Silk fibroins

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). 

Several silk fibroin genes have been cloned and sequenced and they all show a similar sequence pattern variable domains at the N- and C-termini flank a large region of repetitive short sequences of alternating poly-Ala (8 to 10 residues) and Gly-Gly-X repeats (where X is usually Ser, Tyr, or Gin). This middle region varies in length and may comprise up to 800 residues. 

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.)...

The percentages of amino acids in silk fibroin which Poison et al. (224) found by direct visual and indirect photometric analysis of ninhydrin paper-partition chromatograms are shown in Table VII. The percentages obtained for alanine, glycine, and serine appear to be reasonably accurate, inasmuch as they agree closely with those found by other methods. It would be of interest to determine alanine by the microbiological method reported recently by Sauberlich and Baumann (238), in view of the widely different values found for this amino acid by the described ninhydrin-chromatographic procedure and the selec-... 

Table VII. Percentages of Amino Acids in TnSIrMtRnlnr Direct Color Analysis Silk Fibroin literature ...

Several examples have been described in which a chiral natural polymer, such as silk fibroin or chitosan, act as chiral ligand and support at the same time. In such cases, the chiral ligand (the monomer or monomers coordinating... 

Resilin has a greater percentage of acidic residues than collagen, elastin, and silk fibroin and contains fewer non-polar residues (i.e., Gly, Ala, Val, He, Leu, Pro, Met, and Phe) than silk fibroin and elastin. The significant content of acidic residues might account for resilin s hydrophilicity as well as its low isoelectric point, as indicated through swelling experiments [141, 145]. Resilin contains little to no... 

Average wing-hinge Prealar arm Collagen (oxhide) Elastin (ox ligamentum nuchae) Silk fibroin Bombyx mori)... 

Results of amino acid analysis performed on resilin from the locust Schistorcerca gregaria, compared with values for collagen, elastin, and silk fibroin. Reproduced from [145] with permission from Elsevier, copyright Elsevier 1961 Includes 106 hydroxyproline and 7 hydroxylysine... 

Matsumoto A, Chen J, Collette AL et al (2006) Mechanisms of silk fibroin sol-gel transitions. J Phys Chem B 110 21630-21638... 

Hyde, PD Ediger, MD, NMR Imaging of Diffusion of Small Organic Molecules in Silk Fibroin Gel, Macromolecules 24, 620, 1991. 

Silk fibroin and collagen illustrate the close linkage of protein stmcture and biologic function. Diseases of collagen mamration include Ehlers-Danlos syndrome and the vitamin C deficiency disease scurvy. 

The bacterial resistance of silk fibroin can be improved by treatment at low pH values in aqueous solutions of metal ions and at high pH values in solutions of metal-amine complexes, using untreated silk or silk pretreated with an aqueous solution of tannic acid (10.183). Suitable metals include Cu, Zn, Ni, Fe and Ag [533]. It is to be expected that treatment with such metal compounds will be subject to restrictions in environmentally sensitive areas. 

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Besides the previously mentioned collagen, a wide variety of natural polymers have been involved in the synthesis of bio-nanohybrid materials with potential application in bone repair and dental prostheses. For instance, some recent examples refer to bionanocomposites based on the combination of HAP with alginate [96,97], chitosan [98,99], bovine serum albumin (BSA) [100], sodium caseinate [101], hyaluronic acid [102], silk fibroin [103,104], silk sericin [105], or polylactic add (PLA) [106,107]. These examples illustrate the increasing interest in the subject of HAP-based biohybrid materials, which has led to almost 400 articles appeared in scientific journals in 2006 alone. 

Amino acid Egg white Egg yolk Casein Animal glue (collagen) Wool (keratin) Silk (fibroin) Garlic... 

Gotoh, Y., Tsukada, M., and Minoura, N. (1993) Chemical modification of silk fibroin with cyanuric chloride-activated polyethylene glycol Analysis of reaction site by 1H-NMR spectroscopy and conformation of the conjugates. Bioconjugate Cbem. 4, 554-559. 

An amperometric urate sensor based on uricase-immobilized silk fibroin membrane was developed by Zhang [256], The biosensor can be used to measure the urate level in human serum or urine and standard additions of uric acid. F or this biosensor, the recoveries of uric acid in human serum and urine are in the range of 94.2 102.6% to 92.5 97.9%, respectively. The relative standard deviations for repeatedly monitoring standard urate solution, human serum, and urine are 2.37, 3.72, and 2.95%, respectively, based on 100 measurements. 

Y.H. Wu, Q.C. Shen, and S.S. Hu, Direct electrochemistry and electrocatalysis of heme-proteins in regenerated silk fibroin film. Anal. Chim. Acta 558, 179-186 (2006). 

Y.Q. Zhang, W.D. Shen, R.A. Gu, J. Zhu, and R.Y. Xue, Amperometric biosensor for uric acid based on uricase-immobilized silk fibroin membrane. Anal. Chim. Acta 369, 123-128 (1998). 

Raw silk was dissolved in hexafluoro-iso-propanol (HFIP) [17, 33]. A typical working concentration for spinning was 2.5% (w/v) silk fibroin in HFIP. The spinning solution was pressed through a small needle (0 80-250 pm) into a precipitation bath (methanol for Bombyx mori silk proteins and acetone for Nephila clavipes silk proteins) and the silk solution immediately precipitated as a fiber. The best performing fibers approached the maximum strength measured for native fibers of Bombyx mori, but did not achieve the mechanical properties of natural spider silk. 

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. 

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