2012

Abstract: Amphiphilic cyclodextrins (CDs) are good candidates to functionalize natural membranes, as well as synthetic vesicles. In this paper, we fully describe and compare the insertion properties of the permethylated mono-cholesteryl alpha-CD (TASC) and its mono-and di-cholesteryl beta-CD analogues (TBSC and TBdSC) in dipalmitoyl-L-alpha-phosphatidylcholine (DPPC) mono-and bi-layers as membrane models from the macroscopic to the molecular scale. By calculating the inverse compressibility moduli and free excess Gibbs energies from the Langmuir isotherms, the influence of the CD type, CD ratio and number of cholesteryl anchors on the membrane properties have been established. TBdSC, with its two cholesteryl residues, seems to be anchored best to the membrane compared to CD derivatives with only one anchor. Furthermore, TASC appears to be more firmly inserted into the membrane than TBSC. The in-plane structure is characterized by Brewster angle microscopy (BAM) at the air-water interface and atomic force microscopy (AFM) of the mono-and bi-layers deposited on mica. Depending on the compression, full miscibility of the cholesteryl CDs and the phospholipids is observed at low surface pressures and a clear demixing tendency is apparent during compression. CD-modified bilayers are stable and are subject to a gel-liquid phase transition upon heating. Due to their bulky CD moiety, the amphiphilic CDs exhibit a distinct fluidizing effect, shifting the DPPC's gel-liquid transition. The structure of the mixed TASC/DPPC mono- and bi-layers perpendicular to the surface is investigated with Angstrom resolution by neutron reflectivity. In this way a molecular model of the insertion has been established, which suggests that the CD cavities partly protrude into the subphase, which should leave them accessible for complex formation.

Abstract: The binding of cationic surfactants with varying alkyl chain length to a regiorandom conjugated polyanion, poly(3-thiophene acetic acid) (PTAA), is studied in an aqueous buffer by using absorption and emission spectroscopies, photon correlation spectroscopy, isothermal titration calorimetry, and cryogenic transmission electron microscopy. We study the mixed solutions as a function of composition ratio R of surfactant molecules to monomer units molar concentrations, at low polymer concentration and in a very wide composition range (10(-6) < R < 10(2)) below the critical micellar concentration. Upon surfactant binding, the molecularly dispersed chains first collapse progressively and then form new structures as the mixed aggregates get enriched in surfactant. The collapse leads to a strong decrease of the conjugation length and to a blue shift of the absorption spectra by 30 to 50 nm. The new structures are responsible for a new intense emission band at about 600 nm, red-shifted by nearly 130 nm from the initial emission maximum of the polymer (~472 nm). As the surfactant tail becomes shorter, the blue shift of the absorption spectra and the intensity raise of the new emission are delayed to larger composition ratios while their variations become smoother functions of the surfactant concentration. These particular spectroscopic properties of PTAA seem related to its unique combination of a strongly hydrophobic backbone, a large ratio of contour length to persistence length, and an overall good aqueous solubility. Our results show that such features are well suited to design a colorimetric biosensor at small composition ratio, and a fluorescent biomarker at large composition ratio.

Abstract: Using giant unilamellar vesicles (GUVs) made from POPC. DPPC, cholesterol and a small amount of a porphyrin-based photosensitizer that we name PE-porph, we investigated the response of the lipid bilayer under visible light, focusing in the formation of domains during the lipid oxidation induced by singlet oxygen. This reactive species is generated by light excitation of PE-porf in the vicinity of the membrane, and thus promotes formation of hydroperoxides when unsaturated lipids and cholesterol are present. Using optical microscopy we determined the lipid compositions under which GUVs initially in the homogeneous phase displayed Lo-Ld phase separation following irradiation. Such an effect is attributed to the in situ formation of both hydroperoxized POPC and cholesterol. The boundary line separating homogeneous Lo phase and phase coexistence regions in the phase diagram is displaced vertically towards the higher cholesterol content in respect to ternary diagram of POPC:DPPC:cholesterol mixtures in the absence of oxidized species. Phase separated domains emerge from sub-micrometer initial sizes to evolve over hours into large Lo-Ld domains completely separated in the lipid membrane. This study provides not only a new tool to explore the kinetics of domain formation in mixtures of lipid membranes, but may also have implications in biological signaling of redox misbalance. (C) 2011 Elsevier B.V. All rights reserved.

Abstract: Inspired by recent atomic force microscope (AFM) images of plasmids deposited on oppositely charged supported lipid bilayers from salt free solution, we propose a model for strongly adsorbed supercoiled cyclic stiff polyelectrolytes. We discuss how the excess linking number Lk of the deposited cycle is shared between writhe Wr and twist Tw at equilibrium and obtain the typical number of self-crossings in the deposited cycle as a function of surface charge density. The number of crossings at equilibrium is simply determined by the crossing penalty which is a local quantity and by the excess linking number. The number of crossings is well defined despite versatile plasmid shapes. For moderate numbers of crossings the loops are rather small and localized along the primary cycle, as expected from entropic loops. In the regime of many crossings, the cycle takes the shape of a regular flat ply ruled by local stiffness. The model allows for a semiquantitative comparison with the AFM images of deposited plasmids which are strongly charged.