Synthesis, crystal structure, and molecular conformation of N‐Boc‐ L ‐Phe‐dehydro‐Leu‐ L ‐Val‐OCH 3

The peptide N‐Boc‐ L ‐Phe‐dehydro‐Leu‐ L ‐Val‐OCH 3 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   2 α –C   2 βdistance of 1.323(9) Å in dehydro‐Leu is an appropriate double bond length. The bond angle C α –C β –C γ in the dehydro‐Leu residue is 129.4(8)°. The peptide backbone torsion angles are θ 1 = −168.6(6)°, ω 0 = 170.0(6)°, ϕ 1 = −44.5(9)°, ψ 1 = 134.5(6)°, ω 1 = 177.3(6)°, ϕ 2 = 54.5(9)°, ψ 2 = 31.1(10)°, ω 2 = 171.7(6)°, ϕ 3 = 51.9(8)°, ψ   3 T= 139.0(6)°, θ T = −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 i th 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 χ 1 = 171.6(6)°, χ   1 2,1= −102.1(9)°, and χ   1 2,2= 78.6(10)°. The side‐chain conformational angles of dehydro‐Leu residue are χ 2 = 2.7(13)°, χ   2 2,1= −107.3(11)°, and χ   2 2,2= 131.3(10)°. The torsion angles χ   3 1,1and χ   3 1,2that 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.