Synthesis, crystal structure, and molecular conformation of peptide N-Boc-L-Pro-dehydro-Phe-L-Gly-OH

The peptide N-Boc-L-Pro-dehydro-Phe-L-Gly-OH was synthesized by the usual workup procedure and finally coupling the N-Boc-L-Pro-dehydro-Phe to glycine. The peptide crystallizes in monoclinic space group P21 with a = 8.951(4) Å, b = 5.677 (6) Å, c = 21.192(11) Å, β = 96.97(4)°, V = 1069(1) Å3, Z = 2, dm = 1.295(5) Mgm−3, and dc = 1.297(4) Mgm−3. The structure was determined by direct methods using SHELXS86. The structure was refined by the block-diagonal least-squares procedure to an R value of 0.074 for 1002 observed reflections. The Cα2-Cβ 2distance of 1.33(2) Å is an appropriate double bond length. The angle Cα-Cβ -Cϒ is 133(1)°. The peptide backbone torsion angles are θ1 = −167(1)°, ω0 = 179(1)°, Φ1 = −48(1)°, ψ1 = 137(1)°, ω1 = 175(1)°, Φ2 = 65(2)°, ψ2 = 15(2)°, ω2 = −179(1)°, and Φ3 = −166(1)°. These values show that the Boc group has a trans-trans conformation while the peptide backbone adopts a β-turn II conformation, which is stablized by an intramolecular hydrogen bond of length of 3.05(1) Å. The structures of dehydro-Phe containing peptides suggest that the dehydro-Phe promotes the β-turn II conformation. The five-membered pyrrolidine ring of the Pro residue adopts an ideal C-exo conformation with torsion angles χ11 = −24(1)°, χ21 = 34(1)°, χ31 = −30(1)°, χ41 = 15(1)°, and θ01 = 6(1)°. The side chain torsion angles in dehydro-Phe are χ12 = −1(2)°, χ2,12 = −176(1)°, andχ2,22 = 8(2)°. The Plane of Cα2-Cβ 2-Cϒ2 is rotated with respect to the plane of the phenyl ring at 7(1)°, which indicates that the atoms of the side chain of dehydro-Phe are essentially coplanar. The molecules form a 21 screw axis related hydrogen-bonded rows along the b axis