Dissolution Behavior and Structural Changes of Silk Fibroin in Calcium Chloride-Ethanol-Water Solvent Systems
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Abstract:
Conditions for the transformation of insoluble silk fibroin into soluble regenerated silk fibroin and the effect of structural changes in silk fibroin on its solubility were examined to enable preparation of soluble silk fibroin for use in food products. The effects of solvent system, temperature, salt concentration, and ethanol concentration on the dissolution of silk fibroin, as assessed by dissolution time and the nitrogen solubility index of the regenerated silk fibroin, were analyzed. The results showed that insoluble silk fibroin could be dissolved in calcium chloride-ethanol-water systems; soaking of silk fibroin in a solution of 4 mol/L calcium chloride and 30% (V/V) ethanol and heating at 95 ℃for 9 min led to complete dissolution, and the nitrogen solubility index of the regenerated silk fibroin reached 91%. Structural changes in silk fibroin upon dissolution were also examined using low-field nuclear magnetic resonance (NMR) spectroscopy, circular dichroism spectroscopy, X-ray diffractometry, and scanning electron microscopy. In the calcium chloride-ethanol-water system, interactions between silk fibroin and water gradually increased in strength, the silk fibroin fibers swelled and gradually broke into a lamellar shape, and the crystal structure of the silk fibroin was destroyed. In solution, the structure of the regenerated silk fibroin changed from a β-sheet structure to a random coil structure, resulting in an improvement in solubility.
