Silkworm silk fibroin

A third polymorph, silk III, has also been described based on the interfacial behavior of the silkworm silk fibroin and the partitioning behavior at an air-water interface [44, 45]. Silk III, a structure stabilized at interfaces optimizes the surfactancy of the silk in the core repeats of glycine, alanine, and serine. 

Spider and silkworm silks ( fibroins ) are generally characterized as having crystalline regions encoded by repetitive amino... 

Protein-Labeling Strategies Silkworm Silk Fibroin... 

Silkworm silk fibroin is composed of a crystalline motif that is made up of multiple (Alanine (Ala)-Glycine (Gly)-Serine (Ser)-Gly-Ala-Gly) repeats and a Tyrosine (Tyr)-rich r on [164]. C FI CP/MAS NMR has been utilized extensively to study these fibroins as chemical shifts are sensitive to the local environment and can thus report on the secondary structure. One of the first soHd-state NMR studies of silkworm fibroin dimorphs, silk I and silk II, was undertaken by Saito et al. [159] using CP/MAS... 

NMR experiments. Silk I is the dried form of the fibroin produced by the middle silk gland of silkworms, while silk II is obtained after spinning. Silk I is essentially a precursor to silk II, with the structural transition from silk I to silk II only affecting the crystaUine region of the silkworm silk fibroin [164]. Hence, attempts to impart crystaUinity via macroscopic orientation often led to the conversion of silk I to silk II [153]. CP/MAS NMR allowed... 

This protein fiber is commonly known due to its luster and comfort of wear and is widely used in the fashion industry. The best-known source that is also commonly used to produce biomaterials is the Bombyx mori silkworm. Silk fibroin can also be extracted Ifom several other silk-spinning insects or spiders and mussels to name the most significant. 

The formation of silkworm fibers may be related to formation of supramolecular elongated structures starting from micellar structures [188,189]. Spherical micelles (100 to 200 nm diameter) were observed in aqueous solution of reconstituted silkworm silk fibroin. Aggregation of these micelles into larger structures upon increasing fibroin concentration was observed. Shearing of these solutions produced a fibrillar structure with morphological features typical of silkworm fibers. 

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). 

Films or membranes of silkworm silk have been produced by air-drying aqueous solutions prepared from the concentrated salts, followed by dialysis (11,28). The films, which are water soluble, generally contain silk in the silk I conformation with a significant content of random coil. Many different treatments have been used to modify these films to decrease their water solubiUty by converting silk I to silk II in a process found usehil for enzyme entrapment (28). Silk membranes have also been cast from fibroin solutions and characterized for permeation properties. Oxygen and water vapor transmission rates were dependent on the exposure conditions to methanol to faciUtate the conversion to silk II (29). Thin monolayer films have been formed from solubilized silkworm silk using Langmuir techniques to faciUtate stmctural characterization of the protein (30). ResolubiLized silkworm cocoon silk has been spun into fibers (31), as have recombinant silkworm silks (32). 

Silk fibers or monolayers of silk proteins have a number of potential biomedical applications. Biocompatibility tests have been carried out with scaffolds of fibers or solubilized silk proteins from the silkworm Bombyx mori (for review see Ref. [38]). Some biocompatibility problems have been reported, but this was probably due to contamination with residual sericin. More recent studies with well-defined silkworm silk fibers and films suggest that the core fibroin fibers show in vivo and in vivo biocompatibility that is comparable to other biomaterials, such as polyactic acid and collagen. Altmann et al. [39] showed that a silk-fiber matrix obtained from properly processed natural silkworm fibers is a suitable material for the attachment, expansion and differentiation of adult human progenitor bone marrow stromal cells. Also, the direct inflammatory potential of silkworm silk was studied using an in vitro system [40]. The authors claimed that their silk fibers were mostly immunologically inert in short and long term culture with murine macrophage cells. 

Asakura, T., Suzuki, H., and Watanabe, Y. (1983). Conformational characterization of silk fibroin in intact Bombyx mod and Philosamia cynthia ricini silkworms by 13C NMR spectroscopy. Macromolecules 16, 1024—1026. 

Hossain, K. S., Ochi, A., Ooyama, E., Magoshi,J., and Nemoto, N. (2003). Dynamic light scattering of native silk fibroin solution extracted from different parts of the middle division of the silk gland of the Bombyx mori silkworm. Biomacromolecules 4, 350-359. 

