The addition of water to spider dragline silk results in fiber contraction to 50% its initial length and significant changes to the mechanical properties of the silk. This event has been termed supercontraction. A decrease in strength and increase in elasticity have been reported when the silk is in contact with water. Two-dimensional wide-line separation (WISE) nuclear magnetic resonance (NMR) is implemented to correlate 13C chemical shifts with mobility by observing the corresponding 1H line widths and line shapes in water-saturated spider dragline silk. The WISE NMR spectrum of the native silk exhibits 1H line widths that are ∼40 kHz for all carbon environments characteristic of a rigid organic system. In contrast, the water-saturated case displays a component of the 1H line that is narrowed to ∼5 kHz for the glycine Cα and a newly resolved alanine helical environment while the alanine Cβ corresponding to the β-sheet conformation remains broad. These results indicate that water permeates the amorphous, glycine-rich matrix and not the crystalline, polyalanine β-sheets. A delay time is added to the WISE NMR pulse sequence to monitor spin diffusion between the amorphous, mobile region and the crystalline domains. The time required for spin diffusion to reach spatial equilibrium is related to the length scale of the polyalanine crystallites. This technique is employed to measure crystalline domain sizes on the nanometer length scale in water-solvated spider dragline silk. These results provide further insight into the structure of spider silk and mechanism of supercontraction.
ASJC Scopus subject areas
- Colloid and Surface Chemistry