Lamellar Structure in Alanine–Glycine Copolypeptides Studied by Solid-State NMR Spectroscopy: A Model for the Crystalline Domain of Bombyx mori Silk Fibroin in Silk II Form

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Bombyx mori silk fibroin (SF) fibers with excellent mechanical properties have attracted widespread attention as new biomaterials. However, the structural details are still not conclusive. Here, we propose a lamellar structure for the crystalline domain of the SF fiber based on structural analyses of the Ala Cβ peaks in the 13 C cross-polarization/magic angle spinning NMR spectra of (Ala–Gly) m ( m = 9, 12, 15, and 25) and 13 C selectively labeled (Ala–Gly) 15 model peptides. Namely, three Ala Cβ peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of β-sheet and a β-turn, which are interpreted as a lamellar structure formed by repetitive folding using β-turns every eighth amino acid, for which the basic structure is (Ala–Gly) 4 in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using 13 C solid-state spin–lattice relaxation time observations and the rotational echo double resonance experiments, respectively.