Interruption of a 3_10-helix by a single Gly residue in a poly-Aib motif: A crystallographic study

The structural influence of a single Gly residue inserted into an Aib₁₆ homooligomer was studied in the solid state by X-ray crystallography. The peptides N₃Aib₈GlyAib₈PheNH₂ (1) and CbzPheAib₈GlyAib₈ (2) were found to adopt well-defined helical structures, which are broadly 3₁₀ helical. Indeed, 2 is the longest crystallographic 3₁₀ helix thus far reported. However, in the region of the central Gly residue, a loosening of the 3₁₀ structure is observed in both peptides, with 1 clearly showing local adoption of an α-helical structure in the region of residues 7-9.