Structure of alanine and glycine residues of Samia cynthia ricini silk fibers studied with solid-state 15N and 13C NMR

The structure of silk fibroin fiber from a wild silkworm, Samia cynthia ricini, whose amino acid sequence is similar to the spider (major ampullate) silk, was determined with solid-state NMR. The N-15 and C-13 labelings for the dominant amino acids, alanine and glycine, residues of S.c.ricini silk fibers, were performed by oral administration of [N-15]alanine, [1-C-13]alanine, [N-15]glycine, or [1-C-13]glycine to the fifth instar larvae of the silkworm, S.c.ricini. The blocks of the oriented silk fibers, stretched by about 10 times the original length of the sample, were prepared, and the N-15 and C-13 solid-state NMR were observed by changing the angles of the oriented silk fiber axis and the magnetic field. All of the oriented spectra of [N-15]glycine silk fibroin fiber and [1-C-13]glycine silk fibroin fiber were slightly broader than the corresponding spectra of alanine isotope-labeled silk fibers. The fraction of alanine residues in oriented domains in the silk fiber samples was 75%, and that of glycine residues was 65%. The specific orientations of NH, NC', C'O, and C'N bonds for alanine and glycine residues in the oriented domain were determined from the angle-dependent spectra. The conformational space for the alanine and glycine residues was substantially reduced with these bond orientations, and the best fit torsion angles, phi and psi, within the reduced conformational space were determined. The torsion angles of these two residues were within antiparallel beta-sheet structural region.