Structure and solubility of natural silk fibroin

The solubility of silk fibroin in aqueous-salt, aqueous-organic, and organic media is analyzed. Factors affecting the formation of the secondary structural organization of fibroin in solutions and in the solid state after the recovery from solutions are analyzed.     

Determination of molecular weight of silk fibroin by non-gel sieving capillary electrophoresis

A simple non-gel sieving capillary electrophoresis (NGSCE) method was established to determine the MW of silk fibroin using CE. The background electrolyte with a pH of 8.8 was based on three components: polyethylene glycol, tris(hydroxymethyl)aminomethane, and sodium dodecyl sulfate (SDS). NGSCE showed a good linear relationship with satisfactory reproducibility between the migration time and the MW of standard proteins. It was found that the regenerated silk fibroin had an MW around 83 kDa with a wide MW distribution (MWD). This absolute value is lower than the result obtained from SDS-polyacrylamide gel electrophoresis due to the different principles of the methods, but their similar MWD shapes indicated that NGSCE could be a feasible, highly sensitive, rapid method for determination of the MW of silk fibroin.     

Wet-spinning of Regenerated Silk Fiber fromAqueous Silk Fibroin Solutions: Influence ofCalcium Ion Addition in Spinning Dope on thePerformance of Regenerated Silk Fiber简

Ca（II） ions are added in the spinning dope to adjust the solidification rate of regenerated silk fibroin （RSF） solution during the wet-spinning process since Ca（II） ions are proved to be favorable to maintain the stable silk fibroin network in our previous work. The results show that when Ca（II）/RSF ratios are 1/50 and 1/20, the resulted RSF fibers exhibit good performance with the breaking energy more than 70 kJ/kg. However, higher Ca（II）/RSF ratio （for example, 1/10） hinders the solidification of spinning dope and results in poor RSF fibers. These observations together with earlier papers from this laboratory confirm that to produce tough silk fibers the spinning conditions must allow sufficient time for the adjustment of silk fibroin molecular chains.     

Biomaterial coatings by stepwise deposition of silk fibroin

A completely aqueous, stepwise deposition process with Bombyx mori silk fibroin for the assembly of nanoscale thin film coatings is reported the first time. The focus of this work was to develop an understanding of the control of this deposition process and to characterize the films formed from a physicochemical perspective. The deposition process was monitored by UV spectrophotometry and research quartz crystal microbalance. Both absorbance and film thickness correlated linearly with the number of silk fibroin layers deposited, analogous to multilayered materials fabricated from conventional polyelectrolytes. The polymer adsorption process was stable and reproducible, with control of a single layer thickness ranging from a few to tens of nanometers, determined by the concentrations of silk fibroin, salt concentration in the dipping solution, and method of rinsing. The driving force for the assembly of silk fibroin onto the substrate was primarily hydrophobic interactions, while some electrostatic interactions were also involved. The difference with this approach from traditional polyelectrolyte layer-by-layer techniques is that an intervening drying step is used to control the structure and stability of the self-assembled silk fibroin. The assembled films were stable under physiological conditions and supported human bone marrow stem cell adhesion, growth, and differentiation. This approach offers new options to engineer biomaterial coatings as well as bulk materials with control of both interfacial properties conducive to specific cellular or tissue responses and the potential to entrap and deliver labile molecules or other components due to the all-aqueous process described.     

Thin-layer chromatography of the sericin,fatty waxes,and fibroin of natural silk

The method of thin-layer and high-performance thin-layer chromatography is proposed for the investigation of the isolation of sericin, fatty waxes, and fibroin from a natural silk cocoon. The chromatographic fractionation of fibroin according to molecular masses has been studied.Institute of the Chemistry and Physics of Polymers, Academy of Sciences of the Republic of Uzbekistan, Tashkent. fax (3712) 44 26 61. Translated from Kbimiya Prirodnykh Soedinii, No. 4,pp. 588–590, July–August, 1995. Original article submitted October 17, 1994.     

Proteomic profiling of the photo-oxidation of silk fibroin: implications for historic tin-weighted silk

The stability of silk proteins to ultraviolet light is an issue of significant concern in both the appearance retention of silk-derived products and the preservation of historic silk textiles. Until now, evaluation of silk degradation has only been performed at the holistic, rather than molecular level. This article describes the first proteomic profiling of silk photo-oxidation, characterizing protein primary level modification leading to coloration changes, and evaluating the effects of tin weighting on photodegradation. Heavy-chain fibroin, the main proteinaceous component of the silk thread, is a repetitive, highly crystalline protein with a content rich in tyrosine. Photoproducts of tyrosine were characterized and the levels of oxidative modification at the protein primary structural level correlated with changes in coloration and tensile strength. The effect of tin as a weighting agent used on historical fabrics was examined. Tin-weighted fabrics were evaluated following two treatments (pink and dynamite) and proteomic analysis revealed a significant increase in oxidatively modified amino acid residues within the pink-treated silk. These findings offer new insight into the molecular-level oxidation of silk proteins under UV exposure, and the effects of silk treatments in either exacerbating or ameliorating this degradation.     

