Farooq A. Khan

Direct binding of halide ions by valinomycin

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 1H nuclear magnetic resonance (NMR) spectroscopy. The binding selectivity for halide ions is found to be in the order Cl− >Br− ∼F− ≫I− based on electrospray ionisation mass spectrometry experiments in methanol. 1H NMR studies in acetone-d 6 and CD3CN reveal the binding selectivity of Cl− >Br− ≫F− ∼I− . 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− 1 and 57 ± 2 M− 1 were obtained via NMR titrations in acetone-d 6 for chloride and bromide ions, respectively.

Nitration of Polycyclic Aromatic Hydrocarbons Using a Supported Catalyst

Abstract We report the use of catalyst, sulfuric acid supported on silica-gel, as promising method for facile, high yielding, regioselective syntheses of mononitrated polycyclic aromatic hydrocarbons, 6-nitrochrysene, 1-nitropyrene, 1-nitronaphthalene, 2-nitrofluorene, 3-nitrofluoranthene, and 9-nitroanthracene.

Analysis of mixtures of C60 and C70 by Raman spectrometry

A non-destructive method to determine the percent composition and relative purity of bulk solid phase mixed fullerene samples using Raman spectroscopy is described. This ratiometric method has a significant advantage in ease of use over other methods, in that the samples are analyzed in a short time, as solids in suspension, with minimal sample pre-preparation, using easy to operate instrumentation. The ratio of unique vibrational bands vs. percent composition is used to construct a calibration curve, and a detection limit estimated in the presence of chemical noise.

The crystal structure of a valinomycin•2Ca2+ complex and the multi-step solution equilibria in acetonitrile characterised by 1H NMR, UV–Vis, and mass spectrometry

Abstract Valinomycin is a well-known potassium-selective ion carrier, but the nature of its direct interaction with calcium ion, another biologically important cation, is also of considerable interest. We have determined the first crystal structure of a valinomycin–calcium complex, which crystallises exclusively as a 1:2 valinomycin:Ca2+ complex, [VCa2(OTf)2(H2O)2]2+ in the solid state (V = valinomycin, OTf = triflate). Along with the 1:2 complex, the solution phase 1:1 and 2:1 complexes have been characterised in acetonitrile by 1H NMR and UV–Vis titration experiments. The molar absorptivity curves and binding equilibrium constants for these complexes have been determined by the global analysis of the UV–Vis titration data using the program SIVVU™.