Orientation of silk III at the air-water interface
Introduction
A new crystal structure of Bombyx mori silk fibroin was observed in films taken from the air-water interface of aqueous fibroin solutions [1], [2], [3]. The crystal structure, silk III, involves an approximately hexagonal packing of silk molecules in a left handed threefold helical chain conformation. Because this conformation separates the serine and alanine residues, creating a hydrophilic column of residues parallel to the helical axis, surfactant behavior of fibroin at the air-water interface is believed to play a role in selecting the conformation and subsequent crystal structure at the interface. However, much of the data used to characterize the structure was for LB films compressed to 16.7 mN/m. These films possess a uniaxial crystallite orientation similar to a sedimented mat, with the crystallite c (or chain) axes predominantly oriented perpendicular to the plane of the film. If the fibroin is behaving as a surfactant, assuming a threefold helical conformation that separates its hydrophobic and hydrophilic residues, the chain axis of the fibroin molecules and of any resulting crystallites should lie in the plane of the film. While it is possible that compression on the Langmuir trough could reorient the fibroin molecules at the interface, data on the orientation of uncompressed films are needed for comparison.
Solid film formation has been observed previously on uncompressed surfaces of aqueous fibroin solution, but only a cursory examination of the morphology with no supporting diffraction data was reported [4]. In the current study, electron diffraction and TEM morphology data have been obtained for uncompressed surface films. The electron diffraction data clearly indicates a silk III crystal structure with the helical axes lying in the plane of the uncompressed film. The relative intensities in the diffraction patterns are altered in a systematic fashion, consistent with this orientation, i.e. the intensities of 00l reflections are enhanced and the intensity of hk0 reflections are attenuated. The oriented crystalline textures observed are uniform, and can be controlled by varying the surface compression treatments.
Section snippets
Experimental
Bombyx mori cocoons were degummed in order to remove sericin, yielding pure fibroin. This was achieved by boiling cocoon silk for about 1.5 h in distilled water with 1.1 wt.% CaCO3 and 6.6 weight percent sodium dodecyl sulfate (SDS). After this initial treatment, the fibroin was rinsed with distilled water and then boiled a second time in distilled water with 0.4 weight percent CaCO3 for about 1 h. Amino acid analysis has been used to assess the protein composition of fibroin prepared in this
Results and discussion
The silk protein films which form at interfaces are dominated by surface phenomena. The regenerated silk solutions used to prepare our thin films exhibit a marked tendency to foam, indicating surfactancy, and the residue sequence in the crystallizable regions of B. mori fibroin suggests a mechanism for the surface activity of this molecule. The six residue repeating sequence of the crystallizable portion of silk fibroin, [Gly-Ala-Gly-Ala-Gly-Ser]n, in a sterically reasonable, left handed
Conclusions
A plausible explanation of the orientation behavior in the compressed silk LB films is possible which is consistent with our observations. The silk excess layer that forms at the air-water interface results in aggregates of the threefold helical structure with an asymmetric shape. These aggregates possess the orientation one would intuitively expect, with the chain axis lying in the plane of the interface. When the trough is compressed these aggregates realign so that they each occupy a minimum
Acknowledgements
Funding from the National Science Foundation in the form of a CAREER grant (SPG), DMR 9624306, is gratefully acknowledged. Use of central facilities in the NSF funded Materials Research Science and Engineering Center (MRSEC) at the University of Massachusetts Amherst is also acknowledged as is the use of the facilities of the W.M. Keck Electron Microscopy Laboratory.
References (9)
- et al.
Biochem. Biophys. Acta
(1955) - Zhang W, Gido SP, Muller WS, Fossey SA, Kaplan DL. Electron Microsc Soc Am Proc...
- et al.
Macromolecules
(1996) - et al.
Biopolymers
(1997)
Cited by (88)
Radiation-processed silk fibroin micro- /nano-gels as promising antioxidants: Electron beam treatment and physicochemical characterization
2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Semi-crystalline SF materials generally exhibit two following structural organizations either based on α-helix (silk I) or β-sheet (silk II) [47,119,120]. Under specific interfacial conditions, a particular helical structure named silk III has been observed [121]. The extent and nature of the crystallinity depends considerably on the conditions of dissolution, reconstitution and post-treatments applied to the final material.
Controlling the structure and properties of semi-crystalline cellulose/silk-fibroin biocomposites by ionic liquid type and hydrogen peroxide concentration
2022, Carbohydrate Polymer Technologies and ApplicationsMicroporosity engineered printable silk/graphene hydrogels and their cytocompatibility evaluations
2022, Materials Today AdvancesSoil microbes as biopolymers to enhance soil mechanical properties
2022, Microbial Resource Technologies for Sustainable DevelopmentEfficient development of silk fibroin membranes on liquid surface for potential use in biomedical materials
2021, International Journal of Biological MacromoleculesCitation Excerpt :The contact angles of SFM-1 and SFM-2 are 52.5 and 42.3° respectively, suggesting good hydrophilicity of the membranes. The alternative arrangement of hydrophilic and hydrophobic domains along the SF molecular chain is responsible for the good hydrophilicity [32,33]. Theoretically, SFM-2 should be more hydrophobic than SFM-1 because SF denaturation causes the exposure of a large number of hydrophobic groups.