Several compounds possessing diverse chemical structures have been shown to inhibit the action of protein kinase C (PKC). A general property of some of these compounds is that they inhibit bilayer to hexagonal phase interconversion in phosphatidylethanolamine bilayers. In addition, a large number of PKC inhibitors are positively charged. To clarify the relationship between charge and enzyme inhibition, the effect of the cationic amphiphile sphingosine was studied at different pH's. Inhibition by sphingosine was found to be pH dependent. Above pH 7.75, sphingosine has little or no inhibitory effect. In fact, at pH 8.5 sphingosine slightly enhances enzyme activity above that which occurs when the enzyme is stimulated by diacylglycerol and phosphatidylserine. After correcting for electrostatic repulsion, the intrinsic pK for sphingosine in Triton micelles is 8.5. Inhibition of PKC by sphingosine at physiological pH's therefore correlates with the presence of positive charge. In an attempt to optimize the structural features necessary for inhibition of PKC, a number of compounds which incorporate both positive charge as well as bilayer stabilizing ability were designed. These compounds possess a hydrophobic backbone which does not perturb hydrocarbon packing and have a tertiary or quaternary nitrogen functionality in the head group. All designed amphiphiles inhibit PKC activity; the potency of the amphiphile correlates with the presence of positive charge. Quaternary ammonium bilayer stabilizers are 10 fold more potent than their tertiary amine precursors, generally inhibiting in the 10 - 60 (mu)M range using the Triton mixed micelle assay. Aside from charge, factors such as the structure of the amine containing head group and its length from the hydrocarbon moiety did not markedly influence inhibitor potency. In contrast, the hydrocarbon backbone did influence potency. Cationic amphiphiles containing asteroid backbone were more potent than their straight chain analogues. These amphiphiles do not appear to alter the partitioning of PKC from the aqueous phase to the membrane surface. A number of bilayer stabilizing compounds possessing carboxylate and sulfate anions in addition to the quaternary nitrogen functionality were also designed. Although inhibitor potency correlated with the amount of charge present on the amphiphile, charge could not account for all the observed effects. Changes in the position of the charged functionalities and hydrocarbon length resulted in marked differences in amphiphile potency. Some inhibited in the submicromolar range. The results of these studies suggest that a combination of positive charge and a bilayer stabilizing structural characterisitic provides a basis for the rational design of PKC inhibitors. / Thesis / Master of Science (MS)

Bilayer lipid membranes (BLMs) are formed from phospholipid molecules which self-assemble into a lipid bilayer with 4 to 9 nm thickness when submerged in an aqueous solution. This is due to their amphiphilic nature, meaning that one part of the molecule is hydrophilic, or attracted to water, and one part is hydrophobic, or repelled by water. They are the primary structural component of cell membranes in living organisms and therefore are useful for modeling the properties of cell membranes since they share many of the same chemical and physical properties. The objective of this dissertation is to investigate the maximum pressure that can be withstood by a BLM formed over a porous substrate, which will be referred to as the failure pressure. This work represents the first time that this quantity has been measured and reported. The failure pressure is investigated in several complementary ways, including mechanical, electrical, and optical measurements and modeling. The phospholipids that are tested include 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphatidylcholine (SOPC) and mixtures of SOPC and cholesterol (CHOL), which was chosen because cholesterol is known to increase the strength of SOPC BLMs. A new test methodology was developed to measure the failure pressure of BLMs. A custom test fixture was used to pressurize BLMs while measuring the applied pressure with a high degree of precision and repeatability. The BLMs were tested in an electrolyte solution over substrates that contained a single pore and also substrates that contained an array of many pores. SOPC BLMs were tested over single pore substrates with pore sizes ranging from 5 to 20 microns, and this resulted in failure pressures from 67 to 19 kPa, respectively. For single pore tests, the addition of 50 mol% cholesterol to SOPC resulted in a 56% higher failure pressure on average than SOPC alone. For multi pore substrates, SOPC BLMs were tested using pore sizes between 0.05 and 10 microns, which yielded bulk failure pressures of 380 to 1.5 kPa, respectively. For multi pore tests, SOPC/CHOL-50 mol% resulted in a 47% higher bulk failure pressure on average. A model was developed to predict the pressurization curve of BLMs and was applied to both the single and multi pore tests. It was found that the failure pressure of BLMs follows a distribution which was well modeled by a Weibull distribution with a positive skew. Parameters such as the Weibull shape parameter were determined by fitting the model to the experimental pressurization curves and it was found that the shape of the Weibull distribution was nearly the same for every pore size. Using the pressurization model it was estimated that the percentage of failed BLMs that were pressurized over a multi pore substrate ranged from 4% to 33%. The model also coupled the bulk failure pressure of BLMs formed over multiple pores to the failure pressure of a single BLM, showing that the bulk failure pressure of multiple BLMs is smaller than the failure pressure of a single BLM because it represents the failure of only the weakest BLMs in a group. Electrical impedance was measured before and after pressurization of the BLMs, and these measurements were modeled by assuming that the BLMs act as a resistor and a capacitor configured in parallel. In general, the impedance magnitude dropped by two to three orders of magnitude after BLM pressurization, which was a result BLMs failing and opening conductive pathways through the subsequently empty pores. It was found that normalized conductance values for SOPC BLMs were between G / A = 4 x 10^-12 and 2 x 10^-8 S/cm^2, and normalized capacitance values varied between C / A = 3 x 10^-14 and 1 x 10^-10 F/cm^2. In the literature these values ranged from G / A = 10^-1 to 10^-9 S/cm^2 and C / A = 10^-6 to 10^-8 F/cm^2, having a wide range of values due to the many variations of experimental test procedures. Visual images of BLMs were produced using fluorescence microscopy. Images were recorded before and after pressurization of SOPC BLMs formed over a multi pore substrate. As predicted by the pressurization model, it was found that some but not all BLMs fail after pressurization. It was also found that BLMs fail over the center of a pore, and leave remnants around the perimeter of the pore. / Ph. D.

