MECHANICAL STRESS AND THERMAL TREATMENTS INDUCED ALPHA-HELIX TO BETA-SHEET TRANSITION IN SILK FIBROIN FILMS
Bombyx mori consisting of the heavy and light chains and forming the filaments of the silk fibre is the interior structural protein which gives silk its mechanical strength, softness, and elasticity. Silk fibroin (SF) finds application in the fields of the biomedical and biotechnological engineering. Silk fibroin stands as an exemplar of fibrous proteins containing crystalline beta-sheets (silk-II) and β-turns (silk-II). Exposure to organic solvents, mechanical stressor, and thermal treatments can induce non-reversible crystallization. This study compares the structural and thermal properties of silk fibroin films obtained from different raw materials: silk fibroin processed (annealing treatment, freezing, and freeze-drying) and not processed. The presence of crystalline and amorphous structures was evaluated using ATR/FTIR spectroscopy and thermal properties of SF films were studied by DSC measurements. Accordingly, to ATR/FTIR results, the presence of beta-sheet (silk-II) structure is present in the samples that were processed by annealing treatment, freezing, and freeze-drying as well as Bombyx mori cocoon. Difference in exothermic peaks (around 200°C) in DSC curves reveals that samples not processed present a higher content of amorphous structures than samples processed. The degradation peak around 300°C was observed for all samples. The analysis also points out that water annealing treatment, freezing, and freeze-drying induce conformational changes in secondary structures of SF films. It can be concluded that the presence of crystalline structures in SF films varies with the process. This fact represents a progress in the development of processing alternatives to control SF materials structure and physio mechanical properties.