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

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 3(10)-helical fold. Contrary to expectation the structural analysis revealed an unfolded amino terminus, with Ala(1) adopting an extended beta-conformation (Phi=-93 degrees, psi=112 degrees). Residues 2-5 form a 3(10)-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.