Folding in Place: Design of β-Strap Motifs to Stabilize the Folding of Hairpins with Long Loops

Despite their pivotal role in protein function and antibody binding affinity, β-hairpins bearing long non-canonical loops are a challenge to modern synthesis because of the large entropic penalty associated with their folding. Little is known about the contribution and impact of stabilizing motifs on the folding of β-hairpins of variable length and plasticity. Here we report a direct comparison between these b-straps thermodynamics and their thermal stability behavior using several local spectroscopic probes of the folding/unfolding landscape. The judicious cooperative interactions crafted in β-Strap RW(VW)•••(WV/H)WE (strap = strand + cap) greatly stabilized hairpins with up to 10-residue loops lacking an innate nucleating-turn locus (T_m up to 52 ^oC; 88 ± 1% folded at 291 K). The present design of novel β-straps aims to provide the foundation to study new classes of long hairpins and ultimately offer an attractive alternative to macrocyclic peptides for the mimicry of functional loops from proteins and antibodies.