Silk fibers formation

It is now clear from the study of silk fiber formation in lepidoptera and spiders (Akai, 1998 Iizuka, 1966 Kerkam et aL, 1991 Knight and... 

Work, R. W. (1977). Mechanisms of major am-pullate silk fiber formation by orb-web-spinning spiders. Trans. Am. Mictvse. Soc. 96,170-189. 

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. 

Other additional phenomena may contribute to the progressive enrichment in /(-sheet structure. For example, silk glands produce multiple proteins and the interactions of these proteins facilitate fiber formation and contribute to the size and amounts of crystallinity (Craig, 2003 Lee, 2004 Sehnal and Zurovec, 2004 Sponner et al., 2005b). Furthermore,... 

In summary, the formation of silk fibers involves superstructures such as, possibly, micelles and/or molecular rods (Akai, 1998 Jin and Kaplan, 2003 Knight and Vollrath, 2002) that are dependent on the packing and conformation of the individual protein units. The relationship, however, between the shapes of these superstructures and the various forms of protein conformation remains elusive (Valluzzi and Jin, 2004). Seeking to clarify this issue we will examine, in Section II.B, the role of shape and extended network formation modulating solubility, stability, and assembly. 

Fig. 3. Solubility of silk proteins in solution as a function of time. Low solubility corresponds to protein aggregation. The fast and slow aggregations are observed in vitro (Dicko et al., 2004a), whereas the stable helical conformation (storage structure) is observed in vivo (Dicko et al., 2004b,d). This illustrates the inherent instability of silk protein in solution and shows the /(-sheet polymorph structure as the most stable form. In other words, the spiders actively control and modulate the unavoidable silk protein aggregation prior to fiber formation.

Individually, the silk molecules will adopt a shape and conformation dictated by the competition to hydrate polar and nonpolar moieties (Hossain et al., 2003 Jin and Kaplan, 2003). In addition, to achieve fiber formation and optimal axial stiffness, the system must organize and lock the molecules in their extended configuration (Donald and Windle, 1992)... 

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). 

Pseudomorphs on a bronze Shang Dynasty halberd (ca. 1300 b.c.) were subjected to mineralogical analysis to determine their structure and composition. X-Ray diffraction, scanning electron microscopy, and energy dispersive analysis of x-rays were used in these analyses. Photomicrographs of pseudomorphs also were studied for fiber, yam, and fabric formations that give evidence of textiles. A model describing the process of silk pseudomorph formation was proposed. 

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 drawing shows a small portion of a single layer of protein molecules (polypeptide chains) in a silk fiber. These molecules are attached to adjacent molecules in the layer by the formation of hydrogen bonds, which extend laterally from the NH group of one chain to the oxygen atom of an adjacent chain. The fiber consists of many of these hydrogen-bonded layers superimposed upon one another. 

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 first kinetic study of free-radical formation in mechanically loaded polymers was by Zhurkov and co-workers who studied the stepwise loading of nylon 6 and silk fibers (26). The concentration of free radicals was monitored by ESR as a function of applied stress and time. They found that the rate of radical formation (R) is a function only of stress (a) ... 

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). 

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... 

Figure 5 Mechanism of fiber formation in the spicier major ampullate gland. Schematic process of silk secretion and fiber formation, from left to right. Silk protein is represented in green, and the gland cells/wall in orange. Flow is from left to right. Adapted from Michael Ellison, Clemson University.

The formation mechanism of Cuj(BTC)2 nanoparticles upon silk fiber is illustrated in Fig. 11. It is found that formation of Cu3(BTC)2 MOF on silk fiber surface was increased in presence of ultrasound irradiation. In addition, increasing the concentration cause an increase in antimicrobial activity [1]. Figure 12 shows the SEM micrograph of Cu3(BTC)2 MOF on silk surface. 

FIGURE 11 Schematic representation of the formation mechanism of Cu3(BTC)j nanoparticles upon silk fiber. 

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