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β‐Alanine containing peptides: A novel molecular tool for the design of γ‐turns

In the present paper we describe the synthesis, purification, single crystal x‐ray analysis, and solution conformational characterization of the cyclic tetrapeptide cyclo‐ (L‐Pro‐γ‐Ala‐L‐Pro‐γ‐Ala). This peptide was synthesized by classical solution methods and the cyclization of the free tetrapeptide was accomplished in good yields in diluted methylene chloride solution using N, N‐dicyclohexyl‐carbodiimide (DCCI). The compound crystallizes in the orthorombic space group P212121 from ethyl acetate. All peptide bonds are trans. The molecular conformation is stabilized by two intramolecular hydrogen bonds between the CO and NH groups of the two β‐alanine residues. These hydrogen bonds take part in a C7 structure in which both proline residues occupy the 2 position of an inverse γ‐turn. The two β‐alanine residues have a typical folded conformation (around the Cα‐Cβ bond) observed in other cyclic peptides containing this residue. A detailed 1H‐nmr analysis in CD3CN solution has been carried out. The molecule assumes a twofold symmetry in solution with a molecular conformation consistent with that observed in the solid state.