The first non‐helical Aib‐containing hexapeptide: The crystal structure of Z‐Gly‐Aib‐Gly‐Aib‐Gly‐Aib‐OtBu

Abstract

The synthetic peptide Z‐Gly‐Aib‐Gly‐Aib‐Gly‐Aib‐OtBu was crystallized from a mixture of ethyl acetate and n‐hexane. The crystals belong to the centrosymmetric space group Pbca. There are three molecules in the asymmetric unit. The three molecules differ mainly in the Z‐group conformation. The first Gly residue adopts a fully extended conformation, residues 2 and 3 lie in the left‐handed helical region, residues 4 and 5 in the right‐handed helical region, and residue 6 again in the left‐handed helical region of the Ramachandran plot. There are only two of four possible intramolecular hydrogen bonds formed, namely, between Aib4 and Gly1 forming a β‐turn of type III′ and between Aib6 and Gly3 forming a β‐turn of type I. The inverted molecules (by space group symmetry) lie in the regions with opposite handedness and form β‐turns of type III and I′. In contrast to all known long synthetic and naturally occurring Aib‐containing peptides that fold as 310‐ or α‐helix, Z‐(Gly‐Aib)3‐OtBu folds in a quite flat structure from which only the protecting groups bulge out.

Rational design and topochemical synthesis of polymorphs of a polymer

“…Short peptides composed of 2-aminoisobutyric acid (Aib) residues are known to adopt β-turn structures. 55–57 Also, Aib-containing tripeptides are known to adopt multiple conformations. 58–61 In view of these, we designed a tripeptide-monomer N 3 -(Aib) 3 -CH 2 –CCH (1) with an aim to get multiple polymorphs and to polymerize them topochemically.…”