Conformation of di-n-propylglycine residues (Dpg) in peptides: crystal structures of a type I' β-turn forming tetrapeptide and an α-helical tetradecapeptide

Abstract

The crystal structures of two oligopeptides containing di-n-propylglycine (Dpg) residues, Boc-Gly-Dpg-Gly-Leu-OMe (1) and Boc-Val-Ala-Leu-Dpg-Val-Ala-Leu-Val-Ala-Leu-Dpg-Val-Ala-Leu-OMe (2) are presented. Peptide 1 adopts a type I'β - turn conformation with Dpg(2)-Gly(3) at the corner positions. The 14-residue peptide 2 crystallizes with two molecules in the asymmetric unit, both of which adopt α-helical conformations stabilized by 11 successive 5 → 1 hydrogen bonds. In addition, a single 4 → 1 hydrogen bond is also observed at the N-terminus. All five Dpg residues adopt backbone torsion angles (Φ, ψ) in the helical region of conformational space. Evaluation of the available structural data on Dpg peptides confirm the correlation between backbone bond angle N-C^α - C'(τ) and the observed backbone Φ, ψ values. For τ > 106°, helices are observed, while fully extended structures are characterized by τ < 106°. The mean τ values for extended and folded conformations for the Dpg residue are 103.6° ± 1.7° and 109.9° ± 2.6°, respectively.