Nanofiber Formation in Spider Dragline-Silk as Probed by Atomic Force Microscopy and Molecular Pulling
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Nanofiber Formation in Spider Dragline-Silk as Probed by Atomic Force Microscopy and Molecular Pulling Emin Oroudjev, Cheryl Y. Hayashi1, Jason Soares2, Steven Arcidiacono2, Stephen A. Fossey2 and Helen G. Hansma Department of Physics, University of California Santa Barbara, CA 93106 U.S.A. 1 Department of Biology, University of California Riverside, California 92521 U.S.A. 2 US Army Natick R&D Center Natick, MA 01760 U.S.A. ABSTRACT Despite the extensive interest in its remarkable materials properties, the structure of spider dragline silk is not yet known. Indirect structural information is thus of particular interest. New data are presented here from two probe microscopy techniques – atomic force microscopy (AFM) and single-molecule force spectroscopy, or ‘molecular pulling.’ Using a soluble synthetic protein from dragline silk, segmented silk nanofibers have been observed by AFM. Molecular pulling (Force Spectroscopy) has revealed sawtooth-like rupture peaks, indicative of the sacrificial bonds and hidden lengths seen in molecules of other self-healing biomaterials. A model is presented for the folding of single draglinesilk molecules and the arrangement of these molecules into nanofibers. The relationship is discussed between these molecular / nanoscale results and the materials properties of natural and artificial dragline silks.
INTRODUCTION Spider dragline silk is stronger than Kevlar, and it stretches better than nylon. This combination of properties is seen in no other fiber [2]. Therefore scientists have been “scheming for more than 100 years” [2] to capture dragline-silk’s remarkable properties in a form that we can use. Progress was made recently with a recombinant dragline silk protein that was expressed in mammalian cell lines. This protein was spun in a manner similar to the spider under mild conditions from a concentrated aqueous solution into fine fibers with good modulus and toughness but low tenacity (i.e., it breaks easily) [3]. We present here the elastic properties of molecules and the structure of nanofibers from a related bioengineered dragline-silk protein. These molecular and nano-scale studies of dragline silk are a complement to the whole-fiber elastic properties reported previously.
EXPERIMENTAL DETAILS The recombinant dragline-silk protein used here has been named both pS(4+1) [1] and [(SpI)4/SpII)1]4 [4]. We refer to it here as [(SpI)4/SpII)1]4, to be consistent with the nomenclature of spider-silk researchers. [(SpI)4/SpII)1]4 was prepared as described
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previously [1, 5] and was supplied by S. A. Fossey, J. Soares and S. Arcidiacono (U.S. Army Natick R&D Center, Natick, MA 01760). The amino-acid (aa) sequence of [(SpI)4/SpII)1]4 is presented below [5]: SGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGPGGYGPGQQTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGYGGLGSQGTSGRGGLGGQ GAGAAAAAAAAAAGGAGQGGY
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