Molecular Dynamics of Spider Dragline Silk Fiber Investigated
by <sup>2</sup>H MAS NMR
- Publication date
- Publisher
Abstract
The molecular dynamics of the proteins
that comprise spider dragline
silk were investigated with solid-state <sup>2</sup>H magic angle
spinning (MAS) NMR line shape and spin–lattice relaxation time
(<i>T</i><sub>1</sub>) analysis. The experiments were performed
on <sup>2</sup>H/<sup>13</sup>C/<sup>15</sup>N-enriched N. clavipes dragline silk fibers. The silk protein
side-chain and backbone dynamics were probed for Ala-rich regions
(β-sheet and 3<sub>1</sub>-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<sup>9</sup> s<sup>–1</sup>), while
the
backbone remains essentially static (<10<sup>2</sup> s<sup>–1</sup>). 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 3<sub>1</sub>-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 3<sub>1</sub>-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 3<sub>1</sub>-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 3<sub>1</sub>-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