Effect of the Charged Lipid DMPG on the Thickness and Contact Angle of Foam Films

Interactions in lipid stabilised foam films

The interaction between lipid bilayers in water has been intensively studied over the last decades. Osmotic stress was applied to evaluate the forces between two approaching lipid bilayers in aqueous solution. The force-distance relation between lipid mono- or bilayers deposited on mica sheets using a surface force apparatus (SFA) was also measured. Lipid stabilised foam films offer another possibility to study the interactions between lipid monolayers. These films can be prepared comparatively easy with very good reproducibility. Foam films consist usually of two adsorbed surfactant monolayers separated by a layer of the aqueous solution from which the film is created. Their thickness can be conveniently measured using microinterferometric techniques. Studies with foam films deliver valuable information on the interactions between lipid membranes and especially their stability and permeability. Presenting inverse black lipid membrane (BLM) foam films supply information about the properties of the lipid self-organisation in bilayers. The present paper summarises results on microscopic lipid stabilised foam films by measuring their thickness and contact angle. Most of the presented results concern foam films prepared from dispersions of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) and some of its mixtures with the anionic lipid -- 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG). The strength of the long range and short range forces between the lipid layers is discussed. The van der Waals attractive force is calculated. The electrostatic repulsive force is estimated from experiments at different electrolyte concentrations (NaCl, CaCl₂) or by modification of the electrostatic double layer surface potential by incorporating charged lipids in the lipid monolayers. The short range interactions are studied and modified by using small carbohydrates (fructose and sucrose), ethanol (EtOH) or dimethylsulfoxide (DMSO). Some results are compared with the structure of lipid monolayers deposited at the liquid/air interface (monolayers spread in Langmuir trough), which are one of most studied biomembrane model system. The comparison between the film thickness and the free energy of film formation is used to estimate the contribution of the different components of the disjoining pressure to the total interaction in the film and their dependence on the composition of the film forming solution.

Headgroup-dependent lipid self-assembly on zirconium phosphate-terminated interfaces

We report on the self-assembly of selected phospholipids on a Zr phosphate-terminated thiol self-assembled monolayer (SAM) formed on a planar Au surface. The gold substrates were first reacted with 6-mercapto-1-hexanol and then treated with POCl(3) and ZrOCl(2)(aq) prior to exposure to phospholipids. The phospholipids used for adlayer formation were 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA), 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), 1,2-dimyristoyl-sn-glycero-3-phosphatidylethanolamine (DMPE), 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG), and 1,2-dimyristoyl-sn-glycero-3-[phospho-L-serine] (DMPS), and deposition was accomplished through vesicle fusion. The resulting interfaces were characterized using optical ellipsometry and water contact angle measurements, and cyclic voltammetry was used to interrogate the quality of the phospholipid adlayers. Our data indicate that the strongest lipid-interface interaction is with DMPA, whereas DMPC produces a slightly less organized adlayer. Phospholipids DMPE, DMPG, and DMPS were all found to interact relatively weakly with the zirconated interface, and we understand these results in the context of steric and hydrogen bonding effects in the adlayer that are dominated by the phospholipid headgroup.

Ionic binding of phospholipids to interfaces: dependence on metal ion identity

We report on the deposition of 1,2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA) on selected metal-phosphate-terminated self-assembled monolayers (SAMs) constructed on Au. The phosphate-terminated SAMs were reacted with Zr(4+), Cu(+), Cu(2+), Fe(3+), Zn(2+), Ni(2+), Ca(2+), and Mg(2+), with subsequent exposure of the resulting interface to DMPA unilamellar vesicles. The resulting interface was characterized using X-ray photoelectron spectroscopy (XPS), optical ellipsometry, water contact angle measurements, and cyclic voltammetry (CV). The strongest lipid-metal ion interfacial interactions are with Zr(4+) and Fe(3+), with Ca(2+), Cu(+), Ni(2+), Zn(2+), and Mg(2+) producing somewhat less well organized adlayers. Cu(2+) did not bind strongly to the interfacial phosphate moiety, yielding a lipid bilayer structure. These results can be understood in the context of the strength of the metal bisphosphate complex that forms between the phospholipid and the chemically modified interface.

