Conformational Transitions of Thai Silk Fibroin Secondary Structures

Secondary structures of silk fibroin (SF) related directly to their gelling, degradability, mechanical properties which affects its applications. The current works investigated the changes of Thai SF's secondary structure during the process of film fabrication. Thai SF from domestic silk cocoons, Bombyx mori (Nangnoi Sisaket 1) were dissolved into water soluble protein using two solvent systems, 9.3 M LiBr solution, and Ajisawa's reagent (CaCl2: water: EtOH at 1: 8: 2 by mole). The SF solution was subjected to liquid nitrogen followed by lyophilization to preserve the protein's structure. Films were casted from the SF solutions and were immersed in ethanol to regenerate the protein's original conformations. Attenuated totally reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) was used to quantify relative contents of protein's secondary structures altered by each step of the processes. Degummed silk fibers had the highest beta sheet crystallinity of 59.1%. Random coils and beta sheet structures of the SF prepared with Ajisawa's reagent were at 41.5% and 18.2%, and those prepared with LiBr solution were at 45.8% and 14.4%, respectively. The non-treated SF films consisted of 33.8 - 35.0% and 23.2 - 25.9% of beta sheets and random coils, respectively, despite of their preparation processes. Regeneration of the SF using ethanol immersion increased the beta sheet contents by 10 - 12% compared to the non-treated ones. Relative contents of beta turn, alpha helix, and tyrosine side chains were unchanged. The EtOH-regenerated Thai SF films contained 11-17% lower in beta sheet structures than the native fiber, suggesting the silk II formation was partially regained.