Silk fibroin model polypeptide

CRAMPS NMR study of a silk fibroin model polypeptide... 

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.)...

Some natural fibrous proteins and sequential model polypeptides have been used as follows (1) Tussah Antheraea perni silk fibroin [a-helix form], (2) Bombyx mori silk fibroin [silk I and II forms], (3) poly(L-alanyL-glycine) [Ala-Gly]i2 [silk I and II], (4) collagen fibril [triple-strand helix], and (5) poly(L-prolyl-L-alanyL-glycine) [Pro-Ala-Gly [triple-strand helix]. 

Now it is clear that the H chemical shift reflects the conformation of model polypeptide [Ala-Gly] 12 and natural silk fibroins such as Tussah Antheraea pernyi and Bombyx mori silk fibroins. It is confirmed that the well-defined [Ala-Gly] 12 is a suitable model for the structural study of natural silk fibroins (silk I and silk II forms) using high-resolution solid-state NMR. As a result, the H peak assignment of the silk fibroins on the basis of the conformation-dependent H chemical shifts of model polypeptides can be determined utilizing H CRAMPS NMR and H- C 2D HETCOR NMR, as described in this section. The chemical shift results of model polypeptides [Ala-Gly] 12 synthesized by Shoji et al. play an important role in determining new structures for silk I and silk II forms, as very recently proposed by Lazo and Downing. ... 

Such conformation-dependent displacements of the chemical shifts for particular amino-acid residues depending upon their secondary structures can be conveniently utilized as a means to elucidate the local conformations of the respective amino-acid residues for any given proteins or peptides, since all the chemical shifts of the amino-acid residue, adopting unfolded conformations in solution, turn out to be independent of all neighboring residues except for the proline residue. Therefore, the transferability of these parameters for the particular residues from the simple model polypeptides to more complicated proteins is excellent and can be applied to any type of proteins, as far as the amino acid residues under consideration are virtually static (rigid) as in silk fibroin, collagen, and synthetic transmembrane peptides of bR in the solid. ... 

The silkworms can produce strong and stiff fibers at room temperature and from an aqueous solution (1). Therefore, it is important to know the structure of the silk fibroin in silkworm in order to understand the mechanisrn of fiber formation at the molecular level. Two crystalline forms, silk I and silk n, have been reported as the dimorphs of silk fibroin from B. mori based on several spectroscopic investigations (2). The silk II structure (tiie structure of silk fiber after spinning) was first proposed by Marsh et al. (3) to be an anti-parallel p-sheet, which was subsequently supported by other researchers (1). However, flie deterniination of the silk I stmcture was difficult because any attaupts to induce (sientation of the silk fibroin or the model polypeptides with silk I form for studio by X-ray and electron difl action, causes the silk I form to readify convert to the more... 

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