High-resolution 13C NMR study of free and metal-complexed ionophores in the solid state: Conformation and dynamics of the macrocyclic ring and the effect of intermolecular short contact of methyl groups on spin–lattice relaxation times and displacements of chemical shifts

High-resolution 13C NMR spectra of 15 samples of uncomplexed and metal-complexed tetranactin and nonactin were recorded in the solid state, revealing characteristic displacements of peaks due to complex formation and the effect of crystalline packing on the 13C chemical shifts and spin–lattice relaxation times of the methyl groups. The C-1 13C chemical shifts of uncomplexed and complexed tetranaction and nonactin are well related to the variation of nearby torsion angles characteristic of the macrocyclic conformation, as determined by x-ray diffraction. The existence of short intermolecular contact of methyl groups (<3.8 A) at the surface of the molecules results in either prolonged 13C spin–lattice relaxation times in the laboratory frame (T1C) or substantial upfield displacement of peaks (up to 6 ppm). In addition, significantly reduced T1C values in uncomplexed nonactin (one order of magnitude smaller than those of other compounds) was ascribed to the presence of a puckering motion of the tetrahydrofuran ring and fluctuation of the macrocyclic ring in the solid state (with a time scale of 10−8s). Finally, how the conformations of these compounds in the solid are retained in chloroform solution was examined in view of the differences in the 13C chemical shifts between the solid and solution.