Crystal and molecular structure of tert.-butyloxycarbonyl-L-hydroxy-prolyl-α-aminoisobutyryl-α-aminoisobutyryl-L-phenylalaninol

The crystal structure of the synthetic tetrapeptide, Boc‐Hyp‐Aib‐Aib‐Phol, an analogue of the C‐terminal tetrapeptide in the antibiotic Antiamoebin I, was determined as part of a study of the conformation of peptaibophol antibiotics. The crystals are orthorhombic, space group P212121, with cell parameters a = 16.576 (1) Å, b= 17.657 (1) Å, c = 10.435 (1) Å, V = 3053.9 (2) Å3, Z = 4, Dc= 1.163 g · cm‐3. The three amino acids form a single turn of a 310‐helix, stabilized by two intramolecular hydrogen bonds. The Aib residues adopt the usual conformation in the region between the 310‐ and α‐helices. The terminal hydroxy methyl group of the phenylalaninol residue is disordered. The position of the benzyl side chain of the amino alcohol relative to the backbone corresponds to a conformation also observed in phenylalanine residues.