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The role of elastomeric network chains in the mechanics of spider silks Savage, Kenneth Neal

Abstract

Orb-weaving spiders produce capture-webs from two mechanically distinct silk types, major ampullate silk (MA) and flagelliform silk (FL), constructed from proteins with a block-copolymer structure consisting of crystal-forming domains and glycine-rich domains. These fibroins form polymer networks crosslinked by β-sheet crystals, with network chains formed by the glycine-rich domains. There are two major variables in the design of the network chains: in MA silks the network chains can be proline-rich or proline-deficient, and in FL silks there is a 3 to 4-fold difference in chain length. When dry, the network chains are stiff due to inter- and intra-chain hydrogen bonds. Hydration disrupts these hydrogen bonds, and the silk swells, in a process termed supercontraction, and becomes rubber-like. When tethered in the web, supercontraction causes significant stress to develop within the fibre. We compare MA silks from Argiope (proline-rich) and Nephila (proline-deficient) and demonstrate that both silks can withstand the stresses developed during supercontraction. We conclude that hydrated silks can be used to study the functional design of silk fibroins. Mechanical and optical tests on dry and supercontracted MA silks from Araneus (proline-rich) and Nephila (proline-deficient) reveal that the silks are mechanically indistinguishable in the dry state, but are dramatically different the hydrated state. In Araneus the network chains are kinetically-free and amorphous in the hydrated state, but there is semi-crystalline structure in Nephila’s network chains. Thermoelastic measurements on hydrated silks reveal that Araneus MA and FL silks exhibit rubber-like, entropic elasticity, consistent with networks of random, amorphous chains. The elasticity of hydrated Nephila MA silk is largely due to bond energy elasticity, associated with the deformation of stable secondary structures. Although the entropic-elastic mechanism in the proline-rich Araneus MA and FL silks is consistent with a network of kinetically-free, random chains, it does not exclude the possibility that β-spirals provide an alternative molecular mechanism. Mechanical tests on FL silks with different network chain lengths, however, reject the β-spiral as a model for the elasticity of hydrated spider silks. These results indicate the importance of fibroin sequence design in determining the material properties of spider silks.

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