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J Am Chem Soc. 2008 Dec 17;130(50):17070-4. doi: 10.1021/ja806654t.

Silk fiber assembly studied by synchrotron radiation SAXS/WAXS and Raman spectroscopy.

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European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble Cedex, France.


We have characterized the steps involved in silk assembly from the protein solution into beta-type fibers by a combination of small-angle and wide-angle X-ray scattering and Raman spectroscopy. The aggregation process was studied in a concentric flow microfluidic cell, which allows mimicking the spinning duct. The fibroin molecule in solution shows an elongated shape with a maximum diameter of 38 nm. During the pH-driven initial assembly step, large-scale aggregates of fibroin molecules with a maximum diameter of about 260 nm are formed. Raman spectroscopy on the dried, fibrous material shows a principally alpha-helical silk I secondary structure, which is transformed gradually into beta-type silk II by increasing immersion times in water. The formation of crystalline beta-sheet domains within the fiber is confirmed by wide-angle X-ray scattering. The assembly process resembles the peptide condensation-ordering model proposed for amyloid cross-beta formation.

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