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 <i>in situ</i> 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

Interaction of DNA with Cationic Lipid MixturesInvestigation at Langmuir Lipid Monolayers

Four different binary lipid mixtures composed of a cationic lipid and the zwitterionic colipids DOPE or DPPC, which show different DNA transfer activities in cell culture models, were investigated at the soft air/water interface to identify transfection efficiency determining characteristics. Langmuir films are useful models to investigate the interaction between DNA and lipid mixtures in a two-dimensional model system by using different surface sensitive techniques, namely, epifluorescence microscopy and infrared reflection–absorption spectroscopy. Especially, the effect of adsorbed DNA on the properties of the lipid mixtures has been examined. Distinct differences between the lipid composites were found which are caused by the different colipids of the mixtures. DOPE containing lipid mixtures form fluid monolayers with a uniform distribution of the fluorescent probe in the presence and absence of DNA at physiologically relevant surface pressures. Only at high nonphysiological pressures, the lipid monolayer collapses and phase separation was observed if DNA was present in the subphase. In contrast, DPPC containing lipid mixtures show domains in the liquid condensed phase state in the presence and absence of DNA in the subphase. The adsorption of DNA at the positively charged mixed lipid monolayer induces phase separation which is expressed in the morphology and the point of appearance of these domains

The Complete Phase Diagram of Monolayers of Enantiomeric <i>N</i>‑Stearoyl-threonine Mixtures with Preferred Heterochiral Interactions

Langmuir monolayers of chiral amphiphiles are well-controlled model systems for the investigation of phenomena related to stereochemistry. Here, we have investigated mixed monolayers of one pair of enantiomers (l and d) of the amino-acid-based amphiphile N-stearoyl-threonine. The monolayer characteristics were studied by pressure–area isotherm measurements and grazing incidence X-ray diffraction (GIXD) over a wide range of mixing ratios defined by the d-enantiomer mole fraction xD. While the isotherms provide insights into thermodynamical aspects, such as transition pressure, compression/decompression hysteresis, and preferential homo- and heterochiral interactions, GIXD reveals the molecular structural arrangements on the Ångström scale. Dominant heterochiral interactions in the racemic mixture lead to compound formation and the appearance of a nonchiral rectangular lattice, although the pure enantiomers form a chiral oblique lattice. Miscibility was found to be limited to mixtures with 0.27 ≲ xD ≲ 0.73, as well as to both outer edges (xD ≲ 0.08 and xD ≳ 0.92). Beyond this range, coexistence of oblique and rectangular lattices occurs, as is clearly seen in the GIXD patterns. Based on the results, a complete phase diagram with two eutectic points at xD ≈ 0.25 and xD ≈ 0.75 is proposed. Moreover, N-stearoyl-threonine was found to have a strong tendency to form a hydrogen-bonding network between the headgroups, which promotes superlattice formation

CaCO₃ Mineralization under β-Sheet Forming Peptide Monolayers

In biominerals, proteins are key elements in the controlled nucleation and growth of the mineral phase. We report here on the coupled evolution of the organic and inorganic structures during the nucleation and growth of CaCO₃ under a monolayer of acidic β-sheet forming peptides that mimic the natural proteins found in nacre. The investigation is carried out using in situ analytical techniques (X-ray diffraction and IR spectroscopy) to provide molecular scale structural information over the whole course of the mineralization process. Mineralization is shown to coexist with β-sheet order while inducing other conformational changes to the peptide assembly. Peptides promote the growth of unoriented vaterite crystals; no templating effect of the β-sheet order is observed

Polyoxometalate Surfactants as Unique Molecules for Interfacial Self-Assembly

Whereas, commonly, Langmuir monolayers are structurally dominated by the aliphatic chains, we present here the first case of monolayers where the chains merely serve anchoring at the air/water interface and the organization is dictated by the hydrophilic head group self-assembling in a hexagonal lattice. These head groups are polyoxometalates known for their multifunctional potential. The chain length has been systematically varied, allowing for a general study of the impact of the chain length on the supramolecular structure. These model structures are studied here by a combination of modern techniques, the leading ones being X-ray reflectivity and grazing incidence X-ray diffraction. The quantitative structural insights offered in this Letter might represent a starting point for the rational design and study of a new class of emulsions, including an organic tail and a multifunctional inorganic polar head

Investigation of Binary Lipid Mixtures of a Three-Chain Cationic Lipid with Phospholipids Suitable for Gene Delivery

In the present work, we characterize binary lipid mixtures consisting of a three-chain amino-functionalized cationic lipid (DiTT4) with different phospholipids, namely, 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphoethanolamine (DOPE), 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphoethanolamine (DMPE), or 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine (DMPC). The mixing behavior was investigated by differential scanning calorimetry (DSC). Additionally, aqueous dispersions of the binary mixtures were characterized by means of dynamic light scattering (DLS), laser Doppler electrophoresis, and transmission electron microscopy (TEM) to get further information about particle size, charge, and shape. The complex formation between different binary lipid mixtures and plasmid DNA (pDNA) was investigated by zeta-(ζ)-potential (laser Doppler electrophoresis) and DLS measurements, and the lipid/DNA complexes (lipoplexes) were screened for efficient DNA transfer (transfection) in cell culture. Finally, efficient lipid compositions were investigated with respect to serum stability. This work provides a detailed characterization of the cationic lipid mixtures as foundation for further research. Efficient gene transfer in the presence of serum was demonstrated for selected lipoplexes showing their capability to be used as high-potency gene delivery vehicles

