Solid-state NMR determination of the secondary structure of Samia cynthia ricini silk

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Abstract
Silks are fibrous proteins that form heterogeneous, semi-crystalline solids. Silk proteins have a variety of physical properties reflecting their range of functions. Spider dragline silk, for example, has high tensile strength and elasticity1, whereas other silks2 are better suited to making housing, egg sacs or the capture spiral of spiders' webs. The differing physical properties arise from variation in the protein's primary and secondary structure, and their packing in the solid phase. The high mechanical performance of spider dragline silk, for example, is probably due to a β-sheet conformation of poly-alanine domains3, embedded as small crystallites within the fibre. Only limited structural information can be obtained from diffraction of silks3,4,5,6, so further characterization requires spectroscopic studies such as NMR7,8,9,10,11. However, the classical approach to NMR structure determination12 fails because the high molecular weight13, repetitive primary structure13 and structural heterogeneity of solid silk means that signals from individual amino-acid residues cannot be resolved. Here we adapt a recently developed solid-state NMR technique14,15 to determine torsion angle pairs (ϕ, Ψ) in the protein backbone, and we study the distribution of conformations in silk from the Eri silkworm, Samia cynthia ricini. Although the most probable conformation in native fibres is an anti-parallel β-sheet, film produced from liquid directly extracted from the silk glands appears to be primarily α-helical.