X-ray diffraction study of nanocrystalline and amorphous structure within major and minor ampullate dragline spider silks

Nanoscale Structural Features in Major Ampullate Spider Silk

Spider major ampullate silk is often schematically represented as a two-phase material composed of crystalline nanodomains in an amorphous matrix. Here we are interested in revealing its more complex nanoscale organization by probing Argiope bruennichi dragline-type fibers using scanning X-ray nanodiffraction. This allows resolving transversal structural features such as an about 1 μm skin layer composed of around 100 nm diameter nanofibrils serving presumably as an elastic sheath. The core consists of a composite of several nm size crystalline nanodomains with poly(l-alanine) microstructure, embedded in a polypeptide network with short-range order. Stacks of nanodomains separated by less ordered nanosegments form nanofibrils with a periodic axial density modulation which is particularly sensitive to radiation damage. The precipitation of larger β-type nanocrystallites in the outer core-shell is attributed to MaSp1 protein molecules.

Recombinant spider silk from aqueous solutions via a bio-inspired microfluidic chip

Spiders achieve superior silk fibres by controlling the molecular assembly of silk proteins and the hierarchical structure of fibres. However, current wet-spinning process for recombinant spidroins oversimplifies the natural spinning process. Here, water-soluble recombinant spider dragline silk protein (with a low molecular weight of 47 kDa) was adopted to prepare aqueous spinning dope. Artificial spider silks were spun via microfluidic wet-spinning, using a continuous post-spin drawing process (WS-PSD). By mimicking the natural spinning apparatus, shearing and elongational sections were integrated in the microfluidic spinning chip to induce assembly, orientation of spidroins, and fibril structure formation. The additional post-spin drawing process following the wet-spinning section partially mimics the spinning process of natural spider silk and substantially contributes to the compact aggregation of microfibrils. Subsequent post-stretching further improves the hierarchical structure of the fibres, including the crystalline structure, orientation, and fibril melting. The tensile strength and elongation of post-treated fibres reached up to 510 MPa and 15%, respectively.

Exploration of the tight structural-mechanical relationship in mulberry and non-mulberry silkworm silks

The Bombyx mori silkworm is well known as it has been bred by our ancestors with mulberry tree leaves for thousands of years. However, Bombyx mori is not the only silkworm that can produce silk, many other kinds of silkworms can also make silks for commercial use. In this research, we compare the mechanical properties of five different commercial silk fibres including domesticated mulberry Bombyx mori, non-mulberry semi-domesticated eri Samia ricini, and wild tropical tasar Antheraea mylitta and muga Antheraea assamensis. The results demonstrate that the non-mulberry silk fibres have a relatively high extensibility as compared to the mulberry silk fibres. In the meantime, the non-mulberry silk fibres show comparatively unique toughness to the mulberry silk fibres. Synchrotron radiation FTIR microspectroscopy, synchrotron radiation wide angle X-ray diffraction, and Raman dichroism spectroscopy are used to analyze the structural differences among the five species of silk fibres comprehensively. The results clearly show that the mechanical properties of both mulberry and non-mulberry silk fibres are closely related to their structures, such as β-sheet content, crystallinity, and the molecular orientation along the fibre axis. This study aims to understand the differences in the structural and mechanical properties of different mulberry and non-mulberry silk fibres, which are of importance to the related research on understanding and utilizing the non-mulberry silk as a biomaterial. We believe these investigations not only provide insight into the biology of silk fibroins from the non-mulberry silkworms but also offer guidelines for further biomimetic investigations into the design and manufacture of artificial silk protein fibres with novel morphologies and associated material properties for future use in different fields like bioelectronics, biomaterials and biomedical devices.

