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NA2, Self-assembling & Biomimetic Systems

The self-assembly of surfactant molecules in monolayers at the interface between oil and water reduces the interfacial tension drastically. This mechanism is used in all industry of cleaning and washing processes.
Surfactants are also used to stabilize emulsions, they are a basic to the detergents, cosmetics, shampoos, and food industries e.g. nutraceuticals and plant protection. The self-assembly of amphiphilic molecules plays also an important role in biological systems, where the membrane of cells and cell organelles form by self-assembly of lipid molecules. Self-assembling amphiphilic systems are therefore very important for pharmaceutical and biological applications. Besides classical surfactant, new polymeric materials (amphiphilic diblocks) are now available in industry and may play a very important role in applications very soon.

The SoftComp Scientific Activity

Scientifically SoftComp covers a broad range of scientific activities with more than 120 researchers actively involved. Therefore it is impossible to describe their important results. Instead we present a few highlights I an exemplary way.

Network Area 2 Highlights

Toxin-induced invaginations

Pathogens have developed many strategies to enter into cells. This is the case for bacteria secreting toxins, such as the Shiga toxin, which can penetrate in cells using non-conventional pathways. In vivo, long tubular structures are observed when cells, depleted in ATP, are in contact with Shiga toxins. Similar invaginations have been reproduced using giant unilamellar vesicles (GUVs) containing the toxin lipid receptors. We have shown that if the membrane tension is low, tubular invaginations are observed on the membrane, whereas for higher tension, toxin clustering is detected but tension inhibits membrane deformation. We have proposed that these toxins can induce a reorganization of lipids and a compression of the external leaflet of the bilayer, creating thus a negative spontaneous curvature. This cargo-induced deformation can represent a new type of endocytosis.

Figure: Interaction of Shiga Toxin with membranes containing their lipid receptor Gb3. Tubular invaginations form when Shiga toxin (red) interacts with its lipid receptor Gb3 either in vivo (from W. Römer and L. Johannes, Curie Institute) or in vitro with GUV containing Gb3.

Polypeptide/synthetic graft copolymers

Hybrid peptide-synthetic copolymers are interesting materials that combine the functionality and structural diversity of polypeptides with the self-assembling ability of graft and block copolymers. We developed graft copolymers based on a water-soluble poly(N,N-dimethylacrylamide) (DMAM) backbone and having poly(L-Lysine) (PLL) grafts. The resulting copolymers exhibit very interesting stimuli responsive behaviour related to the conformational transitions of PLL grafts (random coil, α-helix, ß-strand). At pH lower than the pKa, of PLL (i.e. pH < 10.5) the grafts are in the coil conformation and the copolymer is perfectly soluble in aqueous solvents. At pH > pKa the PLL grafts adopt the α-helix conformation without any significant change in the properties of the solution. However a slight increase in temperature (T > 40°C) induces ß-sheet formation and gelation of the system. At this high pH the α-helix-to-ß-sheet transition is irreversible, the system remains in the gel state after decreasing temperature. Such gels are insensitive to the ionic strength but they melt after decreasing pH below pKa. The sol-gel transition can be repeated several times by applying the pH-temperature cycle.

Figure: Schematic representation of the behaviour of hybrid copolymers based on PDMAM backbone and PLL grafts.

Double transient network in entangled wormlike micelles bridged by telechelic polymers
The linear viscoelasticity of a new type of transient network --- bridged wormlike micelles --- has been surveyed by the group at the University of Montpellier. This composite material is obtained by adding telechelic copolymers (water-soluble chains with hydrophobic stickers at each extremity) to a solution of entangled wormlike micelles (WM),(Fig. 5). The structure of such networks has been characterized recently. For comparison, naked WM and WM decorated by amphiphilic copolymers are also investigated. While these latter systems exhibit almost the same single ideal Maxwell behavior, it is found that solutions of bridged WM can be described as blends of two Maxwell-fluid components, characterized by two markedly different characteristic times, tfast and tslow, and two elastic moduli, Gfast and Gslow, with Gfast >> Gslow. It is shown that the slow mode is related to the viscoelasticity of the transient network of entangled WM, and the fast mode to the network of telechelic active chains (i.e. chains that do not form loops but bridge two micelles). The dependence of the viscoelasticity with the surfactant concentration, f, and the sticker-to-surfactant molar ratio, b, has been studied in detail. In particular, it has been shown that Gfast is proportional to the number of active chains in the material, fb. Simple theoretical expectations allow then to evaluate the bridges/loops ratio for the telechelic polymers.

Figure 5: Sketch of the structure of the double transient network formed by addition of telechelic polymers (green) to a solution of entangled wormlike micelles (grey) in the one-phase region of the phase diagram, as inferred from structural investigations.

Dynamics of CIS-Polyisoprene in Self-Assembled Nano-Structured Phases
As an extension of the work carried out 1n polymer blends with dynamical asymmetry, we have started to investigate dynamics in self-structured systems where nano-confined phases emerge. Nowadays it is well known that different complex nanostructures can be obtained by self-assembly using diblock-copolymers of different components (Fig.6), molecular weight and composition. However, how the dynamics of the copolymer components is modified by the nano-confinement in the segregated phases is still poorly understood.

