27 research outputs found

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

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    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

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

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    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
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