Mathias Lösche

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers

Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and ‘smart’ ceramics such as lead zirconate titanate. But the strains achievable in these materials are small—less than 0.1 per cent—so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material—the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV m-1 have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV m-1. This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.

Ferromagnetism in oriented graphite samples

We have studied the magnetization of various well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite, and natural graphite to investigate the recently reported ferromagneticlike signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers---to minimize the diamagnetic background---show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.

Fluorescence microscope to observe dynamical processes in monomolecular layers at the air/water interface

An experimental setup is described to microscopically observe surfactant monolayers at the air/water interface. It basically consists of a film balance, incorporated into a fluorescence microscope, detecting the emission of dye probes. The apparatus is stable enough to observe dynamical processes over more than 5 h, with a spatial resolution of less than 2 μm and is sensitive enough to use probe concentrations as low as 0.1 mol%. Examples of surface textures appearing during lipid phase transitions demonstrate the good performance characteristics.

Structural properties of phosphatidylcholine in a monolayer at the air/water interface: Neutron reflection study and reexamination of x-ray reflection measurements.

Neutron reflectivities of phosphatidylcholine monolayers in the liquid condensed (LC) phase on ultrapure H(2)O and D(2)O subphases have been measured on a Langmuir film balance. Using a dedicated liquid surface reflectometer, reflectivities down to R = 10(-6) in the momentum transfer range Q(z) = 0-0.4 A(-1) were accessed.In a new approach, by refining neutron reflectivity data from chain-perdeuterated DPPC-d(62) in combination with x-ray measurements on the same monolayer under similar conditions it is shown that the two techniques mutually complement one another. This analysis leads to a detailed conception of the interface structure. It is found that in the LC phase (which is analogous to the L(beta), phase in vesicle dispersions) the head group is interpenetrated with subphase water (4 +/- 2.5 molecules per lipid) and the average tilt angle of the hydrophobic chains from the surface normal is 33 +/- 3 degrees.

Molecular-scale structural and functional characterization of sparsely tethered bilayer lipid membranes

Surface-tethered biomimetic bilayer membranes (tethered bilayer lipid membranes (tBLMs)) were formed on gold surfaces from phospholipids and a synthetic 1-thiahexa(ethylene oxide) lipid, WC14. They were characterized using electrochemical impedance spectroscopy, neutron reflection (NR), and Fourier-transform infrared reflection-absorption spectroscopy (FT-IRRAS) to obtain functional and structural information. The authors found that electrically insulating membranes (conductance and capacitance as low as 1 μS cm−2 and 0.6 μF cm−2, respectively) with high surface coverage (>95% completion of the outer leaflet) can be formed from a range of lipids in a simple two-step process that consists of the formation of a self-assembled monolayer (SAM) and bilayer completion by “rapid solvent exchange.” NR provided a molecularly resolved characterization of the interface architecture and, in particular, the constitution of the space between the tBLM and the solid support. In tBLMs based on SAMs of pure WC14, the hexa(ethylene oxide) tether region had low hydration even though FT-IRRAS showed that this region is structurally disordered. However, on mixed SAMs made from the coadsorption of WC14 with a short-chain “backfiller,” ß-mercaptoethanol, the submembrane spaces between the tBLM and the substrates contained up to 60% exchangeable solvent by volume, as judged from NR and contrast variation of the solvent. Complete and stable “sparsely tethered” BLMs (stBLMs) can be readily prepared from SAMs chemisorbed from solutions with low WC14 proportions. Phospholipids with unsaturated or saturated, straight or branched chains all formed qualitatively similar stBLMs.

