Hydration-Induced Beta-Sheet Crosslinking Of Alpha-Helical-Rich Spider Prey-Wrapping Silk

Due to its moderate strength (approximate to 700 MPa) and impressive extensibility before breaking (approximate to 60-80%), orb-weaving spider aciniform (AC) prey-wrapping silks are actually the toughest of the spider silks but are remarkably understudied. The previous results indicate that native AC silk fibers are an alpha-helix rich coiled-coil/beta-sheet hybrid nanofiber, and that conversion of disordered or helical domains to beta-sheet aggregates is surprisingly minimal and overall beta-sheet content is low (approximate to 15%). In this work, it is demonstrated through scanning electron microscopy that native AC silk fibers undergo matted cross-linking upon exposure to moisture that increases silk stiffness. The unique molecular mechanism of water-induced cross-linking is revealed with solid-state NMR (SSNMR) methods; water-induced morphological changes are correlated with an increase in AC silk protein beta-sheet content, and additionally a minor unfolding of coiled-coil regions is observed. Continued and increased beta-sheet cross-linking is observed upon application of mechanical shear. The size of these beta-sheet domains to be 4-6 nm using Wide-Line Separation SSNMR is determined. The observation that merely water treatment can be used to convert a protein-based material from a flexible/extensible alpha-helix-rich fiber to a rigid crossed-linked beta-sheet mat is a novel observation that should provide new avenues in bioinspired materials design.