The all-hydrocarbon i,i+4 stapling system using an oct-4-enyl crosslink is one of the most widely employed chemical tools to stabilize an α-helical conformation of a short peptide. This crosslinking system has greatly extended our ability to modulate intracellular protein-macromolecule interactions. The helix-inducing property of the i,i+4 staple has shown to be highly dependent on the length and the stereochemistry of the oct-4-enyl crosslink. Here we show that changing the double bond position within the i,i+4 staple has a considerable impact not only on the formation of the crosslink but also on α-helix induction. The data further increases the understanding of the structure-activity relationships of this valuable chemical tool.

In this study we examined two ester-containing cross-links, hex-2-enyl acetate and hex-2-enyl propionate, as new cross-linking systems for helix stabilization of short peptides. We demonstrated that these hexenyl ester cross-links can be readily installed via a ruthenium-mediated ring-closing metathesis reaction of L-aspartic acid 4-allyl ester or L-glutamic acid 5-allyl ester at position i and (S)-2-(4'-pentenyl)alanine at position i+4 using second generation Hoveyda-Grubbs catalyst at 60 o C. Between these two cross-links, we found that the hex-2-enyl propionate significantly stabilizes the α-helical conformations of short model peptides. The helixstabilizing effects of the hex-2-enyl propionate tether appear to be as powerful as Verdine's i,i+4 allhydrocarbon stapling system, which is one of the most widely used and the most potent helix-stabilizing crosslinking systems. Furthermore, the hex-2-enyl propionate bridge is reasonably robust against non-enzymatic hydrolytic cleavage at a physiological pH. While extended studies for probing its chemical scopes and biological applications are needed, we believe that this new helix-stabilizing system could serve as a useful chemical tool for understanding protein folding and designing conformationally-constrained peptide drugs.