A New Cyclic Pseudopeptide Composed of (l)-Proline and 3-Aminobenzoic Acid Subunits as a Ditopic Receptor for the Simultaneous Complexation of Cations and Anions

The synthesis and receptor properties of a cyclic pseudopeptide composed of (l)-proline and the nonnatural amino acid 3-aminobenzoic acid in an alternating sequence are described. The structure of cyclo[(l)Pro-AB]3 was determined in the solid state by X-ray crystallography and in solution by one- and two-dimensional NMR techniques and FT-IR spectroscopy. The cyclic peptide preferentially adopts conformations comparable with the cone conformation of calixarenes. Similar to calixarenes, cyclo[(l)Pro-AB]3 is able to bind cations by cation−π interactions with its aromatic subunits. In some complexes, the peptide NH groups interact additionally with anions and the cyclic peptide thus behaves as a ditopic receptor. The structure of the ternary complex between cyclo[(l)Pro-AB]3 and N-methylquinuclidinium iodide was determined by X-ray crystallography. Spectroscopic investigations show that this complex has a similar geometry in solution. Stability constants of complexes of the cyclopeptide with various ion pairs...