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UBC Theses and Dissertations

Using birefringence as a tool to investigate biomechanical properties of an elastic biopolymer protein Didier, David Wesley


Egg capsules from Busycon canaliculatum were examined to determine the microscopic mechanical properties. Previous macroscopic studies of this whelk’s egg capsules had determined that the protein polymer of these egg capsule cases have an interesting two-phase stress/strain curve and that this material fully recovered from yield strains. The previous study also demonstrated that stress/strain curves had a large hysteresis, and that these properties could be reversibility suppressed using heat or acid. It was hypothesized that these properties were intrinsic to the macromolecules and not related to the crisscross laminar structure of the capsule walls. A careful study of fiber samples teased out of the capsule walls confirmed that these traits were indeed properties of the macromolecular structure. The microscopic stress/strain tests produced results that were very similar to the macroscopic studies results. The microscopic study revealed two new areas (toe region and tertiary modulus) on the stress/strain graphs. The initial toe region represents the taking up of slack in the fibrous protein matrix as the fibers are loaded. The tertiary modulus showed up at strains that would have caused failure in the laminar structure of the capsule walls in the macroscopic study. This study also incorporated birefringence measurements as an instantaneous measure of order in the sample during stress/strain tests. This data was used to hypothesize about what was happening to the protein molecules are they were strained. Using this data and x-ray diffraction data from an unpublished related study a model was developed to explain the properties of this organic polymer. It is believed that the two-phase stress/strain curves are produced by stress-induced changes to the alpha-helical components of whelk egg capsule protein. Straining Alpha-helixes produces the initial stiff region. At the yield point the H-bonds in the alpha-helixes are over-strained and begin to pop-open, resulting in a loss of overall order (measured using birefringence) in the sample as alpha-helical structures are lost. Pulling on the primary molecular chains that form the alpha-helixes produces the tertiary modulus. The alpha-helixes spontaneously reform on recoil, allowing the protein to fully recover from the strain. beta-sheet x-ray signals have been detected in this material under strain (unpublished results) but the fact that it totally recovers on recoil indicates that no stable beta-sheets were formed during these tests.

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