Exceptionally water-soluble [CuII2] complexes for investigating monosaccharide-metal ion interactions

Abstract

The design, synthesis and characterization of two new dinuclear copper(II) complexes [Cu2(phpda)(µ-Cl)]·2H2O (1) and [Cu2(phpda)(µ-NO3)]·2H2O (2) (H3phpda = N,N′ -bis[2-pyridylmethyl]-2‑hydroxy-1,3-propanediamine-N,N′-dipropionic acid) as carbohydrate binding models have been reported. The FTIR, UV–Vis, ESI-MS, microanalysis and molar conductance data have been utilized to characterize the synthesized complexes. The structures of 1 and 2 have been established by density functional theory (DFT) calculations that exhibit a self-aggregated closely packed arrangement with average inter-metallic Cu···Cu separation of 3.2484 Å. In aqueous alkaline media, both complexes have been studied for their binding interactions with biologically important monosaccharides, d-glucose and d-glucosamine by employing UV–Vis spectroscopy and DFT calculations. The binding-assisted steady decrease of absorbances of 1 and 2 accompanied by a significant red/blue shift of absorption maxima of the complexes upon UV–Vis titration of 1 and 2 with d-glucose and d-glucosamine has been observed, suggesting 1:1 complex/monosaccharide interactions. Evaluation of binding constant values of monosaccharide-bound copper(II) complexes has been accomplished by utilizing UV–Vis titration data, and the values are found to be 2.145 × 103 M−1, 2.652 × 103 M−1, 1.388 × 103 M−1 and 1.793 × 103 M−1 for 1/D-glucose, 1/D-glucosamine, 2/D-glucose and 2/D-glucosamine assemblies, respectively. Fukui functions (fk+) on the copper sites, including HOMO and LUMO in both complexes have been obtained from DFT calculations to understand the likely copper sites involved in the binding events with monosaccharides. However, a comparative assessment of their binding constant values showed that d-glucosamine interacts with 1 and 2 more preferentially than d-glucose.