Natural Protein Fibers

Keywords

Amino acid • Primary structure • a-helix • p-sheet • Nanofibril

Structurally, five types (coiled coil, p-strand, cross p-sheet, collagen triple helix, and polyglycine) of silk have been identified that vary in the amount of crystalline and amorphous regions and the arrangement (sequence and orientation) of the amino acids along the axis of the fiber. Each type of structure has a specific sequence of amino acids. For instance, the coiled-coil structure has seven amino acid residues, and the protein chains form a right-handed a-helix with 3.2 amino acids per turn. In the coiled-coil silks produced by some insect species such as honeybees and weaver ants, each fibroin contains 210 amino acid residues in the coiled-coil region with alanine-rich cores [07Sut]. A structural model for the coiled — coil silk is shown in Fig. 35.1. Coiled-coil silks were also found to contain unusually high levels of alanine and large hydrophobic residues. The high levels of alanine were required to stabilize the helices and facilitate coiled-coil formation [07Sut]. In a p-strand structure, alternating amino acid side chains form opposite faces of the sheet and in a cross-p sheet, the protein chains form p-strands of uniform length and alternating turns at which the direction of the protein chain reverses. In a collagen triple helix, three 32 helices intertwine and form a superhe­lix, and finally in a polyglycine structure, the protein chains form a right-handed helix with three amino acids per turn. Figure 35.2 illustrates the five different types of structures discussed here.

Morphologically, silks are composed of nanofibrils, similar to those seen in cotton. Atomic force microscopic images have shown that Bombyx mori and wild silk such as Antheraea pernyi are composed of nanofibrils or bundles of nanofibrils that are arranged in helices and in different layers. Mean width of the fibrils was found to be 90-170 nm, and the average angle was between 30 and 50°. No correlation was found between the fibril size and fiber properties, but the cross­angle influenced size of the fibers with larger fibers having higher angle.

Figure 35.3a shows an AFM image of the nanofibrils along the fiber axis, and Fig. 35.3b shows the layered structure in an A. pernyi fiber [00Put]. Similar AFM studies for Samia cynthia ricini fibroin have showed that fibroin molecules self — assemble to form highly ordered rodlike structures (0.4 nm long) and that the rods attach end to end due to the electrostatic interactions between the hydrophilic amino acid residues [03Ino]. Such an assembly leads to the formation of a textile like nanofabric as seen in Fig. 35.4. The nanofabric was 2 qm in width and 10 qm in length, and the height of the warp and weft was 0.4 nm which corresponds to the height of a single fibroin molecule. The distance between the adjacent threads was about 50-80 nm which confirms that the fabric was formed by an end-to-end assembly of the fibroin molecules. Fibroin molecules were also found to assemble in a comblike structure as seen in Fig. 35.5. In the case of Samia cynthia ricini, about 10 % of amino acids were found to be charged residues, and these residues are located in the non-helical part allowing the molecules to form aggregates through electrostatic interactions leading to an ordered structure [03Ino].

The structure and properties of silks have been extensively studied, and consid­erable variations in the secondary structures have been reported. Table 35.1 lists the P-sheet contents in various silks determined using different techniques. As seen in the table, there is considerable variation in the p-sheet contents that have been reported. This variation could be due to the inherent differences in the silks that were studied, instrumental differences, and interpretation of results. Particularly unique in terms of protein structure is the wild silkworm A. pernyi that is similar to the spidroins in spider silks than the common B. mori silk [11Fu]. To better understand the mechanism of fiber formation and its influence and structure and properties, silk fibers were forcibly extruded from A. pernyi silkworms, and the

structure and properties of the drawn fibers were studied. This approach avoided the formation of secondary structure during regeneration and was therefore direct evidence to the relationship between structure and properties. Unlike B. mori silk but similar to that of silk produced by major ampullates, the artificially produced A. pernyi silk fibers showed an obvious yield point during tensile testing. Immedi­ately after forced reeling, fibers had strength similar to that of forcibly reeled spider silk but a breaking strain between 0.3 and 0.6, better than that of spider silks. However, the reeled fibers did not show major contraction in water that happens when spider silk is immersed in water. Tensile properties of the fibers reeled from the silk before and water contraction are shown in Table 35.2.

2000 nm

1000 nm

Fig. 35.3 AFM images demonstrating the fibrillar structure along the fiber axis (left) and the layered structure (right) in A. pernyi silk fibers [00Put]. Reproduced with permission from Elsevier

Fig. 35.4 Tapping-mode image of S. c. ricini wild silk fibroin on a mica surface. The sample solution was cast over a mica surface 24 h after filtering. Scale bar: 500 nm [03Ino]. Reproduced with permission from the American Chemical Society

References

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[03Ino] Inoue, S., Tsuda, H., Tanaka, T., Kobayashi, M., Magoshi, Y., Magoshi, J.: Nano Lett. 3 (10), 1329 (2003)

[07Sut] Sutherland, T. D., Weisman, S., Trueman, H. E., Sriskantha, A., Trueman, J. W.H., Haritos, V. S.: Mol. Biol. Evol. 24(11), 2424 (2007)

[10Sut] Sutherland, T. D., Young, J. H., Weisman, S., Hayashi, C. Y., Merritt, D. J.: Annu. Rev. Entomol. 55, 171 (2010)

[11Fu] Fu, C., Porter, D., Chen, X., Vollrath, F., Shao, Z.: Adv. Funct. Mater. 21, 729 (2011) [11Lin] Ling, S., Qi, Z., Knight, D. P., Shao, Z., Chen, X.: Biomacromolecules 12, 3344 (2011)