NMR-STUDY OF SILK-I STRUCTURE OF BOMBYX-MORI SILK FIBROIN WITH N-15-NMR AND C-13-NMR CHEMICAL-SHIFT CONTOUR PLOTS

The metastable slate silk I structures of Bombyx mori silk fibroin in the solid state were studied on the basis of N-15- and C-13-nmr chemic al shifts of Ala, Ser, and Gly residues. The N-15 cross-polarization m agic 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 N-15 chemical shifts of Gly, Ala, and Ser residues. For comparison, N-15 CP/MAS nmr chemica l shifts of Ala were measured for [N-15]Ala Philosamia cynthia ricini silk fibroin with antiparallel beta-sheet and alpha-helix forms. The C -13 CP/MAS nmr chemical shifts of Ala, Ser, and Gly residues of B. mor i silk fibroin with the silk I and silk II forms, as well as C-13 CP/M AS nmr chemical shifts of Ala residue of P. c. ricini silk fibroin wit h beta-sheet and alpha-helix forms, are used for the examination of th e silk I structure. Both silk I and alpha-helix peaks are shifted to a lower field than silk II (beta-sheet) for the C alpha carbons of the Ala residues, while both C beta carbon peaks are shifted to higher fie ld. However, the silk I peak of the N-15 nucleus of the Ala residue is shifted to lower field than the silk II peak, but the alpha-helix pea k is shifted to high field. Thus, the difference in the structure betw een the silk I and or-helix is reflected in a different manner between the C-13 and N-15 chemical shifts. The C alpha and C beta chemical sh ift contour plots for Ala and Ser residues, and the C alpha plot for t he Gly residue, were prepared from the Protein Data Bank data obtained for 12 proteins and used for discussing the silk I structure quantita tively from the conformation-dependent chemical shifts. The plots repo rted by Le and Oldfield for N-15 chemical shifts were also used for th e purpose. All these chemical shift data support Fossey's model (Ala: phi = -80 degrees phi = 150 degrees Gly: phi = -150 degrees phi = 80 d egrees) and do not support Lotz and Keith's model (Ala: phi = -104.6 d egrees, phi = 112.2 degrees Gly: phi = 79.8 degrees, phi = 49.7 degree s or Ala: phi = -124.5 degrees, phi = 88.2 degrees Gly: phi = -49.8 de grees, phi = -76.1 degrees) as the silk I structure. (C) 1997 John Wil ey & Sons, Inc.