Molecular dynamics simulations were performed to study the interactions of bioactive catechins and theaflavins commonly found in tea with lipid bilayers, as a model for cell membranes. Previously, multiple experimental studies rationalized the anticarcinogenic, antibacterial, and other beneficial effects of these compounds in terms of physicochemical molecular interactions with cell membranes. To contribute toward understanding the molecular role of tea polyphenols on the structure of cell membranes, simulation results are presented for seven catechins and three theaflavins in lipid bilayer systems which are both pure (POPC) and representative of HepG2 cancer cells (POPC and POPE). Our simulations show that the catechins and theaflavins evaluated have a strong affinity for the lipid bilayer \textit{via} hydrogen bonding to the bilayer surface, with many of the catechins able to penetrate beneath the surface. Epigallocatechin-gallate (EGCG) and Theaflavin-3,3'-digallate showed the strongest interaction with the lipid bilayers based on the number of hydrogen bonds formed with lipid headgroups. The simulations also provide insight into the functional characteristics of the tea compounds that distinguish them as effective compounds to potentially alter the lipid bilayer properties. The results on the hydrogen-bonding effects may contribute to a better understanding of proposed multiple molecular mechanisms of the action of catechins and theaflavins in microorganisms, cancer cells, and tissues. / Ph. D.

This thesis reports the discovery of a wide negative di↵erential resistance (NDR) region in a graphite-silicone composite that was utilized to create a strain-tuned flexible oscillator. Encoding the strain into frequency mimics the behavior of mechanoreceptor neurons in the skin and demonstrates a flexible and electronically active material suitable for state of the art bio-electronic applications. The NDR was investigated over a range of composite filling fractions and temperatures; alongside theoretical modelling to calculate the tunneling current through a graphite-silicone barrier. This led to the understanding that the NDR is the result of a semi-metal to insulator transition of embedded graphene bilayers within the graphite nanoparticles. The transition, brought about by a transverse bias across specifically orientated particles, opens a partial band-gap at the Fermi level of the bilayer. NDR in a flexible material has not been observed before and has potential for creating a flexible active device. The electromechanical properties of the composite were considered through a bend induced bilayer strain. The piezoresistance was found to be dominated by transient resistance spiking from the breaking of conduction lines, which then reform according to the viscoelasticity of the polymer matrix. The resistance spiking was embraced as a novel method for sensitive di↵erential pressure detection, used in the development of two applications. Firstly, it was employed for the detection of ultrasound waves and found to have an acoustic pressure detection threshold as low as 48 Pa. A commensurability was observed between the composite width and ultrasound wavelength which was shown to be consistent with the formation of standing waves, described by Bragg’s law. Secondly, a differential pressure array of 64 composite pixels was fabricated and demonstrated to image pressures under 3.8 kPa at a resolution of 10 dpi. The NDR active region was incorporated into an LC circuit where it was demonstrated to sustain oscillations of up to 12.5 kHz. The composite was then strained and an intrinsic frequency was observed which had a linear dependence on the strain with a frequency shift of 84 Hz / % strain. Lastly the composite was used in a strain-tuned amplifier circuit and shown to provide a gain of up to 4.5. This thesis provided the groundwork for a completely flexible electronically active device for futuristic bio-electronic skins with resolutions and sensitivities rivalling those of human tactile sensing.

