Silk fiber structure

In addition to traditional X-ray techniques to study silk (Bram etal., 1997 Lotz and Cesari, 1979 Riekel et al., 1999a Warwicker, 1960), other structural tools have helped unravel various aspects of silk protein conformation. These include solid-state NMR (Asakura et al., 1983, 1988, 1994 Beek et al., 2000, 2002) studies of native and regenerated silk together with and studies of isotopically edited silks, which have dramatically improved the model of structure distribution within silk fibers (Beek et al., 2000, 2002). 

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). 

The P structure is one of the most important secondary structures in proteins. It occurs in about 80% of the soluble globular proteins whose structures have been determined. In many cases almost the entire protein is made up of P structure. Single strands of extended polypeptide chain are sometimes present within globular proteins but more often a chain folds back on itself to form a hairpin loop. A second fold may be added to form an antiparallel "P meander"102 and additional folds to form P sheets. Beta structures are found in silk fibers (Box 2-B) as well as in soluble proteins. 

In view of the various level of structural organization, it is worthwhile to draw a comparison between natural silk fibers (silkworm silk and spider silk) and man-made silk-based materials. 

Zarkoob, S., Reneker, D.H., Eby, R.K., Hudson, S.D., Ertley, D., and Adams, W.W. Structure and morphology of nano electrospun silk fibers. Abstracts of Papers of the American Chemical Society (1998), 216, U122-U122. 

A model for the process of silk pseudomorph formation on bronze is proposed, including solution of copper ions, transport inside the swollen silk, reaction with available anions, and deposition of the resulting products. The maintenance of the outer physical structure of the fiber is attributed to the network of the polymer chains of silk and the maintenance of some structural integrity during degradation. The presence of green and black pseudomorphs of equal size in the same location on the halberd led to a discussion of the possibility of difierential replacement of dyed silk fibers and undyed silk fibers. 

The protein secondary structure of silk fibroin [60] was studied with near-lR spectroscopy, using silk fibers that had been very carefully selected from naturally generated fibers. The isolation of individual fibers allowed the trapping from Nature of a protein with a particular secondary structure. A spider is able to generate different fibers for different uses, with each fiber having its own secondary structural composition. In the case of silk, an individual fiber may well have a particular composition secondary structure, and in this case it is possible to use near-lR spectra to perform a characterization. This is quite remarkable because the use of a relatively prominent amide-1 band in the mid-lR represents a major challenge. 

Possible (, 4>) pairs are compared with the observed bond orientations. In addition, the calculated bond orientations, 0 directly correspond to the observed orientations when the Ca(i - 1)—Ca i + 1) direction is parallel with respect to the FAS such as in the oriented /3-sheet structure which appears in the silk fiber. The most accurate (<, ip) values can be found from the combination of the calculated orientations which satisfy the observed bond orientations. Fig. 8.7 shows a combination of the bond orientations, c> CN for the case of the Ala site of B. mori silk fiber. 

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