Electrostatic interactions in phospholipid membranes. I: Influence of monovalent ions

Electrostatic interactions in monolayers and vesicles of acidic phospholipids are studied by thermodynamical and optical techniques in conjunction with numerical calculations. A nonmonotonic ionic strength dependence with an extremum at 0.1 M (NaCl) is observed for the phase transition temperature of vesicles as well as for the surface pressure of monolayers at low molecular density. This finding is in accordance with the calculations predicting the dominance of charge screening by monovalent counterions only for concentrations above 0.1 M. For lower salt content, however, its increase causes an elevation of the degree of dissociation and thus also electrostatic repulsion. This leads to a higher surface pressure, a lower transition temperature and a smaller size of solid domains observed in the liquid/solid coexistence range of monolayers. This supports the previously published idea, that finite size and repulsion of the domains arise from a different surface charge density in fluid and solid lipid phases.

Structure of functional Staphylococcus aureus α-hemolysin channels in tethered bilayer lipid membranes

We demonstrate a method for simultaneous structure and function determination of integral membrane proteins. Electrical impedance spectroscopy shows that Staphylococcus aureus alpha-hemolysin channels in membranes tethered to gold have the same properties as those formed in free-standing bilayer lipid membranes. Neutron reflectometry provides high-resolution structural information on the interaction between the channel and the disordered membrane, validating predictions based on the channels x-ray crystal structure. The robust nature of the membrane enabled the precise localization of the protein within 1.1 A. The channels extramembranous cap domain affects the lipid headgroup region and the alkyl chains in the outer membrane leaflet and significantly dehydrates the headgroups. The results suggest that this technique could be used to elucidate molecular details of the association of other proteins with membranes and may provide structural information on domain organization and stimuli-responsive reorganization for transmembrane proteins in membrane mimics.

Single wall carbon nanotubes enter cells by endocytosis and not membrane penetration

BackgroundCarbon nanotubes are increasingly being tested for use in cellular applications. Determining the mode of entry is essential to control and regulate specific interactions with cells, to understand toxicological effects of nanotubes, and to develop nanotube-based cellular technologies. We investigated cellular uptake of Pluronic copolymer-stabilized, purified ~145 nm long single wall carbon nanotubes (SWCNTs) through a series of complementary cellular, cell-mimetic, and in vitro model membrane experiments.ResultsSWCNTs localized within fluorescently labeled endosomes, and confocal Raman spectroscopy showed a dramatic reduction in SWCNT uptake into cells at 4°C compared with 37°C. These data suggest energy-dependent endocytosis, as shown previously. We also examined the possibility for non-specific physical penetration of SWCNTs through the plasma membrane. Electrochemical impedance spectroscopy and Langmuir monolayer film balance measurements showed that Pluronic-stabilized SWCNTs associated with membranes but did not possess sufficient insertion energy to penetrate through the membrane. SWCNTs associated with vesicles made from plasma membranes but did not rupture the vesicles.ConclusionsThese measurements, combined, demonstrate that Pluronic-stabilized SWCNTs only enter cells via energy-dependent endocytosis, and association of SWCNTs to membrane likely increases uptake.

Size-dependent neurotoxicity of β-amyloid oligomers

The link between the size of soluble amyloid beta (Abeta) oligomers and their toxicity to rat cerebellar granule cells (CGC) was investigated. Variation in conditions during in vitro oligomerization of Abeta(1-42) resulted in peptide assemblies with different particle size as measured by atomic force microscopy and confirmed by dynamic light scattering and fluorescence correlation spectroscopy. Small oligomers of Abeta(1-42) with a mean particle z-height of 1-2 nm exhibited propensity to bind to phospholipid vesicles and they were the most toxic species that induced rapid neuronal necrosis at submicromolar concentrations whereas the bigger aggregates (z-height above 4-5 nm) did not bind vesicles and did not cause detectable neuronal death. A similar neurotoxic pattern was also observed in primary cultures of cortex neurons whereas Abeta(1-42) oligomers, monomers and fibrils were non-toxic to glial cells in CGC cultures or macrophage J774 cells. However, both oligomeric forms of Abeta(1-42) induced reduction of neuronal cell densities in the CGC cultures.

Bacterial S-Layer Protein Coupling to Lipids: X-Ray Reflectivity and Grazing Incidence Diffraction Studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38-66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness lz approximately 90 A of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to approximately 20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of approximately 131 A.