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Do what spiders can't do -- Super Artificial Spider Silk Fiber Synthesis Route

Time:2021-03-08 Hits:

In nature, spiders can create the strongest silk fibers in the world, and they can even create fibers with better mechanical properties than artificial silk fibers at any time and under any circumstances. Spiders' abilities and the silk they create have long been of interest to researchers. Today, scientists have developed artificial spider silk fibers that are used in a variety of applications, from high-performance textiles to sustainable components in sports equipment and machines. The production method of natural spider silk mainly relies on the farm reproduction of spiders to collect silk, but this collection mode still needs industrialization and efficiency. On the other hand, synthetic spider silk fibers still face some challenges, such as the spider silk proteins (spidroins) must be produced in a foreign host. Although scientists have identified a number of suitable hosts, such as some prokaryotic microorganisms, eukaryotes, plants, and even transgenic animals, their yields are low, and the spider silk proteins they produce have low water solubility. Although the subsequent spinning process can indeed make a great improvement in the manufacturing method and the synthetic fiber structure, its mechanical properties are still inadequate compared to the pure natural silk fiber. So Jan Johansson and Anna Rising from the Karolinka Institute in Sweden have identified the key factors that affect the properties of spider silk fibers, and by comparing the advantages and disadvantages of different solutions, they have devised a roadmap for synthesizing new super artificial spider silk fibers. Their forward-looking observations are published in the latest issue of the journal ACS Nano, entitled "Doing what Spiders cannot-a road map to supreme artificial silk fibers."

In general, each spider can create seven different kinds of silk fibers, each of which has different mechanical properties. These silk fibers are composed primarily of spider silk proteins, which share globular terminal domains of repeating regions containing 110-130 amino acids. These terminal domains in silk proteins are unique and regulate the solubility of silk and control fiber formation, while the repeat domains control the mechanical properties of silk. Among them, the hardest silk fiber (Dragline) is mainly composed of ampullary silk protein (MASPs), and its repeat region mainly contains iterative poly-Ala segment and glycine-rich repeat region (Gly). In the silk glands of spiders, the concentration of silk protein is as high as 50% (W/V). Although we have not yet fully understand how will the spider silk protein remain in such a high concentration and does not produce sediment, but today is known, the storage mode is mainly to repeat area in a random way coiled stored within the silk glands, and alpha helix terminal domain can make silk protein to keep certain hydrophilicity, it mainly attached to help in the surface layer of silkgland internal storage. The synthesis of spider silk fibers mainly takes place in the delivery tube, including the decrease of pH value and the increase of relative shear stress, which leads to the pultrusion and finally the formation of spider silk protein. What is more interesting is that the N and C terminal domains of spider silk proteins act as "lock and trigger" during synthesis. The n-terminal domain dimerizes at lower pH, in a way that mimics the coupling of the network, locking the spider silk protein. Under the influence of pH value and shear stress, the C-terminal region formed β-lamellar nucleus, which promoted the transformation of the amino acid repeating region into β-lamellar nucleus. Eventually, this "lock and trigger" mechanism turns the liquid silk protein into a solid fiber.

The main components of fibrin in spider silk

Several key factors affecting spider silk fibers
The macroampullary silk protein is anisotropic in the nanoscale structure, and its laminated β-lamellar nuclei form a crystalline domain (β-crystal), which is embedded in the amorphous matrix. Its main function is to control the tensile strength of spider silk fibers. This structure is the key to determine its mechanical properties. Although the specific relationship between the degree of crystallization and the mechanical properties is not clear, it is found that the volume of the crystal domain itself plays a decisive role in achieving high tensile strength, reducing power and toughness through kinetic simulation.
Silk proteins are secretory proteins, so they must pass through the endoplasmic omentum to enter the secretory pathway. The need to transport silk proteins to the endoplasmic omentum precludes the use of side chains that mediate the reactions between β-crystalline segments and layers, which is a major consideration for synthetic silk fibers.
Polyalanine is significantly better than single alanine in water repellency and silk fiber manufacture. Predictions from the Zipper Database based on polyamino acids (Poly-Ala, Poly-Val, Poly-Ile) indicate that more water-repellent polyamino acids do indeed produce more stable β-lamellar nuclear interactions. The reason the spider chose not to use these two amino acids as the macroampullary silk protein may be because of its relatively strong water repellency and the fact that such repeating regions may become trapped when passing through the endoplasmic omentum. Of course, today's technology has enabled the synthesis of artificial spider silk fibers in eukaryotic systems by binding amino acid chains such as valine and isoleucine to other chains and expressing them directly inside the cell, avoiding the need to cross the endoplasmic reticulum. However, the high hydrophobicity of these two amino acid chains makes it difficult for the host to produce fibers in the nucleus. Of course, making fibers artificially with organic solvents and spinning might reduce the solubility requirements, but it would also require more sophisticated synthetic methods.

Roadmap for synthesizing new super artificial spider silk fibers
Based on known theories and methods, the authors have summarized a roadmap for the synthesis of super artificial spider silk fibers. First, the methods used in bioinformatics (in silico) can be used to synthesize polyamino acid (poly-val, poly-ile) repeat regions and β-crystals from variants that increase sequence space. These resulting crystals can in turn be evaluated for stability and hardness by molecular dynamics simulations. Since spiders themselves are unable to make silk using non-polar materials that are stretched over long distances, the newly introduced element will undoubtedly bring some advantages in synthetic silk. Of course, the repeated regions formed by the new synthesized β-crystals also need to be evaluated by molecular dynamics simulations and experimental tests. Finally, the new synthetic silk fibers produced by spinning can be tested for mechanical properties and structures to compare with the silk fibers produced by spiders.

Roadmap for synthesizing new super artificial spider silk fibers

The thesis links: https://doi.org/10.1021/acsnano.0c08933

(Source: Frontiers in Polymer Science)