Molecular Orientation Enhancement of Silk by the Hot-Stretching-Induced Transition from α-Helix-HFIP Complex to β-Sheet

Abstract

Enhancing the molecular orientation of the regenerated silk fibroin (RF) up to a level comparable to the native silk is highly challenging. Our novel and promising strategy for the poststretching process is (1) creating at first an α-helix-HFIP complex with a hexagonal packing as an intermediate state and then (2) stretching it at a high temperature to induce the helix-to-sheet structural phase transition. Here we show for the first time the significantly high stretching efficiency of the proposed technique compared with the conventional wet-stretching techniques and the successful achievement of higher crystalline orientation and higher Young's modulus compared even with the native silk. The detailed structural analysis based on the time-resolved simultaneous measurement of stress-strain curve, synchrotron X-ray scatterings, and FTIR has revealed the structural transition mechanism from the hexagonally packed α-helix-HFIP complex to the highly oriented β-sheet crystalline state as well as the critical level of crystal orientation needed for the helix-to-sheet transition.

Orientational Mapping Augmented Sub-Wavelength Hyper-Spectral Imaging of Silk

“…In the UV-visible and IR spectral domains, near-field techniques using sharp nano-tips and plasmonic enhancement are used to reach nanoscale spatial resolutions, usually at the expense of polarisation information. However the application of polarised light permits analysis of the molecular orientation and chirality, which define mechanical, thermal, and optical properties 6 . At different wavelengths it is possible to access orientational information of hierarchical structures which underpins mechanical material properties and could be harnessed by engineering their artificial counterparts 4 .…”

“…The field of bio-medical applications could be one of the main beneficiaries of high-spatial resolution techniques with a focus on sensors and bio-materials. In protein based materials, the molecular ordering, orientation, and conformation define their properties 6 . Silk was the material of choice in this study due to its bio-compatibility and bio- degradability 7 , 8 .…”

A study of the extraordinarily strong and tough silk produced by bagworms

“…The PAB of the Saturniidae silkworm or spider dragline silks is considered to form a helical conformation in their silk glands 44,45 . It is reasonably speculated that the polyalanine helices in the neighbouring H-Fib proteins (or spidroins) are gathered around each other, probably forming a hexagonal packing via hydrophobic–hydrophobic self-assembly interactions 10,46,47 . The long PAB sequence of 22 residues (corresponding to 6 helical pitches) in the bagworm silk (Fig.…”

“…6d). We made an assumption, which is widely believed, that the polyalanine helical conformation changes to β -sheet crystals due to the stress-induced helix-sheet transition 10,47 . We can assign the lengths of phases I and II to the amorphous and crystalline phases, respectively (Supplementary Note 1).…”

“…To realise their practical application as structural materials, stronger and tougher silks with more elastic deformation behaviours are required. Towards this goal, many trials are being conducted worldwide to produce ideal silk products, for instance, by controlling the hierarchical structure of regenerated silk proteins 10,11 , producing transgenic (or genome-edited) artificial silk proteins 12–14 , and searching for superior un-explored silks in nature 8,15,16 . When we search for superior un-explored silks, spiders provide a major hint that the silks used for dangling should be strong and tough 17–19 !…”