Conformation transition of silk fibroin induced by blending chitosan

The conformation of silk fibroin in silk fibroin/chitosan (SF/CS) blend membrane was analyzed by infrared spectrum, X-ray diffractometry, and Raman spectrum. The results demonstrated that the SF could show β-sheet conformation when the SF content in blend membranes was 10% (w/w) and 60–80% (w/w), while the pure SF membrane showed random coil conformation. A mechanism of the conformation transition was suggested in that the SF chain could use the rigid CS chain as a mold plate to stretch itself to form a β-sheet structure according to the strong hydrogen bond between CS and SF. Therefore, a new concept, named “Polymer-Induced Conformation Transition,” was proposed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2293–2296, 1997     

Microrheological studies of regenerated silk fibroin solution by video microscopy

We have carried out studies on the rheological properties of regenerated silk fibroin (RSF) solution using video microscopy. The degummed silk from the Bombyx mori silkworm was used to prepare RSF solution by dissolving it in calcium nitrate tetrahydrate‐methanol solvent. Measurements were carried out by tracking the position of an embedded micron‐sized polystyrene bead within the RSF solution through video imaging. The time dependent mean squared displacement (MSD) of the bead in solution and hence the complex shear modulus of this solution was calculated from the bead's position information. We present here the results of rheological measurements of the silk polymer network in solution over a frequency range, whose upper limit is the frame capture rate of our camera at full resolution. By examining the distribution of MSD of beads at different locations within the sample volume, we demonstrate that this probe technique enables us to detect local inhomogeneities at nanometre length scales, not detectable either by a rheometer or from diffusing wave spectroscopy. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2555–2562, 2007     

Nucleation of hydroxyapatite crystals by self-assembled Bombyx mori silk fibroin

Bombyx mori silk fibroin (SF) is known to be capable of facilitating nucleation of the hydroxyapatite crystals (HAps). To find out how SF mediates the nucleation of HAps, self-assembly of SF in 1.5 simulated body fluid (SBF) was observed in this study through design of a co-solution of SF and 1.5-times SBF (SF/1.5 SBF). After the co-solution of SF/1.5 SBF was incubated at 37.2 °C up to 7 days, SEM, X-ray, and Fourier transform infrared (FTIR) observations indicated that nucleation of HAps was increased. In addition, the structure of SF was transited from random coil into β-sheet indicated by FTIR spectra. The β-sheet assembly of SF in 1.5 SBF was also supported by CD spectra. Atomic force microscopy provided detailed progress of the self-assembly that SF incubated in 1.5 SBF was self-assembled in the form from dot, through rod to final net. Therefore, this study suggested that nucleation of HAps of SF was controlled by its molecular self-assembly. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Influence of an iodine treatment on the structure and physical properties of Bombyx mori silk fibroin fiber

Silk fibroin (SF) fiber from the Bombyx mori silkworm was treated with a 1.23 N iodine/potassium iodide (I₂–KI) aqueous solution, and the structure and physical properties were investigated to elucidate the effects of the iodine treatment. The SF fiber absorbed polyiodide ions such as I and I by immersion in the I₂–KI solution, and the weight gain of the SF fiber increased with the treatment time; it became saturated at about 20 wt % after 40 h. The results of the weight gain, Fourier transform infrared spectroscopy, and X‐ray diffraction measurements suggested that polyiodide ions mainly entered the amorphous region. Moreover, a new sharp reflection in the meridional direction, corresponding to a period of 7.0 Å, was observed and indicated the possibility of the formation of a mesophase structure of β‐conformation chains. Dynamic viscoelastic measurements showed that the molecular motion of the crystalline regions at about 220 °C was enhanced and shifted to lower temperature by the introduction of polyiodide ions. This indicated that the iodine component weakened the hydrogen bonding between the SF molecules forming the β‐sheet structure and caused molecular motion of the crystal to occur more easily with heating. With heating above 270 °C, the iodine component introduced intermolecular crosslinking to SF, and the melt flow of the sample was inhibited. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3418–3426, 2006     

Interfacial rheology of natural silk fibroin at air/water and oil/water interfaces

The interfacial viscoelastic behavior of natural silk fibroin at both the air/water and oil/water interfaces is reported. This natural multiblock copolymer is found to be strongly amphiphilic and forms stable films at these interfaces. The result is an interfacial layer that is rheologically complex with strong surface elastic moduli that are only slightly frequency-dependent. The kinetics of surface viscoelastic evolution are reported as functions of time for various concentrations of the spread films. Films deposited by Langmuir-Blodgett deposition were studied by scanning electron microscopy (SEM) to reveal a fibrous structure at the interface. The production of stable O/W emulsions by silk fibroin further confirms the generation of the elastic films at the oil/water interfaces.     

