Crystal and molecular structure of isoleucinomycin, cyclo[‐(D‐Ile‐Lac‐Ile‐D‐Hyi)₃‐](C₆₀H₁₀₂N₆O₁₈)

The crystal structure of a synthetic analog of valinomycin, cyclo[‐(D‐Ile‐Lac‐Ile‐D‐Hyi)₃‐] (C₆₀H₁₀₂N₆O₁₈), has been determined by x‐ray diffraction procedures. The crystals are orthorhombic, space group P2₁2₁2₁, with cell parameters a = 11.516, b = 15.705, c = 39.310 Å, and Z = 4. The atomic coordinates for the C, N, O atoms were refined in the anisotropic thermal motion approximation and for the H atoms in the isotropic approximation. Values of standard (R) and weighted (R_w) reliability factors after refinement are 0.073 and 0.056, respectively. The structure is completely asymmetric. The cyclic molecular backbone is stabilized by six intramolecular hydrogen bonds NH…OC, five bonds being of the 4→1 type and one being of the 5→1 type. The side chains are located on the molecular periphery. The conformational state of isoleucinomycin in the crystal is intermediate between the corresponding crystalline states of valinomycin and meso‐valinomycin. The observed conformation suggests that complexation could proceed via entry of the ion at the face possessing the L‐Lac residues, the less crowded face.