Hybrid Silk Fibers Dry-Spun from Regenerated Silk Fibroin/Graphene Oxide Aqueous Solutions

Regenerated silk fibroin (RSF)/graphene oxide (GO) hybrid silk fibers were dry-spun from a mixed dope of GO suspension and RSF aqueous solution. It was observed that the presence of GO greatly affect the viscosity of RSF solution. The RSF/GO hybrid fibers showed from FTIR result lower β-sheet content compared to that of pure RSF fibers. The result of synchrotron radiation wide-angle X-ray diffraction showed that the addition of GO confined the crystallization of silk fibroin (SF) leading to the decrease of crystallinity, smaller crystallite size, and new formation of interphase zones in the artificial silks. Synchrotron radiation small-angle X-ray scattering also proved that GO sheets in the hybrid silks and blended solutions were coated with a certain thickness of interphase zones due to the complex interaction between the two components. A low addition of GO, together with the mesophase zones formed between GO and RSF, enhanced the mechanical properties of hybrid fibers. The highest breaking stress of the hybrid fibers reached 435.5 ± 71.6 MPa, 23% improvement in comparison to that of degummed silk and 72% larger than that of pure RSF silk fiber. The hybrid RSF/GO materials with good biocompatibility and enhanced mechanical properties may have potential applications in tissue engineering, bioelectronic devices, or energy storage.

Secondary Structure Transition and Critical Stress for a Model of Spider Silk Assembly

Spiders spin their silk from an aqueous solution to a solid fiber in ambient conditions. However, to date, the assembly mechanism in the spider silk gland has not been satisfactorily explained. In this paper, we use molecular dynamics simulations to model Nephila clavipes MaSp1 dragline silk formation under shear flow and determine the secondary structure transitions leading to the experimentally observed fiber structures. While no experiments are performed on the silk fiber itself, insights from this polypeptide model can be transferred to the fiber scale. The novelty of this study lies in the calculation of the shear stress (300-700 MPa) required for fiber formation and identification of the amino acid residues involved in the transition. This is the first time that the shear stress has been quantified in connection with a secondary structure transition. By study of molecules containing varying numbers of contiguous MaSp1 repeats, we determine that the smallest molecule size giving rise to a "silk-like" structure contains six polyalanine repeats. Through a probability analysis of the secondary structure, we identify specific amino acids that transition from α-helix to β-sheet. In addition to portions of the polyalanine section, these amino acids include glycine, leucine, and glutamine. The stability of β-sheet structures appears to arise from a close proximity in space of helices in the initial spidroin state. Our results are in agreement with the forces exerted by spiders in the silking process and the experimentally determined global secondary structure of spidroin and pulled MaSp1 silk. Our study emphasizes the role of shear in the assembly process of silk and can guide the design of microfluidic devices that attempt to mimic the natural spinning process and predict molecular requirements for the next generation of silk-based functional materials.

Protein secondary structure of Green Lynx spider dragline silk investigated by solid-state NMR and X-ray diffraction

In this study, the secondary structure of the major ampullate silk from Peucetia viridans (Green Lynx) spiders is characterized by X-ray diffraction and solid-state NMR spectroscopy. From X-ray diffraction measurement, β-sheet nanocrystallites were observed and found to be highly oriented along the fiber axis, with an orientational order, fc≈0.98. The size of the nanocrystallites was determined to be on average 2.5nm×3.3nm×3.8nm. Besides a prominent nanocrystalline region, a partially oriented amorphous region was also observed with an fa≈0.89. Two-dimensional (13)C-(13)C through-space and through-bond solid-state NMR experiments were employed to elucidate structure details of P. viridans silk proteins. It reveals that β-sheet nanocrystallites constitutes 40.0±1.2% of the protein and are dominated by alanine-rich repetitive motifs. Furthermore, based upon the NMR data, 18±1% of alanine, 60±2% glycine and 54±2% serine are incorporated into helical conformations.

