Crystal structure and molecular conformation of the peptide N-Boc-L-gly-dehydro-Phe-NHCH₃

Abstract

The peptide N-Boc-L-Gly-dehydro-Phe-NHCH₃ was synthesized by the combination of N-Boc-L-Gly-dehydro-Phe azlactone and methylamine. The peptide crystallizes in orthorhombic space group P2₁2₁2₁ with a = 5.679(2) Å, b = 16.423(9) Å, c = 19.198(10) Å, V = 1791(2) ų, Z = 4, dm = 1.212(5) Mg m⁻³, dc = 1.237(1) Mg m⁻³. The structure was determined by direct methods using SHELXS 86. The structure was refined by full-matrix least squares procedure to an R value of 0.049 for 1509 observed reflections. The molecular dimensions are, in general, in good agreement with the standard values. The bond angle C^α-C^β-C^γ; in the dehydro-Phe residue is 133.6(5)°. The peptide backbone torsion angles are θ¹ = −171.4(4)°, ω₀ = 178.2(4)°, Φ₁ = −57.2(6)°, ψ₁ = 141.2(4)°, ω₁ = −174.4(4)°, Φ₂ = 71.5(6)°, ψ₂ = 7.2(6)°, and ω₂ = −179.8(5)°. These values show that the backbone adopts the β-bend type II conformation. The Boc group has a trans-trans conformation. The side-chain torsion angles in dehydro-Phe are χ₂ = 1.6(9)°, χ^2,1₂ = 0.5(9)°, and χ^2,2₂ = 179.8(6)°. The plane of C^α₂-C^β₂-C^γ₂ is rotated with respect to the plane of the phenyl ring at 0.5(6)° , which indicates that the atoms of the side chain of the dehydro-Phe residue are essentially coplanar. As a result of the β-bend in the structure, an intramolecular hydrogen bond is formed between the oxygen of the ith residue and the NH of the (i + 3)th residue at a distance of 2.940(5) Å. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions.