Solvent-induced Beta-hairpin to helix conformational transition in a designed peptide

An octapeptide containing a central Aib-Gly- segment capable of adopting -turn conformations compatible with both hairpin ( II or I ) and helical ( I) structures has been designed. The effect of solvent on the conformation of the peptide Boc-Leu-Val-Val-Aib-Gly-Leu-Val-Val-OMe (VIII; Boc: t-butyloxycarbonyl; OMe: methyl ester) has been investigated by NMR and CD spectroscopy. Peptide VIII adopts a well-defined -hairpin conformation in solvents capable of hydrogen bonding like (CD3)2SO and CD3OH. In solvents that have a lower tendency to interact with backbone peptide groups, like CDCl3 and CD3CN, helical conformations predominate. Nuclear Overhauser effects between the backbone protons and solvent shielding of NH groups involved in cross-strand hydrogen bonding, backbone chemical shifts, and vicinal coupling constants provide further support for the conformational assignments in different solvents. Truncated peptides Boc-Val-Val-Aib-Gly-Leu-Val-Val-OMe (VII), Boc-Val-Val-Aib-Gly-Leu-Val-OMe (VI), and Boc-Val-Aib-Gly-Leu-OMe (IV) were studied in CDCl3 and (CD3)2SO by 500 MHz 1H-NMR spectroscopy. Peptides IV and VI show no evidence for hairpin conformation in both the solvents. The three truncated peptides show a well-defined helical conformation in CDCl3. In (CD3)2SO, peptide VII adopts a -hairpin conformation. The results establish that peptides may be designed, which are poised to undergo a dramatic conformational transitio