexes 1 and 2 to predict the possible metal centers involved in the binding process with BSA protein.