Insertion of Methylene Units into the Turn Segment of Designed β-Hairpin Peptides

Abstract

The effect of insertion of methylene groups into the turn segment of β-hairpin peptides has been investigated in the model sequence Boc-Leu-Val-Val-DPro-δ-Ava-Leu-Val-Val-OMe. This sequence is related to the previously well-characterized model β-hairpin octapeptide, Boc-Leu-Val-Val-DPro-Gly-Leu-Val-Val-OMe. Replacement of Gly by δ-Ava (δ-aminovaleric acid) formally corresponds to expansion of the turn segment from a two-residue loop to a three-residue loop. Backbone proton chemical shifts, vicinal coupling constants, and circular dichroism spectra for the two peptides are virtually indistinguishable. Nuclear Overhauser effects corresponding to short cross-strand interproton distances confirm that the registry of the β-hairpin structure is maintained in the δ-Ava peptide. Restrained molecular dynamics simulations, using experimental constraints, yield two structural families that are consistent with the NOE data. Both families correspond to β-hairpin conformations and differ only in the backbone torsion angles at the δ-Ava residue.