Silk fibers fibroin, formation

Xie F et al (2006) Effect of shearing on formation of silk fibers from regenerated Bombyx mori silk fibroin aqueous solution. Int J Biol Macromol 38(3-5) 284-288 Li C et al (2006) Electrospun silk-BMP-2 scaffolds for bone tissue engineering. Biomaterials 27(16) 3115-3124... 

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... 

Silk fibers formed by the secretion of the major ampullate glands of the spider Nephila clavipes also involve the linear supramolecular aggregation of a globular protein (fibroin). The formation of rodlike aggregates is consistent with the observation of a precursor liquid crystalline phase before fiber solidification [135,136]. Details of this supramolecular polymerization are still unclear. 

In a recent study, Jin and Kaplan (2003) demonstrate the formation of silk fibroin aggregates in the presence of polyethylene glycol, and present a step by step model for fiber formation based on the principle of micelle formation, and driven by dehydration as well as flow elongation. During this process, hydrophobic chains are exposed to the solvent, but because of the molecules high free energy, water solvation is unfavorable and phase separation followed by aggregation predominates. 

Zhao, C.H., Yao, J.M., Masuda, H., Kishore, R., and Asakura, T. "Structural characterization and artificial fiber formation of Bombyx mori silk fibroin in hexafluoro-iso-propanol solvent system". Biopolymers 69(2), 253-259 (2003). 

Li et al. [83] fabricated silk fibroin fiber scaffolds containing bone morphogenetic protein 2 (BMP-2) and/or nanoparticles of hydroxyapatite (nHAp) by electrospinning. These scaffolds were used in vitro to study bone formation from hMSCs. The results showed that the incorporation of BMP-2 and/or nHAp into silk... 

Magoshi, J. Magoshi, Y. Nakamura, S. Crystallization, liquid crystal, and fiber formation of silk fibroins. J. Appl Polym. Sci. Appl. Polym. Symp. 41 187-204 (1985). 

In contrast to the a-helical structure of the a-K. discussed above, the -K. have -pleated sheet structure. The most prominent representative of this class is silk fibroin (iff, 365,000, 2 subunits). Here the chains run antiparallel rather than parallel, and form a zig-zag structure. The formation of hydrogen bonds between the -CH(=0) and -NH- groups of neighboring chains stabilizes the pleated sheet structure. Together with weak hydrophobic interactions, the hydrogen bonds link pairs of polypeptides into a three-dimensional protein complex. These are additionally stabilized, in silk, by a water-soluble protein, sericin. The resultant fiber is very resistant and flexible, but only slightly elastic. The amino acid sequence which repeats over long stretches of the chain is, for silk fibroin, (Gly-Ser-Gly-Ala-Gly-Ala-) . 

The formation of silkworm fibers may be related to formation of supramolecular elongated structures starting from micellar structures [188,189]. Spherical micelles (100 to 200 nm diameter) were observed in aqueous solution of reconstituted silkworm silk fibroin. Aggregation of these micelles into larger structures upon increasing fibroin concentration was observed. Shearing of these solutions produced a fibrillar structure with morphological features typical of silkworm fibers. 

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