A combined extended and helical backbone for Boc-(Ala-Leu-Ac₇c-)₂-OMe

Abstract

The structure of the peptide Boc-Ala-Leu-Ac₇c-Ala-Leu-Ac₇c-OMe (Ac₇c,1-aminocycloheptane-1-carboxylic acid) is described in crystals. The presence of two Ac₇c residues was expected to stabilize a 3₁₀-helical fold. Contrary to expectation the structural analysis revealed an unfolded amino terminus, with Ala(1) adopting an extended β-conformation (φ = -93°, ψ = 112°). Residues 2-5 form a 3₁₀-helix, stabilized by three successive intramolecular hydrogen bonds. Notably, two NH groups Ala(1) and Ac₇c(3) do not form any hydrogen bonds in the crystal. Peptide assembly appears to be dominated by packing of the cycloheptane rings that stack against one another within the molecule and also throughout the crystal in columns.