Abstract
Formation of the α-helical conformation in the poly-l-alanine (PA) sequence regions, subsequent structural transition to β-sheet during natural spinning, and presence of residual α-helices in Samia cynthia ricini (S. c. ricini) native silk fiber have been experimentally proven. However, the aggregation state of the residual α-helices, and their influence on the mechanical deformation behavior in native fiber remain unclear. Here we show that the α-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 β-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 S. c. ricini native silk, a direct connection between the plateau region and the α-helix to β-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 α-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.
Highlights
Towards establishing a sustainable society, effective utilization of Saturniidae wild-silkworm silks is receiving increased attention because of their excellent material properties and abundant yields in nature [1]
Our results revealed that the residual α-helices in S. c. ricini native silk aggregate with a hexagonal packing and their strong contribution to the characteristic fiber physical properties was clarified
Some S. c. ricini larvae from each batch reared were let to spin cocoons whence native fibers analysed was obtained as described in the subsequent sections
Summary
Towards establishing a sustainable society, effective utilization of Saturniidae wild-silkworm silks is receiving increased attention because of their excellent material properties and abundant yields in nature [1]. Ricini native fiber exhibit a characteristic plateau region in their stress-strain curve unlike that from B. mori that show a distinct yielding point [21,28,29,30]. This unique feature, observed in other wild silks [21,29], is often interpreted to be a result of some structural transitions, such as helical to zigzag conformations in poly(ethylene oxide) [31], α-helix to β-sheet conformations in keratin [32,33], and amorphous to crystalline states in natural rubber [34].
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