An Improved Turn Structure for Inducing β-Hairpin Formation in Peptides.

Although β-hairpins are widespread in proteins, there is no tool to coax any small peptide to adopt a β-hairpin conformation, regardless of sequence. Here, we report that δ-linked γ(R)-methyl-ornithine (δ MeOrn) provides an improved β-turn template for inducing a β-hairpin conformation in peptides. We developed a synthesis of protected δ MeOrn as a building block suitable for use in Fmoc-based solid-phase peptide synthesis. The synthesis begins with l-leucine and affords gram quantities of the Nα -Boc-Nδ -Fmoc-γ(R)-methyl-ornithine building block. X-ray crystallography confirms that the δ MeOrn turn unit adopts a folded structure in a macrocyclic β-hairpin peptide. CD and NMR spectroscopy allow comparison of the δ MeOrn turn template to the δ-linked ornithine (δ Orn) turn template that we previously introduced and to the popular d-Pro-Gly turn template. These studies show that the folding of the δ MeOrn turn template is substantially better than that of δ Orn and is comparable to d-Pro-Gly.