American Chemical Society
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Molecular Dynamics of Spider Dragline Silk Fiber Investigated by 2H MAS NMR

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journal contribution
posted on 2015-03-09, 00:00 authored by Xiangyan Shi, Gregory P. Holland, Jeffery L. Yarger
The molecular dynamics of the proteins that comprise spider dragline silk were investigated with solid-state 2H magic angle spinning (MAS) NMR line shape and spin–lattice relaxation time (T1) analysis. The experiments were performed on 2H/13C/15N-enriched N. clavipes dragline silk fibers. The silk protein side-chain and backbone dynamics were probed for Ala-rich regions (β-sheet and 31-helical domains) in both native (dry) and supercontracted (wet) spider silk. In native (dry) silk fibers, the side chains in all Ala containing regions undergo similar fast methyl rotations (>109 s–1), while the backbone remains essentially static (<102 s–1). When the silk is wet and supercontracted, the presence of water initiates fast side-chain and backbone motions for a fraction of the β-sheet region and 31-helicies. β-Sheet subregion 1 ascribed to the poly­(Ala) core exhibits slower dynamics, while β-sheet subregion 2 present in the interfacial, primarily poly­(Gly-Ala) region that links the β-sheets to disordered 31-helical motifs, exhibits faster motions when the silk is supercontracted. Particularly notable is the observation of microsecond backbone motions for β-sheet subregion 2 and 31-helicies. It is proposed that these microsecond backbone motions lead to hydrogen-bond disruption in β-sheet subregion 2 and helps to explain the decrease in silk stiffness when the silk is wet and supercontracted. In addition, water mobilizes and softens 31-helical motifs, contributing to the increased extensibility observed when the silk is in a supercontracted state. The present study provides critical insight into the supercontraction mechanism and corresponding changes in mechanical properties observed for spider dragline silks.