Crystal and molecular structure of Boc‐ D ‐Ala‐ΔPhe‐Gly‐ΔPhe‐ D ‐Ala‐OMe: A 3 10 ‐helical dehydropeptide

The crystal and molecular structure of the pentapeptide Boc‐ D ‐Ala‐ΔPhe‐Gly‐ΔPhe‐ D ‐Ala‐OMe, containing two dehydrophenylalanine residues, was determined by x‐ray diffraction. The molecule crystallizes in the orthorombic P2 1 2 1 2 1 space group, with a = 10.439(3), b = 15.319(3) and c = 21.099(4) Å. In the solid state, the conformation of the pentapeptide is characterized by the presence of two type III′ β‐turns. Thus the peptide assumes a left‐handed 3 10 ‐helical conformation, the left sense being due to the D configuration of the alanine residues. The two unsaturated residues are located in the ( i + 1) position of the first β‐turn and in the ( i + 2) position of the second β‐turn, respectively. In the crystal, the helical molecules are linked head to tail by hydrogen bonds. Lateral hydrogen bonds are also formed between molecules related by a twofold screw symmetry. This gives rise to a typical mode of packing characterized by infinite helical “chains,” smiliar to the packing found in other oligopeptides that adopt a 3 10 ‐helical structure.