Cross-/! silks structure

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

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). 

Insects and arachnoids produce well-known amyloids. Silk and spider webs, like P-keratin, also differ from amyloids in being fibrous P-sheet proteins composed of peptide strands that are parallel, rather than perpendicular, to the direction of the fibril axis. For the process of silk formation by spiders, it has been proposed that fibrils in the silk gland have an initial cross-P structure (Kenney et al. 2002 Table 3) that, when stretched, assume parallel P-structures. However, X-ray diffraction for a peptide derived from the central domain of the A class of chorion proteins, derived frovaAntheraea polyphemus eggshells, displayed P-sheets perpendicular to the fibril axis, the same cross-P structure that occurs in amyloid proteins (Iconomidou et al. 2000 Table 3). The stability and strength of the amyloid fibres provides mechanical and biological protection for the oocyte and developing embryo from a variety of environmental and predatory hazards. 

Matting agents are used to produce coatings with a matt, semi-matt, or silk finish. They include natural mineral products such as talc or diatomites and synthetic materials such as pyrogenic silicas or polyolefin waxes. Matting can also be obtained by special formulations that exploit the incompatibility between binder components and their cross-linked structures. 

Demura M, Minami M, Asakura T, Cross TA. Structure of Bombyx mori silk Fibroin based on solid state NMR orientational constraints and fiber diffraction unit cell parameters,/Am Chem Soc, 120,1300-1308. 

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. 

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., 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. 

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. 

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... 

When the a helices in wool are stretched, intrahelix hydrogen bonds are broken as are some of the interhelix disulhde bridges maximum stretching yields an extended P sheet structure. The Cys cross-links provide some resistance to stretch and help pull the a helices back to their original positions. In silk, the P sheets are already maximally stretched to form hydrogen bonds. Each P pleated sheet resists stretching, but since the contacts between the sheets primarily involve van der Waals forces, the sheets are somewhat flexible. 

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