Silk fibroin, proposed structure

Figure 36.3. Pleated sheet structure (hefa arrangement) proposed by Pauling for silk fibroin. Chains contracted to make room for small side chains. Adjacent chains head in opposite directions hydrogen bonding between adjacent chains.

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. ... 

We now consider the contribution of water molecules in the protein structure to the relaxation above 250 K. It is interesting to compare our H pulsed NMR data obtained for B. mori silk fibroin, dried and undried, with those reported by Andrew et for other proteins. A moder - proposed for silk fibroin films... 

Although the film (Os. 40°C) has the same water content as the film (24h, 23°C), its Tj, is considerably larger than that of the latter film. This means that water molecules trapped exclusively in the interior of the film become more mobile when the surface structure of the film changes from random coil or silk 1 to silk 11. On the other hand, water molecules distributed uniformly over the film are less mobile. This is the case of the film (Os, 40°C). This finding supports the heterogeneous hydrated structure of the silk fibroin film prepared with the aqueous methanol solution proposed previously. 

The ESR spectra were analyzed quantitatively and the fraction of the fast (t. = 10 ), slow (r,. = 10 ), and very slow = I0 ") motions of the spin-label site (Tyr side-chain) were determined. A model is proposed for the heterogeneous structure of the swollen silk fibroin membrane. The ESR spectrum was complex, indicating the heterogeneous structure of the swollen membrane. 

On the basis of - C NMR and ESR data, a model for the heterogeneous structure of the swollen silk fibroin membrane treated with methanol is proposed as shown in Fig. 34. 

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... 

In terms of the silk II secondary structure, since it is easier to handle than silk I, fiber diffraction studies had long proposed it to be mainly of an anti-paraUel P-sheet character [168]. Saito et al. [159], using an approach similar to what they used with silk I, agreed with the P-sheet conformer model. However, it was also recognized that some disorder existed in the crystalline Ala-Gly region [169]. To characterize the fibroin more precisely, Asakura et al. [164] performed CP/NMR experiments on isotopicaUy... 

The pleated sheet structure applies to proteins of the silk fibroin-/3-keratin group. For silk fibroin itself it may be assumed that the structure has been proved correct by Pauling s calculations (antiparallel chains). A pleated sheet structure with parallel chains is proposed for stretched hair (/3-keratin). Investigations on some other natural representatives have not yet been concluded. Possibly, amorphous or differently arranged sections occur in combination with pleated-sheet areas. 

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