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) ų, Z = 4, D_c= 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.