Here, the effect of nanoscopic confinement on local and global polymer dynamics has been studied by broadband dielectric spectroscopy (BDS). As a template for the confinement spherical and cylindrical micelles, spontaneously formed by self-assembly of highly asymmetric (composition of one of the components of 18%) poly(isoprene)-poly(dimethyl siloxane) (PI-PDMS) diblock copolymers, were chosen. The minority component, PI, which forms the core, displays a dipole moment normal and parallel to the polymer backbone allowing both the global dynamics (normal mode) and local segmental relaxation (a-process) to be observed simultaneously rendering possible a comparable study over a large dynamical range in an extended temperature range of 223 – 323 K. However, it is worthy of remark that the experimental observation of both relaxation processes of PI in the system was a difficult task pushing the dielectric techniques to the limit of the resolution, taking into account the low concentration of PI and the dielectric strength. In addition to the dielectric measurements, temperature modulated scanning calorimetry investigation was carried out yielding information about glass transition and crystallization behaviour. The structure was analyzed by small angle x-ray scattering (SAXS),(Fig.7). This work is a good example of collaborative research between two SoftComp partners (Univ. Pais Vasco, San Sebastian and FZJ-Richter, Jülich), involving well-controlled chemical synthesis, and very different experimental techniques with different expertise required.

The results show that for low molecular weight of PI (4300 gram/mol), cylinders (radius of about 5nm) in a hexagonal lattice were formed (Fig.8). At higher molecular weights we observe spherical structures with a radius in between 10-13 nm. Dielectric spectroscopy showed that the dynamics of PI is considerably faster and broader in the micellar cores compared to normal bulk behaviour (Fig.9a). Surprisingly, this concerned both the local segmental a-relaxation and the global chain orientation dynamics indicating that the dynamics is modified over a wide spatial scale (Fig.9b). Theoretical interpretation is in progress. We speculate that this behaviour is induced by the interfacial dynamics between the two blocks. However this hypothesis needs to be further verified by neutron spin echo measurements and likely by molecular dynamics simulations on coarse-grain models.

Figure 6: Diblock copolymer

Figure 7: Small angle X-ray Scattering

Figure 8: Hexagonal Cylinders

Figure 9a: cis-PI Dynamics by Broadband Dielectric Spectroscopy - Segmental and global scales

Figure 9b: cis-PI Dynamics by Broadband Dielectric Spectroscopy - Both scales strongly affected - interface fluctuations?

Simulating the Transport Properties of the Stratum Corneum.

The stratum corneum, the top-most layer of the skin, has been simulated using a ceramide-2 bilayer as a model system. In order to understand the transport of molecules across this layer, which has important practical consequences, the energy profile of pore formation in such a bilayer in water has been calculated by applying a mechanical constraint to the lipid density in the centre of the bilayer. The simulations reveal that, because of the high rigidity of the ceramide chains, the resulting small hydrophobic pores are not filled with water, but instead remain void (Figure 10). A simple thermodynamic analysis, based on the relevant surface tensions, confirms that water will only enter the pore once a critical radius, proportional to the membrane thickness, has been surpassed. The permeability barrier of the membrane is thus effectively increased.
We have tested how a ‘classical’ penetration enhancer would modify the baseline structure properties of the ceramide bilayer. Preliminary results of membranes in an aqueous solvent containing a classic penetration enhancer, shows that the added component molecules tend to form a thin protective layer covering the previously exposed tails in the interior of the pore, as well as making the membrane more flexible. These effects promote the formation of a water-filled hydrophilic pore, and hence facilitate the transport of molecules (e.g. benefit agents) across the membrane. This is a first step towards a better understanding of the surprising permeation properties of the skin. (Unilever and University of Twente)

Figure 10: Effect of the constraint on ceramide bilayer. For each constraint value, a section of the bilayer and a top-view are displayed showing the presence of void of increasing size in the bilayer.

Red blood cells in capillary flow.
Figure 3 shows snapshots of typical shapes of red blood cells in capillary flow. For small flow velocities, the red blood cells assumes its characteristic discocyte shape. However, at higher flow velocities, a transition to a parachute shape has been predicted on the basis of molecular dynamics simulations with a new hybrid model, which combines a particle-based mesoscopic simulation technique for the solvent with a dynamically-triangulated surface model for the membrane. The parachute shape owes its stability in flow to the composite nature of the red-blood-cell membrane, which consists of a spectrin polymer network anchored to a lipid bilayer. It is very important to understand the effect of membrane elasticity on the flow behaviour, because the reduced deformability of red blood cells in diseases such as diabetes mellitus and sickle cell anemia is responsible for the increase of the apparent blood viscosity and blocking of micro-vascular flows (FZ Jülich, Gompper group).

Figure 3: Red blood cells in capillary flow, above low flow below higher flow.

Microscopic view on a new mechanism of fusion of two wormlike micelles

Figure 1 displays a microscopic view on a new mechanism of fusion of two wormlike micelles under shear flow. The results were obtained by computer simulation and shed some light on the mechanism behind the strong shear thinning behaviour of such systems which are important candidate materials to be employed in tertiary oil recovery. The upper picture shows two entangled worm like micelles hindering each other to flow. Under shear stress the micelles gradually merge and are transformed into an H shaped structures (lower picture)where the two different wormlike micelles have fully merged. In this state the entanglement effect is removed and the system flows easily (Twente/Schlumberger).

Figure 1: Entangling and merging wormlike micelles under shear.

Phase separation in giant micelles

Figure 2 shows the phase separation in giant micelles containing a mixture of two lipids and cholesterol (sphingomeylin/Cholesterol/
DOPC). Such mixtures are biomimetic models for multicomponent biological membranes. The different phases are made visible by fluorescence markers through fluorescence microscopy. Taken at different compositions the results allow the determination of the phase diagram shown at the bottom of Figure 2 (Marie Curie/

Figure 2: Phase separation in giant vesicles. (a) Large domains are seen in fluorescence microscopy. (b) Obtained phase diagram.