Formation of air-stable supported lipid monolayers and bilayers

We have devised a means of depositing planar, air-stable supported lipid adlayers on modified Au substrates. Using the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), we form planar supported adlayer structures by vesicle fusion. Lipid bilayer formation proceeds on a hydroxythiol-terminated Au surface. Phospholipid monolayers form on hydroxythiol-terminated gold surfaces that have been treated with POCl3 and ZrOCl2(aq) prior to lipid deposition, providing an interface that interacts strongly with the DMPC phosphocholine headgroup. We use FTIR, cyclic voltammetry, optical ellipsometry, and water contact angle measurements to confirm the presence of lipid bilayers or monolayers on the modified Au substrates. For the zirconated surface, we observe the conversion of an initial partial lipid bilayer to a lipid monolayer, over a ca. 20 min time period, on the basis on ellipsometric thickness and contact angle data. 31P NMR measurements show the complexation of the phospholipid headgroup to a Zr-phosphate surface.

Dried foam films with a triple bilayer structure induced by ionic liquids

Free-standing films with a triple bilayer structure were formed by drying micrometre-scale foam films of zwitterionic surfactants prepared from the aqueous solutions containing a certain concentration range of ionic liquids.

Structure, surface interactions, and compressibility of bacterial S-layers through scanning force microscopy and the surface force apparatus

Two-dimensional crystalline bacterial surface layers (S-layers) are found in a broad range of bacteria and archaea as the outermost cell envelope component. The self-assembling properties of the S-layers permit them to recrystallize on solid substrates. Beyond their biological interest as S-layers, they are currently used in nanotechnology to build supramolecular structures. Here, the structure of S-layers and the interactions between them are studied through surface force techniques. Scanning force microscopy has been used to study the structure of recrystallized S-layers from Bacillus sphaericus on mica at different 1:1 electrolyte concentrations. They give evidence of the two-dimensional organization of the proteins and reveal small corrugations of the S-layers formed on mica. The lattice parameters of the S-layers were a=b=14 nm, gamma=90 degrees and did not depend on the electrolyte concentration. The interaction forces between recrystallized S-layers on mica were studied with the surface force apparatus as a function of electrolyte concentration. Force measurements show that electrostatic and steric interactions are dominant at long distances. When the S-layers are compressed they exhibit elastic behavior. No adhesion between recrystallized layers takes place. We report for the first time, to our knowledge, the value of the compressibility modulus of the S-layer (0.6 MPa). The compressibility modulus is independent on the electrolyte concentration, although loads of 20 mN m-1 damage the layer locally. Control experiments with denatured S-proteins show similar elastic properties under compression but they exhibit adhesion forces between proteins, which were not observed in recrystallized S-layers.

Spectroscopic Study of Lysopalmitoylphosphatidylcholine Newton Black Films

Lysopalmitoylphosphatidylcholine (LPC) black films have been studied by confocal Raman spectroscopy and their spectra analyzed and compared to their counterparts obtained from LPC in the solid state and aqueous solution. It appears that LPC is able to form stable and highly ordered black films, despite the presence of only one hydrophobic chain in this molecule. A complementary infrared study of LPC Gibbs monolayers suggests that the whole LPC polar head is perpendicular to the air/water interface. Such an orientation could explain the high order and the close packing observed in black films.

Phospholipid black foam films: dynamic contact angles and gas permeability of DMPC+DMPG black films

The behavior of black foam films from aqueous dispersions of dimyristoylphosphatidyl-choline (DMPC) with addition of the soluble phospholipid dimyristoylphosphatidylglycerol (DMPG) has been studied in dynamic conditions. The dynamic contact angles theta and the gas permeability coefficient K have been measured using the diminishing bubble method. The DMPC vesicle suspension in water is obtained through sonication and DMPG is dissolved in it. Two solutions with different NaCl concentrations (0.1 M and 0.5 M) have been studied. The behavior of the dynamic contact angles is very different for DMPC black films with, and without DMPG, respectively. They follow very different time dependence during spontaneous or forced variations of the bubble size. The gas permeability coefficient is significantly reduced by the DMPG addition. The NaCl concentration also influences this specific behavior. It seems that the electrically charged DMPG anions, which determine a significant electrostatic disjoining pressure, play an important role for this specific behavior. The results are discussed in connection with data regarding the thickness and structure of these black foam films. Films from DMPC+DMPG solutions in ethanol plus water mixed solvent have been studied as well, but no quantitative results could be obtained.