Methanol-Water-Dependent Structural Changes of Regenerated Silk Fibroin Probed Using Terahertz Spectroscopy

The mechanism of β-sheet crystallization in silk fibroin remains unclear, due to the incomplete information of protein assembly and structural state. The emerging terahertz (THz) spectroscopy (<10 THz) has been taken as an important tool to detect new aspects of biomolecular structure and is used for the first time to analyze the methanol-water (MeOH) induced structural changes of Bombyx mori silk fibroin. Mid-infrared spectroscopy (IR) and X-ray diffraction (XRD) results show that silk fibroin initially exists in a typical silk I form and reassemble into a predominant silk II (antiparallel β-sheet crystal) structure after MeOH treatment. The samples treated with MeOH-H₂O mixed solutions show a predominant silk I structure without any silk-II-related peaks. As the MeOH concentration approaches 40 vol%, the absorbance of the β-sheet-related IR bands and the XRD peaks gradually increase, indicating a formation of β-sheet crystal during this process. THz spectrum shows the absorption capacity below 3 THz as well as the absorbance at 5.1 THz and 7.9 THz is indeed affected by the MeOH-H₂O treatment, implying a MeOH-H₂O-dependent change of intermolecular H-bonds in silk fibroin. The THz spectrum for silk fibroin gives additional information to the existing studies on the MeOH-H₂O induced β-sheet crystallization of silk fibroin, which may help us understanding the structural changes of natural silk.