Crystal structure and molecular conformation of the tripeptide, N‐Boc‐ L ‐Phe‐Dehydro‐Phe‐ L ‐Val‐OCH 3

The tripeptide, N‐Boc‐ L ‐Phe‐dehydro‐Phe‐ L ‐Val‐methyl ester (C 29 H 37 N 3 O 6 ) was synthesized as a model peptide to produce a specific β‐turn with the help of dehydro‐Phe. This peptide crystallizes in the triclinic space group Pl with a = 6.085(2) Å, b = 9.515(4) Å, c = 13.243(5) Å, α = 105.16(3)°, β = 92.69(3)°, γ = 104.9(3)°, and Z = 1. The structure was solved by direct methods using MULTAN 80. The structure was refined by the block‐diagonal least‐squares method to an R value of 0.075 for 2438 observed reflections. The bond lengths and angles, in general, are in good agreement with the standard values. The peptide backbone adopts the β‐conformation as a result of deprotonation at the C α and C β atoms in the dehydro‐Phe residue. The backbone conformations around the C α atoms are cis , and the corresponding moieties are nonplanar. The backbone segments between the C α atoms have extended trans conformations with highly planar configurations. The chain conformation angles—ϕ 1 , ψ 1 , ω 1 , ϕ 2 , ψ 2 , ω 2 , ϕ 3 , and ψ   T 2 —are −45.4(11)°, −45.6(6)°, − 179.3(4)°, −46.5(5)°, −38.0(6)°, 165.3(9)°, 51.7(5)°, and 43.0(6)°, respectively. The phenyl rings of Phe and dehydro‐Phe are essentially planar and their planes are inclined with respect to each other at 69.7(5)°. The C β C γ bonds in Phe and dehydro‐Phe are trans with respect to the NC α bonds, with a X 1 torsion angle (about C α C β ) of −176.3(1)° and −179.4(5)°, respectively. The C′C α bonds are cis and trans with respect to the C β C γ bonds, with torsion angles of −56.8(7)° and −170.0(16)° in Phe and dehydro‐Phe, respectively. The β‐turn conformation is stabilized by an intramolecular hydrogen bond, N 3 H 3 … O 2 , of a distance of 3.116(6) Å. The molecules are held in the crystal structure by a network of hydrogen bonds and the van der Waals forces.