| 1. |
- Akesson, Anna, et al.
(author)
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Composition and structure of mixed phospholipid supported bilayers formed by POPC and DPPC
- 2012
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In: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 8:20, s. 5658-5665
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Journal article (peer-reviewed)abstract
- In this paper we present a systematic study of the morphology and composition of supported lipid bilayers (SLBs) formed by vesicle fusion using a wide variety of surface sensitive techniques that give information about the lateral as well as vertical structure and bilayer fluidity. SLBs of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) mixtures at five different bulk vesicle compositions were formed in such a way that the phase separation boundaries were crossed. For all compositions studied, the SLBs were systematically enriched with POPC compared to the nominal vesicle composition. Nevertheless, gel-fluid domain coexistence was observed for SLB compositions in which phase separation was expected based on the bulk phase diagram. The probable causes for the compositional difference in the SLBs are discussed in terms of the phase behaviour of the mixture and its effect on the membrane formation process by vesicle fusion.
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| 2. |
- Akesson, Anna, et al.
(author)
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The effect of PAMAM G6 dendrimers on the structure of lipid vesicles
- 2010
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 12:38, s. 12267-12272
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Journal article (peer-reviewed)abstract
- Dendrimers are polymers with unique properties that make them promising in a variety of applications such as potential drug and gene delivery systems. PAMAM dendrimers, in particular, have been widely investigated and are efficiently translocated into the cell. The mechanism of translocation, however, is still unknown. Recently it was proposed that PAMAM dendrimers are able to open holes in lipid bilayers by stealing lipid from the bilayer and forming "dendrisomes''. The present work intends to contribute in the clarification of this question: why are dendrimers able to translocate into the cell? We create simple models for cell membranes by using small lipid vesicles that present a single lipid phase at physiologically relevant conditions. We then follow the effect that dendrimers have on the structure of the vesicles by using a combination of various techniques: dynamic light scattering, cryo-TEM and small angle X-ray scattering. We discuss our results with respect to the previous findings and reflect on their possible implications for real translocation in living cells.
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| 3. |
- Cárdenas, Marité, et al.
(author)
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Structure of DNA-Cationic Surfactant Complexes at Hydrophobically Modified and Hydrophilic Silica Surfaces as Revealed by Neutron Reflectometry
- 2011
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:20, s. 12506-12514
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Journal article (peer-reviewed)abstract
- In this article, we discuss the structure and composition of mixed DNA-cationic surfactant adsorption layers on both hydrophobic and hydrophilic solid surfaces. We have focused on the effects of the bulk concentrations, the surfactant chain length, and the type solid surface on the interfacial layer structure (the location, coverage, and conformation the e DNA and surfactant molecules). Neutron reflectometry is the technique of choice for revealing the surface layer structure by means of selective deuteration. We start by studying the interfacial complexation of DNA with dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) on hydrophobic surfaces, where we show that DNA molecules are located on top of a self-assembled surfactant monolayer, with the thickness of the DNA layer and the surfactant DNA ratio determined by the surface coverage of the underlying cationic layer. The surface coverages of surfactant and DNA are determined by the bulk concentration of the surfactant relative to its critical micelle concentration (cmc). The structure of the interfacial layer is not affected by the choice of cationic surfactant studied. However, to obtain similar interfacial structures, a higher concentration in relation to its cmc is required for the more soluble DTAB surfactant with a shorter alkyl chain than for CTAB. Our results suggest that the DNA Molecules Will spontaneously form a relatively dense, thin layer on top of a surfactant monolayer (hydrophobic surface) or a layer of admicelles (hydrophilic surface) as long as the surface concentration of surfactant is great enough to ensure a high interfacial-charge density. These findings have implications for bioanalytical and nanotechnology applications, which require the deposition of DNA layers with well controlled structure and composition.
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| 4. |
- Lind, Tania Kjellerup, et al.
