Biomedical subjects

Barbara J Frisken

Publications and source records attributed to Barbara J Frisken.

7 recordsLinked to original sources

PubMed2006

Ergosterol in POPC membranes: physical properties and comparison with structurally similar sterols.

The physical properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/ergosterol bilayers in the liquid-crystalline phase were determined using deuterium nuclear magnetic resonance ((2)H NMR) and vesicle extrusion. For the (2)H NMR experiments, the sn-1 chain of POPC was perdeuterated, and spectra were taken as a function of ergosterol concentration and temperature. Analysis of the liquid-crystalline spectra provides clear evidence that two types of liquid-crystalline domains, neither of which is a liquid-ordered phase, having distinct average chain conformations coexist in 80:20 and 75:25 POPC/ergosterol membranes over a wide temperature range (from -2 to at least 31 degrees C). Adding ergosterol to a concentration of 25 mol % increases POPC-d(31) chain ordering as measured by the NMR spectral first moment M(1) and also increases the membrane lysis tension, obtained from vesicle extrusion. Further addition of ergosterol had no effect on either chain order or lysis tension. This behavior is in marked contrast to the effect of cholesterol on POPC membranes: POPC/cholesterol membranes have a linear dependence of chain order on sterol concentration to at least 40 mol %. To investigate further we compared the dependence on sterol structure and concentration of the NMR spectra and lysis tension for several POPC/sterol membranes at 25 degrees C. For all POPC/sterol membranes investigated in this study, we observed a universal linear relation between lysis tension and M(1). This suggests that changes in acyl chain ordering directly affect the tensile properties of the membrane.

Deuterium↗

PubMed2006

Microrheology and structure of a yield-stress polymer gel.

The small-scale rheology of Carbopol ETD 2050, a polymer gel with a yield stress, is studied as a function of polymer concentration by measuring the diffusion of submicron-sized spherical fluorescent particles suspended in gel. Dynamic light scattering is used to determine the mean-squared displacement (of the particles as a function of lag time t. Fluorescence microscopy is used to track the particle trajectories directly, from which and the van Hove correlation function are determined. From our results we calculate the microrheological viscous and elastic moduli of the material. The two techniques cover complementary ranges of tau and and give results that agree well. The microrheological moduli are substantially smaller than the bulk values as determined by conventional shear rheometry. The bulk viscoelastic behavior is dominated by the elastic modulus, while at low enough concentrations and high enough frequencies the microrheological response is predominantly viscous. These results will be discussed in the context of the gel structure.

Journal Article↗

PubMed2006

Direct determination of the number-weighted mean radius and polydispersity from dynamic light-scattering data.

We compare results for the number-weighted mean radius and polydispersity obtained either by directly fitting number distributions to dynamic light-scattering data or by converting results obtained by fitting intensity-weighted distributions. We find that results from fits using number distributions are angle independent and that converting intensity-weighted distributions is not always reliable, especially when the polydispersity of the sample is large. We compare the results of fitting symmetric and asymmetric distributions, as represented by Gaussian and Schulz distributions, respectively, to data for extruded vesicles and find that the Schulz distribution provides a better estimate of the size distribution for these samples.

Journal Article↗

PubMed2005

CTP:phosphocholine cytidylyltransferase binds anionic phospholipid vesicles in a cross-bridging mode.

