Beta transition and stress-induced phase separation in the spinning of spider dragline silk
Spider dragline silk is formed as the result of a remarkable transformation in which an aqueous dope solution is rapidly converted into an insoluble protein filament with outstanding mechanical properties. Microscopy on the spinning duct in Nephila edulis spiders suggests that this transformation involves a stress-induced formation of anti-parallel beta-sheets induced by extensional flow. Measurements of draw stress at different draw rates during silking confirm that a stress-induced phase transition occurs.
Available from: Juan Guan
- "Of course, post-processing conditions during reeling, such as postdraw (stretching the fibres during reeling) or wet-reeling (dipping the silk through water during reeling), would also be expected to create additional order . Adding tension to the fibre before it is fully set creates stress-induced molecular alignment , allowing hydrogen bonding during drying to ''lock'' the order into place . Indeed, in spider spinning, such post-processing additions affect the mechanical properties by increasing the order within the fibre  ; this is also seen in synthetic polymer spinning    . "
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ABSTRACT: The forced reeling of silkworms offers the potential to produce a spectrum of silk filaments, spun from natural silk dope and subjected to carefully controlled applied processing conditions. Here we demonstrate that the envelope of stress-strain properties for forced reeled silks can encompass both naturally spun cocoon silk and unnaturally processed artificial silk filaments. We use dynamic mechanical thermal analysis (DMTA) to quantify the structural properties of these silks. Using this well established mechanical spectroscopic technique, we show high variation in the mechanical properties and the associated degree of disordered hydrogen-bonded structures in forced reeled silks. Furthermore we show this disorder can be manipulated by a range of processing conditions and even ameliorated under certain parameters, such as annealing under heat and mechanical load. We conclude that the powerful combination of forced reeling silk and DMTA has tied together native/natural and synthetic/unnatural extrusion spinning. The presented techniques therefore have the ability to define the potential of Bombyx-derived proteins for use in fibre-based applications and serve as a roadmap to improve fibre quality via post-processing.
Available from: Maxime Boulet-Audet
- "Instead, they are now firmly locked into a polymer network, responding passively and collectively to stresses and strains imparted on the bulk material. Whether one agrees with this view or not, it is well established that silk molecules have two different states linked by a distinct, one-way transition3435. We may assume that 400 M years of selective tweaking and tuning will have evolved a ‘spinning' process that is highly advanced. The three major independent evolutionary pathways (in insects, arachnids and crustaceans36) that have led to more or less the same proteins and processes suggest to us that silks can provide insights into protein denaturation that are likely to be fundamental and of generic relevance. "
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ABSTRACT: Here we present a set of measurements using Differential Scanning Fluorimetry (DSF) as an inexpensive, high throughput screening method to investigate the folding of silk protein molecules as they abandon their first native melt conformation, dehydrate and denature into their final solid filament conformation. Our first data and analyses comparing silks from spiders, mulberry and wild silkworms as well as reconstituted 'silk' fibroin show that DSF can provide valuable insights into details of silk denaturation processes that might be active during spinning. We conclude that this technique and technology offers a powerful and novel tool to analyse silk protein transitions in detail by allowing many changes to the silk solutions to be tested rapidly with microliter scale sample sizes. Such transition mechanisms will lead to important generic insights into the folding patterns not only of silks but also of other fibrous protein (bio)polymers.
Available from: Craig A Vierra
- "To be able to produce synthetic spider silk composite fibers, scientists must first understand the natural spinning process. Biophysical studies of the liquid contents stored within the major ampulla, also referred to as the spinning dope, reveal these proteins are unfolded and have a disordered secondary structure (Knight et al., 2000; Lefevre et al., 2008). The spinning dope has been shown to represent a highly concentrated aqueous mixture of major ampullate spidroins. "
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