Abstract
The peptide N‐Ac‐dehydro‐Phe‐L‐Val‐L‐Val‐OCH~3~ (C~22~H~31~N~3~O~5~) was synthesized by the usual workup procedure and finally by coupling the N‐Ac‐dehydro‐Phe‐L‐Val‐OH to valine methyl ester. It was crystallized from its solution in acetonitrile‐water mixture at 4°C. The crystals belong to the space group P1 with a = 8.900(3) Å, b = 11.135(2) Å, c = 12.918(2) Å, α = 90.36(1)°, β = 110.14(3)°, γ = 90.10(3)°, V = 1207.7(6) Å,^3^ Z = 2, d~m~ = 1.156(5) Mgm^−3^, d~c~ = 1.148(5) Mgm^−3^. The structure was determined by direct methods using SHELXS86. The structure was refined by full‐matrix least‐squares procedure to an R value of 0.077 for 3916 observed reflections. The molecular dimensions and conformations of the two crystallographically independent molecules are in good agreement. In the dehydro residues, the average C^α^–C^β^ distance is 1.31(2) Å whereas the bond angle C^α^–C^β^–C^γ^ is 132(1)°. The average backbone torsion angles are ω~0~ = 169(1)°, ϕ~1~ = −40(1)°, ψ~1~ = −50(1)°, ω~1~ = −177(1)°, ϕ~2~ = 54(1)°, ψ~2~ = 46(1)°, ω~2~ = −174(l)°, ϕ~3~ = 103(1)°, ψ = −139(1)°, and θ = −176(1)°. The acetyl group is in the trans conformation, while the backbone adopts a right‐handed and left‐handed helical conformation alternatingly. The two crystallographically independent molecules are held together by three hydrogen bonds: N~21~–H~21~–O′~12~ = 2.911(12) Å, N~22~–H~22~–O′~11~ = 2.941(12) Å and N~23~–H~23~–O~11~ = 3.005 (9) Å. The crystal structure is stabilized by van der Waals forces and three additional hydrogen bonds: N~11~–H~11~–O~2~, = 2.896(15) Å, N~12~–H~12~–O′~21~ = 2.947(14) Å and N~13~–H~13~–O~21~ = 2.987(15) Å.