Due to its moderate strength (~700 MPa) and impressive extensibility before breaking (~60-80%), orb-weaving spider aciniform (AC) prey-wrapping silks are actually the toughest of all the spider silks but are understudied. Our previous results indicate that native AC silk fibers are an α-helix-rich coiled-coil / β-sheet hybrid nanofiber, and that conversion of disordered or helical domains to β-sheet aggregates is surprisingly minimal and overall β-sheet content in the fiber is very low (~15%) particularly when compared to other spider and silkworm silks. We have recently demonstrated through Scanning Electron Microscopy (SEM) that native AC silk fibers undergo changes where AC silk fiber bundles cross-link forming a rigid mat as a response to moisture. The unique molecular mechanism of this water-induced fiber cross-linking is revealed with 1D and 2D solid-state NMR (ssNMR) methods. Water-induced morphological changes are correlated with an increase in AC silk protein β-sheet content for Ala, Ser, and Gly residues, and additionally we observe a minor unfolding of Val- and Leu-rich domains from α-helical coiled-coil hierarchical regions. Further, we observe a continued and increased structural change upon application of mechanical agitation where the biomaterial can be further cross-linked by applying moderate shear. We determine the size of these formed β-sheet domains to be between 4-6 nm using Wide-line Separation (WISE) ssNMR Spectroscopy. These results reveal a mechanism for water-induced β-sheet fiber cross-linking and stiffening of AC silks that should be of broad interest to scientists investigating protein-based materials inspired by nature.

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

ACS Spring Meeting 2021

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