Crystal structure and conformation of a highly constrained linear tetrapeptide Boc-Leu-dehydro Phe-Ala-Leu-OCH3

The conformation of a tetrapeptide containing a dehydro amino acid, Δ^zPhe, in its sequence has been determined in the crystalline state using X-ray crystallographic techniques. The tetrapeptide, Boc-Leu-Δ^zPhe-Ala-Leu-OCH₃, crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules in a unit cell of dimensions a = 11.655(1) Å, b = 15.698(6) Å and c = 18.651(3) Å V = 3414.9 Å and Dcalc =1.12 g/cm ^?3. The asymmetric unit contains one tetrapeptide molecule, C₃₀H₄₆N₄O₇, a total of 41 nonhydrogen atoms. The structure was determined using the direct methods program SHELXS86 and refined to an R-factor of 0.049 for 3347 reflections (13.0(I). The linear tetrapeptide in the crystal exhibits a double bend of the Type III-I, with Leu¹ (<φ=?54.1°, Ψ=?34.5°) and Δ^zPhe² (φ=?59.9°, Ψ=?17.1°) as the corner residues of Type III turn and Δ^zPhe² (φ=?59.9°, Ψ=?17.1°) and Ala³ (φ=?80.4°, Ψ= 0.5°) residues occupying the corners of Type I turn, with Δ^zPhe as the common residue in the double bend. The turn structures are further stabilized by two intramolecular 4→1 type hydrogen bonds.