Insertion of methylene units into the turn segment of designed β-hairpin peptides

The effect of insertion of methylene groups into the turn segment of beta-hairpin peptides has been investigated in the model sequence Boc-Leu-Val-Val-(D)Pro-delta-Ava-Leu-Val-Val-OMe. This sequence is related to the previously well-characterized model beta-hairpin octapeptide, Boc-Leu-Val-Val-(D)Pro-Gly-Leu-Val-Val-OMe. Replacement of Gly by delta-Ava (delta-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 beta-hairpin structure is maintained in the delta-Ava peptide. Restrained molecular dynamics simulations, using experimental constraints, yield two structural families that are consistent with the NOE data. Both families correspond to beta-hairpin conformations and differ only in the backbone torsion angles at the delta-Ava residue.