Molecular Dynamics of Spider Drag line Silk Fiber Investigated by 2H MAS NMR

The molecular dynamics of the proteins that comprise spider dragline silk were investigated with solid-state H-2 magic angle spinning (MAS) NMR line shape and spinlattice relaxation time (T-1) analysis. The experiments were performed on H-2/C-13/N-15-enriched N. clavipes dragline silk fibers. The silk protein side-chain and backbone dynamics were probed for Ala-rich regions (beta-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 (>10(9) s(-1)), while the backbone remains essentially static (<10(2) 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 beta-sheet region and 31-helicies. beta-Sheet subregion 1 ascribed to the poly(Ala) core exhibits slower dynamics, while beta-sheet subregion 2 present in the interfacial, primarily poly(Gly-Ala) region that links the beta-sheets to disordered 31-helical motifs, exhibits faster motions when the silk is supercontracted. Particularly notable is the observation of microsecond backbone motions for beta-sheet subregion 2 and 31-helicies. It is proposed that these microsecond backbone motions lead to hydrogen-bond disruption in beta-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.