Graduate Student Ashley Baxter

"The Consequences of Photosensitized Lipid Oxidation on Lipid Bilayer Formation and Structure"

Lipid membranes comprise the boundary of cells and organelles, providing structure, support, and protection. Model membrane systems can be formed through a variety of a techniques and are advantageous for studying lipid bilayer membranes in a controlled system. Reactive oxygen species (ROS) are found and produced endogenously within cells and organelles; however, unsaturated lipids are particularly susceptible to oxidation via ROS. Oxidation of these unsaturated fatty acid tails changes their chemical structure, rendering them more polar, which causes a number of physical changes to the lipid bilayer membrane. The consequences of lipid oxidation include an increase in the area per lipid molecule which causes lateral membrane expansion, a decrease in lipid order, membrane thinning, increased membrane permeability, and others.1 Studies have also shown the induction of highly curved membrane structures, such as buds or tubes, upon lipid oxidation. Furthermore, lipid oxidation has been linked with many degenerative diseases as well as aging.
The first half of my work investigates how the formation process of a supported lipid bilayer (SLB) is altered by lipid photo-oxidation through use of the lipid-conjugated fluorophore TopFluor-PC (TF-PC) as a photosensitizer.2 My studies using quartz crystal microbalance with dissipation monitoring (QCM-D) show that the traditional pathway of SLB formation is disrupted when vesicles have been photo-oxidized, suggesting a structural change to the lipids comprising the vesicles. This is further supported by the attenuation of the observed affects when an antioxidant such as α-tocopherol (vitamin E) or saturated phospholipids are incorporated into the lipid vesicles. The second half of my work uses fluorescence microscopy to observe the consequences of oxidation on a SLB, namely the formation of membrane tubules followed by the formation vesicles from these tubes.3 Characterization of these defects shows a direct correlation between the length of the tube and the diameter of the resulting vesicle. Using geometric modeling, I can calculate the diameter of individual membrane nanotubes. Additionally, the presence of the antioxidant α-tocopherol attenuates the membrane defects brought about by photo-oxidation, and membranes made of fully-saturated lipids show no evidence of tubulation or vesiculation.
This work elucidates the physiological changes caused by membrane photo-oxidation, and investigates the importance and relevance of lipid oxidation as it pertains to biological and biochemical systems.

(1) Jurkiewicz, P.; Olzyṅska, A.; Cwiklik, L.; Conte, E.; Jungwirth, P.; Megli, F. M.; Hof, M. Biochim. Biophys. Acta Biomembr. 2012, 1818, 2388-2402.
(2) Baxter, A. and Wittenberg, N. Langmuir. 2019, 35, 11542-11549.
(3) Baxter, A.; Jordan, L. R.; Kullappan, M.; Wittenberg, N. Langmuir. 2021, 37, 5753-5762.