Formation of Silk

Silk is a protein fiber spun by silkworm, which is the caterpillar or larva of the domesticated silkmoth, Bombyx mori. In addition to silkworm, many other insects or non-insects (eg., spider) can spin silk fibers. However, the silk of Bombyx mori is the mainstay of commercial silk production and consumption. This section will only discuss the formation of silk fiber by silkworm. 

The silkworm spins the silk in a figure-8 pattern aroimd itself to form a complete cocoon. The cocoon then can be immersed in hot water to loosen the fibers, which are pulled from the water for use. 

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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. 

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. 

A comparable folding mechanism was found in silks. A seminal study by Li et al. (2001) found that in vitro formation of silk fibrils is conformation dependent and occurred via a nucleation mechanism. Although now established as amyloidogenic (Kenney et al., 2002), the nature of the silk fibril assembly remains unclear. Noteworthy is the evidence for a cross-nucleation ability of silk proteins, supporting the amyloidogenicity of silk (Lundmark et al., 2005). 

Zhang, Y.Q., Shen, W.D., Xiang, R.L., Zhuge, L.J., Gao, W.J., and Wang, W.B. "Formation of silk fibroin nanoparticles in water-miscible organic solvent and their characterization". 

B-M. Min., L. Jeong., Y.S. Nam., J-M. Kim,J. Y. Kim, W.H. Park. 2004. Formation of silk fibroin matriees with different texture and its eellular response to normal human keratinocytes. InternationalJournal of Biological Macromolecules, 34.pp.281-288. 

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... 

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). 

Sir Joseph Swan, as a result of his quest for carbon fiber for lamp filaments (2), learned how to denitrate nitrocellulose using ammonium sulfide. In 1885 he exhibited the first textiles made from this new artificial sHk, but with carbon fiber being his main theme he failed to foUow up on the textile possibihties. Meanwhile Count Hilaire de Chardoimet (3) was researching the nitrocellulose route and had perfected his first fibers and textiles in time for the Paris Exhibition in 1889. There he got the necessary financial backing for the first Chardoimet silk factory in Besancon in 1890. His process involved treating mulberry leaves with nitric and sulfuric acids to form cellulose nitrate which could be dissolved in ether and alcohol. This collodion solution could be extmded through holes in a spinneret into warm air where solvent evaporation led to the formation of soHd cellulose nitrate filaments. 

Although it is evident from the above discussion that the fatty acids present in the usual vegetable or animal fats do not contribute to the carbohydrate stores in the animal body, there is ample proof that such may be the case in plants and lower organisms. This change has been confirmed in the castor bean where the R. Q. has been found to vary from 0.30 to 0.58 during the period of germination.1660 This could be correlated with the disappearance of fat and the formation of carbohydrate.167 There also seems to be evidence that silk worms are able to build carbohydrate at the expense of fat.168... 

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... 

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.

Several studies (Asakura etal, 2001 Dicko etal, 2005 Heslot, 1998 Lazo and Downing, 1999 Valluzzi et al, 2002) have focused their effort toward the understanding of silk stability and to describing the secondary structure population present in the prespun silk. One can, however, hypothesize that the formation of the gel state involves the competition between... 

Critically, another important aspect of having heterogeneities is the formation of superstructures such as fibrils. Fibrils or any other structures based on silk crystallization will need heterogeneity to nucleate and grow. 

Inoue, S., Tsuda, H., Tanaka, T., Kobayashi, M., Magoshi, Y., and Magoshi, J. (2003). Nanostructure of natural fibrous protein In vitro nanofabric formation of Sarnia cynthia ricini wild silk fibroin by self-assembling. Nano Lett. 3, 1329-1332. 

The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye Cl Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124] ... 

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