Thesis (Ph. D.)--University of Wisconsin--Madison, 1960. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

No description available.

Thesis (Ph. D.)--Brigham Young University. Dept. of Chemistry and Biochemistry, 2008. / Includes bibliographical references (p. 76-96).

No description available.

Dissertation (Ph.D.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Thesis (Ph.D.) - Simon Fraser University, 2004. / Theses (Dept. of Chemistry) / Simon Fraser University. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1968. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Two failure modes related to water management in Proton Exchange Membrane fuel cells (dehydration and flooding) were investigated using electrochemical impedance spectroscopy as a diagnosis tool. It was hypothesised that each failure mode corresponds to changes in the overall stack impedance that are observable in different frequency ranges. This hypothesis was corroborated experimentally. The experimental implementation required new testing hardware and techniques. A four-cell stack capable of delivering individually conditioned reactants to each cell was designed, built, tested, and characterised under a variety of operating conditions. This stack is the first reported prototype of its type. The stack was used to perform galvanostatic, impedance measurements in situ. The measurements were made at three different temperatures (62, 70 and 80°C), covering the current density range 0.1 to 1.0 A cm−2 , and the frequency range 0.1 to 4 × 105 Hz. The recorded data represent the first reported set of measurements covering these ranges. The failure modes were simulated on individual cells within the stack. The effects on individual cell and stack impedance were studied by measuring the changes in stack and cell impedances under flooding or dehydration conditions. Dehydration effects were measurable over a wide frequency range (0.5 to 105 Hz). In contrast, flooding effects were measurable in a narrower frequency range (0.5 to 102 Hz). Using these results, separate or concurrent impedance measurements in these frequency ranges (or narrow bands thereof) can be used to discern and identify the two failure modes quasi-instantaneously. Such detection was not possible with pre-existing, do techniques. The measured spectra were modelled by a simple equivalent circuit whose time constants corresponded to ideal (RC) and distributed (Warburg) components. The model was robust enough to fit all the measured spectra (for single cells and the stack), under normal and simulated-failure conditions. Approximate membrane conductivities were calculated using this model. The calculations yielded a range from 0.04 to 0.065 S cm−1 (under normal humidification), and conductivities that deviated from these nominal range under flooding or dehydrating conditions. The highest conductivity value (was ∼0.10 S cm−1) was measured under flooding conditions at j = 0.4 A cm−2. The lowest conductivity (∼0.02 S cm−1) corresponded to a dehydrated cell at j = 0.1 A cm−2. These values fall within the ranges of published data for modern proton exchange membranes. The phenomenological and numerical results reported in this work represent the first demonstration of these techniques on a PEMFC stack under real operating conditions. They are also the basis of ongoing research, development, and intellectual property protection. / Graduate

The ubiquitously expressed pannexin 1 (Panx1) ion- and metabolite-permeable channel is capable of mediating ATP release in a multitude of cells and tissues. This leads to activation of nearby purinergic (P2X/P2Y) receptors in an autocrine/paracrine manner. Stimulation of P2 receptors also triggers Panx1 activation, leading to the formation of a positive feedback loop. Although the focus of Panx1 research has primarily been on its expression at the cell surface, there is robust and stable expression of Panx1 on intracellular membranes. Whether intracellular Panx1 was the consequence of direct diversion from the secretory pathway or internalization from the cell surface was unknown at the onset of my studies. I postulated that Panx1 internalization to these membranes would require a ubiquitous constitutively or episodically released stimulus to allow stable intracellular expression. ATP, a potent signalling molecule released via exocytosis (constitutive or regulated) or large pore channels, fit this criterion. My hypothesis was that ATP triggered Panx1 internalization to intracellular compartments. Upon elevation of extracellular ATP, I observed P2X7R-mediated non-canonical internalization of Panx1 via macropinocytosis. This involved upstream cholesterol-dependent P2X7R-Panx1 clustering via a physical interaction between P2X7R-Panx1 ectodomains and possible contribution of phospholipid (PA, PIP, PIP2) interactions localized to the Panx1 C-terminus. Physical P2X7R-Panx1 interaction may promote Panx1 association with actively endocytosing regions of the membrane. Internalized Panx1 was targeted to slow recycling Rab14/Rab11-positive endosomes in an Arf6-dependent mechanism. The data I presented here provides an additional negative feedback layer to P2X7R-Panx1 crosstalk in the many cell types where they are co-expressed. Further, this is the first evidence demonstrating that Panx1 surface expression is labile to changes in the cellular environment, which contributes to the understanding of the regulation of Panx1 and associated behaviours through trafficking mechanisms. / Graduate / 2018-06-20

The compound 3-phenylindole (3PI) is a particularly active antimicrobial which interacts with phospholipids in fungal mycelia membranes, and which strongly inhibits the uptake of phosphate into fungal mycelia. The physical effects of 3PI on ion transport across bilayer lipid membranes composed of phosphatidylcholine/cholesterol have been investigated using three lipophilic ions and one ion/carrier complex. It was found that 3PI increased the electrical conductivity induced by the lipophilic cation (tetraphenylarsonium) and by the positively charged complex (nonactin-K('+)) by several orders of magnitude whereas 3PI decreased the conductivity induced by the two lipophilic anions (tetraphenylborate, dipicrylamine) by a factor of less than ten. These conductivity changes are explained as a combination of changes in the electrostatic and in the non-electrostatic properties of the bilayer. The electrostatic potential of the bilayer interior was shown to decrease in the presence of 3PI, a phenomenon which was also confirmed by measurements of the surface potential of phosphatidylcholine/cholesterol monolayers. The changes in non-electrostatic properties of the bilayer are qualitatively discussed in terms of increased bilayer fluidity or decreased bilayer thickness brought about by the presence of 3PI. The partition coefficient of the lipophilic anions, as determined by the voltage-step transient current technique, decreased slightly when 3PI was present in the aqueous phase. From the voltage dependence of the normalized steady state conductivity it was shown that 3PI did not kinetically limit tetraphenylarsonium transport but that it did kinetically limit nonactin-K('+) transport at concentrations greater than 25 (mu)M 3PI. The theory of carrier-mediated transport predicts the occurrence of transient currents in the kinetically limited regime but no transients were detected for nonactin-K('+) in the presence of 80 (mu)M 3PI. A method for analyzing the adsorption of neutral lipophilic molecules onto lipid monolayers has been presented, and by this method it was found that the partition coefficient of 3PI onto phosphatidylcholine/cholesterol monolayers was 1.3 x 10('-4) m and that the maximum adsorbed surface number density of 3PI was 1.1 x 10('-6) moles/m('2). From the experimental changes in monolayer surface potential as a function of adsorbed surface number density, a value for the normal component of the dipole moment of 3PI was obtained.