Aggregation State of Residual α-Helices and Their Influence on Physical Properties of S. c. ricini Native Fiber

Formation of the alpha-helical conformation in the poly-l-alanine (PA) sequence regions, subsequent structural transition to beta-sheet during natural spinning, and presence of residual alpha-helices in Samia cynthia ricini (S. c. ricini) native silk fiber have been experimentally proven. However, the aggregation state of the residual alpha-helices, and their influence on the mechanical deformation behavior in native fiber remain unclear. Here we show that the alpha-helices form an ordered aggregation state with a hexagonal packing in the aqueous solution, some of which remain during natural spinning. X-ray scattering and differential scanning calorimetry (DSC) analyses revealed occurrence of a structural transition of the residual alpha-helices to the beta-sheet structure, accompanied by disappearance of the plateau region in the force-strain curve, due to heat-treatment at similar to 220 degrees C. On the basis of X-ray scattering before and after tensile stretching of S. c. ricini native silk, a direct connection between the plateau region and the alpha-helix to beta-sheet structural transition was confirmed. Our findings demonstrate the importance of the PA sequence regions in fiber structure formation and their influence on the tensile deformation behavior of S. c. ricini silk, features believed to be essentially similar in other saturniid silks. We strongly believe the residual ordered alpha-helices to be strategically and systematically designed by S. c. ricini silkworms to impart flexibility in native silk fiber. We anticipate that these knowledge forms a basis for fruitful strategies in the design and development of amino acid sequences for artificial silks with desired mechanical properties.