The authors are grateful to Dr. S. Amiya and Mr. K. Enomoto at Kuraray Corp. for their helpful discussion in artificial spinning. T.A. acknowledges support from the Program for Promotion of Basic Research Activities for Innovative Biosciences, Japan.

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-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 C 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 -turn, 13% -sheet (parallel Ala residues), and 25%-sheet (alternating Ala residues). The remaining fraction of 44% amorphous Tyr-rich region, 22% in both distorted-turn and distorted -sheet. Such a heterogeneous structure including distorted-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 C peak of 13C CP/MAS NMR spectra of the silk fibroin samples with different stretching ratios.

There are many kinds of silks spun by silkworms and spiders, which are suitable to study the structureproperty relationship for molecular design of fibers with high strength and high elasticity. In this review, we mainly focus on the structural determination of two well-known silk fibroin proteins that are from the domesticated silkworm, Bombyx mori, and the wild silkworm, Samia cynthia ricini, respectively. The structures of B. mori silk fibroin before and after spinning were determined by using an appropriate model peptide, (AG)15, with several solid-state NMR methods; 13C two-dimensional spin-diffusion solid-state NMR and rotational echo double resonance (REDOR) NMR techniques along with the quantitative use of the conformation-dependent 13C CP/MAS chemical shifts. The structure of S. c. ricini silk fibroin before spinning was also determined by using a model peptide, GGAGGGYGGDGG(A)12GGAGDGYGAG, which is a typical repeated sequence of the silk fibroin, with the solid-state NMR methods. The transition from the structure of B. mori silk fibroin before spinning to the structure after spinning was studied with molecular dynamics calculation by taking into account several external forces applied to the silk fibroin in the silkworm.

The 13C CP/MAS NMR, X-ray diffraction, and Raman spectroscopies were used for monitoring the structural transition of Samia cynthia ricini (S. c. ricini) silk fibroin induced by stretching. Here the silk fibroin was obtained from the aqueous solution stored in the silk gland. All of these spectroscopic data indicate that the structural transition from R-helix to â-sheet occurs with increasing the stretching ratio, especially between the stretching ratios, 4 and 6. The 13C chemical shifts of Ala Câ peak in the 13C CP/MAS NMR spectrum change significantly depending on R-helix, random coil, and two kinds of â-sheet structure, which make it possible to clarify the local structure and structural transition. Actually, the fraction of individual structure in the silk fibroin samples was determined from decomposition of the Ala Câ peak by assuming a Gaussian line shape. To examine the conformational change of Ala residues of the polyalanine region in S. c. ricini silk fibroin observed with these spectroscopic methods, MD simulations for four peptide molecules, AGGAGG(A)12GGAGAG, with R-helix conformation were performed in the presence of water molecules under different tensile strengths, and then additional MM calculations were performed after removal of water molecules. The change in the conformational character of Ala residues induced by stretching is explicable by these calculations.