NMR study of silk I structure of Bombyx mori silk fibroin with 15N- and 13C-NMR chemical shift contour plots

The metastable state silk I structures of Bombyx mori silk fibroin in the solid state were studied on the basis of 15N- and 13C-nmr chemical shifts of Ala, Ser, and Gly residues. The 15N cross-polarization magic angle spinning (CP/MAS) nmr spectra of the precipitated fraction after chymotrypsin hydrolysis of B. mori silk fibroin with the silk I and silk II forms were measured to determine the 15N chemical shifts of Gly, Ala, and Ser residues. For comparison, 15N CP/MAS nmr chemical shifts of Ala were measured for [15N] Ala Philosamia cynthia ricini silk fibroin with antiparallel β-sheet and α-helix forms. The 13C CP/MAS nmr chemical shifts of Ala, Ser, and Gly residues of B. mori silk fibroin with the silk I and silk II forms, as well as 13C CP/MAS nmr chemical shifts of Ala residue of P. c. ricini silk fibroin with β-sheet and α-helix forms, are used for the examination of the silk I structure. Both silk I and α-helix peaks are shifted to a lower field than silk II (β-sheet) for the Cα carbons of the Ala residues, while both Cβ carbon peaks are shifted to higher field. However, the silk I peak of the 15N nucleus of the Ala residue is shifted to lower field than the silk II peak, but the α-helix peak is shifted to high field. Thus, the difference in the structure between the silk I and α-helix is reflected in a different manner between the 13C and 15N chemical shifts. The Cα and Cβ chemical shift contour plots for Ala and Ser residues, and the Cα plot for the Gly residue, were prepared from the Protein Data Bank data obtained for 12 proteins and used for discussing the silk I structure quantitatively from the conformation-dependent chemical shifts. The plots reported by Le and Oldfield for 15N chemical shifts were also used for the purpose. All these chemical shift data support Fossey's model (Ala: ϕ = −80°, φ = 150°, Gly: ϕ = −150°, φ = 80°) and do not support Lotz and Keith's model (Ala: ϕ = −104.6°, φ = 112.2°, Gly: ϕ = 79.8°, φ = 49.7° or Ala: ϕ = −124.5°, φ = 88.2°, Gly: ϕ = −49.8°, φ = −76.1°) as the silk I structure. © 1997 John Wiley & Sons, Inc.