Acetylation of Bombyx mori silk fibroin and their characterization in the dry and hydrated states using 13C solid-state NMR

Abstract

Acetylation of Bombyx mori silk fibroin (SF) was accomplished by chemical reaction of the side chain OH groups in Ser and Tyr residues with acetic anhydrate to increase hydrophobic character of SF. A combination of three kinds of 13C solid-state NMR techniques was used to elucidate the effect of acetylation in the dry and hydrated states for [3-13C]Ser, [3-13C]Tyr and [3-13C]Ala enriched-SF powder. The mobilities of Tyr and Ala residues in the amorphous region of acetylated 13C-labeled SF powder increased slightly and only very small amounts of the Ala residue with high mobility was observed by hydration. On the other hand, there are essentially no effect of water on the β-sheet region of these residues and acetylated Ser residues. A similar tendency toward the effect of hydration for acetylated SF powder was observed for acetylated SF fiber. The remarkable improvement of the dimensional stability in water was attained although the tensile strengths of fibers were slightly lower by the acetylation of SF fiber. Thus, acetylation proved very effective to achieve water-proofing of SF materials and the effect of acetylation on the local structure and dynamics of SF could be clarified using 13C solid-state NMR.