Spontaneous vesicles, disks, threadlike and spherical micelles found in the solubilization of DMPC liposomes by the detergent DTAC

Effect of Block Sequence on the Solution Self-Assembly of Symmetric ABCBA Pentablock Polymers in a Selective Solvent

Solution self-assembly of multiblock polymers offers a platform to create complex functional self-assembled nanostructures. However, a complete understanding of the effect of the different single-molecule-level parameters and solution conditions on the self-assembled morphology is still lacking. In this work, we have used dissipative particle dynamics to investigate the solution self-assembly of symmetric ABCBA linear pentablock polymers in a selective solvent and examined the effect of the block sequence, composition, and polymer concentration on the final morphology and polymer conformations. We confirmed that block sequence has an effect on the self-assembled morphologies, and it has a strong influence on polymer conformations that give place to physical gels for the sequence where the solvophilic block is located in the middle of the macromolecule. Our results are summarized in terms of morphology diagrams in the composition-concentration parameter space.

Transition kinetics of mixed lipid:photosurfactant assemblies studied by time-resolved small angle X-ray scattering

HYPOTHESIS

Photoswitchable surfactants are used in the design of many light-responsive colloids and/or self-assemblies. Photo-isomerization enables to control molecular equilibrium, and triggers transient reorganizations with possibly out-of-equilibrium intermediate states that have been overlooked. Here, we address this question by an in depth structural investigation of intermediate lipid-surfactant assemblies that occur during fast isothermal photo-triggered transition in lipid:surfactant mixtures.

EXPERIMENTS

The structural parameters of mixed assemblies of azobenzene-containing cationic surfactant (AzoTMA) and dioleoylphosphatidylcholine (DOPC) lipids were studied by light scattering and time-resolved small angle X-ray scattering. Structural and compositional information about the assemblies and unimers in bulk were determined at the photostationary states, as well as at intermediate kinetic states formed during UV or blue light illumination.

FINDINGS

DOPC:AzoTMA systems form mixed assemblies representative of phospholipid:cationic surfactant mixtures, that evolve from spheroid, to rod-like micelles, and vesicles with increasing DOPC fraction. Transient assemblies detected during the photo-triggered kinetics are similar to the ones found in stationary states. But changes of AzoTMA unimers in bulk can be considerably faster than mass reorganizations of the mixed assemblies, suggesting that out-of-equilibrium conditions are transiently reached. Mass reorganization of the surfactant-enriched assemblies is much faster than in the lipid enriched ones, providing insight into the role of lipids in a slow reorganization of the assemblies.

Aggregation/disaggregation of chlorophyll a in model phospholipid-detergent vesicles and micelles

The photosynthetic pigments of higher plants exist in complex oligomeric states, which are difficult to study in vivo. To investigate aggregation processes of chlorophyll a (Chl a), we used an in vitro reconstitution procedure, with this pigment incorporated into liposomes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), micelles and pre-micelle media of the detergent n-dodecyltrimethylammonium chloride (DTAC), and mixed, spontaneous, DMPC-DTAC vesicles and micelles. Chl a oligomers were characterized by UV-visible absorption, steady-state and time-resolved fluorescence, and fluorescence lifetime imaging microscopy. Equivalent diameters of the colloidal structures were obtained by fluorescence correlation spectroscopy. In DMPC liposomes and DMPC-DTAC vesicles and micelles, three fluorescence lifetimes indicated the coexistence of Chl a monomers (≈5 ns) and oligomers (≈1-2 to ≈0.1 ns). The increase in DTAC amount, in the mixed system, induces a progressive solubilization of DMPC liposomes (from vesicles to micelles) and simultaneous disruption of Chl a aggregates; in pure DTAC micelles, mostly monomers were found. The present work aims for a better understanding of chlorophyll-chlorophyll (Chl-Chl), Chl-lipid, and Chl-detergent interactions in spontaneous colloidal micro- and nanostructures.

