Structure of Bombyx mori Silk Fibroin Based on the DFT Chemical Shift Calculation

Despite extensive investigation of Bombyx mori silk fibroin structures by many experimental methods, e.g., X-ray diffraction, molecular modeling calculation, and solid static ¹³C NMR spectroscopy, a set of fingerprint structural parameters remains unavailable both for the silk I and the silk II forms. In this study, a density functional theory (DFT) approach was used to assess available structural parameters on the basis of the comparison of calculated¹³C chemical shifts or shielding tensors with experimental data. The results indicate that:  (i) the silk I form, first proposed in this work, is a 3₁-helixlike conformation with torsion angle ranges of 〈φ〉 = −59 ± 2°, 〈ψ〉 = 119 ± 2°, and 〈ω〉 = 178 ± 2° for the alanine residue and 〈φ〉 = −78 ± 2°, 〈ψ〉 = 149 ± 2°, and 〈ω〉 = 178 ± 2° for the glycine residue in the silk fibroin; and (ii) the silk II structures independently determined by Marsh, Fossey, and Asakura are considered to be more rational than those determined by other authors. The resulting torsion angle ranges for the silk II form are 〈φ〉 = −143 ± 6°, 〈ψ〉 = 142 ± 5°, and 〈ω〉 = 178 ± 2° both for Ala and Gly residues.