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

Chauhan, V. S. ; Singh, T. P. ; Haridas, M. ; Kumar, A. ; Viterbo, D. (1987) Crystal structure and molecular conformation of the tripeptide, N-Boc-L-Phe-Dehydro-Phe-L-Val-OCH3 Biopolymers, 26 (6). pp. 819-829. ISSN 0006-3525

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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 Cα; and Cβ atoms in the dehydro-Phe residue. The backbone conformations around the Cα 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 Cβ-Cγ bonds in Phe and dehydro-Phe are trans with respect to the N-Cα bonds, with a X1 torsion angle (about Cα-Cβ) of −176.3(1)° and −179.4(5)°, respectively. The C'-Cα bonds are cis and trans with respect to the Cβ-Cγ 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.

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