WISE NMR Characterization of Nanoscale Heterogeneity and Mobility in Supercontracted Nephila clavipes Spider Dragline Silk

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 C_α and a newly resolved alanine helical environment while the alanine C_β 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.