Silk fibroin molecular weight

Silk (qv) suture is made from the threads spun by the silkworm Bombjx mori. The fiber is composed principally of the protein fibroin and has a natural coating composed of sericin gum. The gum is usually removed before braiding the silk yams to make sutures in a range of sizes. Fine silk sutures may be made by simply twisting the gum-coated silk yams to produce the desired diameter. White silk is undyed. Silk is either dyed black with logwood extract or blue with D C Blue No. 9. The suture may be uncoated or coated either with high molecular weight polydimethylsiloxane or with wax. 

The core filament of B. mori silk, fibroin, is composed of a heavy chain fibroin (H-fibroin, 391 kDa) and a light chain fibroin (L-fibroin, 28 kDa), as well as P25 protein (25 kDa). These three constituents assemble into the secretory units in the ratio of 6 6 1 (H L P25) (Inoue et al., 2000 Shimura et al., 1976). H-fibroin and L-fibroin link together via a disulfide bond and the P25 is thought to act as a kind of chaperon to assist the transport and secretion of the insoluble H-fibroin (Sehnal and Zurovec, 2004 Tanaka et al., 1999 Zhou et al., 2000). H-fibroin has a much higher molecular weight and takes up 90% weight of the core filament. The properties of the core filament are mainly attributed to H-fibroin, which is often referred to as fibroin. 

Several workers subsequently confirmed these observations. Stein et al. (1944) examined the hydrolytic products with the manometric methods developed by Van Slyke and co-workers (1941) and demonstrated that dipeptides represented the overwhelming proportion of the products formed when silk fibroin was hydrolyzed for 96 hr at 40°C in concentrated HCl. After 40 hr, the hydrolyzate contained about 25% free amino acids with the remainder of the residues existing as di- and tripeptides. Bull and Hahn (1948), using a spread monolayer technique for estimating molecular weights, examined partial acid hydrolysis of egg albumin. By this method, immediate cleavage of about fifty bonds was observed when egg albumin was dissolved in 7.6 N HCl at 60°C. The remainder of the bonds were hydrolyzed at much slower rates. 

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

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