The peptide N‐Boc‐L‐Phe‐dehydro‐Leu‐L‐Val‐OCH3 was synthesized by the usual workup procedure and finally by coupling the N‐Boc‐L‐Phe‐dehydro‐Leu‐OH to valine methyl ester. It was crystallized from its solution in methanol–water mixture at 4°C. The crystals belong to the triclinic space group P1 with a = 5.972(5) Å, b = 9.455(6) Å, c = 13.101(6) Å, α = 103.00(4)°, β = 97.14(5)°, γ = 102.86(50)°, V = 690.8(8) Å, Z = 1, dm = 1.179(5) Mg m−3 and dc = 1.177(5) Mg m−3. The structure was determined by direct methods using SHELXS86. It was refined by block‐diagonal least‐squares procedure to an R value of 0.060 for 1674 observed reflections. The C 2α–C 2β distance of 1.323(9) Å in dehydro‐Leu is an appropriate double bond length. The bond angle Cα–Cβ–Cγ in the dehydro‐Leu residue is 129.4(8)°. The peptide backbone torsion angles are θ1 = −168.6(6)°, ω0 = 170.0(6)°, ϕ1 = −44.5(9)°, ψ1 = 134.5(6)°, ω1 = 177.3(6)°, ϕ2 = 54.5(9)°, ψ2 = 31.1(10)°, ω2 = 171.7(6)°, ϕ3 = 51.9(8)°, ψ 3T = 139.0(6)°, θT = −175.7(6)°. These values show that the backbone adopts a β‐turn II conformation. As a result of β‐turn, an intramolecular hydrogen bond is formed between the oxygen of the ith residue and NH of the (i + 3)th residue at a distance of 3.134(6) Å. The Boc group has a trans–trans conformation. The side‐chain torsion angles of the Phe residue are χ1 = 171.6(6)°, χ 12,1 = −102.1(9)°, and χ 12,2 = 78.6(10)°. The side‐chain conformational angles of dehydro‐Leu residue are χ2 = 2.7(13)°, χ 22,1 = −107.3(11)°, and χ 22,2 = 131.3(10)°. The torsion angles χ 31,1 and χ 31,2 that define the conformation of the valyl side chain are −166.16(6)° and 69.1(9)°, respectively. The crystal structure is stabilized by hydrogen bonds along the a and b axes, while the van der Waals forces are the stabilizing interactions in the c direction. Copyright © 1990 John Wiley & Sons, Inc.
