Aggregation State of Residual -Helices and Their Influence on Physical Properties of Native Fiber.

Formation of the -helical conformation in the poly-l-alanine (PA) sequence regions, subsequent structural transition to -sheet during natural spinning, and presence of residual -helices in () native silk fiber have been experimentally proven. However, the aggregation state of the residual -helices, and their influence on the mechanical deformation behavior in native fiber remain unclear. Here we show that the -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 α-helices to the -sheet structure, accompanied by disappearance of the plateau region in the force-strain curve, due to heat-treatment at ~220 °C. On the basis of X-ray scattering before and after tensile stretching of native silk, a direct connection between the plateau region and the -helix to -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 silk, features believed to be essentially similar in other saturniid silks. We strongly believe the residual ordered -helices to be strategically and systematically designed by 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.