Rigid urea and self-healing thiourea ethanolamine monolayers

A series of long-tail alkyl ethanolamine analogs containing amide-, urea-, and thiourea moieties was synthesized and the behavior of the corresponding monolayers was assessed on the Langmuir–Pockels trough combined with grazing incidence X-ray diffraction experiments and complemented by computer simulations. All compounds form stable monolayers at the soft air/water interface. The phase behavior is dominated by strong intermolecular headgroup hydrogen bond networks. While the amide analog forms well-defined monolayer structures, the stronger hydrogen bonds in the urea analogs lead to the formation of small three-dimensional crystallites already during spreading due to concentration fluctuations. The hydrogen bonds in the thiourea case form a two-dimensional network, which ruptures temporarily during compression and is recovered in a self-healing process, while in the urea clusters the hydrogen bonds form a more planar framework with gliding planes keeping the structure intact during compression. Because the thiourea analogs are able to self-heal after rupture, such compounds could have interesting properties as tight, ordered, and self-healing monolayers

Bilayer Properties of 1,3-Diamidophospholipids

A series of 1,3-diamido phosphocholines was synthesized, and their potential to form stable bilayers was investigated. Large and giant unilamellar vesicles produced from these new lipids form a wide variety of faceted liposomes. Factors such as cooling rates and the careful choice of the liposome preparation method influence the formation of facets. Interdigitation was hypothesized as a main factor for the stabilization of facets and effectively monitored by small-angle X-ray scattering measurements

Against the rules: pressure induced transition from high to reduced order

Envisioning the next generation of drug delivery nanocontainers requires more in- depth information on the fundamental physical forces at play in bilayer membranes. In order to achieve this, we combine chemical synthesis with physical–chemical analytical methods and probe the relationship between a molecular structure and its biophysical properties. With the aim of increasing the number of hydrogen bond donors compared to natural phospholipids, a phospholipid compound bearing urea moieties has been synthesized. The new molecules form interdigitated bilayers in aqueous dispersions and self-assemble at soft interfaces in thin layers with distinctive structural order. At lower temperatures, endothermic and exothermic transitions are observed during compression. The LC1 phase is dominated by an intermolecular hydrogen bond network of the urea moieties leading to a very high chain tilt of 52°. During compression and at higher temperatures, presumably this hydrogen bond network is broken allowing a much lower chain tilt of 35°. The extremely different monolayer thicknesses violate the two-dimensional Clausius–Clapeyron equation

Monolayer properties of 1,3-diamidophospholipids

While nature provides an endless variety of phospholipids presenting hydrolyzable ester linkages for the 1,2-positioned hydrocarbon tails, we designed and synthesized 1,3-diamidophospholipids which contain stable fatty acid amides. These new phospholipids form faceted unilamellar vesicles with mechanosensitive properties. Aiming to understand the mechanism responsible for this behavior at a molecular level, we investigated the 1,3-diamidophospholipid family in monolayers, a simplified model membrane system. Langmuir isotherms combined with in situ grazing incidence X-ray diffraction (GIXD), specular X-ray reflectivity (XR), and infrared reflection–absorption spectroscopy (IRRAS) allowed the characterization of the monolayers from a structural and thermodynamical point of view. The existence of strong headgroup interactions due to the formation of a hydrogen-bonding network was clearly revealed by IRRAS and by the high rigidity of the monolayers. GIXD showed that only the longer chain compounds of the series (Pad-PC-Pad (1,3-dipalmitamidopropan-2-phosphocholine) and Sad-PC-Sad (1,3-distearamidopropan-2-phosphocholine) were able to form ordered monolayers. The chains are strongly tilted in a rigid lattice formed due to these hydrogen-bonding interactions between the headgroups. The thermodynamical analysis leads to a critical temperature of the monolayer which is clearly different from the main phase transition temperature in bulk, indicating that there must be a different structural arrangement of the 1,3-diamidophospholipids in monolayers and in bilayers

Polymer-capped magnetite nanoparticles change the 2D structure of DPPC model membranes

A panoply of new iron oxide nanoparticles (NPs) has been designed and synthesized in the last years as valuable medical nano-tools. Generally, their coating enables them to bind, adsorb, or carry compounds such as drugs, proteins, enzymes, or antibodies, which can be then directed to a special tissue or organ using an external magnetic field. Nevertheless, despite the important number of newly synthesized NPs and their multiple biomedical applications, the knowledge of their interaction with cells or model membranes is still scarce. In this physical-chemical study, we investigated the interaction of Fe 3 O 4 @MEO 2 MA 90 -co-OEGMA 10 NPs with monolayers of dipalmitoylphosphatidylcholine (DPPC). Lipid monolayers have been chosen due to their general acceptance as excellent versatile model systems, able to mimic the outer leaflet of a cellular membrane. As a prerequisite of understanding the interaction between the NPs and the model membrane, we previously studied and reported the interfacial properties of the NPs at the air/water interface Maximum insertion pressure experiments have been performed by injecting a defined amount of NPs underneath DPPC monolayers compressed to different initial lateral pressures. The surface area was kept constant and the increase in the surface pressure was monitored in time. The obtained equilibrium surface pressure of ~ 25 mN/m is independent of the initial surface pressure. This value corresponds to the equilibrium surface pressure of the NPs adsorbed at the bare air/water interface. The linear extrapolation gives a value of 25.5 mN/m. No change in the surface pressure was observed when the NPs were injected underneath DPPC monolayers compressed to surface pressures higher than 25 mN/m. These findings demonstrate that the insertion process of the NPs into the DPPC monolayer is controlled by the surface activity of the NPs, and therefore, as previously reported, by the surface activity of the copolymer. The structure of the LC phase of DPPC was investigated by GIXD at BW1. The results show clearly the existence of only one liquid-crystalline phase, with a changed structure, compared to pure DPPC. BAM experiments showed an enrichment of the polymer chains at the domain boundary. However, since only one homogeneous monolayer structure is observed, it is obvious that the NPs changed the DPPC structure in a highly cooperative way. The most unexpected result is that up to a lateral pressure of ~35 mN/m, which is higher than the MIP, the polymer capped NPs have a strong influence on the DPPC structure. The phase-separated parts of the mixed DPPC-NPs layer react differently on the layer compression. The soft polymer part can be easily compressed to a higher density, whereas the rigid DPPC-NPs part containing some polymer chains is almost incompressible

Cross-linking reactions in Langmuir monolayers of specially designed aminolipids – a toolbox for the customized production of amphiphilic nanosheets

Synthetic amino lipids, already known as highly efficient gene therapy tool, are used in a novel way to create cross-linked stable one-molecule-thin films envisioned for future (bio)-materials applications. The films are prepared as Langmuir monolayers at the air/water interface and cross-linked ‘in situ’ via dynamic imine chemistry. The cross-linking process and the film characteristics are monitored by various surface-sensitive techniques such as grazing incidence X-ray diffraction, X-ray reflectivity, and infrared reflection–absorption spectroscopy. After transfer onto carbon grids, the cross-linked films are investigated by transmission and scanning electron microscopy. The obtained micrographs display mechanically self-supported nanosheets with area dimensions over several micrometers and, thus, an undeniable visual proof of successful cross-linking. The cross-linking process at the air/water interface allows to obtain Janus-faced sheets with a hydrophobic side characterized by aliphatic alkyl chains and a hydrophilic side characterized by nucleophilic groups like amines, hydroxyl groups and imine