Structural investigation of poly‐ L , D ‐proline, a synthetic ion channel across bilayer membranes: Crystal structure and molecular conformation of two tetra‐ D , L ‐proline derivatives

The crystal structures and molecular conformations of two tetraproline derivatives with alternating configurations Boc(D‐Pro,L‐Pro)2OH and Boc(D‐Pro,L‐Pro)2OCH3 are investigated in connection with the ability of the homologous polymer to selectively increase (as an ion channel) the ion permeability across bilayer membranes. Both tetramers are characterized by the cis‐trans alternating conformation of the peptide bonds, which formally transforms in a turn of the poly‐D,L‐proline channel after a cis‐trans change of the central peptide residue.