The Structure of Silk

There are all sorts of silk that are found in nature. The stuff that is usually found in textiles comes from silkworms (Bombyx mori). They are not really worms, but the larvae of moths. They emerge from very small eggs with an incredible lust for mulberry leaves, which they consume until they are ready to. pupate and weave a cocoon around themselves. Unlike spiders, which spin silk from their rear end, silkworm silk is actually hardened saliva, which comes out of the mouth. The larva has a small spinneret on its lip, through which the silk emerges. The cocoon is formed from a single strand of silk that 

FIGURE 9-12 Schematic diagram illustrating the case when two amide carbonyl groups are too dose. 

FIGURE 9-14 Schematic diagram illustrating the pitch of a helix. 

FIGURE 9-15 Unraveling silk fibers from cocoons (the white things floating in the baths). 

Notice that if the chain is arranged in a zig-zag or extended chain conformation, as found in the crystal structures of polyethyl- 

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Lotz, B., Brack, A., and Spach, G. (1974). Beta structure of periodic copolypeptides of L-alanine and glycine. Their relevance to the structure of silks./. Mol. Biol. 87, 193-203. 

Marsh, R. E., Corey, R. B., and Pauling, L. (1955b). An investigation of the structure of silk fibroin. Biochim. Biophys. Acta 16, 1-34. 

First, a review of the properties and the structure of silk is given followed by a discussion on the relationship between molecular composition, assembled protein structure, and mechanical properties. Second, artificial spinning of silk proteins and their bioapplications are emphasized. Finally, the potential role of silk proteins in biomineralization is introduced and discussed. 

Figure 6 Schematic representation of major ampullate gland (a). The various parts of the gland and their function, as well as the structure of silk under each part, are shown (Lewis, 2006). Also shown is a model of chain folding, micelle formation, globule formation and curing, and shear processing of silk proteins. This model is motivated by experiments in vitro (Jin and Kaplan, 2003) (b).

The active site of the catalyst has been considered to be metallic palladium deposited in the -structure of silk fibroin as an a mmetric chelate. However, as a result of the examination of the reproducibility of these atymmetric hydrogenation reactions, the asymmetric modification Cpommii ) of palladium by S-a-amino acid was suspected, the latter being concomitantly formed by the l rdrolysis of silk protein with hydrochloric acid from palladium(II) chloride in the course of the catalyst preparation (4). 

FIGURE 23.19 The structure of silk. The amino acid side groups (shown as R) must be small to fit into a sheet-like structure. Glycine (R=H) and alanine (R=CH3) predominate. The repeating unit is shown as an inset. 

Since the Ala-Gly sequence is the major one in Bombyx mori silk, the structure of this sequential polypeptide is of importance in understanding the structure of silk. In early studies of )3-(AlaGly) it was found that the X-ray powder diffraction pattern and the IR spectrum resembled that from die crystalline component of B. mori silk. The proposed APPS structure of silk (Marsh et al., 1955b) led to a more detailed analysis of the X-ray pattern of )3-(AlaGly) (Fraser et al., 1965, 1966). This, together with conformational energy analysis (Colonna-Cesari et al., 1975),... 

The structure of silk fibroin is almost entirely antiparallel p-pleated sheet. 

Notice that almost every other residue is a glycine and that between them lie either alanine or serine residues. This repeating structure results in simple, tightly organized structures, such as the structure of silk fibroin shown in Figure 6.12. 

The structure of silk fibroin from a wild silkworm was examined by using solution and solid state NMR methods. The structural transition of the silk fibroin in aqueous solution was monitored by using solution NMR spectroscopy as a function of temperature. The torsion angles of several Ala and Gly residues in the model peptide, GGAGGGYGGDGG(A)i2GGA-GDGY-GAG, were determined by the conformation-dependent chemical shifts, REDOR and 2D spin-diffusion techniques in the solid state. 

The chemist chiefly responsible for the development of Neoprene was Wallace Hume Carothers (1896-1937), working with the Du Pont Company in the United States. It was Carothers who was also responsible for the introduction of the first polyamide, nylon 66. It was after a carefol analysis of the structure of silk that Carothers decided to synthesise polymeric amides. Nylon 66 was obtained by reacting hexamethylenediamine with adipic acid, and has been very successful as a synthetic fibre. Another class of linear condensation polymers which has found wide application as fibres is that of the polyesters. 

The structure of silk has been thoroughly studied and, as natural fibres go, is rather simple. In the first place, the composition is strongly biased towards amino acids with the smallest side chains about 60% are glycine, the smallest of all, in which a mere... 

Chakravarty, A. (2013). Humanitarian Relief Chain Rapid Response under Uncertainty International Journal of Production Economics. doi.org/10.1016/j.ijpe.2013.10.007 Chandler, D. (2010). New analysis of the structure of silks explains paradox of super-strength. 

In order to functionalize silk fibers without disturbing their performance, the structure of silk fibers needs to be understood. Although the Bombyx mori silkworm silk structure has been investigated for some time, the hierarchical structure did... 

To characterize the structure of silk, various techniques, namely X-ray diffraction (XRD), vibrational spectroscopy, and cross-polarization magic-angle spinning nuclear magnetic resonance, and so on, have been adopted. Hu et al. reported a study of self-assembled yS-pleated sheets in Bombyx mori silk fibroin films using... 

Vollrath, Porter, D., Dicko, C., The Structure of Silk , in Handbook of Textile Fibre Structure, Volume II Natural, Regenerated, Inorganic and Specialist Fibres, editors Eichhom, S.J., Hearle, J.W.S., Jaffe, M., and Kikutani, T, Woodhead Publishing Limited, 2009. 

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