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

Silks are fibrous proteins that form heterogeneous, semi-crystalline solids . Silk proteins have a variety of physical properties reflecting their rang e of functions. Spider dragline silk, for example, has high tensile strengt h and elasticity(1), whereas other silks(2) are better suited to making hou sing, egg sacs or the capture spiral of spiders' webs. The differing physic al properties arise from variation in the protein's primary and secondary s tructure, and their packing in the solid phase. The high mechanical perform ance of spider dragline silk, for example, is probably due to a beta-sheet conformation of poly-alanine domains(3), embedded as small crystallites wit hin the fibre. Only limited structural information can be obtained from dif fraction of silks(3-6), so further characterization requires spectroscopic studies such as NMR7-11. However, the classical approach to NMR structure d etermination(12) fails because the high molecular weight(13), repetitive pr imary structure(13) and structural heterogeneity of solid silk means that s ignals from individual amino-acid residues cannot be resolved. Here we adap t a recently developed solid-state NMR technique(14,15) to determine torsio n angle pairs (phi, Psi) in the protein backbone, and we study the distribu tion of conformations in silk from the Eri silkworm, Samia cynthia ricini. Although the most probable conformation in native fibres is an anti-paralle l beta-sheet, film produced from liquid directly extracted from the silk gl ands appears to be primarily alpha-helical.