Tuning the beta-turn segment in designed peptide beta-hairpins: construction of a stable type I' beta-turn nucleus and hairpin-helix transition promoting segments.

Designed octapeptides Boc-Leu-Val-Val-Aib-(D)Xxx-Leu-Val-Val-OMe ((D)Xxx = (D)Ala, 3a;(D)Val, 3c and (D)Pro, 5a) and Boc-Leu-Phe-Val-Aib-(D)Ala-Leu-Phe-Val-OMe (3b) have been investigated to construct models of a stable type I' beta-turn nucleated hairpin and to generate systems for investigating helix-hairpin conformational transitions. Peptide 5a, which contains a central Aib-(D)Pro segment, is shown to adopt a stable type I' beta-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 beta-hairpin conformation. Replacement of (D)Pro5 by (D)Ala/(D)Val (3a-c) results in sequences that form beta-hairpins in hydrogen bonding solvents like CD(3)OH and DMSO-d(6). However, in CDCl(3) 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 CDCl(3) and a significant population of both helices and hairpins in CD(3)OH and DMSO-d(6). The coexistence of multiple conformations is established by the simultaneous observation of diagnostic NOEs. Control over stereochemistry of the central beta-turn permits generation of models for robust beta-hairpins and also for the construction of systems that may be used to probe helix-hairpin conformational transitions.