Crystal structure and molecular conformation of the tripeptide, N-Boc-L-Phe-Dehydro-Phe-L-Val-OCH₃

Abstract

The tripeptide, N-Boc-L-Phe-dehydro-Phe-L-Val-methyl ester (C₂₉H₃₇N₃O₆) 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 Cα atoms have extended trans conformations with highly planar configurations. The chain conformation angles-Φ₁, ψ₁, ω₁, Φ₂, ψ₂, ω₂, φ₃, and ψ-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 N-C^α bonds, with a X₁ torsion angle (about C^α-C^β) of −176.3(1)° and −179.4(5)°, respectively. The C'-Cα 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, N₃-H₃ ... O₂, 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.