Design of Peptides with α,β-Dehydro-Residues: Syntheses and Crystal Structures of (i) N-Tertiarybutyloxycarbonyl-L-Ala-ΔPhe-L-Ala-OCH3 and (ii) N-Tertiarybutyloxycarbonyl-L-Leu-ΔPhe-L-Leu-OCH3

In order to contribute to design rules with α,β-dehydro amino acid residues, two peptides (i) Boc-Ala-ΔPhe-Ala-OCH3 and (ii) Boc-Leu-ΔPhe-Leu-OCH3 were synthesized and their crystal structures were determined. Peptide (i) with Ala residues on both sides of ΔPhe adopted a type II β-turn conformation with dihedral angles of two corner residues, φ1 = −62.6(4)∘, ψ1 = 138.9(5)∘, φ2 = 76.3(4)∘ and ψ2 = 13.1(3)∘, while the peptide (ii) with Leu residues formed an unfolded conformation with dihedral angles, φ1 = −81.9(5)∘, ψ1 = −28.3(4)∘, φ2 = 56.7(5)∘ and ψ2 = 42.6(4)∘. The structure of peptide (i) was stabilized by an intramolecular 4→1 hydrogen bond between Ala3 NH and BOC carbonyl oxygen atom, whereas that of peptide (ii) was stabilized by van der Waals forces involving the side chains of two Leu residues.