The conformations of monensin-A metal complexes in solution determined by NMR spectroscopy

The structures in solution of the Na+, K+ and Rb+ 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.