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.