The crystal structure of Z-(Aib)10-OH at 0.65 Å resolution: three complete turns of 310-helix.

The synthetic peptide Z-(Aib)10-OH was crystallized from hot methanol by slow evaporation. The crystal used for data collection reflected synchrotron radiation to sub-atomic resolution, where the bonding electron density becomes visible between the non-hydrogen atoms. Crystals belong to the centrosymmetric space group P1. Both molecules in the asymmetric unit form regular 310 -helices. All residues in each molecule possess the same handedness, which is in contrast to all other crystal structure determined to date of longer Aib-homopeptides. These other peptides are C-terminal protected by OtBu or OMe. In these cases, because of the missing ability of the C-terminal protection group to form a hydrogen bond to the residue i-3, the sense of the helix is reversed in the last residue. Here, the C-terminal OH-groups form hydrogen bonds to the residues i-3, in part mediated by water molecules. This makes Z-(Aib)10-OH an Aib-homopeptide with three complete 310-helical turns in spite of the shorter length it has compared with Z-(Aib)11-OtBu, the only homopeptide to date with three complete turns.