Chain-Folded Lamellar Stacking Structure of the Crystalline Fraction of <i>Bombyx mori</i> Silk Fibroin with Silk II Form Studied by 2D ¹³C–¹³C Homonuclear Correlation NMR Spectroscopy

The structure of <i>Bombyx mori</i> silk fibroin (SF) is a subject of significant interest due to its remarkable physical properties; however, its atomic-level structure is still not conclusive. We previously proposed a lamellar stacking structure for the crystalline fraction (Cp) with β-turns occurring every eighth amino acid. In this study, we took the following steps: At first, a model of the chain-folded lamellar stacking structure in antipolar and antiparallel β-sheet layers was constructed. Then, dipolar-assisted rotational resonance solid-state NMR spectra were observed to determine the effective internuclear distance (<i>r</i>_{<i>j</i>,<i>k</i>}^eff) for the uniformly ¹³C-labeled Cp fraction sample. By comparing the experimentally obtained <i>r</i>_{<i>j</i>,<i>k</i>}^eff (obs) values with the calculated <i>r</i>_{<i>j</i>,<i>k</i>}^eff (calc) values from our structural model, a fairly good correlation between the observed and calculated values of the internuclear distances was obtained with a standard deviation of 0.37 Å. This supports the existence of the chain-folded lamellar stacking structure in the SF fiber. These findings contribute to our understanding of the atomic-level structure of SF and its exceptional properties.