A silk composite fiber reinforced by telechelic-type polyalanine and its strengthening mechanism

Antiparallel β-sheets play a key role in determining the physical properties of fibroins, e.g., degradation and mechanical properties, and are typically formed by poly(A) motifs from spider dragline silks and GAGAGS motifs from Bombyx mori silkworm silks. To explore the interaction between these two motifs within the same system, a telechelic-type polyalanine (TPA) was prepared through chemoenzymatic synthesis and doped in silkworm silk fibroins to fabricate silk composite fibers. Interestingly, when TPA was added at suitable ratios of 1 and 3 wt%, the mechanical properties of the composite fibers were largely improved by approximately 42% and 51% compared with those of silk-only fibers in terms of tensile strength and toughness, respectively. As revealed by wide-angle X-ray diffraction (WAXD), silk composite fibers achieved the highest crystallinity at a TPA ratio of 1 wt%, largely contributing to their tensile strength. Evidenced by simultaneous stretching during WAXD measurement, TPA did not compete with the silk matrix by forming its own crystallization. Ultimately, a strengthening mechanism of nucleus-dependent crystallization was discussed to show the favorable heterogeneous nucleation created from TPA molecules for the promotion of crystallization in silk fibroins. Interestingly, regularly packed and aligned granules within composite fibers were detected by AFM to further support the enhanced mechanical performance. This work envisions sophisticated control of β-sheet crystals to better understand the structure–property relationship

Papain-Catalyzed, Sequence-Dependent Polymerization Yields Polypeptides Containing Periodic Histidine Residues

His-containing polypeptides, including polyHis, are attractive materials due to the unique characteristics of the imidazole ring of the His residue. In particular, His-containing polypeptides with repetitive sequences have a variety of distinctive features based on their periodic structure. In this study, chemoenzymatic polymerization of ethyl ester monomers with sequences His, GlyHis, HisGly, and GlyHisGly with hydrophobic side chains on the imidazole ring was performed using papain as a catalyst. Sequence dependence in chemoenzymatic polymerization was observed for GlyHis- and HisGly-based monomers: GlyHis-based monomers did not undergo polymerization, whereas polymerization of HisGly-based monomers afforded polypeptides with a degree of polymerization from 6 to 38 and from 5 to 31 and a number-average degree of polymerization of 16.4 and 12.4 for poly(HisGly) and poly[His(Bu)Gly], respectively. The difference in polymerizability of these dipeptide monomers was supported by a docking simulation between these monomers and papain, where the ester group of the HisGly-based monomer was closer to the catalytic center of papain than that of the GlyHis-based monomer. Infrared spectroscopy and synchrotron wide-angle X-ray diffraction measurements indicated that poly(HisGly) formed a β-sheet structure whose crystallinity was 41.6%, whereas the other tripeptide-based polypeptides were more amorphous showing 19.6–30.7% of crystallinity. Poly(HisGly) exhibited the highest thermal stability among all of the polypeptides in the thermogravimetric analysis, reflecting the difference in the secondary structures

Chemoenzymatic Polymerization of l-Serine Ethyl Ester in Aqueous Media without Side-Group Protection

Poly(l-serine) (polySer) has tremendous potential as a polypeptide-based functional material due to the utility of the hydroxyl group on its side chain; however, tedious protection/deprotection of the hydroxyl groups is required for its synthesis. In this study, polySer was synthesized by the chemoenzymatic polymerization (CEP) of l-serine ethyl ester (Ser-OEt) or l-serine methyl ester (Ser-OMe) using papain as a catalyst in an aqueous medium. The CEP of Ser-OEt proceeded at basic pH ranging from 7.5 to 9.5 and resulted in the maximum precipitate yield of polySer at an optimized pH of 8.5. A series of peaks detected by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry revealed that the formed precipitate consisted of polySer with a degree of polymerization ranging from 5 to 22. Moreover, infrared spectroscopy, circular dichroism spectroscopy, and synchrotron wide-angle X-ray diffraction measurements indicated that the obtained polySer formed a β-sheet/strand structure. This is the first time the synthesis of polySer was realized by CEP in aqueous solution without protecting the hydroxyl group of the Ser monomer

Ultra-SAXS observation of fibril-sized structure formation after the necking of poly(ethylene terephthalate) and poly(phenylene sulfide) fibers

Fibril-sized structures, which exhibit a density fluctuation of several tens to hundreds of nanometers, are important because they influence many properties of fibers and films, particularly the strength and thermal shrinkage of synthetic fibers. We analyzed the formation of fibril-sized structures after necking using ultra-small-angle X-ray scattering (USAXS), which enables observation of the void and craze formations. Continuous laser-heated drawing and undulator synchrotron radiation were adopted to measure the structural development of poly(ethylene terephthalate) (PET) and poly(phenylene sulfide) (PPS) fibers. Both fibers showed a clear increase in the meridional streak intensity just after necking and an increase in the equatorial streak after necking. These increases were distinctive for PPS. Moreover, a layer-lined streak appeared after necking only for PET. The intensity of the meridional streak increased with an increase in the draw ratio, whereas the intensity of the layer-lined scattering decreased with an increase in the draw ratio.ArticlePOLYMER JOURNAL.51(2):211-219(2018)journal articl

