Conformation of valinomycin in its barium perchlorate complex from X-ray crystallography and N.M.R.

Abstract

The conformation of the ionophore antibiotic valinomycin in its barium perchlorate complex has been determined in the solid state by X-ray crystallography and in solution using proton (¹H) and carbon-13 (¹³C) nuclear magnetic resonance spectroscopy. All our results indicate that both in solution and in the solid state, the valinomycin molecule in the complex stabilizes in a flat open structure, without internal hydrogen bonds, with two bariums per molecule. In the crystals each barium is liganded with three consecutive amide carbonyls and is separated from the other by 4.57 Å. The other atoms coordinating to barium ions belong to perchlorate groups and solvent molecules. The crystal structure contains infinite layers of valinomycin molecules interconnected by perchlorate groups. The solvent molecules in the structure are sandwiched between successive layers. In solution the valinomycin molecules exhibit three-fold symmetry in the n.m.r. time scale. This observation, along with a careful consideration of steric factors as well as the conformational differences between chemically equivalent residues in the solid state, suggests the presence of many conformers in solution, all clearly related to the one observed in the crystals, interconverting rapidly in the n.m.r. time scale.