Conformations of the li-antamanide complex and na-[phe, val]antamanide complex in the crystalline state

Antamanide, a cyclic decapeptide isolated from the poisonous mushroom Amanita phalloides, preferably complexes with Na⁺, but in less polar solvents, e.g., acetonitrile, also with Li⁺ or K⁺. The selectivity of complexation makes it an important model for the study of conformational requirements of ion binding. The conformations of the lithium antamanide complex and the Na-[Phe⁴, Val⁶]antamanide complex have been established by x-ray diffraction analyses of single crystals. The two compounds are isostructural, but not isomorphous. The complexes are folded into a globular shape with an approximate 2-fold axis. Two of the peptide linkages are in the cis conformation, Pro²-Pro³ and Pro⁷-Pro⁸. There are only two intramolecular hydrogen bonds. Four C==O groups have their O atoms directed inward to form four Li-O or Na-O ligands. The fifth ligand to the metal ion is provided by a solvent molecule. The conformation found in the crystalline state is different from any of the conformations proposed for the sodium antamanide complex in solution on the basis of nuclear magnetic resonance data.