Molecular Order in Silk Secretions
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MOLECULAR ORDER IN SILK SECRETIONS CHRISTOPHER VINEY,* KEVEN KERKAM,t LISA GILLILAND,§ DAVID KAPLAN,: AND STEPHEN FOSSEYt *Molecular Bioengineering Program, Center for Bioengineering WD-12, Univ. of Washington, Seattle, WA 98195 tDept. of Materials Science and Engineering FB-10, Univ. of Washington, Seattle, WA 98195 §Oncogen/Bristol Myers Squibb, 3005 First Avenue, Seattle, WA 98121 :US Army Research, Development and Engineering Center, Natick, MA 01760 ABSTRACT Transmitted polarized light microscopy of various natural silk secretions reveals their ability to form nematic liquid crystalline phases. Observations of microstructure, together with a simple secondary structure analysis of known amino acid sequences in silk proteins, suggest that the rodlike structures forming the nematic phase are supramolecular aggregates, rather than individual rigid molecular segments. The optical birefringence of dragline fiber produced by controlled silking depends on the linear haul-off velocity, and can exceed the birefringence of naturally spun fibers; this suggests the possibility of in-vitro spinning of silk to obtain values of strength and stiffness even greater than those achieved in vivo. BACKGROUND Natural silk fibers (and the process by which they are spun) exhibit a spectrum of impressive characteristics that cannot at present be achieved with artificial polymers. These characteristics include the unusual combination of high strength, stiffness and toughness of Nephila clavipes (golden orb weaver) dragline silk. A few synthetic polymers such as Kevlar® have a slightly higher strength than Nephila dragline, but their toughness is significantly lower [1,2]. (This comparison is necessarily qualitative, since it uses data from multiple sources that do not specify initial sample length.) Other desirable materials properties of silk are its durability in a variety of chemical and biological environments, and its biodegradability by proteolytic enzymes as is evident from the fact that spiders can recycle their web silk [3]. The way in which silk secretions are processed into fiber is also sophisticated. The mechanism of silk synthesis (molecules are assembled according to a DNA-encoded template) means that the chemical structure of the product is insensitive to changes in the composition of the raw material. Natural silk fibers are spun at ambient temperatures from aqueous solutions, whereas synthetic polymers are spun at elevated temperatures and / or from more aggressive solvents. The concept of a water-soluble polymer that converts to insoluble fiber upon shearing through a spinneret is novel to industrial polymer processing. The fact that some spiders and insect larvae can produce solid silk structures under water [4,5] clearly demonstrates that solidification is not a process of "drying out"; rather, it is one in which the pattern of inter- and intramolecular bonding is altered. Finally, cribellate spiders can produce silk filaments that are a mere 0.0Iltm in diameter [6] - very much finer than any polymer fibers spun ind
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