Elastin is a polymeric protein of the extracellular matrix that imparts the characteristics of extensibility and elastic recoil to tissues. Recombinant polypeptides based on the domain structures and sequences of human elastin self-assemble into organized fibrous structures, with physical properties similar to those of native polymeric elastin. Elastin self-assembly is initiated by a temperature-induced phase separation, called coacervation. Previous to this work, coacervation temperature had been the only parameter available to measure propensity for self-assembly. A variety of techniques were developed using spectrophotometry, microscopy, and rheometry to differentiate the stages of self-assembly, thereby enabling independent observation and quantitation of each stage, and allowing investigations into properties of polypeptides and solution conditions affecting these stages.
Kinetic analysis of self-assembly yielded two additional parameters: coacervation velocity and maturation velocity. Examining the effects of agitation, salt concentration, temperature, polypeptide concentration, size of a polypeptide, hydrophobic domain sequence, and cross-linking domain structure on the kinetics demonstrated that coacervation and maturation are independent stages of self-assembly involving distinct mechanisms. Microscopic observations showed that protein-rich droplets of coacervate grew by coalescence to a stable droplet size, which correlated to differences in maturation velocities between polypeptides. Coacervate droplet growth appeared limited by the formation of organized polypeptide at the surface of the droplets, decreasing surface fluidity. Many of the general principles of the physical chemistry of colloids and emulsions appeared to apply to the formation, growth and stabilization of coacervates of the elastin-like polypeptides.
Self-assembly in the presence of non-elastin, matrix-associated proteins showed that these proteins maintained the coacervate as small droplets, which sometimes flocculated into fibre-like structures. Rheometry demonstrated a second temperature-induced transition above the coacervation temperature, which resulted in gelation and viscoelastic characteristics similar to microgels.
Together, these observations have resulted in a greater level of understanding of the entire self-assembly process, and provided a comprehensive model of elastin-like polypeptide self-assembly that relates to in vivo assembly of elastic fibres.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/17743 |
| Date | 23 September 2009 |
| Creators | Cirulis, Judith |
| Contributors | Keeley, Fred |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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