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

The structures in solution of the Na+, K+, Rb+ and Cs+ salts of narasin have been determined by the use of NOESY distance restraints and molecular modelling. As the size of the metal ion increases the same set of six oxygen atoms remain co-ordinated to the metal but the metal to oxygen distances increase according to the known atomic radii of the ions. Increasing ion size is accommodated by the molecule in two principal ways. As the size of the metal ion increases there is an increase in the H-C(12)-C(13)-H dihedral angle. There is a "head to tail" hydrogen bond (OH(11). . .O(1)) in the Na+, K+ and Rb+ salts which is much weaker in the Cs+ salt. There is a relatively strong hydrogen bond between OH(4) and O(2) in all four salts and a much weaker hydrogen bond between OH(8) and O(2) seen only in the Na+ salt. The relative tightness of the binding increases as the metal ion size increases.