Abstract
Silk fibroin (SF)-based materials are exposed to both natural and artificial ultraviolet (UV) light during preparation or administration. However, the effects of UV irradiation on SF films prepared under different conditions have not yet been described in detail. In this study, four SF films with different molecular weight (MW) distribution were fabricated using SF solutions, which were prepared by dissolving degummed SF for 0.5–24 h. We observed UV (365 nm) irradiation on SF films induced the increase of yellowness and absorbance at 310 nm of SF films, indicating the formation of new photo-products and di-tyrosine bonds by photo-oxidation. Due to di-tyrosine cross-links between SF chains, UV-irradiated SF films were not fully dissociated in urea solution. In addition to formation of new products, UV reduced the crystallinity of SF films by breaking hydrogen bonds of β-sheet conformation. Unlike the UV-induced decomposition of physical interactions, UV did not affect the covalent bonds (i.e., peptide bonds). Through these experiments, we could expect that SF with higher MW was more susceptible and SF with lower MW was more resistant to UV-induced photo-oxidation and photo-degradation. These results provide useful information about UV-induced aging of SF-based materials under natural sunlight and UV irradiating conditions.
Highlights
Silk fibroin (SF) is one of well-known natural protein materials obtained from silkworms
Amorphous regions of relatively hydrophilic blocks composed of random/α-helix conformations contribute to the elasticity of SF
Investigation on the effect of UV irradiation on the physicochemical properties of SF films prepared under different conditions was carried out
Summary
Silk fibroin (SF) is one of well-known natural protein materials obtained from silkworms. The primary structure of SF is highly unique and composed of repetitive glycinealanine-glycine-alanine-glycine-serine (GAGAGS) [1] These repeating sequences form the crystalline region with β-sheet conformation maintained by various molecular interactions such as hydrogen bonds, van der Waals interaction, and hydrophobic interaction. During the solubilization process of degummed SF, the structure of SF can be affected by various conditions such as the type of solvent, dissolution time, and temperature, resulting in differences in physical and chemical features of SF. Peak shift from 150 kDa to 110 kDa and new peak formation indicating 16 kDa were observed when dissolution time was 180 min [6] Such a different MW distribution resulted in different particle size, rheological properties, and degree of transparency. Spectroscopic and structural properties of UV-irradiated SF films were investigated
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