Crystal and molecular structure of the depsipeptide ionophore Hexadecaisoleucinomycin, cyclo‐[‐(D‐Ile‐L‐Lac‐L‐Ile‐D‐Hyi)₄‐] (C₈₀H₁₃₆N₈O₂₄)

The crystal structure of a synthetic depsipeptide ionophore hexadecaisoleucinomycin, cyclo[‐(D‐Ile‐L‐Lac‐L‐Ile‐D‐Hyi)₄‐] (C₈₀H₁₃₆N₈O₂₄), has been determined by single crystal x‐ray diffraction techniques. The crystals are orthorhombic, space group P2₁2₁2₁, number of molecules per unit cell z = 4, and cell parameters a = 11.195, b = 17.853, c = 54.835 Å. The values of the standard ( R) and weighted ( R_w ) discrepancy factors after refinement are 0.122 and 0.135, respectively. The structure is characterized by an elongated bracelet form with a twofold axis of pseudosymmetry. It is stabilized by eight intramolecular 4 → 1 hydrogen bonds between the amide CO and NH groups. The ester carbonyls are directed toward the inside of the molecule, their oxygen atoms forming an ellipsoidal internal cavity. The side chains are located on the molecular periphery. The conformational states of hexadecaisoleucinomycin in solution are discussed in the light of the data obtained.