Silk fibroin composition

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

Figure 14.9 Spicier fibers are composite materials formed by large silk fibroin polypeptide chains with repetitive sequences that form p sheets. Some regions of the chains participate in forming 100-nm crystals, while other regions are part of a less-ordered mesh-work in which the crystals are embedded. The diagram shows a model of the current concepts of how these fibers are built up, which probably will be modified and extended as new knowledge is gained. (Adapted from F. Vollrath, Sci. Am. p. 54-58, March 1992 and A.H. Simmons, Science 271 84-87, 1996. Photograph courtesy of Science Photo Library.)...

Lucas, F., and Rudall, K. M. (1968). Variety in composition and structure of silk fibroins Some new types of silk from the hymenoptera. In Symposium on Fibrous Proteins, Australia 1967 (W. G. Crewther, Ed.), pp. 45-55. Butterworths, Sydney. 

Pioneering work in fibroin wet spinning can be traced back to 1930s. After that, little work has been done until the late 1980s, when more research was done to investigate the spinning dope systems, and structure and properties of the artificial fibroin silk. The composition of the dope is very important to the properties of the final fiber. Several kinds of solvents, such as LiBr—EtOH, Ca(NOo,)2—MeOH, formic acid, HFIP, hexafluoro acetone (HFA), and so on, are used to prepare the spinning dope (Table 4). Very recently, an ionic liquid was used as dope solvent (Phillips et al., 2005). 

Altman et al. utilized a composite silk fibroin/chitosan scaffold for seeding and in vivo delivery of human ADSCs in a murine cutaneous wound model, and the delivery technique conferred physiological benefits to accelerated wound closure. ADSC seeded on a silk fibroin/chitosan scaffold differentiated into fibrovascular, endothelial, and epithelial components of restored tissue and enhanced the wound healing process [219]. 

The protein secondary structure of silk fibroin [60] was studied with near-lR spectroscopy, using silk fibers that had been very carefully selected from naturally generated fibers. The isolation of individual fibers allowed the trapping from Nature of a protein with a particular secondary structure. A spider is able to generate different fibers for different uses, with each fiber having its own secondary structural composition. In the case of silk, an individual fiber may well have a particular composition secondary structure, and in this case it is possible to use near-lR spectra to perform a characterization. This is quite remarkable because the use of a relatively prominent amide-1 band in the mid-lR represents a major challenge. 

Assuming a molecular weight of 30,000 for silk fibroin (Coleman and Howitt, 1946) its composition is given by the following formula ... 

These results are in agreement with the suggestion of Meyer et al. (1940), that silk fibroin is composed of two parts, an amorphous part of complex amino acid composition and a crystalline part built up simply of Gly.Ala units linked together. This theory was based on the observation that the X-ray data could be fitted by a unit cell containing four parallel Gly.Ala residues. It was considered that some of the alanine residues in the crystalline part could be replaced by serine since its molecular dimensions are similar. Clearly the idea of a long chain containing glycine and alanine alternately is untenable in view of the results of Levy and Slobodiansky since in such a case the yields of Ala.Gly and Gly.Ala should be equal. However such a sequence could be broken up to a certain extent by serine residues. 

The amino acid composition and primary structure of B. ntori and S.c. ricini 104 silk fibroin... 

Studies of structure-property relationships are very active. It is well known that such dynamics as the several orders of the scale of the motions significantly influence the properties of polymers including silk fibroins. However, previous study of silk fibroin from the viewpoint of dynamics is limited and therefore, in this review, we describe mainly our work on the study of the dynamics of silk fibroins from B. mori and Samia cynthia ricini (S.c. ricini). The latter silkworm is a wild one and the NMR study was performed as a comparison with B. mori silk because of the amino acid composition changes. 

THE AMINO ACID COMPOSITION AND PRIMARY STRUCTURE OF B. mori AND S.c. ricini SILK FIBROIN... 

The amino acid composition of the silk fibroins from B. mori and Sx, ricini (in mole percent) is listed in Table 1.- ... 

Figure 4 shows the C NMR spectrum of the liquid silk from S.c. ricini mature larva.-" The spectrum of S.c. ricini liquid silk is sharp, indicating very rapid segmental motion of the main chain characterized by a very small correlation time on the order of 10 " s at room temperature (as in the case of B. mori silk fibroin). Assignment was performed by reference to chemical shift data of the pentapeptides, Gly-Gly-X-Gly-Gly, where X equals the specified residue.- "- and the amino acid composition. Solution - C and N NMR studies of S.c. ricini silk fibroin in aqueous solution indicate that about 70% of Ala residues form a-helices while, the conformation of the other Ala residues is random coil." - - ... 

Gui-Bo, Y., et al., 2010. Study of the electrospun PLA/silk fibroin-gelatin composite nano-fibrous scaffold for tissue engineering. Journal of Biomedical Materials Research Part A 93A (1), 158-163. 

Noshiki, Y., Nishiyama, Y., Wada, M., Kuga, S., Magoshi, J. Mechanical properties of silk fibroin-microcrystalline cellulose composite films. J. Appl. Polym. Sci. 86,3425-3429 (2002)... 

Jung, R., Jin, H.-J. Preparations of silk fibroin/bacterial cellulose composite films and their mechanical properties. Key Eng. Mater. 342-343, 741-744 (2007)... 

The different production history and composition in terms of amino acid percentages found in silk reflects into a different X-ray pattern, described by an arrangement of the chains in what is termed as anti-parallel P-sheet and shown in Figure 9.6.8. The same configuration, which is stable for silk fibroin, is also achieved metastably by oc-keratins when stretched in a wet environment. ... 

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