NephilaclavipesSpider Dragline Silk Microstructure Studied by Scanning Transmission X-ray Microscopy

Abstract

Nephila clavipes dragline silk microstructure has been investigated by scanning transmission X-ray microscopy (STXM), a technique that allows quantitative mapping of the level of orientation of the peptide groups at high spatial resolution (<50 nm). Maps of the orientation parameter P2 have been derived for spider silk for the first time. Dragline silk presents a very fine microstructure in which small, highly oriented domains (average area of 1800 nm2, thus clearly bigger than individual beta-sheet crystallites) are dispersed in a dominant, moderately oriented matrix with several small unoriented domains. Our results also highlight the orientation of the noncrystalline fraction in silk, which has been underestimated in numerous structural models. No evidence of either a regular lamellar structure or any periodicity along the fiber was observed at this spatial resolution. The surface of fresh spider silk sections consists of a approximately 30-120 nm thick layer of highly oriented protein chains, which was found to vary with the reeling speed, where web building (0.5 cm/s) and lifeline (10 cm/s) spinning speeds were investigated. While the average level of orientation of the protein chains is unaffected by the spinning speed, STXM measurements clearly highlight microstructure differences. The slowpull fiber contains a larger fraction of highly oriented domains, while the protein chains are more homogeneously oriented in the fastpull fiber. In comparison, cocoon silk from the silkworm Bombyx mori presents a narrower orientation distribution. The strength-extensibility combination found in spider dragline silk is associated with its broad orientation distribution of highly interdigitated and unoriented domains.

Secondary Structure Transition and Critical Stress for a Model of Spider Silk Assembly

“…22,23 Up to a limit of 50mm/s, XRD results show that the higher reeling speed also helps the orientation and formation of the silk network, while no influence on the secondary structure composition could be determined. [24][25][26] Premature invitro self-assembly poses one of the issues involved with the synthetic production of silk fibers.…”

Structure of silk by raman spectromicroscopy: From the spinning glands to the fibers

“…As a result, only a few techniques can provide structural information about silk monofilaments, including microscopic methods (atomic force microscopy, [1][2][3][4][5][6] scanning 3,5 and transmission electron microscopy, 7,8 and scanning transmission X-ray microscopy 9,10 ) and synchrotron X-ray diffraction. 11,12 Nevertheless, some of these techniques require a certain degree of treatment of the fiber that might affect its structure and/or they provide no molecular (i.e., spectroscopic) information.…”

X-ray microscopy

“…ÂÍ, ËÔÒÑÎßÊÖâ ÓÂÊÎËÚËâ Ä NEXAFS-ÔÒÇÍÕÓÂØ ×ÂÊ Fe(III) Ë Fe(II), ÂÄÕÑÓÞ ÓÂÃÑÕÞ [196], ËÊÖÚÂâ ÃËÑÏËÐÇÓÂ- [227].°Ó ËÇÐÕÂÙËÑÐÐÂâ ÚÖÄÔÕÄËÕÇÎßÐÑÔÕß NEXAFS-ÔÒÇÍÕ-ÓÑÄ ËÔÒÑÎßÊÑÄÂÎÂÔß Ä ÔÒÇÍÕÓÑÏËÍÓÑÔÍÑÒËË AEÎâ ËÔÔÎÇAEÑ-ÄÂÐËÌ ÑÓËÇÐÕÂÙËË ÏÑÎÇÍÖÎ Ä ÑÓÅÂÐËÚÇÔÍËØ ÐÂÐÑÏÂÕÇÓËÂ-ÎÂØ [228,229], ÃÎÑÚÐÑÌ ÔÕÓÖÍÕÖÓÞ ÑÓÅÂÐËÚÇÔÍËØ ÒÎÈÐÑÍ [230,231], ÑÓÅÂÐËÚÇÔÍËØ ÒÑÎÖÒÓÑÄÑAEÐËÍÑÄ [232], ÕÓÂÐÊË-ÔÕÑÓÑÄ [233,234], ÑÓËÇÐÕÂÙËË ÏÑÎÇÍÖÎ Ä ËÔÍÖÔÔÕÄÇÐÐÞØ Ë ÒÓËÓÑAEÐÞØ ÄÑÎÑÍÐÂØ [226,235,236],  ÕÂÍÉÇ ÔÕÓÖÍÕÖÓÞ ÉËAEÍËØ ÍÓËÔÕÂÎÎÑÄ [227,237], ÖÅÎÇÓÑAEÐÞØ ÐÂÐÑÕÓÖÃÑÍ [177]. ³ÒËÔÑÍ ÎËÕÇÓÂÕÖÓÞ…”