Crystal and molecular structure of Boc-D-Ala-ΔPhe-Gly-ΔPhe-D-Ala-OMe: A 310-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 P212121 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 310-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 310-helical structure.