Synthesis, and crystal and molecular structure of the 3 10 ‐helical α,β‐dehydro pentapeptide Boc‐Leu‐Phe‐Ala‐ΔPhe‐Leu‐Ome

α,β‐Dehydro amino acid residues are known to constrain the peptide backbone to the β‐bend conformation. A pentapeptide containing only one α,β dehydrophenylalanine (ΔPhe) residue has been synthesized and crystallized, and its solid state conformation has been determined. The pentapeptide Boc‐Leu‐Phe‐Ala‐ΔPhe‐Leu‐OMe (C 39 H 55 N 5 O 8 , M w = 721.9) was crystallized from aqueous methanol. Monoclinic space group was P2 1 , a = 10.290(2)°, b = 17.149(2)°, c = 12.179(2) Å, β = 96.64(1)° with two molecules in the unit cell. The x‐ray (Mo K α , λ = 0.7107A) intensity data were collected using a CAD4 diffractometer. The crystal structure was determined by direct methods and refined using least‐squares technique. R = 4.4% and R w = 5.4% for 4403 reflections having |F 0 | ≥ 3σ(|F 0 |). All the peptide links are trans and the pentapeptide molecule assumes 3 10 ‐helical conformation. The mean ϕ,ψ values, averaged over the first four residues, are −64.4°, −22.4° respectively. There are three 4 → 1 intramolecular hydrogen bonds, characteristic of 3 10 ,‐helix. In the crystal, the peptide helices interact through two head‐to‐tail. NHO intermolecular hydrogen bonds. The peptide molecules related by 2 1 , screw symmetry form a skewed assembly of helices. © 1995 John Wiley & Sons, Inc.