Internal Secondary Structural Conformational States of Silk Fibroin Studied by Raman Spectroscopy with Band Deconvolution Analysis

Silk fibroin (SF) is an extensively utilized biofiber recognized for its excellent mechanical properties across various applications. However, the relationship between its internal structure and mechanical behavior remains inadequately understood. In this work, we employed polarized Raman spectra to investigate how SF's secondary structures respond to external tensile loads. Our results show considerable variations in the amide III band, particularly the 1210 cm^-1 random coil subpeak, under tensile stimuli. Subsequent loading-unloading experiments that plotted the peak intensity ratio of <i>I</i>_{1210 cm^-1}/<i>I</i>_{1226 cm^-1} against tensile strain produced multiple hysteresis loops, suggesting irreversible structural changes during mechanical cycling. With additional evidence from other characteristic bands, we demonstrate that tensile elongation facilitates the transition of random coils to β-sheets within the SF network, which intrinsically contributes to the fiber's stiffening behavior. This work provides valuable insight into SF's deformation mechanism and highlights the significance of the previously underappreciated amide III band.