The synthetic peptide Z-Gly-Aib-Gly-Aib-OtBu was dissolved in methanol and crystallized in a mixture of ethyl acetate and petroleum ether. The crystals belong to the centrosymmetric space group P4/n that is observed less than 0.3% in the Cambridge Structural Database. The first Gly residue assumes a semi-extended conformation (φ ±62°, ψ ∓131°). The right-handed peptide folds in two consecutive β-turns of type II' and type I or an incipient 310 -helix, and the left-handed counterpart folds accordingly in the opposite configuration. In the crystal lattice, one molecule is linked to four neighbors in the ab-plane via hydrogen bonds. These bonds form a continuous network of left- and right-handed molecules. The successive ab-planes stack via apolar contacts in the c-direction. An ethyl acetate molecule is situated on and close to the fourfold axis.
Keywords: Gly-Aib peptides; achiral peptides; centrosymmetry; crystal structure; semi-extended conformation; α-aminoisobutyric acid.
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