The α‐helical conformation of the undecapeptide boc‐ l ‐Ala‐[Aib‐Ala]2‐Glu(OBzl)‐Ala‐[Aib‐Ala]2‐OMe: Synthesis, X‐Ray crystal structure, and conformation in solution

As models of the helical N‐terminal part of alamethicin the undecapeptides Boc‐L‐Ala‐[Aib‐Ala]2‐Glu(OBzl)‐Ala‐[Aib‐Ala]2‐OMe (1) and Boc‐L‐Ala‐[Aib‐Ala]2‐Gly‐Ala‐[Aib‐Ala]2‐OMe (2) were synthesized. 1 was examined by X‐ray crystallography using direct methods for solution of the phase problem. The unit cell (P21) contains two antiparallel α‐helices with nine residues being part of the helix. Continuing chains of head‐to‐tail linked α‐helices are arranged in the crystal lattice. CD and 13C NMR spectra revealed a pronounced stabilizing effect of the α‐aminoisobutyric acid residues for the α‐helical conformation in solution.