Tuning the β-turn segment in designed peptide β-hairpins: Construction of a stable type I′ β-turn nucleus and hairpin-helix transition promoting segments Ч

Abstract

Designed octapeptides Boc-Leu-Val-Val-Aib-^DXxx-Leu-Val-Val-OMe (DXxx = ^DAla, 3a;^DVal, 3c and ^DPro, 5a) and Boc-Leu-Phe-Val-Aib-^DAla-Leu-Phe-Val-OMe (3b) have been investigated to construct models of a stable type I′ β-turn nucleated hairpin and to generate systems for investigating helix-hairpin conformational transitions. Peptide 5a, which contains a central Aib-DPro segment, is shown to adopt a stable type I′ β-turn nucleated hairpin structure, stabilized by four cross-strand hydrogen bonds. The stability of the structure in diverse solvents is established by the observation of all diagnostic NOEs expected in a β-hairpin conformation. Replacement of ^DPro5 by ^DAla/^DVal (3a-c) results in sequences that form β-hairpins in hydrogen bonding solvents like CD₃OH and DMSO-d₆. However, in CDCl₃ evidence for population of helical conformations is obtained. Peptide 6b (Boc-Leu-Phe-Val-Aib-Aib-Leu-Phe-Val-OMe), which contains a centrally positioned Aib-Aib segment, provides a clear example of a system, which exhibits a helical conformation in CDCl₃ and a significant population of both helices and hairpins in CD₃OH and DMSO-d₆. The coexistence of multiple conformations is established by the simultaneous observation of diagnostic NOEs. Control over stereochemistry of the central β-turn permits generation of models for robust -hairpins and also for the construction of systems that may be used to probe helix-hairpin conformational transitions.