The antibiotic valinomycin is a potassium-selective ionophore, which increases the transport of potassium ions across cell membranes and thereby causes damage to bacteria cells. Valinomycin has been extensively studied as an ionophore for cations. We report for the first time the direct binding of anions to valinomycin using electrospray ionisation mass spectrometry and 1 H nuclear magnetic resonance (NMR) spectroscopy. The binding selectivity for halide ions is found to be in the order Cl 2 . Br 2 , F 2 q I 2 based on electrospray ionisation mass spectrometry experiments in methanol. 1 H NMR studies in acetone-d 6 and CD 3 CN reveal the binding selectivity of Cl 2 . Br 2 q F 2 , I 2 . NMR studies and density functional theory (DFT) calculations support a bracelet-like structure for the binding of a chloride ion to valinomycin. Association constants of 531^45 M 21 and 57^2 M 21 were obtained via NMR titrations in acetone-d 6 for chloride and bromide ions, respectively.

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