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

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Abstract
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 C–C distance of 1.33(2) Å is an appropriate double bond length. The angle C–C–C 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 Cγ‐exo conformation with torsion angles χ = −24(1)°, χ = 34(1)°, χ = −30(1)°, χ = 15(1)°, and θ = 6(1)°. The side chain torsion angles in dehydro‐Phe are χ = −1(2)°, χ = −176(1)°, and χ = 8(2)°. The Plane of C–C–C 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.

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