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

Despite their pivotal role in defining
antibody affinity and protein
function, β-hairpins harboring long noncanonical loops remain
synthetically challenging because of the large entropic penalty associated
with their conformational folding. Little is known about the contribution
and impact of stabilizing motifs on the folding of β-hairpins
with loops of variable length and plasticity. Here, we report a design
of minimalist β-straps (strap = strand + cap) that offset the
entropic cost of long-loop folding. The judicious positioning of noncovalent
interactions (hydrophobic cluster and salt-bridge) within the novel
8-mer β-strap design RW­(V/H)­W···WVWE stabilizes
hairpins with up to 10-residue loops of varying degrees of plasticity
(Tm up to 52 °C; 88 ± 1% folded
at 18 °C). This “hyper” thermostable β-strap
outperforms the previous gold-standard technology of β-strand-β-cap
(16-mer) and provides a foundation for producing new classes of long
hairpins as a viable and practical alternative to macrocyclic peptides.