Heterogeneous Structure of Silk Fibers from Bombyx mori Resolved by 13C Solid-State NMR Spectroscopy

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Abstract
The molecular conformation of silk fibrion is characterized by solid-state 13C NMR before spinning (silk I structure) and after spinning (silk II structure). We compare native silk fibers with the quasi-crystalline Cp-fraction and a synthetic model peptide (Ala-Gly)15, both of which can be converted either into silk I by dialysis from 9 M LiBr or into silk II by treatment with formic acid. Our results demonstrate that silk II fibers are intrinsically heterogeneous, consisting of β-sheets, distorted β-turns, and distorted β-sheets. This higher-order heterogeneity is revealed by the 13C-NMR Cβ-peak of Ala, indicating that the Ala side chains are stacked partially in parallel and partially face-to-face, at a ratio of 1:2.