Helix and hairpin nucleation in short peptides using centrally positioned conformationally constrained dipeptide segments

Abstract

The effect of incorporation of a centrally positioned Ac₆c-Xxx segment where Xxx = ^LVal/^DVal into a host oligopeptide composed of L-amino acid residues has been investigated. Studies of four designed octapeptides Boc-Leu-Phe-Val-Ac₆c-Xxx-Leu-Phe-Val-OMe (Xxx = ^DVal 1, ^LVal 2) Boc-Leu-Val-Val-Ac₆c-Xxx-Leu-Val-Val-OMe (Xxx = ^DVal 3, ^LVal 4) are reported. Diagnostic nuclear Overhouse effects characteristic of hairpin conformations are observed for Xxx = ^DVal peptides (1 and 3) while continuous helical conformation characterized by sequential N_iH ↔ N_i+1H NOEs are favored for Xxx = ^LVal peptides (2 and 4) in methanol solutions. Temperature co-efficient of NH chemical shifts are in agreement with distinctly different conformational preferences upon changing the configuration of the residue at position 5. Crystal structures of peptides 2 and 4 (Xxx = ^LVal) establish helical conformations in the solid state, in agreement with the structures deduced from NMR data. The results support the design principle that centrally positioned type I β-turns may be used to nucleate helices in short peptides, while type I′ β-turns can facilitate folding into β-hairpins.