Rossle, M., Panine, P., Urban, V. S., and Riekel, C. (2004). Structural evolution of regenerated silk fibroin under shear Combined wide- and small-angle x-ray scattering experiments using synchrotron radiation. Biopolymers 74, 316-327. Rousseau, M. E., Lefevre, T., Beaulieu, L., Asakura, T., and Pezolet, M. (2004). Study of protein conformation and orientation in silkworm and spider silk fibres using Raman microspectroscopy. Biomacromolecules 5, 2247-2257. 

Terry, A. E., Knight, D. P., Porter, D., and Vollrath, F. (2004). PH induced changes in the rheology of silk fibroin solution from the middle division of Bombyx mori silkworm. Biomacromolecules 5, 768-772. 

Silk is produced from the spun threads from silkworms (the larvae of the moth Bombyx mori and related species). The main protein in silk, fibroin, consists of antiparallel pleated sheet structures arranged one on top of the other in numerous layers (1). Since the amino acid side chains in pleated sheets point either straight up or straight down (see p. 68), only compact side chains fit between the layers. In fact, more than 80% of fibroin consists of glycine, alanine, and serine, the three amino acids with the shortest side chains. A typical repetitive amino acid sequence is (Gly-Ala-Gly-Ala-Gly-Ser). The individual pleated sheet layers in fibroin are found to lie alternately 0.35 nm and 0.57 nm apart. In the first case, only glycine residues (R = H) are opposed to one another. The slightly greater distance of 0.57 nm results from repulsion forces between the side chains of alanine and serine residues (2). 

The beta arrangement, or pleated sheet conformation (see Figure 14.2), is predominant when small pendant groups are present in the chain, as in silk fibroin. The silk fibroins, which are spun by various species of silkworms, are monofilament polypeptides with extensive secondary interchain bonding. The crystalline portion of the fibroin is a polymer of a hexapeptide. The... 

It has been proposed, as for other silks, that the peptide chains in the stalks are aligned perpendicular to the long dimension of the fiber and are folded back on themselves many times to form a (3 sheet with only 8 residues between folds.3 The chains of silk fibroin, the major protein of silkworm silk, contain 50 repeats of the sequence 13... 

From the viewpoint of zootaxa, the silkworm and the spider belong to insect and arachnid of arthropod, respectively. Their silk proteins (fibroin for silkworm silk and spidroin for spider major ampullate silk) do not have any genetic heritage in common and their amino acids sequence compositions are different too. However, the silkworm and spider employ a similar spinning process to produce silk. Furthermore, the silkworm silk and the major ampullate silk have a number of similar structural characteristics, both at the level of the secondary protein structure and the condensed silk morphology. Therefore, for the sake of convenience, they are discussed together in some parts of this text. 

Although the amino acid sequence as well as the secondary structure of fibroin differs from those of spidroin, the fibers spun from these proteins, that is, silkworm silk and spider silk have comparable mechanical properties. These may be attributed to the structural characteristics, both at the molecular and filament level. The superior mechanical properties of silk-based materials, such as films, coatings, scaffolds, and fibers produced using reconstituted or recombinant silk proteins, are determined by their condensed structures. 

As to fibers, it was reported that the inferior mechanical properties of silk from cocoons compared to spider silk result from the silkworm spinning process. If silkworm silk is processed at a constant pulling speed rather than constant force pulling, it possesses excellent properties, approaching the spider dragline silk (Shao and Vollrath, 2002). This suggests that the silkworm silk has the potential to produce better fibers, and the regenerated fibroin, which is easy to harvest, has the possibility to be fabricated into a reconstituted super-fiber. 

Since silk fibroin of cultured silkworms contains relatively large amount of achiral glycine residue, silk fibroin cS wild lkworms with a h er content of chiral alanine residue was examined as the carrier of palladium and optical yields of S<+) 14.4% and R-(+) 32.1% were obtained in reaaions (1) and (2), re ctively (3). l n amino groups of silk fibroin of cultured silk worm were acetylated and used as the carrier, the optical yield in reaction (2) was R-(+) 65,9%. 

The fibers of silk that are spun by silkworms and spiders consist mainly of the protein called silk fibroin. Different species of silkworms and spiders produce different kinds of silk fibroin, which differ somewhat from one another in their molecular architecture. All of the kinds of silk fibroin contain long zigzag polypeptide chains lying parallel to one another, as shown in the adjacent drawing. The chains extend in the "direction of the fiber or thread (vertically in the drawing). 

---