Domain formation and permeabilization induced by the saponin α-hederin and its aglycone hederagenin in a cholesterol-containing bilayer

Saponins and triterpenic acids have been shown to be able to interact with lipid membranes and domains enriched with cholesterol (rafts). How saponins are able to modulate lipid phase separation in membranes and the role of the sugar chains for this activity is unknown. We demonstrate in a binary membrane model composed of DMPC/Chol (3:1 mol/mol) that the saponin α-hederin and its aglycone presenting no sugar chain, the triterpenic acid hederagenin, are able to induce the formation of lipid domains. We show on multilamellar vesicles (MLV), giant unilamellar vesicles (GUV), and supported planar bilayers (SPB) that the presence of sugar units on the sapogenin accelerates domain formation and increases the proportion of sterols within these domains. The domain shape is also influenced by the presence of sugars because α-hederin and hederagenin induce the formation of tubular and spherical domains, respectively. These highly curved structures should result from the induction of membrane curvature by both compounds. In addition to the formation of domains, α-hederin and hederagenin permeabilize GUV. The formation of membrane holes by α-hederin comes along with the accumulation of lipids into nonbilayer structures in SPB. This process might be responsible for the permeabilizing activity of both compounds. In LUV, permeabilization by α-hederin was sterol-dependent. The biological implications of our results and the mechanisms involved are discussed in relation to the activity of saponins and triterpenic acids on membrane rafts, cancer cells, and hemolysis.

Pairing of cholesterol with oxidized phospholipid species in lipid bilayers

We claim that (1) cholesterol protects bilayers from disruption caused by lipid oxidation by sequestering conical shaped oxidized lipid species such as 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PZPC) away from phospholipid, because cholesterol and the oxidized lipid have complementary shapes and (2) mixtures of cholesterol and oxidized lipids can self-assemble into bilayers much like lysolipid–cholesterol mixtures. The evidence for bilayer protection comes from molecular dynamics (MD) simulations and dynamic light scattering (DLS) measurements. Unimodal size distributions of extruded vesicles (LUVETs) made up of a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and PZPC containing high amounts of PZPC are only obtained when cholesterol is present in high concentrations. In simulations, bilayers containing high amounts of PZPC become porous, unless cholesterol is also present. The protective effect of cholesterol on oxidized lipids has been observed previously using electron paramagnetic resonance (EPR) and electron microscopy imaging of vesicles. The evidence for the pairing of cholesterol and PZPC comes mainly from correlated 2-D density and thickness plots from simulations, which show that these two molecules co-localize in bilayers. Further evidence that the two molecules can cohabitate comes from self-assembly simulations, where we show that cholesterol-oxidized lipid mixtures can form lamellar phases at specific concentrations, reminiscent of lysolipid–cholesterol mixtures. The additivity of the packing parameters of cholesterol and PZPC explains their cohabitation in a planar bilayer. Oxidized lipids are ubiquitously present in significant amounts in high- and low-density lipoprotein (HDL and LDL) particles, diseased tissues, and in model phospholipid mixtures containing polyunsaturated lipids. Therefore, our hypothesis has important consequences for cellular cholesterol trafficking; diseases related to oxidized lipids, and to biophysical studies of phase behaviour of cholesterol-containing phospholipid mixtures.

Morphological and nanomechanical behavior of supported lipid bilayers on addition of cationic surfactants

The addition of surfactants to lipid bilayers is important for the modulation of lipid bilayer properties (e.g., in protein reconstitution and development of nonviral gene delivery vehicles) and to provide insight on the properties of natural biomembranes. In this work, the thermal behavior, organization, and nanomechanical stability of model cationic lipid-surfactant bilayers have been investigated. Two different cationic surfactants, hexadecyltrimethylammonium bromide (CTAB) and a novel derivative of the amino acid serine (Ser16TFAc), have been added (up to 50 mol %) to both liposomes and supported lipid bilayers (SLBs) composed by the zwitterionic phospholipid DPPC. The thermal phase behavior of mixed liposomes has been probed by differential scanning calorimetry (DSC), and the morphology and nanomechanical properties of mixed SLBs by atomic force microscopy-based force spectroscopy (AFM-FS). Although DSC thermograms show different results for the two mixed liposomes, when both are deposited on mica substrates similar trends on the morphology and the mechanical response of the lipid-surfactant bilayers are observed. DSC thermograms indicate microdomain formation in both systems, but while CTAB decreases the degree of organization on the liposome bilayer, Ser16TFAc ultimately induces the opposite effect. Regarding the AFM-FS studies, they show that microphase segregation occurs for these systems and that the effect is dependent on the surfactant content. In both SLB systems, different microdomains characterized by their height and breakthrough force Fb are formed. The molecular organization and composition is critically discussed in the light of our experimental results and literature data on similar lipid-surfactant systems.