Synthetic peptide helices in crystals: structure and antiparallel and skewed packing motifs for α-helices in two isomeric decapeptides

Abstract

The isomeric decapeptides Boc-Aib-Ala-Leu-Ala-Aib-Aib-Leu-Ala-Leu-Aib-OMe (II) and Boc-Aib-Ala-Aib-Ala-Leu-Ala-Leu-Aib-Leu-Aib-OMe (III), are predominantly a-helical with little effect on the conformation with interchange of Aib/Ala residues or Aib/Leu residues. The packing motif of helices in crystal II is antiparallel, whereas the helices pack in a skewed fashion in crystal III, with a 40° angle between neighboring helix axes. Crystal III contains a water molecule in a hydrophobic hole that forms hydrogen bonds with two carbonyl oxygens that also participate in 5 →1 type hydrogen bonds. Values for helical torsional angles Φ and Ψ and assume a much wider range than anticipated. Crystal II: C₄₉H₈₈N₁₀O₁₃, space group P2₁ with a = 16.625(2) Å, b = 9.811(5) Å, c = 18.412(3) Å, β = 99.79(1)°, Z = 2, R = 5.7% for 4338 data with |F₀| > 3σ(F). Crystal III: C₄₉H₈₈N₁₀O₁₃. 1/2H₂O, space group P2₁, with a = 11.072(2) Å, b = 34.663(5)Å, c = 16.446(3) Å, β = 107.85(1) °, Z = 4, R = 8.3% for 6087 data with |F₀| > 3σ(F).