Due to its high strength (~700 MPa) and impressive extensibility before breaking (~60-80%), orb-weaving spider aciniform (AC) prey-wrapping silks are actually the toughest of the spider silks but are remarkably uncharacterized. When stored in the gland AC silk protein is highly helical, adopting a compact “beads-on-a-string” hierarchy with no evidence of β-sheet secondary structure. Our previous results indicate that native AC silk fibers is a α-helix rich coiled-coil / β-sheet hybrid nanofiber, and that conversion of disordered or helical domains to β-sheet aggregates is surprisingly minimal. In this work we demonstrate through solid-state NMR, FTIR and SEM techniques that native AC silks exhibit a remarkable water-induced cross-linking. These physical changes are correlated to an increase in β-sheet content from Ala, Ser, Gly and Gln residues, and additionally we observe a clear unfolding of Val- and Leu-rich domains from coiled-coil regions. We hypothesize that exposure to moisture provides disordered regions and semi-stable coiled-coil motifs the necessary degrees of freedom to access lower-energy β-sheet aggregates. We think this cross-linking behavior is an evolutionarily-adapted trait that benefits the spider during extra-oral digestion and feeding. As the spider feeds, digestive fluids are regurgitated onto her prey. The moisture-exposed prey-wrapping silk bundles are fused together through water-induced b-sheet crosslinking, collapsing the inter-fiber space and becoming more rigid, thereby squeezing and further crushing the prey-package. In addition to their intriguing ecological implications, these findings should be of significant interest to polymer engineers and materials chemists; using water as a solvent to convert malleable, flexible and tough protein-based materials into rigid cross-linked sheets may prove instrumental to developing biomaterials tailored for defense, biomedical and space exploration applications.

Water-induced β-sheet crosslinking of α-helix rich spider prey-wrapping silk

2019 Fall National Meeting

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