Development of a Process for the Spinning of Synthetic Spider Silk

Spider silks have unique mechanical properties but current efforts to duplicate those properties with recombinant proteins have been unsuccessful. This study was designed to develop a single process to spin fibers with excellent and consistent mechanical properties. As-spun fibers produced were brittle, but by stretching the fibers the mechanical properties were greatly improved. A water-dip or water-stretch further increased the strength and elongation of the synthetic spider silk fibers. Given the promising results of the water stretch, a mechanical double-stretch system was developed. Both a methanol/water mixture and an isopropanol/water mixture were independently used to stretch the fibers with this system. It was found that the methanol mixture produced fibers with high tensile strength while the isopropanol mixture produced fibers with high elongation.

Tough protein-carbon nanotube hybrid fibers comparable to natural spider silks

Animal silks, especially spider dragline silks, have an excellent portfolio of mechanical properties, but it is still a challenge to obtain artificial silk fibers with similar properties to the natural ones. In this paper, we show how to extrude tough regenerated silk fibers by adding a small amount of commercially available functionalized multiwalled carbon nanotubes (less than 1%) through an environmentally friendly wet-spinning process reported by this laboratory previously. Most of the resulting regenerated silk fibers exhibited a breaking energy beyond 130 MJ m^-3, which is comparable to spider dragline silks (∼160 MJ m^-3). The best of these fibers in terms of performance show a breaking stress of 0.42 GPa, breaking strain of 59%, and breaking energy of 186 MJ m^-3. In addition, we used several advanced characterization techniques, such as synchrotron radiation FTIR microspectroscopy and synchrotron radiation X-ray diffraction, to reveal the toughening mechanism in such a protein-inorganic hybrid system. We believe our attempt to produce such tough protein-based hybrid fibers by using cheap, abundant and sustainable regenerated silkworm protein and commercially available functionalized carbon nanotubes, with simplified industrial wet-spinning apparatus, may open up a practical way for the industrial production of super-tough fiber materials.

Structural hysteresis in dragline spider silks induced by supercontraction: An x-ray fiber micro-diffraction study

Interaction with water causes shrinkage and significant changes in the structure of spider dragline silks, which has been referred to as supercontraction in the literature. Preferred orientation or alignment of protein chains with respect to the fiber axis is extensively changed during this supercontraction process. Synchrotron x-ray micro-fiber diffraction experiments have been performed on Nephila clavipes and Argiope aurantia major and minor ampullate dragline spider fibers in the native dry, contracted (by immersion in water) and restretched (from contracted) states. Changes in the orientation of β-sheet nanocrystallites and the oriented component of the amorphous network have been determined from wide-angle x-ray diffraction patterns. While both the crystalline and amorphous components lose preferred orientation on wetting with water, the nano-crystallites regain their orientation on wet-restretching, whereas the oriented amorphous components only partially regain their orientation. Dragline major ampullate silks in both the species contract more than their minor ampullate silks.

Mechanical and physical properties of recombinant spider silk films using organic and aqueous solvents

Spider silk has exceptional mechanical and biocompatibility properties. The goal of this study was optimization of the mechanical properties of synthetic spider silk thin films made from synthetic forms of MaSp1 and MaSp2, which compose the dragline silk of Nephila clavipes. We increased the mechanical stress of MaSp1 and 2 films solubilized in both HFIP and water by adding glutaraldehyde and then stretching them in an alcohol based stretch bath. This resulted in stresses as high as 206 MPa and elongations up to 35%, which is 4× higher than the as-poured controls. Films were analyzed using NMR, XRD, and Raman, which showed that the secondary structure after solubilization and film formation in as-poured films is mainly a helical conformation. After the post-pour stretch in a methanol/water bath, the MaSp proteins in both the HFIP and water-based films formed aligned β-sheets similar to those in spider silk fibers.

