Structural analysis of silk with 13C NMR chemical shift contour plots.

The polymorphic structures of silk fibroins in the solid state were examined on the basis of a quantitative relationship between the 13C chemical shift and local structure in proteins. To determine this relationship, 13C chemical shift contour plots for C alpha and C beta carbons of Ala and Ser residues, and the C alpha chemical shift plot for Gly residues were prepared using atomic co-ordinates from the Protein Data Bank and 13C NMR chemical shift data in aqueous solution reported for 40 proteins. The 13C CP/MAS NMR chemical shifts of Ala, Ser and Gly residues of Bombyx mori silk fibroin in silk I and silk II forms were used along with 13C CP/MAS NMR chemical shifts of Ala residues of Samia cynthia ricini silk fibroin in beta-sheet and alpha-helix forms for the structure analyses of silk fibroins. The allowed regions in the 13C chemical shift contour plots for C alpha and C beta carbons of Ala and Ser residues for the structures in silk fibroins, i.e. Silk II, Silk I and alpha-helix, were determined using their 13C isotropic NMR chemical shifts in the solid state. There are two area of the phi,psi map which satisfy the observed Silk I chemical shift data for both the C alpha and C beta carbons of Ala and Ser residues in the 13C chemical shift contour plots.