Crystal and molecular structure of the doubly unsaturated dehydropeptide Ac-ΔPhe-Ala-ΔPhe-NH-Me

The dehydropeptide Ac-ΔPhe-L-Ala-ΔPhe-NH-Me, containing two dehydro-phenylalanine (ΔPhe) residues, crystallizes from methanol/water in space group P212121, with a = 12.508 (2), b = 12.746(1) and c = 15.465(9). In the crystalline state, the peptide chain assumes a right-handed 310-helical conformation stabilized by two intramolecular hydrogen bonds, between the N-terminal acetyl group and the NH of ΔPhe3, and between the CO of ΔPhe1 and the NH of the C-terminal methylamide group, respectively. The two consecutive 10-membered rings formed by the hydrogen bonds have torsion angles quite close to the standard values for type III β-bends. ΔPhe1 is located in the (i + 1) position of the first β-bend, while ΔPhe2 is located in the (i + 2) position of the other β-bend. In the crystal, the molecules are linked head to tail by intermolecular hydrogen bonds to form long helical chains. The axes of the helices are parallel to the c axis, but neighboring helices run in antiparallel directions. This crystal packing is similar to the packing motifs frequently observed in Aib-containing peptides