Conformational transition and liquid crystalline state of regenerated silk fibroin in water

The conformational transition of molecular chains of regenerated silk fibroin (SF) aqueous solution is systematically investigated by circular dichroism, Raman, IR, and UV-vis spectroscopies. It is found that an initial random coil conformation of the SF can be readily changed into an ordered beta-sheet structure by optimizing the solution conditions, such as the SF concentration, pH, temperature, or metal-ion content. Circular dichroic spectra quantitatively confirm a steadily decreased content of the random coil conformation but a significantly increased beta-sheet content after an ultrasonic or extruding treatment. Furthermore, the extrusion is more powerful to achieve high beta-sheet content than the ultrasonic. It is interesting that the polarized optical micrographs of the SF aqueous solution extruded by injection illustrate the formation and existence of liquid crystalline state. A study of extrusion in vitro could be used as a model system to understand the natural silk spinning process in silkworm.