(author)
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Continuous flow atomic force microscopy imaging reveals fluidity and time-dependent interactions of antimicrobial dendrimer with model lipid membranes
- 2014
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 8:1, s. 396-408
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Journal article (peer-reviewed)abstract
- In this paper, an amphiphilic peptide dendrimer with potential applications against multi-resistant bacteria such as Staphylococcus aureus was synthesized and studied on model cell membranes. The combination of quartz crystal microbalance and atomic force microscopy imaging during continuous flow allowed for in situ monitoring of the very initial interaction processes and membrane transformations on longer time scales. We used three different membrane compositions of low and high melting temperature phospholipids to vary the membrane properties from a single fluid phase to a pure gel phase, while crossing the phase coexistence boundaries at room temperature. The interaction mechanism of the dendrimer was found to be time-dependent and to vary remarkably with the fluidity and coexistence of liquid-solid phases in the membrane. Spherical micelle-like dendrimer-lipid aggregates were formed in the fluid-phase bilayer and led to partial solubilization of the membrane, while in gel-phase membranes, the dendrimers caused areas of local depressions followed by redeposition of flexible lipid patches. Domain coexistence led to a sequence of events initiated by the formation of a ribbon-like network and followed by membrane solubilization via spherical aggregates from the edges of bilayer patches. Our results show that the dendrimer molecules were able to destroy the membrane integrity through different mechanisms depending on the lipid phase and morphology and shed light on their antimicrobial activity. These findings could have an impact on the efficacy of the dendrimers since lipid membranes in certain bacteria have transition temperatures very close to the host body temperature.
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| 5. |
- Lind, Tania Kjellerup, et al.
(author)
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Formation of Supported Lipid Bilayers by Vesicle Fusion: Effect of Deposition Temperature
- 2014
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 30:25, s. 7259-7263
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Journal article (peer-reviewed)abstract
- We have investigated the effect of deposition temperature on supported lipid bilayer formation via vesicle fusion. By using several complementary surface-sensitive techniques, we demonstrate that despite contradicting literature on the subject, high-quality bilayers can be formed below the main phase-transition temperature of the lipid. We have carefully studied the formation mechanism of supported DPPC bilayers below and above the lipid melting temperature (T-m) by quartz crystal microbalance and atomic force microscopy under continuous flow conditions. We also measured the structure of lipid bilayers formed below or above T-m by neutron reflection and investigated the effect of subsequent cooling to below the T-m. Our results clearly show that a continuous supported bilayer can be formed with high surface coverage below the lipid T-m. We also demonstrate that the high dissipation responses observed during the deposition process by QCM-D correspond to vesicles absorbed on top of a continuous bilayer and not to a surface-supported vesicular layer as previously reported.
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| 6. |
- Mølgaard, Susanne, et al.
(author)
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Cellulose-nanofiber/polygalacturonic acid coatings with high oxygen barrier and targeted release properties
- 2014
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In: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 114, s. 179-182
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Journal article (peer-reviewed)abstract
- A bio-inspired coating consisting of pectin (polygalacturonic acid) and cationic cellulose nanofibers were successfully produced by the layer-by-layer method. The build-up and the morphology of the resulting coatings were studied with spectroscopic ellipsometry and atomic force microscopy, respectively. The coating was able to survive the exposure of a simulated gastric fluid, but was partially degraded upon exposure to pectinase enzyme, which simulate the action of the microbial symbionts present in the human colon. Prior to exposure, the oxygen permeability coefficient of the coating (0.033 ml (STP) mm m-2 day-1 atm-1 at 23 °C and 20% RH) was in the same order of magnitude as for ethylene vinyl alcohol films (0.001-0.01 ml (STP) mm m-2 day-1 atm-1). However, after exposure to the mimicked gastrointestinal (GI) tract conditions, the contribution of coating to the overall barrier properties was not measurable.
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| 7. |
- Wadsater, Maria, et al.
(author)
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Aligning Nanodiscs at the Air-Water Interface, a Neutron Reflectivity Study
- 2011
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:24, s. 15065-15073
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Journal article (peer-reviewed)abstract
- Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer. The proof of concept is hereby presented with the use of nanodiscs composed of a mixture of two different lipid (DMPC and DMPG) types to obtain a net overall negative charge of the nanodiscs. We find that the nanodisc layer has a thickness or 40.9 +/- 2.6 angstrom with a surface coverage of 66 +/- 4%. This layer is located about 15 angstrom below a cationic surfactant layer at the air water interface. The high level of organization within the nanodiscs layer is reflected by a low interfacial roughness (similar to 4.5 angstrom) found. The use of the nanodisc as a biomimetic model of the cell membrane allows for studies of single membrane proteins isolated in a confined lipid environment. The 2D alignment of nanodiscs could therefore enable studies of high-density layers containing membrane proteins that, in contrast to membrane proteins reconstituted in a continuous lipid bilayer, remain isolated front influences of neighboring membrane proteins within the layer.
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