Effects of dimensions and regularity on the mechanical properties of the smectic phase formed during orientation-induced crystallization of poly (ethylene terephthalate)

A fibrillar-shaped metastable smectic phase forms during orientation-induced crystallization of poly(ethylene terephthalate). We investigated the effects of dimensions and regularity of the smectic phase on the mechanical properties by high-precision X-ray measurements. The drawing stress dependence was 76-168 MPa and the spinning speed dependence of 250-2000 m/min. The proportion, persistence length, and thickness of the smectic phase achieved their maximum values at 0.3-0.4 ms after necking. At this time g(II) also reached a minimum value of 3.7%-4.3%. The persistence length increased linearly with the natural draw ratio of as-spun fibers at less than 1500 m/min. The maximum values of the proportion, persistence length, and thickness showed no dependence on the drawing stress; however, the proportion and persistence length increased more rapidly. The increase of d-spacing with drawing stress led to an apparent elastic modulus of approximately 40 GPa for the oriented molecular bundle.ArticlePOLYMER.164:163-173(2019)journal articl

Replicating shear-mediated self-assembly of spider silk through microfluidics

マイクロ流路を利用したクモ糸形成プロセスの再現 --マイクロ流体デバイスによる生物プロセスの精密模倣--. 京都大学プレスリリース. 2024-01-31.The development of artificial spider silk with properties similar to native silk has been a challenging task in materials science. In this study, we use a microfluidic device to create continuous fibers based on recombinant MaSp2 spidroin. The strategy incorporates ion-induced liquid-liquid phase separation, pH-driven fibrillation, and shear-dependent induction of β-sheet formation. We find that a threshold shear stress of approximately 72 Pa is required for fiber formation, and that β-sheet formation is dependent on the presence of polyalanine blocks in the repetitive sequence. The MaSp2 fiber formed has a β-sheet content (29.2%) comparable to that of native dragline with a shear stress requirement of 111 Pa. Interestingly, the polyalanine blocks have limited influence on the occurrence of liquid-liquid phase separation and hierarchical structure. These results offer insights into the shear-induced crystallization and sequence-structure relationship of spider silk and have significant implications for the rational design of artificially spun fibers

Heterometallic Benzenehexathiolato Coordination Nanosheets: Periodic Structure Improves Crystallinity and Electrical Conductivity.

Funder: White Rock FoundationFunder: Jardine FoundationFunder: Cambridge Commonwealth European and International TrustCoordination nanosheets are an emerging class of 2D, bottom-up materials having fully π-conjugated, planar, graphite-like structures with high electrical conductivities. Since their discovery, great effort has been devoted to expand the variety of coordination nanosheets; however, in most cases, their low crystallinity in thick films hampers practical device applications. In this study, mixtures of nickel and copper ions are employed to fabricate benzenehexathiolato (BHT)-based coordination nanosheet films, and serendipitously, it is found that this heterometallicity preferentially forms a structural phase with improved film crystallinity. Spectroscopic and scattering measurements provide evidence for a bilayer structure with in-plane periodic arrangement of copper and nickel ions with the NiCu2 BHT formula. Compared with homometallic films, heterometallic films exhibit more crystalline microstructures with larger and more oriented grains, achieving higher electrical conductivities reaching metallic behaviors. Low dependency of Seebeck coefficient on the mixing ratio of nickel and copper ions supports that the large variation in the conductivity data is not caused by change in the intrinsic properties of the films. The findings open new pathways to improve crystallinity and to tune functional properties of 2D coordination nanosheets.EPSRC-JSPS core-to-core program (EP/S030662/1, JPJSCCA20190005

Tensile Reinforcement of Silk Films by the Addition of Telechelic-Type Polyalanine

An appropriate modification technique for silk materials is needed to effectively improve their physical properties for specific applications. A telechelic-type polyalanine (T-polyA) was synthesized by papain-catalyzed polymerization as a novel reinforcing agent for silk materials. A silk fibroin obtained from Bombyx mori was homogeneously doped with T-polyA, and casting a solution of silk fibroin and T-polyA in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) resulted in a robust and transparent film. Tensile deformation studies of the silk composite film containing T-polyA with prestretching revealed that the tensile strength and toughness were enhanced relative to those of a silk-only film. To determine the capability of T-polyA to reinforce the tensile property of silk films, the secondary structure in the silk composite film was characterized by wide-angle X-ray diffraction (WAXD) analysis. Antiparallel β-sheet structures of T-polyA and GAGAGS motifs of silk fibroin formed independently in the prestretched composite film. In addition, measuring the tensile deformation and performing WAXD analysis simultaneously demonstrated that the β-sheet structures of both T-polyA and the silk fibroin were aligned along the stretching direction and that T-polyA had no significant effect on the final morphology of the silk crystal domains. The silk film was toughened by the addition of T-polyA because of the generation of the T-polyA β-sheet in the amorphous region of the composite film. This work provides novel insight into the design and development of tough silk materials with controlled and aligned β-sheet structures