Refinement of Repeated β-turn Structure for Silk I Conformation of <i>Bombyx mori </i>Silk Fibroin Using ¹³C Solid-State NMR and X-ray Diffraction Methods

Abstract

Refinement of the structural model of silk I (the structure of Bombyx mori silk fibroin before spinning in the solid state) proposed previously was performed on the basis of a detailed analysis of the 13C solid-state NMR data using two-dimensional (2D) spin-diffusion NMR and REDOR of stable isotope-labeled (AG)15, coupled with the X-ray diffraction analysis of the crystalline fraction of B. mori silk fibroin. The repeated β-turn type II structure was proposed, with the torsion angles (φ, ψ) = (−62°, 125°) for Ala residue and (φ, ψ) = (77°, 10°) for Gly residue of poly(Ala-Gly) chain, which satisfies both solid-state NMR and X-ray diffraction data quantitatively. The torsion angles determined in this work were slightly different from the previous angles, (φ, ψ) = (−60°, 130°) for Ala residue and (φ, ψ) = (70°, 30°) for Gly residue. The intra- and intermolecular hydrogen bonding was formed alternatively along the chain. The solid-state NMR is sensitive to the local conformation of a chain, but X-ray diffraction is rather sensitive to intermolecular arrangement as well as the conformation. Thus, the detailed silk I structure with intermolecular arrangement was determined by the use of several solid-state NMR and X-ray diffraction methods complementarily.