Tightly winding structure of sequential model peptide for repeated helical region in Samia cynthia ricini silk fibroin studied with solid-state NMR

摘要

There are many kinds of silks from silkworms and spiders with different structures and properties, and thus, silks are suitable to study the structure-property relationship of fibrous proteins. Silk fibroin from a wild silkworm, Samia cynthia ricini, mainly consists of the repeated similar sequences by about 100 times where there are alternative appearances of the polyalanine (Ala)12–13 region and the Gly-rich region. In this paper, a sequential model peptide, GGAGGGYGGDGG(A)12GGAGDGYGAG, which is a typical sequence of the silk fibroin, was synthesized, and the atomic-level conformations of Gly residues at the N- and C-terminal ends of the polyalanine region were determined as well as that of the central Ala residue using ¹³C 2D spin diffusion solid-state nuclear magnetic resonance (NMR) under off-magic angle spinning. In the model peptide with α-helical conformation, the torsion angle of the central Ala residue, the 19th Ala, was determined to be (φ, ψ) = (−60°, −50°), which was a typical α-helical structure, but the torsion angles of two Gly residues, the 12th and 25th Gly residues, which are located at the N- and C-terminal ends of the polyalanine region, were determined to be (φ,ψ) = (−70°, −30°) and (φ,ψ) = (−70°, −20°), respectively. Thus, it was observed that the turns at both ends of polyalanine with α-helix conformation in the model peptide are tightly wound.