The effect of water on the structure and dynamics of spider silk and silk-like polymers studied by magnetic resonance and x-ray diffraction

Due to its unique combination of tensile strength and elasticity, the dragline silk of the orb-weaving spider Nephila clavipes has attracted much attention. Most importantly, it has a high energy to break that is unparalleled in other fibers. Though the basis for the strength of the silk fiber has been uncovered, the molecular reason of the fiber's large shrinkage in water is unknown. This has been a major hurdle in the practical applications of the fiber, and to any man-made copy of this material. Small-angle X-ray scattering (SAXS) is used to probe of the long-range structures in the semicrystalline silk. Scattering patterns of wet and dry samples indicate that the crystalline regions stack along the fiber axis to form lamellar structures. These structures are sparsely dispersed in a softer matrix with a long spacing of 8.4 nm. This spacing increases reversibly by 4% when fibers are stretched by 10%, and shrinks to 5.8 nm when fibers shrink 45% in length on wetting. Solid-state nuclear magnetic resonance (NMR) experiments are performed to reveal the microscopic details of the dynamics in the silk. Cross-polarization magic-angle spinning 13C NMR demonstrates that a substantial fraction of the glycine, glutamine, tyrosine, serine, and leucine residues experience dramatic increases in the rate of large-amplitude reorientation at the protein backbone when fibers are wet. Variable temperature deuterium NMR measurements were carried out on silk samples that incorporate leucine deuterated at the methyl group. Results show that only a subset of these leucine residues is strongly affected by water. Quantitative analysis and chemical considerations suggest that the highly conserved YGGLGS(N)QGAGR blocks, only found in the dragline silk protein, play a major role in the supercontraction process. Protein sequences are proposed to produce artificial spider silk with similar mechanical properties, but without the undesired phenomenon of supercontraction. The spinning and postspinning processing procedures are investigated by regenerating fibers from dissolved native dragline silk. Tensile tests and structural characterization of the regenerated fibers have correlated the properties of the final material and the processing history of the fiber. An aqueous environment is important in the annealing of the material.