Conformation of the oxalamide group in retro-bispeptides. Three crystal structures

Abstract

We have determined the structures of a range of peptides having the oxalamide (-NH-CO-CO-NH-) unit located at their center. The oxalamide group has the trans conformation in two retropeptides and an approximately orthogonal conformation in the peptide with Pro residues. Torsional angles about the CO-CO bond are 180° in MeO-Aib-CO-CO-Aib-OMe (1). 175 in MeO-L-Leu-CO-CO-L-Leu-OMe (2), and -108 and -106° for the two independent molecules in the crystal of MeO-L-Pro-CO-CO-L-Pro-OMe (3), owing to steric hindrance between CO and the pyrolidine ring. Crystal data are: (1) C₁₂H₂₀N₂O₆, triclinic, space group ₁, a= 6.190(1), b - 10.044(2), c= 11.989(2) A, α= 86.38(2), β= 83.13(2), γ= 80.16(2)°, R= 0.057 for 1646 observed reflections [F_0., >3σ(F₀)]; (2) C₁₆H₂₈N₂O₆, tetragonal, space group P4₁, a= b= 11.121(3), c= 15.775(6) Å, R= 0.058 for 1216 observed reflections [F₀, > 3σ(F0)]; (3) C₁₄H₂₀N₂O₆, monoclinic space group P2₁, a= 9.556(2), b= 17.861(3), c= 9.618(2) Å, β= 104.35(1)°; R= 0.051 for 2100 observed reflections [F₀ > 3σ(F₀)].