Molecular and crystal structures of three monothiated analogues of the terminally blocked ala‐aib‐ala sequence of peptaibol antibiotics

The molecular and crystal structures of three monothiated analogues of the blocked L‐Ala‐Aib‐L‐Ala sequence of peptaibol antibiotics, t‐Boc‐L‐Ala‐Aib‐ψ(CSNH)‐L‐Ala‐OMe, Ac‐L‐Ala‐Aib‐ψ(CSNH)‐L‐Ala‐OMe, and Ac‐ψ(CSNH)‐L‐Ala‐Aib‐L‐Ala‐OMe, determined by x‐ray diffraction analyses, are reported. In all cases the peptide chain is folded with φ,ψ angles close to or slightly distorted from those expected for a type II β‐bend conformation. However, the 4 → 1 H‐bond distance falls within the accepted limits only for Ac‐L‐Ala‐Aib‐ψ(CSNH)‐L‐Ala‐OMe. The structures are compared with those already published for their two oxygenated analogues.