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

Abstract

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.