The α-Helical Conformation of the Aib-Containing Oligopeptide, Boc–(Leu3–Aib)2–OBzl

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Abstract
The octapeptide Boc–(Leu3–Aib)2–OBzl crystallizes in the space group P21, with a=11.450(2), b=27.915(7), c=11.255(2) Å, β=116.95(1)°, and Z=2. The crystal structure has been solved by direct methods and refined to an R value of 0.08. The peptide back-bone, except for the C-terminal Aib residue, folds into a right-handed α-helical conformation, stabilized by four intramolecular (5→1) hydrogen bonds and two intermolecular hydrogen bonds between molecules related by the 21 symmetry along the b direction. In order to determine the helical conformation definitively, the helical parameters (unit height h and unit twist θ) at each amino-acid residue were derived from the bond lengths, bond angles, and torsional angles of the peptide main chain. The average values of these parameters for seven residues (h=1.57 Å and θ=99.0°) showed much better agreement with those for an α-helix (h=1.5 Å and θ=100°) rather than with those for a 310-helix (h=2.0 Å and θ=120°). Peptide helices are arranged in columns in head-to-tail fashion. These columns are packed in parallel with respect to their helical direction. Between neighbouring chains of helices only hydrophobic contacts are found.

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