Silk fibers crystallization

Silk fibroin, which is the main constitueni of silk fibers, has a fo n molecular oonformatkm. A sheet structure is formed by hydrogen bonds between adjoining mole cules. Oriented films of silk fibroin consist of -foim crystals. The temperature dependence of the d constant is similar to that for dry collagen film. 

The crystallite size can be obtained from the wide-angle X-ray scattering (WAXS) pattern of a silk fiber. Radial integration along the equator and meridian on the WAXS gives the radial peak width on the (200), (120), and (002) reflections. According to Scherrer s formula [85], the crystal size is... 

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

Figure 14.9 Spicier fibers are composite materials formed by large silk fibroin polypeptide chains with repetitive sequences that form p sheets. Some regions of the chains participate in forming 100-nm crystals, while other regions are part of a less-ordered mesh-work in which the crystals are embedded. The diagram shows a model of the current concepts of how these fibers are built up, which probably will be modified and extended as new knowledge is gained. (Adapted from F. Vollrath, Sci. Am. p. 54-58, March 1992 and A.H. Simmons, Science 271 84-87, 1996. Photograph courtesy of Science Photo Library.)...

Crystallization is the basis of silk strength. But if crystallization were excessive, then the silk would become very brittle rather than being the flexible fiber that it is. The study of model peptides inspired from silk sequences has shown catastrophic and uncontrollable aggregation suggesting... 

Internal Structure of Silk and Linen. Triangular-shaped silk filaments (called brins and composed of the protein fibroin) are embedded in a second protein (sericin) to form the bave unit. Microscopic examination of fabrics woven with the silk bave unit and subsequently cleaned of sericin reveals paired brin filaments. Fibroin contains 16 amino acids, and more than 80 of the polymer is composed of glycine, alanine, and serine only (6). Their small side groups allow chains of these amino acids to pack together closely to form the crystalline portions of the fiber. The bulkier amino acids, such as tyrosine, disrupt crystal order and produce the amorphous areas of the internal structure of the fiber. 

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