Silk fibroin chemical structure

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. ... 

To extract information on molecular orientation distribution from experimental data, the most widely known technique, the Legendre moment expansion approach can be taken. In this section, this approach will be discussed first, followed by methods to elucidate atomic resolution details of the structures of ordered polymers with orientation-dependent NMR interactions, such as those from chemical shielding, dipole-dipole and quadrupolar coupling. Then, solid-state NMR studies of the torsion angles of the peptide backbone of highly ordered silk fibroin fiber, a protein that has been studied extensively as a model for fibrous proteins, will be described. 

The structure of silk fibroin from a wild silkworm was examined by using solution and solid state NMR methods. The structural transition of the silk fibroin in aqueous solution was monitored by using solution NMR spectroscopy as a function of temperature. The torsion angles of several Ala and Gly residues in the model peptide, GGAGGGYGGDGG(A)i2GGA-GDGY-GAG, were determined by the conformation-dependent chemical shifts, REDOR and 2D spin-diffusion techniques in the solid state. 

Finally, structural studies of two well-known silk fibroin proteins, Bombyx mori and Sarnia cynthia ricini, have been reported. Solid state NMR methods, such as C 2D spin-diffusion NMR and REDOR were used in addition to the quantitative use of the conformation-dependent chemical shifts, measured by C CP MAS. 

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. ... 

Asakura, T. Demura, M. Date, T. Miyashita, N. Ogawa, K. Williamson, M.P. NMR study of silk I structure of Bombyx mori silk fibroin with N-15- and C-13-NMR chemical shift contour plots. Biopolymers. 41 193-203 (1997). 

In the current investigation, a detailed structure for Tussah silk fibroin has been derived which is compatible with both chemical and X-ray data. This structure also has strong impUcations concerning the structure of the (stretched) form of XM>ly-i,-alanine. 

We can now see how the differences in the structures of Bombyz mori and Tussah silk fibroins are related to the striking differences in their chemical compositions. The principal difference between the pseudo structures of the two silks is in the method of packing... 

The question whether silk fibroin filaments are resorbable or permanent is open to interpretation. Having a polypeptide chemical structure, silk fibroin, like any other protein, is susceptible to proteolytic degradation, and will become weaker and eventually over a period of 2 years will be totally resorbed in vzvo. However, given the definition for an absorbable suture in the United States Pharmacopeia as a material that loses most of its tensile strength within 60 days post-implantation silk can therefore be classified as a permanent biomaterial. 

Silkworm silk fibroin is composed of a crystalline motif that is made up of multiple (Alanine (Ala)-Glycine (Gly)-Serine (Ser)-Gly-Ala-Gly) repeats and a Tyrosine (Tyr)-rich r on [164]. C FI CP/MAS NMR has been utilized extensively to study these fibroins as chemical shifts are sensitive to the local environment and can thus report on the secondary structure. One of the first soHd-state NMR studies of silkworm fibroin dimorphs, silk I and silk II, was undertaken by Saito et al. [159] using CP/MAS... 

Silk fibroin has been recognized to be a semi-crystalline polymer.(228,229) Analyses of the small peptide fragments found in partial hydrolyzates of silk fibroin are not in accord with the concept of a regular chemical repeating sequence throughout the molecule.(230) Rather, a structure in which certain types of residues occur in particular portions of the chain is suggested. Specific fission of the poly(pep-fide) chain at tyrosine has allowed for the isolation of two major portions.(231)... 

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