Structure Water-Solubility Relationship in α-Helix-Rich Films Cast From Aqueous and 1,1,1,3,3,3-Hexafluoro-2-Propanol Solutions of S. c. ricini Silk Fibroin

Silk fibroin (SF) produced by the domesticated wild silkworm, Samia cynthia ricini (S. c. ricini) is attracting increasing interest owing to its unique mechanical properties, biocompatibility, and abundance in nature. However, its utilization is limited, largely due to lack of appropriate processing strategies. Various strategies have been assessed to regenerate cocoon SF, as well as the use of aqueous liquid fibroin (LFaq) prepared by dissolution of silk dope obtained from the silk glands of mature silkworm larvae in water. However, films cast from these fibroin solutions in water or organic solvents are often water-soluble and require post-treatment to render them water-stable. Here, we present a strategy for fabrication of water-stable films from S. c. ricini silk gland fibroin (SGF) without post-treatment. Aqueous ethanol induced gelation of fibroin in the posterior silk glands (PSG), enabling its separation from the rest of the silk gland. When dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), the SGF-gel gave a solution from which a transparent, flexible, and water-insoluble film (SGF(HFIP)) was cast. Detailed structural characterization of the SGF(HFIP) as-cast film was carried out and compared to a conventional, water-soluble film cast from LFaq. FTIR and C-13 solid-state NMR analyses revealed both cast films to be alpha-helix-rich. However, gelation of SGF induced by the 40%-EtOH-treatment resulted in an imperfect beta-sheet structure. As a result, the SGF-gel was soluble in HFIP, but some beta-sheet structural memory remains, and the SGF(HFIP) as-cast film obtained has some beta-sheet content which renders it water-resistant. These results reveal a structure water-solubility relationship in S. c. ricini SF films that may offer useful insights towards tunable fabrication of novel biomaterials. A plausible model of the mechanism that leads to the difference in water resistance of the two kinds of alpha-helix-rich films is proposed.