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Recent progress in studies of the mechanism of transport of alkali metal ions by ionophoric antibiotics and the structures of alkali metal salts of the ionophores monensin and narasin is reviewed. The structures obtained from 2D NMR experiments in solution provide considerable insights into the mechanisms of transport.

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The membrane permeabilisation properties of six linear natural 18-residue peptaibols, termed trichorzins PA, have been assessed on liposomes and on mollicutes (trivial name, mycoplasmas), a class of parasitic bacteria characterized by a small genome, the lack of a cell wall, a minute cell size, and the incorporation in their plasma membrane of exogenously supplied cholesterol. The trichorzins PA used in this study (PA II, PA IV-VI, PA VIII, and PA IX) differ between them by amino acid or amino alcohol substitutions at positions 4, 7, and 18, and form slightly amphipathic alpha-helices. They proved bactericidal for mollicutes belonging to the genera Acholeplasma, Mycoplasma, and Spiroplasma, with minimal inhibitory concentrations (3.12

The structures in solution of the Na ϩ , K ϩ and Rb ϩ salts of monensin have been determined by the use of NOESY distance restraints and molecular modelling. The structures are similar to those obtained by X-ray diffraction with the exception of a close carboxylate oxygen-to-metal distance and only five observed close metal-to-oxygen ligating interactions. Molecular dynamics involving the derived structure of the sodium salt at increasing relative permittivity sheds light on the mechanism by which monensin binds to and unbinds from sodium ions in the membrane surface. The series of structures as the size of the metal ion increases also shows that the whole monensin molecule adapts to incorporate changes in the ionic radius including changes in torsion angles, changes in heterocyclic ring conformations and changes in the ligation and hydrogen bonding patterns.