A High Resolution 23Na NMR Study of Sodium Complexes with Ionophores in Solution. Stability of Complexes as Viewed from Displacements of 23Na Chemical Shifts as Referred to Those in the Solid State

Abstract We have recorded 23Na NMR spectra of sodium complexes with naturally occurring and synthetic ionophores in chloroform and methanol solution, to gain insight into conformational stability of these complexes as examined from the displacements of 23Na chemical shifts with respect to those in the solid state. Interestingly, the 23Na chemical shifts of sodium complexes with monensin and tetranactin obtained in chloroform solution are very close to those in the solid, as a result of adopting similar conformation between the solid and solution. For the rest of complexes, however, the 23Na NMR signals are displaced downfield in solution as compared with those of the solid state. Such downfield shifts are well interpreted in terms of the presence of conformational fluctuation and/or sodium ions interacting with solvent or anions. Therefore, downfield displacement is more significant in methanol which can be good ligand molecules. The presence of fluctuation-induced quadrupole coupling constants is consistent with our view about 23Na chemical shifts. These quadrupole interactions were reduced in methanol solution, owing to the presence of exchange with solvents.