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the rate-limiting step in phosphatidylcholine (PC) synthesis, and its activity is regulated by reversible association with membranes, mediated by an amphipathic helical domain M. Here we describe a new feature of the CCTalpha isoform, vesicle tethering. We show, using dynamic light scattering and transmission electron microscopy, that dimers of CCTalpha can cross-bridge separate vesicles to promote vesicle aggregation. The vesicles contained either class I activators (anionic phospholipids) or the less potent class II activators, which favor nonlamellar phase formation. CCT increased the apparent hydrodynamic radius and polydispersity of anionic phospholipid vesicles even at low CCT concentrations corresponding to only one or two dimers per vesicle. Electron micrographs of negatively stained phosphatidylglycerol (PG) vesicles confirmed CCT-mediated vesicle aggregation. CCT conjugated to colloidal gold accumulated on the vesicle surfaces and in areas of vesicle-vesicle contact. PG vesicle aggregation required both the membrane-binding domain and the intact CCT dimer, suggesting binding of CCT to apposed membranes via the two M domains situated on opposite sides of the dimerization domain. In contrast to the effects on anionic phospholipid vesicles, CCT did not induce aggregation of PC vesicles containing the class II lipids, oleic acid, diacylglycerol, or phosphatidylethanolamine. The different behavior of the two lipid classes reflected differences in measured binding affinity, with only strongly binding phospholipid vesicles being susceptible to CCT-induced aggregation. Our findings suggest a new model for CCTalpha domain organization and membrane interaction, and a potential involvement of the enzyme in cellular events that implicate close apposition of membranes.

Anions↗

PubMed2005

Influence of secondary components on the synthesis of self-cross-linked N-isopropylacrylamide microgels.

This work presents systematic studies of cross-linker-free microgels formed by copolymerization of N-isopropylacrylamide (NIPAAm) and various secondary monomer components in water under standard reaction conditions. The sizes, solid densities, and volume phase transitions of these particles have been characterized through static and dynamic laser light-scattering experiments. We find that introducing a hydrophobic component, for example, styrene (St) or methyl methacrylate (MMA), leads to particles with smaller sizes and higher solid densities, while the volume phase transition shifts to lower temperatures. On the other hand, introducing a hydrophilic component such as acrylamide (AAm) or acrylic acid (AA) leads to larger particles with lower solid densities and a volume phase transition that shifts to higher temperatures and is broadened. The molar mass changes little in either case. Introducing a charged component such as sodium styrene sulfonate (NaSS) or poly(sodium styrene sulfonate) (PNaSS) leads to a sharp decrease in molar mass and particle size, and a very broad phase transition. These trends provide good guidance for synthesizing both self-cross-linked and cross-linked copolymerized microgels of different properties.

Journal Article↗

PubMed2003

Engineering asymmetric vesicles.

Vesicles are bilayers of lipid molecules enclosing a fixed volume of aqueous solution. Ubiquitous in cells, they can be produced in vitro to study the physical properties of biological membranes and for use in drug delivery and cosmetics. Biological membranes are, in fact, a fluid mosaic of lipids and other molecules; the richness of their chemical and mechanical properties in vivo is often dictated by an asymmetric distribution of these molecules. Techniques for vesicle preparation have been based on the spontaneous assembly of lipid bilayers, precluding the formation of such asymmetric structures. Partial asymmetry has been achieved only with chemical methods greatly restricting the study of the physical and chemical properties of asymmetric vesicles and their use in potential applications for drug delivery. Here we describe the systematic engineering of unilamellar vesicles assembled with two independently prepared monolayers; this process produces asymmetries as high as 95%. We demonstrate the versatility of our method by investigating the stability of the asymmetry. We also use it to engineer hybrid structures comprised of an inner leaflet of diblock copolymer and an independent lipid outer leaflet.

Fluorescence↗

PubMed2003

The pressure-dependence of the size of extruded vesicles.

Variations in the size of vesicles formed by extrusion through small pores are discussed in terms of a simple model. Our model predicts that the radius should decrease as the square root of the applied pressure, consistent with data for vesicles extruded under various conditions. The model also predicts dependencies on the pore size used and on the lysis tension of the vesicles being extruded that are consistent with our data. The pore size was varied by using track-etched polycarbonate membranes with average pore diameters ranging from 50 to 200 nm. To vary the lysis tension, vesicles made from POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine), mixtures of POPC and cholesterol, and mixtures of POPC and C(16)-ceramide were studied. The lysis tension, as measured by an extrusion-based technique, of POPC:cholesterol vesicles is higher than that of pure POPC vesicles whereas POPC:ceramide vesicles have lower lysis tensions than POPC vesicles.

Ceramides↗