Effects of poloxamer on the gelation of silk fibroin

The influence of the concentration of poloxamer 407, the pH and the temperature on the gelation of silk fibroin (SF) were studied. It was found that the gelation of SF occurred in the presence of poloxamer at pH value of 7.0 while gelation of SF itself did not occur. The gelation time of SF was shortened with increasing the poloxamer concentration and the temperature. The sol‐gel transition of SF became reversible with an addition of poloxamer. From infrared (IR) and circular‐dichroism (CD) spectroscopy measurements, it was found that a conformational change of the SF in the SF/poloxamer system from random coil to β‐structure was accelerated after forming a polymer complex with the poloxamer. The crystallinity of the poloxamer was reduced by SF from X‐ray diffraction measurements.     

Mineralization of HA crystals regulated by terephthaloyl chloride-modified silk fibroin films

Terephthaloyl chloride(DB)-modified silk fibroin(SF) films were immersed into 1.5 times simulated body fluid(1.5 SBF) to regulate the mineralization of hydroxyapatite(HA) crystals at about 36.5℃for 24 h.UV was used to prove that the new bonds form between the DB and SF.The structure and morphology of the SF/HA were investigated by FTIR,ICP,XRD and SEM.The results showed that the apatite deposited on the matrix of SF mainly was HA.HA was self-assembled on the matrix of SF and formed three-dimensional framework when the weight ratio of DB/SF was 0.30.The content of DB affected the structure and morphology of the apatite composites deposited on the SF films.     

Preparation of PCM microcapsules by complex coacervation of silk fibroin and chitosan

Phase change material microcapsules were prepared by complex coacervation of silk fibroin (SF) and chitosan (CHI). n-Eicosane was used as the core material. The effects of SF/CHI ratio, and percentage of cross-linking agent and n-Eicosane content on the properties of microcapsules were studied. The size distribution and the surface morphology of microcapsules were characterized by optical and scanning electron microscopy. The encapsulation of core material was determined by energy dispersive spectrometer analysis. The results indicated that SF/CHI microcapsules were prepared successfully. Microcapsules had smooth outer surface when the ratio of SF to CHI was close to 5. On the other hand, at high SF/CHI ratios (≥14), microcapsules showed a two-layer structure, an inner compact layer, and an outer, more porous, sponge-like layer. The highest microencapsulation efficiency was obtained at a SF/CHI ratio of 20 in the presence of 0.9% cross-linking agent and of 1.5% n-Eicosane content.     

Controlling molecular conformation of regenerated wild silk fibroin by aqueous ethanol treatment

Regenerated Antheraea pernyi silk fibroin film was prepared by dissolution of native silk fiber in aqueous lithium thiocyanate. The influence of aqueous ethanol treatment of the dried regenerated film on molecular conformation was studied by X‐ray diffraction, infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. While the initial regenerated film consisted of α‐helix and random coil components, aqueous ethanol treatment of the film resulted in significant increase in β‐sheet component and improvement of water resistance of the film. This effect was strongly dependent on ethanol concentration, and 40–60% (w/w) ethanol was most effective due to balance of hydrophilic/hydrophobic action of the solvent. Copyright © 2003 John Wiley & Sons, Ltd.     

Nanofibers of silk fibroin controlled by the crystallization of polyethylene glycol in frozen solution

The conformation of silk fibroin(SF) frozen with polyethylene glycol(PEG) at a molecular weight from 2kDa to 20kDa and a mass ratio of PEG:SF from 1:5 to 10:1 was studied by spectral and microscopic methods. It is found that the conformation transition of SF from random coil to b-sheet could be induced by the stress resulting from PEG crystallization at-20 °C, and greatly depended on the cooling rate, PEG:SF mass ratio and PEG molecular weight. These findings provide a new method for the preparation of desired SF nanofibers.     

Structure and microporous formation of cellulose/silk fibroin blend membranes: Part II. Effect of post-treatment by alkali

Blend membranes (RCF1) were prepared from mixture solution of cellulose and silk fibroin (SF) in cuoxam by coagulating with acetone–acetic acid (4:1 by volume). The blend membranes were subjected to post-treatment with 10% NaOH aqueous solution, and their structure and properties were characterized by FT-IR, X-ray diffraction, DSC, SEM and DMTA. In previous work, cellulose/SF blend membranes (RCF2) prepared by coagulating with 10% NaOH aqueous solution formed a microporous structure, in which the SF as a pore former was almost completely removed from the membrane. However, when the blend membranes RCF1 were immersed in 10% NaOH aqueous solution for post-treatment, a strong hydrogen bonding between cellulose and SF inhibited the removal of SF. Although alkali is a good solvent for SF, the blend membranes RCF1 such obtained from cellulose and SF were alkali resistant. The crystallinity and the mean pore size of the blend membranes slightly decreased with increasing post-treatment time. This work provided a cellulose/silk blend membrane, which can be used under alkaline medium.