Natural protein fibers, silks, represented by spiders and silkworm silks, are expected as sustainable structural materials because of their excellent mechanical properties, especially the high-toughness nature with a good balance of strength and extensibility. In addition, quick response to hydration and dehydration with a large structural deformation, called “supercontraction”, showing various potentialities for the applications to sensors and actuators, is also one of them. However, the structural origin of these specific phenomena has not been well understood, because of less understanding of hierarchical structure of silks. Strong motivation in our study is to understand the relationship between the hierarchical structure and such mechanical properties of silks. For this purpose, we have mainly utilized synchrotron X-ray scattering techniques. In this seminar, I will introduce our X-ray scattering studies for clarifying the structural origins of high-toughness nature and supercontraction, and I expect to have a fruitful discussion for the future subject: How can we effectively introduce the neutron scattering techniques to develop the understanding of these topics.
1. High-toughness nature
High-toughness nature of silks is one of the most attractive aspects, and an ultimate trial to produce artificial tough silks, combining the production of recombinant silk proteins and the development of artificial spinning technique, have been competed over the world . For the successful achievement of this, our contribution is to describe the hierarchical structure of native silks as quantitatively as possible from amino acid sequence to fibrillar structure, and to clarify the relationship between hierarchical structure and mechanical property. By combining Wide-angle and Small-angle X-ray scatterings (SWAXS), we successfully described the quantitative fibrillar structure model of silks, connecting from amino acid sequence to crystalline and amorphous phases [2,3]. In addition, simultaneous measurements of tensile test and synchrotron SWAXS, we proposed a key structural factor for the high-toughness nature of silks .
Spider dragline silk shows significant shrink of ∼50 % and more of its original length when the fiber is wetted in the free ends. This behavior is known as “supercontraction” and is generally interpreted as the result of water-induced breaking of interchain hydrogen bonds and subsequent entropy-driven recoiling of oriented amorphous chains. On the other hand, if the both ends of fiber are fixed (under restrained condition), supercontraction generates a significant contraction stress of ∼50 MPa, and the stress generation is repeated cyclically . We found that such a reversible stress generation driven by humidity change is observed not only for spider dragline silk specifically but also for the uniaxially oriented products of regenerated silk fibroin from Bombyx mori silkworm silk and other hydrogen-bonding polymers [5,6]. Synchrotron SWAXS analyses during the reversible humid change under restrained condition revealed an essential structural change, but not detailed phase transition in amorphous phase [5.6]. I will discuss about the experimental proposal to progress the understanding of structural change in amorphous phase by utilizing neutron scattering techniques.
 Saric M. and Scheibel T., Curr. Opin. Biotechnol. 2019, 60, 213-220.  Yoshioka T. and Kameda T., J. Silk Sci. Tech. Jpn. 2019, 27, 95-101.  Yoshioka T. et al., Nat. Commun. 2019, 10, 1469.  Blackledge T. A. et al., J. Exp. Biol. 2009, 212, 1981-89.  Yoshioka T. et al., Macromolecules 2011, 44, 7713-18.  Yoshioka T. et al., Macromolecules 2017, 50, 2803-13.
Dr. Jitae Park
Dr. Theresia Heiden-Hecht