Date of Award
Chemistry and Biochemistry
Cell membrane, Lipid membrane, Lipid rafts, Neutron scattering, Pulmonary surfactant, Vitamin E
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Over the course of this dissertation, the fundamental behaviours and interactions between various analogs of vitamin E and biologically-relevant membranes will be explored. These analogs include the biologically active form, α-tocopherol; the most naturally abundant form γ-tocopherol; an oxidized product, α-tocopheryl quinone; and the synthetic stabile form, vitamin E acetate.
Beginning in Chapter 1, a foundation in membrane biophysics is established from a historical progression of the field. The importance of biomembranes is argued and a deep connection is established between structure and function. Key structural features of lipid assemblies, and role of lipid composition is defined, and with the groundwork of biomembranes laid, the mystery of vitamin E is introduced. This membrane-residing molecule is debated to be the primary antioxidant of the membrane, yet there are some caveats to its effectiveness as an antioxidant and in vitro studies suggests that it may have functions beyond this role. Current relevance of membrane studies on vitamin E are contextualized through the recent epidemic of vaping-related lung injury.
In Chapter 2, small angle X-ray scattering and molecular volumetric measurements establish a basic association between α-tocopherol and oxygen-sensitive lipid environments. Molecular volume contraction and solubility in disordered phases are evidenced as a favourable interaction, with contrasting responses in highly ordered environments. This association is considered innate of an effective membrane antioxidant.
In Chapter 3, the limits of a non-antioxidant role of vitamin E is discovered through understanding of the influence of various tocopherols on membrane organization. Model membranes with established phase behaviour are used as a platform to assess the influence of vitamin E on potential lipid rafts. From probe-free contrast-matched small angle neutron scattering and fluorescence microscopy studies, it is shown that only supraphysiological amount of vitamin E are able to influence these robust, highly stabile domains. In Chapter 4, this hypothesis is revisited by exploring the phase separation phenomenon in a more biologically-representative model emulating nanoscale phase separation. This platform also incorporates polyunsaturated fatty acid lipids that preferentially associate with vitamin E. Complementing contrast-matched small angle neutron scattering studies with Förster resonance energy transfer studies and differential scanning calorimetry, the results of the previous Chapter are largely recaptured. In both of these studies, the division between the effects from oxidized and non-oxidized vitamin E proposes an intriguing dimension to an antioxidant mechanism of vitamin E.
In Chapter 5, focus is shifted to assess the potential implications of inhaling vitamin E acetate found in vape oils. This resulted in the emergence of an epidemic known as E-cigarette or vaping-use associated lung injury. Due to an absence of clinical evidence for the cause of lung injury symptoms, an interaction was suspected between vitamin E acetate and the pulmonary surfactant that is vital for breathing compliance. Employing neutron spin-echo spectroscopy to assess the bending elasticity of synthetic mimics of pulmonary surfactant, it was found that increasing concentrations of vitamin E acetate produced a membrane that was approximately 20 % less resistant to deformation, which indicates it would not be able to resist collapse and alveolar failure under the high surface pressures of respiration.
These findings are extended in Chapter 6 and confirmed by neutron spin-echo spectroscopy with a biologically-derived pulmonary surfactant including a suite of surfactant proteins. The effect of vitamin E acetate on the structure, organization, and elasticity of three mimics of pulmonary surfactant with differing levels of complexity are compared. Though small angle scattering, Förster resonance energy transfer studies, confocal microscopy, and Langmuir monolayers with surface rheology, the mechanisms of pulmonary surfactant functions are assessed and the influence of vitamin E acetate are suggested.
Chapter 7 summarizes key aspects of this work by providing a commentary of vitamin E in membrane systems and an outlook for future research.
DiPasquale, Mitchell, "Structure and Mechanics of Biomembrane Mimics in the Presence of Vitamin E" (2022). Electronic Theses and Dissertations. 9003.