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.