Ion Pairing between Cl- or ClO4 - and Alkali Metal Complexes of Ionophore Antibiotics in Organic Solvents:  A Multinuclear NMR and FT-IR Study

• Published in: Inorganic chemistry 2001-03, Vol.40 (7), p.1654-1662
• Main Authors: Wittenkeller, Lisa, Lin, Wanrong, Diven, Conrad, Ciaccia, Angela, Wang, Fei, Mota de Freitas, Duarte
• Format: Article
• Language: English
• Subjects: Anions; Anti-Bacterial Agents - chemistry; Chlorides - chemistry; Hydrogen-Ion Concentration; Ionophores - chemistry; Magnetic Resonance Spectroscopy; Metals - chemistry; Molecular Structure; Perchlorates - chemistry; Spectroscopy, Fourier Transform Infrared
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic001187y

The extent of ion pairing in chloride and perchlorate salts was studied by measurement of the Cl- and ClO4 - resonances and the observation of the perchlorate stretching frequency by use of nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FT-IR), respectively, for a variety of ionophores in various solutions and in large unilaminar vesicles (LUVs). The NMR line widths of chloride and perchlorate were larger in solutions containing the neutral ionophores valinomycin (Val) and nonactin (Non) than in solutions containing the negatively charged ionophores nigericin (Nig), lasalocid (Las), and monensin (Mon). The viscosity-corrected perchlorate NMR line widths in solutions containing Val and Las were significantly negatively correlated (r 2≥ 0.99) with the dielectric constant of the solvent. Solvents with low dielectric constants favored ion pair formation. From methanolic solutions containing the Li+, Na+, K+, and Cs+ salts of Cl- and ClO4 -, it was determined that the cation with the highest selectivity for the ionophore affords the most ion pairing. A decrease in pH from 7 to 3 had no significant effect on the NMR line widths of chloride and perchlorate in methanolic solutions containing Val, whereas a similar decrease in pH in a methanolic solution containing Mon caused a 2-fold increase in the line widths. The FT-IR difference spectrum of KClO4 in a methanolic solution containing Val showed splitting at the perchlorate stretching frequency. No band splitting was observed in the FT-IR difference spectrum of KClO4 in methanolic solutions containing Las. The efflux of 35Cl in LUVs containing the neutral ionophore Val followed first-order kinetics with an efflux constant of 1.70 × 10-3 min-1, as determined by 35Cl NMR spectroscopy. The induction of increased membrane permeability in LUVs by the ionophore was determined to be negligible for Val and Nig by fluorescence spectroscopy.