Coexistence of Folded and Extended Conformations of a Tripeptide Containing α,α-Di-n-propylglycine in Crystals

Abstract

The crystal structure of the tripeptide Boc-Leu-Dpg-Val-OMe (Dpg, α, α-di-n-propylglycine) reveals the coexistence of two distinct backbone conformations. In molecule A the Dpg residue adopts a fully extended conformation (φ = 76.0°, ψ =180.0°) while in molecule B a left handed helical conformation (φ = 62.8°, ψ = 39.6°) is observed. Molecule B adopts a folded structure corresponding to a highly distorted Type II β-turn conformation, which lacks an intramolecular 4 → 1 hydrogen bond. In contrast, molecule A has an open, extended conformation. The results demonstrate that both fully extended and helical conformations are energetically accessible to the Dpg residue. © 1994 Academic Press. All rights reserved.