Crystal structure and molecular conformation of the cyclic hexapeptide cyclo-(Gly-Aib-Gly)₂

We have synthesized and crystallized the cyclic peptide (Gly-Aib-Gly)₂. Its structure has been determined by conventional X-ray diffraction methods. In the crystal it adopts a conformation with one β-turn (type I) and its mirror image at the other side of the ring. All conformational angles are similar to those reported for these amino acid residues. In particular the Aib residue has a conformation intermediate between α- and 3₁₀-helical conformations. The ring is an adequate model for the β-turn conformation. A molecule of formic acid is found in the crystal which shows a very short hydrogen bond with one of the glycine carbonyl groups.