Synthesis,
Structure, Spectroscopic Characterization,
and Protein Binding Affinity of New Water-Soluble Hetero- and Homometallic
Tetranuclear [Cu<sup>II</sup><sub>2</sub>Zn<sup>II</sup><sub>2</sub>] and [Cu<sup>II</sup><sub>4</sub>] Clusters
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Abstract
Two
new water-soluble hetero- and homometallic tetranuclear clusters,
Na<sub>4</sub>[Cu<sub>2</sub>Zn<sub>2</sub>(ccdp)<sub>2</sub>(μ-OH)<sub>2</sub>]·CH<sub>3</sub>OH·6H<sub>2</sub>O (<b>1</b>) and K<sub>3</sub>[Cu<sub>4</sub>(ccdp)<sub>2</sub>(μ-OH)(μ-OH<sub>2</sub>)]·14H<sub>2</sub>O (<b>2</b>), have been
synthesized in methanol–water at room temperature by exploiting
the flexibility, chelating ability,
and bridging potential of a carboxylate-rich dinucleating ligand, <i>N</i>,<i>N</i>′-bis(2-carboxybenzomethyl)-<i>N</i>,<i>N</i>′-bis(carboxymethyl)-1,3 diaminopropan-2-ol
(H<sub>5</sub>ccdp). Complex <b>1</b> is obtained through the
self-assembly of two monoanionic [CuZn(ccdp)]<sup>−</sup> fragments,
which are, in turn, exclusively bridged by two μ-OH<sup>–</sup> groups. Similarly, complex <b>2</b> is formed through
the self-assembly of two monoanionic [Cu<sub>2</sub>(ccdp)]<sup>−</sup> species exclusively bridged by one μ-OH<sup>–</sup> and one μ-OH<sub>2</sub> groups. Complexes <b>1</b> and <b>2</b> 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 <b>1</b> reveals that
two Cu<sup>II</sup> centers are in a distorted square-pyramidal geometry,
whereas two
Zn<sup>II</sup> centers are in a distorted trigonal-bipyramidal geometry.
The solid-state structure of <b>2</b> contains two dinuclear
[Cu<sub>2</sub>(ccdp)]<sup>−</sup> units having one Cu<sup>II</sup> center in a distorted square-pyramidal
geometry and another Cu<sup>II</sup> center in a distorted trigonal-bipyramidal
geometry within each dinuclear unit. In the powder state, the high-field
EPR spectrum of complex <b>1</b> indicates that two Cu<sup>II</sup> ions are not spin-coupled, whereas that of complex <b>2</b> exhibits at least one noninteracting Cu<sup>II</sup> 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 <b>1</b> and <b>2</b> 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 <b>1</b> and <b>2</b> to predict the possible
metal centers involved in the binding process with BSA protein