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AMYLOID FIBRILS IN NATURAL ADHESIVES
BACKGROUND
While biological systems are often infamous for their complexity, sometimes nature displays mechanisms that are elegant in their simplicity. Using Atomic Force Microscopy we believe we have identified such a mechanism at work to enhance the mechanical strength of various natural adhesives (1,2,3). The mechanism is simple because although it relies on the self-assembly of protein molecules it appears to be a mechanism independent of amino-acid sequence. This characteristic makes it an ideal target for biomimicry. It has been proposed that 'modular proteins' consisting of repetitive structural domains, similar to the muscle protein titin, could provide a mechanism for mechanical strength and toughness in a wide range of natural materials (4). This is due to the features of 'hidden length' and 'sacrificial bonds' in their structure, which result in an energy rich sawtooth mechanical response when unfolded under a tensile loading force.Recently we speculated, and have subsequently shown, that amyloid fibrils can also show a sawtooth mechanical response under axial tension (5). Amyloid fibrils are normally associated with neurodegenerative diseases, however, recently it has been suggested that amyloid fibrils can readily self-assemble from most polypeptides in vitro under appropriate conditions (6), which are usually denaturing.
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Schematic illustration of proposed mechanisms explaining, left, unraveling of the peptide molecules from the bulk of the fibril and right, the peeling of an intermolecular ß-sheet with an AFM Tip.
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BIOADHESIVES
We have been investigating the nanoscale adhesive properties of several terrestrial algae and marine invertebrates. While the chemical composition of many of these bioadhesives remains essentially unknown, we have been able to provide new insights into the underlying mechanical design for adhesive strength and attachment at the molecular level. We have shown that the adhesives of terrestrial green alga exhibit high mechanical strength and toughness due to the presence of ‘sacrificial bonds’ and ‘hidden length’ within the adhesive molecules (1,2,3), and proposed a mechanism for their adhesive strength based on small protein molecules cross-linked within a quaternary amyloid structure (1). |
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Terrestrial algae growing on anthropogenic substrates in Dublin. |
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This proposed mechanism was based not only on mechanical signatures we had shown (6), but was strengthened by evidence of the presence of amyloid in our adhesives by polarizing (using Congo red), and confocal (using thioflavin T) microscopies (1). Recent results using raman spectroscopy has provided additional support for amyloid structures in our natural adhesives. Emerging evidence exists for the occurrence of non-pathological amyloid occurring in nature (7), and the generic nature of the amyloid fold could provide a widespread mechanism for mechanical strength in other natural adhesives. |
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Representative force-extension curve sawtooth from the adhesive of a terrestrial alga. The regularly spaced, highly ordered peaks have been fitted to the WLC model. |
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APPLICATIONS
While amyloid fibrils are normally associated with neurodegenerative diseases and therefore highly undesirable, there are a number of features associated with these structures that make them an attractive generic mechanism for mechanical strength in a wide range of natural materials and therefore a target for biomimicry (1,2,5).
1) Under a tensile axial load they exhibit the mechanically beneficial features of 'hidden length' and 'sacrificial bonds' in an analogous way to 'modular proteins' such as titin.
2) Their strong ability to readily self-assemble means that the fibrils are effectively self-healing.
3) Their high degree of rotational symmetry provides a mechanism for mechanical strength in multiple directions.
As well as investigating the nanomechanical properties of other bioadhesives, we are exploring the possibility of forming amyloid structures from inexpensive, readily available sources for commercial applications. Initial results indicate that these interesting quaternary structures can be formed readily in the laboratory under denaturing conditions providing a promising starting point for biomimicry and as templates for nanoscale patterns and wires. |
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PROJECT COLLABORATORS
Fabio Rindi, University of Alababma, Alambama, USA.
Professor Ulf Karsten, Dr Rhena Schumann, University of Rostock, Rostock, Germany.
Dr Hugh Byrne, Dublin Institute of Technology, Dublin, Ireland.
Dr Rowena Crockett, EMPA, Zurich, Switzerland.
Professor Nic Spencer, ETH Zurich, Switzerland. |
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RELATED REFERENCES
1) Mostaert A. S., Higgins M. J., Fukuma T., Rindi F. & Jarvis S. P. (2006) Nanoscale mechanical characterisation of amyloid fibrils discovered in a natural adhesive. J. Biol. Phys. DOI 10.1007/s10867-006-9023-y.
2) A.S. Mostaert and S.P. Jarvis, "Beneficial characteristics of mechanically functional amyloid fibrils evolutionarily preserved in natural adhesives", Nanotechnology 18 (2007) 044010.
3) Karsten, U., Schumann, R. & Mostaert, A. S. (2006). Aeroterrestrial algae growing on man-made surfaces – What are the secrets of their ecological success? In: Extremophilic and Enigmatic Algae and Non-photosynthetic Protists. Cellular Origin, Life in Extreme Habitats and Astrobiology Book Series, J. Seckbach, editor, Springer. (in press)
4) Smith, B. L., Schaffer, T. E., Viani, M., Thompson, J. B., Frederick, N. A., Kindt, J., Belcher, A., Stucky, G. D., Morse, D. E. & Hansma, P. K. (1999) Nature 399, 761.
5) Fukuma, T., Mostaert, A. S. & Jarvis, S. P. (2006) Explanation for the mechanical strength of amyloid fibrils. Trib. Lett. 22, 233-237.
6) Fandrich, M. & Dobson, C. M. (2002) The behavior of polyamino acids reveals an inverse side chain effect in amyloid structure formation. EMBO J. 21, 5682.
7) Fowler, D. M., Koulov, A. V., Alory-Jost, C., Marks, M. S., Balch, W. E. & Kelly, J. W. (2006) Functional amyloid formation within mammalian tissue. PLoS 4, 100-107. |
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