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

Abstract

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 Cα and Cβ carbons of Ala and Ser residues, and the Cα 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 β-sheet and α-helix forms for the structure analyses of silk fibroins. The allowed regions in the 13C chemical shift contour plots for Cα and Cβ carbons of Ala and Ser residues for the structures in silk fibroins, i.e. Silk II, Silk I and α-helix, were determined using their 13C isotropic NMR chemical shifts in the solid state. There are two area of the φ,Ψ map which satisfy the observed Silk I chemical shift data for both the Cα and Cβ carbons of Ala and Ser residues in the 13C chemical shift contour plots.