Unravelling the biodiversity of nanoscale signatures of spider silk fibres

Living organisms are masters at designing outstanding self-assembled nanostructures through a hierarchical organization of modular proteins. Protein-based biopolymers improved and selected by the driving forces of molecular evolution are among the most impressive archetypes of nanomaterials. One of these biomacromolecules is the myriad of compound fibroins of spider silks, which combine surprisingly high tensile strength with great elasticity. However, no consensus on the nano-organization of spider silk fibres has been reached. Here we explore the biodiversity of spider silk fibres, focusing on nanoscale characterization with high-resolution atomic force microscopy. Our results reveal an evolution of the nanoroughness, nanostiffness, nanoviscoelastic, nanotribological and nanoelectric organization of microfibres, even when they share similar sizes and shapes. These features are related to unique aspects of their molecular structures. The results show that combined nanoscale analyses of spider silks may enable the screening of appropriate motifs for bioengineering synthetic fibres from recombinant proteins.

Mechanisms of mineral membrane fouling growth modulated by pulsed modes of current during electrodialysis: evidences of water splitting implications in the appearance of the amorphous phases of magnesium hydroxide and calcium carbonate

Experiments revealed the fouling nature evolutions along different electrodialysis (ED) trials, and how it disappears when current pulsation acts repetitively on the interfaces of ion-exchange membranes (IEMs). Fouling was totally controlled on the diluate side of cation-exchange membrane (CEM) by the repetitive pulsation frequency of the higher on-duty ratios applied. They created steady water splitting proton-barriers that neutralized OH(-) leakage through the membrane, decreasing the interfacial pH, and fouling of the concentrate side. The anion-exchange membrane (AEM) on the diluate side was similarly protected, but it was fouled once water splitting OH(-) generation became either intense enough or excessively weak. Interestingly, amorphous magnesium hydroxide (AMH) stemmed on the CEM-diluate side from brucite under intense water splitting OH(-) generation, and/or strong OH(-) leakage electromigration through the membrane. Water dissociation and overlimiting current regimes triggered drastic water molecule removal from crystal lattices through an accelerated cascade water splitting reaction. Also, amorphous calcium carbonate (ACC) appeared on CEM under intense water splitting reaction, and disappeared once intense OH(-) leakage was allowed by the water splitting proton-barrier dissipation. Our findings have implications for membrane fouling control, as well as for the understanding of the growth behavior of CaCO3 and Mg(OH)2 species on electromembrane interfaces.

Structural characterization of nanofiber silk produced by embiopterans (webspinners)

Embiopterans produce silken galleries and sheets using exceptionally fine silk fibers in which they live and breed. In this study, we use electron microscopy (EM), Fourier-transform infrared (FT-IR) spectroscopy, wide angle X-ray diffraction (WAXD) and solid-state nuclear magnetic resonance (ssNMR) techniques to elucidate the molecular level protein structure of webspinner (embiid) silks. Silks from two species Antipaluria urichi and Aposthonia ceylonica are studied in this work. Electron microscopy images show that the fibers are about 90-100 nm in diameter, making webspinner silks among the finest of all known animal silks. Structural studies reveal that the silk protein core is dominated by β-sheet structures, and that the protein core is coated with a hydrophobic alkane-rich surface coating. FTIR spectra of native embiid silk shows characteristic alkane CH₂ stretchings near 2800-2900 cm^-1, which decrease approximately 50% after washing the silk with 2 : 1 CHCl₃ : MeOH. Furthermore, ¹³C ssNMR data shows a significant CH₂ resonance that is strongly affected by the presence of water, supporting the idea that the silk fibers are coated with a hydrocarbon-rich layer. Such a layer is likely used to protect the colonies from rain. FTIR data also suggests that embiid silks are dominated by β-sheet secondary structures similar to spider and silkworm silk fibers. NMR data confirms the presence of β-sheet nanostructures dominated by serine-rich repetitive regions. A deconvolution of the serine Cβ NMR resonance reveals that approximately 70% of all seryl residues exist in a β-sheet structure. This is consistent with WAXD results that suggest webspinner silks are 70% crystalline, which is the highest crystalline fraction reported for any animal silks. The work presented here provides a molecular level structural picture of silk fibers produced by webspinners.

