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Author ORCID Identifier
Campus-Only Access for One (1) Year
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Anthony D. Dinsmore
Jennifer L. Ross
Analytical Chemistry | Materials Chemistry | Statistical, Nonlinear, and Soft Matter Physics
This thesis explores an experimental system probing the effect of energy input (in light-responsive bilayers) on membrane physicomechanical properties and dynamics of response to a trigger. We were inspired by the ability of cell membranes to alter their elastic and permeability properties and shape in response to energy input, change in lipid chemistry, or bilayer composition. Our work demonstrates and sheds new light on the roles of lipid chemical character, light-responsive moieties' incorporation in the membrane, and the lipid bilayer's mechanical properties on membrane response to chemical tuning or energy input.
To observe how lipid chemistry affects membrane physical properties and to develop a chemically robust membrane, we investigated the effect of a small polar functional group placed at the hydrophobic domain of an archaea-type bolalipid. Our results demonstrated a remarkably different membrane material property that is controllable via the membrane composition. Archaea lipids are ester-based robust lipids with promising material science applications. In contrast to “conventional” lipids, bolalipids are composed of two polar heads linked by two hydrophobic acyl tails. We separately characterize the bolalipid membrane with and without the polar moiety.
Without the polar moiety, the membrane shows a rigid and less permeable character. The membrane stiffness and water permeability dropped dramatically when we added the polar unit to the hydrophobic domain. The bolalipids with the polar unit showed an order of magnitude faster solute release.
We then turned to study the light-induced change in lipid bilayer material property using three different systems. One, we sequestered a light-responsive molecule called azobenzene in a liquid lipid bilayer membrane. Upon shining light, azobenzene isomerizes from trans to cis or vice versa, depending on the wavelength. We found that the membrane property, specifically the rigidity for both bending and stretching, changed. We show that the change in material property of such membranes is a function of the azobenzene concentration. As we increased the azobenzene concentration, we observed that the bending rigidity dropped significantly and to a lesser extent, the membrane stretching stiffness dropped. For the case of 20 mol% azobenzene mixed with lipid, we discovered that the cis state (under ultraviolet light excitation) had bending and stretching moduli 28% and 10% lower than the trans-state (under blue excitation), respectively. In the second system, we adopted a covalent approach by using a lipid ("azo-PC") in which the azobenzene is incorporated in one of the fatty acid tails of a phosphocholine lipid. The trans-state had a relatively straight tail while the cis state had a sharp bend (a kink) in it. For the case of vesicles composed of 100 mol% azo-PC, we found that the cis state had area dilation, stretching moduli, toughness, and water permeability that were up to 12% higher, and 2.5, 3.0, and 3.5 times lower than the trans-state, respectively. The azo-PC lipid was then mixed with another phosphocholine lipid (DOPC), and we formed liquid-phase lipid bilayer vesicles. The mixture case showed a significant change in the material property upon light excitation. To our surprise, the stretching modulus and water permeability showed a nonmonotonic response upon UV irradiation as a function of azo- PC mol%. Area dilation plateaued with azo-PC mol%. We discuss our experimental and compare them to insights gained from our collaborators' atomistic molecular dynamic simulation. We show how the membrane thickness mismatch, the different areas per lipid molecule, and the alignment order of the tails contributed to the nonmonotonic response. In the third case, we explored the irradiation-response of a lipid membrane mixture that is in a solid gel phase mixed with azo-PC. We mixed the azo-PC lipid with a saturated phosphocholine lipid (DPPC) that forms solid membranes. Bilayers with 0 or 10 mol% azo-PC showed no response to UV or blue light excitations. For the case of 30 mol% azo- PC, we found that vesicles responded to UV light (cis state) by forming an apparently jagged or crumpled structure and then returning to a smooth structure with blue light (trans). With three repeated cycles of UV and blue, we found an increase in the interior volume and a decrease in the refractive-index contrast between inside and outside the vesicles. These changes indicate a significant and reversible increase of sugar permeability during the UV excitation. For the 50 mol% ratios, we sometimes observed a vesicle deformation phenomenon from sphere-to-ellipse. Finally, at 80 and 100 mol% azo-PC, we find that the membrane area isotopically swells with UV irradiation and de- swells when UV is turned off. We discuss how the membrane rigidity dictates its response to light irradiation and the potential for using solid-phase membranes to tune solute permeability.
All these responses observed for the different systems are robust and repeatable, providing a physical understanding of the photoisomerization-induced change in membrane property and remodeling. Our findings help unify the diverse phenomena observed previously and present new insight into membrane elasticity and photoisomerization-induced higher instabilities. They open the door to a new class of vesicle-based innovative materials that is more than 99% water and encapsulate and release on demand sustainably or fast and show how to drive intentional membrane response and shape change to design responsive systems.
Manafirad, Arash, "Manipulating the Properties of Light-Responsive Active Lipid Bilayer Membranes: Measuring Mechanics and Probing Mechanisms" (2022). Doctoral Dissertations. 2403.
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