Effect of Phase Separated Sericin on the Gelation Behavior of Silk Fibroin

Abstract

The silk spinning process offers several advantages over the synthetic fiber spinning process. However, the mechanism of silk spinning has not been clearly elucidated. Silk consists of two proteins—silk fibroin (SF) and silk sericin (SS). Most studies have focused on the silk spinning mechanism based on the self-assembly of SF. However, the role of SS should not be ignored, because SS accounts for 25% of the total silk protein and surrounds the SF in the middle silk gland, maintaining the phase separation between them. Thus, the primary aim of this study was to investigate the effect of phase-separated SS on the conformational transition of SF. In addition, the metal ion partition between SF and SS was also investigated. Finally, the effects of metal ion diffusion between the phase-separated SF and SS on the gelation kinetics of SF were examined. For the study, an SF hydrogel was prepared in the presence of phase-separated SS over an SF solution. The gelation time of SF increased as the concentration of SS in the upper layer increased. The increased gelation time resulted in slow β-sheet formation in the SF solution in the presence of the SS layer. Moreover, the SF hydrogels prepared with 0.5% and 1.0% SS in the upper layer showed the typical X-ray diffraction pattern of Silk I. Thus, SS not only prevents the premature β-sheet formation of SF, but also induces a Silk I hydrogel structure in the SF, even in the phase-separated state. In the ion diffusion experiment, most metal ions showed a higher affinity for SF, except for the Na+ ion. The amount of diffused metal ions and the presence of phase-separated SS affected the gelation kinetics of SF. Most ions that diffused into the SF solution increased the gelation time of the SF solution. Among these, Ca2+ had the strongest delaying effect. The presence of phase-separated SS further increased the gelation time. However, Cu2+ shortened the gelation time.