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Water permeability of spider dragline silk by solid state NMR Li, Xiang


The remarkable mechanical properties of spider dragline silk can be radically altered by the addition of water. Unconstrained spider silk fibers contract to about half of their original length when immersed in water, a phenomenon named supercontraction. With an eye towards understanding supercontraction and facilitating the study of the permeability of other molecules into silk, we use solid-state NMR to investigate the penetration of deuterium oxide into spider dragline silk. The quadrupolar echo NMR experiments are used to examine the hydrogen-deuterium exchange process in a silk sample that has been soaked in deuterium oxide (D20) and then dried. 1 3C-D Rotational Echo Double Resonance (REDOR) NMR experiments are performed to quantify water permeability of silk's different structural regions. In contrast to the indications from previous studies by X-ray diffraction and NMR, our study reveals that the P-sheet crystalline regions of silk are accessible to water. We also show that the correlation between isotope exchange rate and molecular structure, usually found for proteins in solutions and crystal lattices, also holds true for the insoluble silk proteins; the exchange rate is smaller in the /3-sheet crystalline regions than in the rest of the silk. We further found that in the interphase regions of silk, which connect the /3-sheet with the amorphous regions, water permeation is restricted, a reflection of rigid molecular structure or strong hydrogen-bonding in those regions.

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