[4S04e] 19. Marine Biotechology - Part 3

6:10 PM - 6:30 PM JST(9:10 AM - 9:30 AM UTC)

[4Marine-15] Tailoring Crystalline Structure of Silk-Based Resins via Chemical Modification toward the Control of Marine Biodegradability

○Shota Akioka¹, Momoka Kuwabara¹, Yuri Matsumoto¹, Keiichi Noguchi¹, Yasumoto Nakazawa¹ (1. Tokyo University of Agriculture and Technology (Japan))

Keywords:

silk fibroin,resin,bio-based plastic,structural modification,crystalline structure

[Introduction] Most plastics currently in use lack marine biodegradability and contribute to marine plastic pollution. Silk fibroin (SF), the primary structural protein of silk fibers produced by domesticated silkworms, possesses multifunctional properties and has also been reported to exhibit gradual biodegradability in marine environments. We have been developing SF-based plastics (SF resins), fabricated by hot-pressing SF in a mold, as potential alternatives to petroleum-based plastics. SF consists mainly (~80%) of repetitive amino acid sequences such as GAGAGS and GAGAGY (G: glycine, A: alanine, S: serine, Y: tyrosine). This characteristic primary structure readily forms crystalline structures through intermolecular hydrogen bonding and hydrophobic interactions, conferring excellent mechanical properties and thermal stability. Previous studies in the medical field have shown that the biodegradability of SF (primarily in fiber and film forms) depends on its crystallinity in enzymatic environments and in vivo. However, its biodegradation behavior in marine environments remains poorly understood, especially in resin forms.

[Purpose] We aimed to modify the crystalline structure of SF resin through selective chemical modification of amino acids involved in the crystallization of SF (serine and tyrosine) as a strategy to control their marine biodegradability.

[Methods] SF resin (RSF) was prepared by hot-pressing amorphous SF sponge in a steel mold. Chemically modified resins were fabricated from SF sponges acylated with five types of carboxylic acid anhydrides. The resulting resins were designated as RSF-A (acetic anhydride), RSF-P (propionic anhydride), RSF-B (butyric anhydride), RSF-isoB (isobutyric anhydride), and RSF-Pi (pivalic anhydride). Thermal and structural properties were evaluated using differential scanning calorimetry (DSC), solid-state ¹³C CP/MAS NMR (NMR), and X-ray diffraction (XRD).

[Results & Discussion] DSC analysis revealed that RSF-A and RSF-Pi exhibited lower crystal relaxation temperatures (T_m) than unmodified RSF. Solid-state NMR analysis showed consistent trends in the secondary structure composition, suggesting that the introduced acyl groups inhibited the formation of crystalline structure. In contrast, RSF-B and RSF-isoB exhibited higher T_m than RSF, indicating that the introduced acyl chains promoted crystallization. This suggests that acyl group plays a critical role in tuning crystallinity and potentially marine biodegradability.

[Conclusion] Five types of SF-based resins with different crystalline structures and physical properties were successfully fabricated by introducing acyl groups with varying lengths and steric hindrance. Future evaluation of their marine biodegradability will contribute to the design of bio-based plastics with tunable degradation behavior.

[Funding] This study was supported by JST COI-NEXT Program (JPMJPF2104) and JSPS KAKENHI (23K13573).