Versatility of a Glycosylphosphatidylinositol Fragment in Forming Highly Ordered Polymorphs

Glycosylphosphatidylinositols (GPIs) are often attributed with the ability to associate with the organized membrane microdomains. GPI fragment <b>1</b> forms a highly ordered subgel-phase structure characterized by ordering of both headgroups and alkyl chains in thin layers. While investigating the driving forces behind the formation of these ordered monolayers, we have studied polymorphism of <b>1</b> under different conditions employing surface-sensitive X-ray diffraction methods. Three distinct polymorphs of <b>1</b> (<b>I</b>, <b>II</b>, and <b>III</b>) were identified and characterized by grazing incidence X-ray diffraction. Polymorphs <b>II</b> (a condensed monolayer structure) and <b>III</b> (highly ordered subgel phase) coexist on an 8 M urea solution subphase allowing for a detailed thermodynamic and kinetic analysis of the processes leading to the formation of these polymorphs. They are enantiotropic and can be directly interconverted by changes in temperature or lateral surface pressure. As a consequence, polymorph <b>III</b> nuclei of critical size (or larger) could be formed by density fluctuations in a multicomponent system, and they could continue to exist for a period of time even under conditions that would normally not allow for the nucleation of polymorph <b>III</b>. The processes described here could also lead to the formation of patches of highly ordered structures in a disordered environment of a cell membrane suggesting that GPIs may play a role in the formation of such domains

Synthesis and Biophysical Characterization of an Odd-Numbered 1,3-Diamidophospholipid

Nanomedicine suffers from low drug delivery efficiencies. Mechanoresponsive vesicles could provide an alternative way to release active compounds triggered by the basic physics of the human body. 1,3-Diamidophospholipids with C16 tails proved to be an effective building block for mechanoresponsive vesicles, but their low main phase transition temperature prevents an effective application in humans. As the main phase transition temperature of a membrane depends on the fatty acyl chain length, we synthesized a C17 homologue of a 1,3-diamidophospholipid: Rad-PC-Rad. The elevated main phase transition temperature of Rad-PC-Rad allows mechanoresponsive drug delivery at body temperature. Herein, we report the biophysical properties of Rad-PC-Rad monolayer and bilayer membranes. Rad-PC-Rad is an ideal candidate for advancing the concept of physically triggered drug release

Langmuir Monolayers of an Inclusion Complex Formed by a New Calixarene Derivative and Fullerene

The design of new molecules with directed interactions to functional molecules as complementary building blocks is one of the main goals of supramolecular chemistry. A new <i>p-tert</i>-butylcalix­[6]­arene monosubstituted derivative bearing only one alkyl chain with an acid group (C6A3C) has been synthesized. The C6A3C has been successfully used for building Langmuir monolayers at the air–water interface. The C6A3C molecule adopts a flatlike orientation with respect to the air–water interface. The molecular structure gives the molecule amphiphilic character, while allowing the control of both the dissociation degree and the molecular conformation at the air–water interface. The C63AC has been combined with pristine fullerene (C60) to form the supramolecular complex C6A3C:C60 in 2:1 molar ratio (CFC). The CFC complex retains the ability of C6A3C to form Langmuir monolayers at the air/water interface. The interfacial molecular arrangement of the CFC complex has been convincingly described by in situ UV–vis reflection spectroscopy and synchrotron X-ray reflectivity measurements. Computer simulations complement the experimental data, confirming a perpendicular orientation of the calixarene units of CFC with respect to the air–water interface. This orientation is stabilized by the formation of intermolecular H-bonds. The interfacial monolayer of the CFC supramolecular complex is proposed as a useful model for the well-defined self-assembly of recognition and functional building blocks

Impact of Structural Differences in Galactocerebrosides on the Behavior of 2D Monolayers

The molecular interactions of three biologically important galactocerebrosides have been studied in monolayers formed at the soft air/water interface as 2D model membranes. Highly surface-sensitive techniques as GIXD (grazing incidence X-ray diffraction), IRRAS (infrared reflection–absorption spectroscopy), and BAM (Brewster angle microscopy) have been used. The study reveals that small differences in the chemical structure have a relevant impact on the physical–chemical properties and intermolecular interactions. The presence of a 2-d-hydroxyl group in the fatty acid favored for <b>GalCer C24:0 (2-OH)</b> monolayers a higher hydration state of the headgroup at low lateral pressures (<25 mN/m) and a higher condensation effect above 30 mN/m. An opposite behavior was recorded for <b>GalCer C24:0</b> and <b>GalCer C24:1</b>, for which the intermolecular interactions are defined by the weakly hydrated but strong H-bonded interconnected head groups. Additionally, the 15-<i>cis</i>-double bond in the fatty acid chain (nervonic acid) of <b>GalCer C24:1</b> stabilized the LE phase but did not disturb the packing parameters of the LC phase as compared with the saturated compound <b>GalCer C24:0</b>