Stapling of a 3₁₀-Helix with Click Chemistry

Short peptides are important as lead compounds and molecular probes in drug discovery and chemical biology, but their well-known drawbacks, such as high conformational flexibility, protease lability, poor bioavailability and short half-lives in vivo, have prevented their potential from being fully realized. Side chain-to-side chain cyclization, e.g., by ring-closing olefin metathesis, known as stapling, is one approach to increase the biological activity of short peptides that has shown promise when applied to 3₁₀- and α-helical peptides. However, atomic resolution structural information on the effect of side chain-to-side chain cyclization in 3₁₀-helical peptides is scarce, and reported data suggest that there is significant potential for improvement of existing methodologies. Here, we report a novel stapling methodology for 3₁₀-helical peptides using the copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction in a model aminoisobutyric acid (Aib) rich peptide and examine the structural effect of side chain-to-side chain cyclization by NMR, X-ray diffraction, linear IR and femtosecond 2D IR spectroscopy. Our data show that the resulting cyclic peptide represents a more ideal 3₁₀-helix than its acyclic precursor and other stapled 3₁₀-helical peptides reported to date. Side chain-to-side chain stapling by CuAAC should prove useful when applied to 3₁₀-helical peptides and protein segments of interest in biomedicine.