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.