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¹³C CP/MAS NMR study on structural heterogeneity in Bombyx mori silk fiber and their generation by stretching

Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan

Reprint requests to: Tetsuo Asakura, Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan; e-mail: asakura{at}cc.tuat.ac.jp; fax: 81-42-383-7733.

It is important to resolve the structure of Bombyx mori silk fibroin before spinning (silk I) and after spinning (silk II), and the mechanism of the structural transition during fiber formation in developing new silk-like fiber. The silk I structure has been recently resolved by ¹³C solid-state NMR as a "repeated ß-turn type II structure." Here, we used ¹³C solid-state NMR to clarify the heterogeneous structure of the natural fiber from Bombyx mori silk fibroin in the silk II form. Interestingly, the ¹³C CP/MAS NMR revealed a broad and asymmetric peak for the Ala Cß carbon. The relative proportions of the various heterogeneous components were determined from their relative peak intensities after line shape deconvolution. Namely, for 56% crystalline fraction (mainly repeated Ala-Gly-Ser-Gly-Ala-Gly sequences), 18% distorted ß-turn, 13% ß-sheet (parallel Ala residues), and 25% ß-sheet (alternating Ala residues). The remaining fraction of 44% amorphous Tyr-rich region, 22% in both distorted ß-turn and distorted ß-sheet. Such a heterogeneous structure including distorted ß-turn can be observed for the peptides (AG)_n (n > 9 ). The structural change from silk I to silk II occurs exclusively for the sequence (Ala-Gly-Ser-Gly-Ala-Gly)_n in B. mori silk fibroin. The generation of the heterogeneous structure can be studied by change in the Ala Cß peak of ¹³C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios.

Keywords: Bombyx mori silk fiber; antiparallel ß-sheet structure; poly(Ala-Gly); 13C CP/MAS NMR; silk I and silk II; heterogeneous structure of silk fiber

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