Includes bibliographical references. / 189 leaves, 18, [4] pages : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physical and Inorganic Chemistry, 1991

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 92-102).

Indiana University-Purdue University Indianapolis (IUPUI) / Tissue cells display mechanosensitivity in their ability to discern and respond to changes in the viscoelastic properties of their surroundings. By anchoring and pulling, cells are capable of translating mechanical stimuli into a biological response through a process known as mechanotransduction, a pathway believed to critically impact cell adhesion, morphology and multiple cellular processes from migration to differentiation. While previous studies on polymeric gels have revealed the influence of substrate elasticity on cellular shape and function, a lack of suitable substrates (i.e. with mobile cell-substrate linkers) has hindered research on the role of substrate viscosity. This work presents the successful design and characterization of lipid-bilayer based cell substrates of tunable viscosity affecting cell-substrate linker mobility through changes in viscous drag. Here, two complementary membrane systems were employed to span a wide range of viscosity. Single polymer-tethered lipid bilayers were used to generate subtle changes in substrate viscosity while multiple, polymer-interconnected lipid bilayer stacks were capable of producing dramatic changes in substrate viscosity. The homogeneity and integrity of these novel multibilayer systems in the presence of adherent cells was confirmed using optical microscopy techniques. Profound changes in cellular growth, phenotype and cytoskeletal organization confirm the ability of cells to sense changes in viscosity. Moreover, increased migration speeds coupled with rapid area fluctuations suggest a transition to a different migration mode in response to the dramatic changes in substrate viscosity.

No description available.

Call number: LD2668 .T4 1984 G664 / Master of Science

In this thesis we study the interactions of ions and cationic peptides with a negatively charged lipid bilayer in an ionic solution where the electrostatic interactions are screened. <br /><br /> We first examine the problem of charge renormalization and inversion of a highly charged bilayer with low dielectric constant. To be specific, we consider an asymmetrically charged lipid bilayer, in which only one layer is negatively charged. In particular, we study how dielectric discontinuities and charge correlations among lipid charges and condensed counterions influence the effective charge of the surface. When counterions are monovalent, e. g. , Na⁺, our mean-field approach implies that dielectric discontinuities can enhance counterion condensation. A simple scaling picture shows how the effects of dielectric discontinuities and surface-charge distributions are intertwined: Dielectric discontinuities diminish condensation if the backbone charge is uniformly smeared out while counterions are localized in space; they can, however, enhance condensation when the backbone charge is discrete. In the presence of asymmetric salts such as CaCl₂, we find that the correlation effect, treated at the Gaussian level, is more pronounced when the surface has a lower dielectric constant, inverting the sign of the charge at a smaller value of Ca²⁺ concentration. <br /><br /> In the last chapter we study binding of cationic peptides onto a lipid-bilayer membrane. The peptide not only interacts electrostatically with anionic lipids, rearranging their spatial distributions, but it can also insert hydrophobically into the membrane, expanding the area of its binding layer (i. e. , the outer layer). We examine how peptide charges and peptide insertion (thus area expansion) are intertwined. Our results show that, depending on the bilayer's surface charge density and peptide hydrophobicity, there is an optimal peptide charge yielding the maximum peptide penetration. Our results shed light on the physics behind the activity and selective toxicity of antimicrobial peptides, i. e. , they selectively rupture bacterial membranes while leaving host cells intact.