Crystal structure and molecular conformation of N‐boc‐L‐pro‐dehydro‐leu‐OCH3

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Abstract
The peptide N‐Boc‐L‐Pro‐dehydro‐Leu‐OCH3 was synthesized by coupling dehydroleucine methyl ester with Boc‐Pro‐OH. It was crystallized from its solution in a methanol–water mixture at 4°C and the crystals belong to the orthorhombic space group P212121 with a = 10.239(1) Å, b = 19.276(4) Å, c = 20.319(3) Å, V = 4010(1) Å3, and Z = 8. The structure was solved by direct methods using MULTAN 80. The structure was refined to an R value of 0.095 for 1871 observed reflections. There are two crystallographically independent molecules in the unit cell. The molecular dimensions of the two molecules are very similar. The backbone conformation angles of these molecules are as follows: θ1 = −176(1)°, ω0 = −2(2)°, ϕ1 = −72(1)°, ψ1 = 141(1)°, ω1 = 169(1)°, ϕ2 = −98(1)°, ψ2 = −179(1)°, θT = 179(1)°, θ = 174(1)°, ω10 = −3cm−1, ϕ11 = −64(1)°, ψ11 = 148(1)°, ω11 = 167(1)°, ϕ12 = −67(1)°, ψ12 = 175(1)°, and θ1T = 172(1)°, respectively. Other torsion angles such as θ = 86(1)°, χ2 = −5(2)°, = 131(1)°, and χ12 = −16(2)° are interesting to define the backbone conformation together with the dehydro‐Leu side chain as a conformation similar to that of the β‐bend type II. The central amide bond of Pro‐dehydro‐Leu segment is deviated from the planarity. The Boc group adopts a trans–cis conformation. The side‐chain torsion angles χ1, χ2, χ3, χ4, and χ0 for Pro‐residue in the two molecules are 11(2)°, −11(2)°, 6(2)°, 2(1)°, −9(1)° and −18(1)°, 31(1)°, −31(1)°, 21(1)°, and −1(1)° respectively. The side chain of the Pro residue of molecule A is in the flattened Cγ‐endo conformation while the pyrrolidine ring in miolecule B adopts the well‐known Cγ‐exo conformation. The side‐chain torsion angles χ, χ2,1, and χ2,2 in the leucyl residues of molecules A and B are −5(2)°, −111(2)°, 126(2)° and −16(2)°, −110(2)°, and 133(2)°, respectively. The molecular packing is stabilized by intermolecular hydrogen bonds and van der Waals interactions.

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