Solid-State NMR Analysis Aimed at Elucidating the Atomic-Level Structure and Dynamic Behavior of Silk Using Alanine Residues as Markers.

Understanding the structure, packing, and dynamics of silk fibers from silkworms and spider draglines is essential to explain their excellent mechanical properties. However, their atomic coordinate structural information remains limited. This review focuses on the methyl group of alanine and employs mainly 13C solid-state NMR chemical shifts and spin-lattice relaxation times to elucidate silk structure including the packing structure, and dynamics of alanine methyl groups quantitatively. Wild silkworm (Samia cynthia ricini) silk shows a staggered polyalanine packing, while spider silk exhibits a mixed packing of rectangular and staggered types with glycine-rich segments mainly forming random coils and β-turns. Domestic silkworm silk features an antipolar lamellar structure, folding every eight amino acids via β-turns. Additionally, some alanine Cβ carbons display long relaxation times and short correlation times at short 13C-13C distances, suggesting fast gear-like hopping motion under strong 13C-13C interactions, which stabilizes the staggered stacking of antiparallel β-sheets.