Spider dragline silk is a semicrystalline protein polymer in which alanine-rich crystalline regions are connected by soft glycine-rich linkers that,in dry fibers are quiescent but become motionally active when plasticized by water.Using solid-state NMR to probe molecular motion and orientation of selectively ~13C-labeled glycine residues,we observe a collapse of amino acid chains in hydration-induced local phase transitions and find that the relative abundance of the static and mobile phases can be controlled by fiber strain.Our results suggest a molecular mechanism for silk's mechanical properties based on latent entropic springs that are drawn in performed extended structures,stabilized by interchain hydrogen bonding,and collapsed by hydration.
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