Cross-/! silks

A trademark of amyloid fibrils is their cross-/ structure. This structure is the basis of the repetitive hydrogen-bonding extension of the fibril (Makin et al., 2005). Cross-/ structures are observed in the silk fibers of some insects (Geddes et al., 1968 Hepburn et al., 1979), although none are observed in spiders or lepidoptera (Craig, 1997). This absence has been explained by the possibility that cross-/ silks or a-silks may be converted into collinear /1-silks by stretching the fiber and an increased orientation-function correlated to the speed at which silk is formed (Riekel et al., 2000). 

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

Many other properties have to be considered, especially for apparel fibres, e.g., moisture absorption, ability to dye, drape, texture, weaving characteristics, etc. Many of the properties are influenced by the cross-section profile of the fibre. Thus cotton and some rayons (an artificial synthetic fibre derived from cellulose) are a hollow round fibre silk has a triangular shape giving it a fine lustre and drape. 

A comparable folding mechanism was found in silks. A seminal study by Li et al. (2001) found that in vitro formation of silk fibrils is conformation dependent and occurred via a nucleation mechanism. Although now established as amyloidogenic (Kenney et al., 2002), the nature of the silk fibril assembly remains unclear. Noteworthy is the evidence for a cross-nucleation ability of silk proteins, supporting the amyloidogenicity of silk (Lundmark et al., 2005). 

One aspect of the silk fibril formed in solution remains unclear the apparent absence of the cross-/l structure that characterizes amyloid fibrils. 

A better characterization of fibrils found in various silks could resolve this issue and most likely reveal the conformational criteria involved in the choice of the col I in ear-/) over the cross-/ structures. 

Asakura, T., Kuzuhara, A., Tabeta, R., and Saito, H. (1985). Conformation characteriz-tion of Bombyx mod silk fibroin in the solid state by high-frequency 13C cross polarization-magic angle spinning NMR, x-ray diffraction, and infrared spectroscopy. Macromolecules 18, 1841-1845. 

Asakura, T., Ohgo, K., Ishida, T., Taddei, P., Monti, P., and Kishore, R. (2005). Possible implications of serine and tyrosine residues and intermolecular interactions on the appearance of silk I structure of Bombyx mod silk fibroin-derived synthetic peptides High-resolution 13C cross-polarization/magic-angle spinning NMR study. Biomacromolecules 6, 468-474. 

The throne s incumbent is robed in white linen with a mantle of crimson silk falling from his shoulders. He bears a tall staff surmounted by the triple cross of Melchizadek. Upon his head rests the White Crown of Upper Egypt, encircled by three gold diadems. His face is as old as yesterday and as young as tomorrow. The mouth is generous and warm and the eyes are deep and dark to look into them is to see the starry wisdom mirrored therein. 

More akin to silk yarns, continuous filament POY produces lighter fabrics, typically of 100 % PET. Such yarns have provided a fertile field for imaginative engineering of cross-sectional shapes, fiber sizes and combinations of color and texture. An entire field of specialty filament yarns known as Shingosen has been developed in Japan, providing novel and luxurious fabrics that cannot be duplicated with natural fibers. 

The major textiles before the 1920s were wool (animal hair), cotton (a seed pod), and silk (a protein used for making cocoons). The silk spider also had a clever device in its abdomen for expelling a gel in a sac through a spinneret where reactions with air made a solid fiber with a uniform cross section. DuPont took this idea in spinning hydrolyzed cellulose into rayon fibers and scaling-the process up far beyond the needs of spiders. 

Hexamethylenediamine, 1,6 Hexanediamine (HMDA). H2N(CH2) NH2 mw 116.24, N 24.10% silk-like leaves, mp 42° bp 205° (subl), sol in water si sol in h eth. Prepd by hydrogenation of adiponitrile over Raney Ni or Co chlorination of butadiene then reactn with NaCN hydrogenation. Acute local irritant-mod flame hazard. Used in high polymer synth as a cross-linking agent (Refs 1, 3 4)... 

Besides prinfing and decoration, flexographic, gravure, and lifhographic plates as well as silk screens are used in resist chemistry, which is widely employed in the production of printed circuit boards. A resist is a material that will resist solvent attack. A negative resist is a material that, upon exposure to light cross-links, becomes less soluble in a solvent system that would... 

I Open systems in one dimension here the chains are cross linked to form sheets, axially pleated, with the direction of the chain and the hydrogen bonds roughly at right angles. The type of this is the Astbury / structure of silk and stretched KMEF proteins. 

A spider s orb-web is formed by extrusion of a concentrated protein solution and stretching of the resulting fiber. The cross-strands, which are stronger than steel, resemble silkworm silk. The molecules contain microcrystalline p sheet domains that are rich in Gly-Ala repeats as well as polyalanine segments. The capture spiral is formed from much more elastic molecules that contain many -tum-forming sequences. These assume a springlike p spiral. See Box 2-B. 

The structure of wool is more complicated than that of silk fibroin (Figure 25-13) because wool, like insulin (Figure 25-8) and lysozyme (Figure 25-15), contains a considerable quantity of cystine, which provides —S—S— (disulfide) cross-links between the peptide chains. These disulfide linkages play... 

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