Design of a peptide hairpin containing a central three-residue loop

The construction of a designed β-hairpin structure, containing a central three-residue loop has been successfully achieved in the synthetic nonapeptide Boc-Leu-Phe-Val-DPro-LPro-DAla-Leu-Phe-Val-OMe (2). The design is based on expanding the two-residue loop established in the peptide β-hairpin Boc-Leu-Phe-Val-DPro-LPro-Leu-Phe-Val-OMe (1). Characterization of the registered β-hairpins in peptides 1 and 2 is based on the observation of key nuclear Overhauser effects (NOEs) in CDCl3 and CD3OH. Solvent titration and temperature dependence of NH chemical shifts establish the identity of NH groups involved in interstrand hydrogen bonding. In peptide 2, the antiparallel registry is maintained, with the formation of a DPro-LPro-DAla loop, stabilized by a 5→1 hydrogen bond between Val3 CO and Leu7 NH groups (C13, β-turn) and a 3→1 hydrogen bond between dPro4 CO and DAla6 NH groups (C7, γ-turn). NMR derived structures suggest that in peptide 2, DAla(6) adopts an αL conformation. In peptide 1, the DPro-LPro segment adopts a type II' β-turn. Replacement of DAla (6) in peptide 2 by lAla in peptide 3 yields a β-hairpin conformation, with a central DPro-LPro two-residue loop. Strand slippage at the C-terminus results in altered registry of the antiparallel strands