Abstract
The fabrication of cellulose-spider silk bio-nanocomposites comprised of cellulose nanocrystals (CNCs) and recombinant spider silk protein fused to a cellulose binding domain (CBD) is described. Silk-CBD successfully binds cellulose, and unlike recombinant silk alone, silk-CBD self-assembles into microfibrils even in the absence of CNCs. Silk-CBD-CNC composite sponges and films show changes in internal structure and CNC alignment related to the addition of silk-CBD. The silk-CBD sponges exhibit improved thermal and structural characteristics in comparison to control recombinant spider silk sponges. The glass transition temperature (Tg) of the silk-CBD sponge was higher than the control silk sponge and similar to native dragline spider silk fibers. Gel filtration analysis, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (TEM) indicated that silk-CBD, but not the recombinant silk control, formed a nematic liquid crystalline phase similar to that observed in native spider silk during the silk spinning process. Silk-CBD microfibrils spontaneously formed in solution upon ultrasonication. We suggest a model for silk-CBD assembly that implicates CBD in the central role of driving the dimerization of spider silk monomers, a process essential to the molecular assembly of spider-silk nanofibers and silk-CNC composites.
Highlights
Silks are produced by a variety of insects and spiders and some spiders spin as many as seven different kinds of silks, each tailored to fulfill a certain biological function [1]
Silk-cellulose binding domain (CBD)-cellulose nanocrystals (CNCs) composite materials may be useful in a variety of medical and industrial applications, and this preliminary research is a necessary step toward the end goal of harnessing the attractive properties of the components into a composite with superior properties
The sy2n.1t.hPertoitceinspEixdpreerssisoinlkanadnPdurtifhiceatifounsion spider silk-CBD genes were successfully expressed, and the resu2lt. aRnetsupltrTsohatenesdiynnDsthiwestciuecrssespiiodpneurrsilfikeadndftrhoemfusEio.ncsopliiduerssiinlkg-CNBDi-gNenTeAs wcehrerosumccaestsofgulrlyapexhpyre. ssTehd,eanpdure protein.1.t.hty(PhFTiereiehogletdreueexsrissnepyfuo1Enlrrt)etxa.htnpshersteteipisoecsrxniospotpneroieidansfnsessirdowisnlPeiklurokerf(ia4pfsini7culdkarktii(tfo4Dihen7edakf)fuDrsaoaimn)oandEns.dspciisoldilkliek-ur-CCssiBBinlDkgD-NC(6(Bi56-ND5kTDgkAaeD)ncweahsr)eowrwmee6aer0teroeasgnur6dac0pc4eh0asysnm.fuTdghl/lL4ey,0peruxemspreprgeep/csrtsoLievtd,eeil,rnyaensdpectively the resultant proteins were purified from E. coli using Ni-NTA chromatography
Summary
Silks are produced by a variety of insects and spiders and some spiders spin as many as seven different kinds of silks, each tailored to fulfill a certain biological function [1]. Noishiki et al [23] found that CNC-native silkworm silk films had breaking strengths and ductility about five times greater than those of the constituent materials. The authors attributed these improvements to the flat and ordered surfaces of CNCs, which served as a template for the assembly of silk β-sheets, a process that usually requires shear and elongation stress. We observed that silk-CBD binds CNCs and confers molecular order which is different from that of either the silk proteins or CNCs. Silk-CBD-CNC composite materials may be useful in a variety of medical and industrial applications, and this preliminary research is a necessary step toward the end goal of harnessing the attractive properties of the components into a composite with superior properties
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