Structure and Structural Changes of the Silk Fibroin from Samia cynthia ricini Using Nuclear Magnetic Resonance Spectroscopy

Abstract

Summary: The structure of silk fibroin from a wild silkworm, S. c. ricini, the amino acid sequence of which consists of repeated poly-Ala and Gly-rich regions, was examined by using solution and solid-state NMR methods. The structural transition of the silk fibroin in aqueous solution was monitored by using ¹³C solution NMR spectroscopy as a function of temperature. The fast exchange with respect to the chemical shift between the helix and coil conformations was observed in the poly-Ala region and the slow conformational change from α-helix to random coil was observed for the Gly residue adjacent to the N-terminal Ala residue of the poly-Ala region. The torsion angles of several Ala and Gly residues in the model peptide, GGAGGGYGGDGG(A)₁₂GGA-GDGYGAG, were determined by the conformation-dependent ¹³C chemical shifts, rotational echo double resonance (REDOR) and 2D spin-diffusion NMR methods. The solid-state NMR analysis leads to the precise silk structure before spinning, where the poly-Ala sequence takes a typical α-helix pattern with a tightly winded helical structure at both terminal regions of the poly-Ala sequence. This is expected to stabilize the α-helical structure of the poly-Ala region in S. c. ricini silk fibroin from the silkworm.