In vivo effects of metal ions on conformation and mechanical performance of silkworm silks

Background: The mechanism of silk fiber formation is of particular interest. Although in vitro evidence has shown that metal ions affect conformational transitions of silks, the in vivo effects of metal ions on silk conformations and mechanical performance are still unclear.

Methods: This study explored the effects of metal ions on silk conformations and mechanical properties of silk fibers by adding K⁺ and Cu²⁺ into the silk fibroin solutions or injecting them into the silkworms. Aimed by CD analysis, FTIR analysis, and mechanical testing, the conformational and mechanical changes of the silks were estimated. By using BION Web Server, the interactions of K⁺ and N-terminal of silk fibroin were also simulated.

Results: We presented that K⁺ and Cu²⁺ induced the conformational transitions of silk fibroin by forming β-sheet structures. Moreover, the mechanical parameters of silk fibers, such as strength, toughness and Young's modulus, were also improved after K⁺ or Cu²⁺ injection. Using BION Web Server, we found that potassium ions may have strong electrostatic interactions with the negatively charged residues.

Conclusion: We suggest that K⁺ and Cu²⁺ play crucial roles in the conformation and mechanical performances of silks and they are involved in the silk fiber formation in vivo.

General significance: Our results are helpful for clarifying the mechanism of silk fiber formation, and provide insights for modifying the mechanical properties of silk fibers.