Crystal structure of the α‐helical undecapeptide Boc‐L‐Ala‐Aib‐Ala‐Aib‐Ala‐Glu(OBzl)‐Ala‐Aib‐Ala‐Aib‐Ala‐OMe

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Abstract
The x‐ray structure of Boc‐L‐Ala‐Aib‐Ala‐Aib‐Ala‐Glu(OBzl)‐Ala‐Aib‐Ala‐Aib‐Ala‐OMe(I) represents the first α‐helix determined by direct methods. This undecapeptide is a model of the N‐terminus of alamethicin, and it exhibits voltage‐dependent pores in bilayer membranes at a higher voltage and concentration than alamethicin. The molecule crystallizes in the monoclinic space group P21 with a = 10.602(1), b = 23.884(3), c = 13.622(1) Å, β = 95.61(6)°, and Z = 2. It adopts a right‐handed α‐helical conformation in the solid state with intramolecular 5 → 1 hydrogen bonds. An additional intramolecular hydrogen bond is bifurcated, forming a stronger 4 → 1 interaction (i.e., a β‐turn III) and a weaker 5 → 1 interaction, thus prolonging the α‐helical part up to 9 residues. The α‐helix radius of 2.1 Å, the height per residue (distance Ni … Ni + 4) of 1.53 Å, the resulting length of the α‐helical part of 13.8 Å (9 residues) resp. 15.3 Å (10 residues), the van der Waals radius (4.7 Å), and the minimal diameter of pores formed by aggregation of 3–10 α‐helices were calculated omitting the Glu(OBzl) side chain. In the crystal, the α‐helices are linked head to tail via two hydrogen bridges forming continuous chains. Adjacent helices are oriented in antiparallel with their helix axes and have only van der Waals contacts.