Chain-folded lamellar structure and dynamics of the crystalline fraction of Bombyx mori silk fibroin and of (Ala-Gly-Ser-Gly-Ala-Gly)n model peptides

• Published in: International journal of biological macromolecules Vol. 164; pp. 3974 - 3983
• Main Authors: Asakura, Tetsuo, Ogawa, Tatsuya, Naito, Akira, Williamson, Michael P.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.12.2020
• Subjects: 13C CP/MAS NMR; 13C solid-state NMR relaxation time; Amino Acid Sequence; Animals; Bombyx - chemistry; Bombyx mori silk fibroin; Chemical Phenomena; Fibroins - chemistry; Hydrogen Bonding; Lamellar structure; Models, Molecular; Molecular Conformation; Molecular Structure; Peptides - chemistry; Silk - chemistry; Spectrum Analysis; Lamellar structure; 13C solid-state NMR relaxation time; Bombyx mori silk fibroin; 13C CP/MAS NMR; C solid-state NMR relaxation time; C CP/MAS NMR
• ISSN: 0141-8130; 1879-0003
• DOI: 10.1016/j.ijbiomac.2020.08.220

Solid-state NMR is a powerful analytical technique to determine the composite structure of Bombyx mori silk fibroin (SF). In our previous paper, we proposed a lamellar structure for Ala-Gly copolypeptides as a model of the crystalline fraction in Silk II. In this paper, the structure and dynamics of the crystalline fraction and of a better mimic of the crystalline fraction, (Ala-Gly-Ser-Gly-Ala-Gly)n (n = 2–5, 8), and 13C selectively labeled [3-13C]Ala-(AGSGAG)5 in Silk II forms, were studied using structural and dynamical analyses of the Ala Cβ peaks in 13C cross polarization/ magic angle spinning NMR and 13C solid-state spin-lattice relaxation time (T1) measurements, respectively. Like Ala-Gly copolypeptides, these materials have lamellar structures with two kinds of Ala residues in β-sheet, A and B, plus one distorted β-turn, t, formed by repetitive folding using β-turns every eighth amino acid in an antipolar arrangement. However, because of the presence of Ser residues at every sixth residue in (AGSGAG)n, the T1 values and mobilities of B decreased significantly. We conclude that the Ser hydroxyls hydrogen bond to adjacent lamellar layers and fix them together in a similar way to Velcro®. [Display omitted] •A lamellar structure is proposed for the Silk II crystalline region of B. mori silk fibroin.•13C solid-state NMR was used to determine silk structure and dynamics.•Serines in (AGSGAG)n lock adjacent lamellae together by hydrogen bonding.