Characterizing the secondary protein structure of black widow dragline silk using solid-state NMR and X-ray diffraction

This study provides a detailed secondary structural characterization of major ampullate dragline silk from Latrodectus hesperus (black widow) spiders. X-ray diffraction results show that the structure of black widow major ampullate silk fibers is comprised of stacked β-sheet nanocrystallites oriented parallel to the fiber axis and an amorphous region with oriented (anisotropic) and isotropic components. The combination of two-dimensional (2D) (13)C-(13)C through-space and through-bond solid-state NMR experiments provide chemical shifts that are used to determine detailed information about the amino acid motif secondary structure in black widow spider dragline silk. Individual amino acids are incorporated into different repetitive motifs that make up the majority of this protein-based biopolymer. From the solid-state NMR measurements, we assign distinct secondary conformations to each repetitive amino acid motif and, hence, to the amino acids that make up the motifs. Specifically, alanine is incorporated in β-sheet (poly(Alan) and poly(Gly-Ala)), 3(1)-helix (poly(Gly-Gly-Xaa), and α-helix (poly(Gln-Gln-Ala-Tyr)) components. Glycine is determined to be in β-sheet (poly(Gly-Ala)) and 3(1)-helical (poly(Gly-Gly-X(aa))) regions, while serine is present in β-sheet (poly(Gly-Ala-Ser)), 3(1)-helix (poly(Gly-Gly-Ser)), and β-turn (poly(Gly-Pro-Ser)) structures. These various motif-specific secondary structural elements are quantitatively correlated to the primary amino acid sequence of major ampullate spidroin 1 and 2 (MaSp1 and MaSp2) and are shown to form a self-consistent model for black widow dragline silk.

β-Sheet nanocrystalline domains formed from phosphorylated serine-rich motifs in caddisfly larval silk: a solid state NMR and XRD study

Adhesive silks spun by aquatic caddisfly (order Trichoptera) larvae are used to build both intricate protective shelters and food harvesting nets underwater. In this study, we use (13)C and (31)P solid-state NMR and wide angle X-ray diffraction (WAXD) as tools to elucidate molecular protein structure of caddisfly larval silk from the species Hesperophylax consimilis . Caddisfly larval silk is a fibroin protein based biopolymer containing mostly repetitive amino acid motifs. NMR and X-ray results provide strong supporting evidence for a structural model in which phosphorylated serine repeats (pSX)4 complex with divalent cations Ca(2+) and Mg(2+) to form rigid nanocrystalline β-sheet structures in caddisfly silk. (13)C NMR data suggests that both phosphorylated serine and neighboring valine residues exist in a β-sheet conformation while glycine and leucine residues common in GGX repeats likely reside in random coil conformations. Additionally, (31)P chemical shift anisotropy (CSA) analysis indicates that the phosphates on phosphoserine residues are doubly ionized, and are charge-stabilized by divalent cations. Positively charged arginine side chains also likely play a role in charge stabilization. Finally, WAXD results finds that the silk is at least 7-8% crystalline, with β-sheet interplane spacings of 3.7 and 4.5 Å.

Reproducing natural spider silks' copolymer behavior in synthetic silk mimics

Dragline silk from orb-weaving spiders is a copolymer of two large proteins, major ampullate spidroin 1 (MaSp1) and 2 (MaSp2). The ratio of these proteins is known to have a large variation across different species of orb-weaving spiders. NMR results from gland material of two different species of spiders, N. clavipes and A. aurantia , indicates that MaSp1 proteins are more easily formed into β-sheet nanostructures, while MaSp2 proteins form random coil and helical structures. To test if this behavior of natural silk proteins could be reproduced by recombinantly produced spider silk mimic protein, recombinant MaSp1/MaSp2 mixed fibers as well as chimeric silk fibers from MaSp1 and MaSp2 sequences in a single protein were produced based on the variable ratio and conserved motifs of MaSp1 and MaSp2 in native silk fiber. Mechanical properties, solid-state NMR, and XRD results of tested synthetic fibers indicate the differing roles of MaSp1 and MaSp2 in the fiber and verify the importance of postspin stretching treatment in helping the fiber to form the proper spatial structure.