Abstract
The polyether ionophorous antibiotic monensic acid A (MonH.H2O) reacts with copper(II) ions in the presence of a weak base (Et4NOH) to form mononuclear coordination species of composition [Cu(Mon)2(H2O)2]. The complexation process in solution was studied by synchrotron radiation circular dichroism (SRCD). The isolated complex was characterized using infrared spectroscopy (IR), nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). The copper ion is placed in an octahedral environment composed of two deprotonated antibiotic monoanions and two water molecules. The monensinates are cis-oriented with respect to the metal(II) centre, acting bidentately through a monodentate carboxylate function and a hydroxyl group, both placed at the opposite ends of the ligand structure. The copper(II) cations do not enter the hydrophilic cavity of monensin. Instead it is occupied by water molecules which complete the octahedral inner coordination sphere of the complex and stabilize the “head-to-tail” chelates of the antibiotic by intramolecular hydrogen bonds.
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