Sericin water-soluble

The B. mori silk fiber is made up of two kinds of protein. One is called sericin, a water-soluble protein responsible for the gum-like, sticky coating covering the fiber, and the other is referred to as fibroin, the core filament of silk. The inner part of silk fiber is composed of two monofilaments called brins (Figure la) (Poza et al., 2002 Shao and Vollrath, 2002). 

Biological Degradation. Microbial (enzymatic) attack can occur on silk, but it is the water-soluble sericin, that is the more readily depolymerised. 

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-) . 

A Bombyx mori silkworm silk thread consists of two triangular water-insoluble silk fibroin fibers glued by water-soluble sericin (see Figure 7.1a). Many views concerning the detailed structure of silk fibers have been published so far. Based on solid-state NMR, Asakura et al. [23-26] reported the structural analysis of natural protein fibers with mixed parallel and antiparallel /6-sheet structures. Alanine tripeptide samples with 100% parallel /6-sheet structure and with 100% antiparallel /6-sheet structure were chosen to characterize the /6-sheet structures following solid-state NMR experiments. AU resonances of the tripeptides were assigned by a comparison of the methyl resonances of Ala3 with different... 

Sericin, the protein that binds the pairs of fibroin filaments as they emerge from the silkworm, and which may have a role in dehydrating the fibroin and encouraging its crystallisation, has a markedly different composition and structure to that of fibroin. It is largely amorphous and is rich in serine (—32%), aspartic acid (—14%) and glycine (—13%) there is a much greater proportion of residues with polar and/or bulky side-chains. The predominance of these polar, hydrophilic groups means that sericin is readily soluble in hot water. 

Sericin biopolymer, an amorphous glue-like substance, is made up of 18 amino acids most of which 70% have a polar terminal functional group, such as -OH, -COOH, and -NH2 groups, which are responsible for its solubility as well as water absorption and desorption [72, 143]. Biocompatibility, blodegradablllty, excellent antimicrobial activity, resistance to UV radiation, and moisture absorption ability inherent properties along with its availability have facilitated its potential applications in different fields, including textiles [72, 77,143]. 

In raw form silk consists of some 75% fibroin, 20% sericin and 5% of various salts. The former is crystalline fibrous proteins and the latter is amorphous and soluble in hot water or dilute alkali solutions.The degree at... 

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