Frank Bringezu

The physics and physiology of lung surfactants

Langmuir monolayers have provided experimentally accessible models for studies of lung surfactants at the air-alveolus interface since the medical necessity of lung surfactant was demonstrated by the pioneering work of Avery and Clements in the early 1960s. The fundamental goal of these in vitro studies is a molecular level understanding of the relationships between lung surfactant composition, monolayer morphology, and monolayer physical parameters such as minimum surface tension, spreading, viscosity, etc.

Binding of nonsteroidal anti-inflammatory drugs to DPPC: Structure and thermodynamic aspects

The effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on the phase transition and phase properties of 1,2-dipalmitoylphosphatidylcholine (DPPC) has been investigated in both 2D (monolayers at the air/water interface) and 3D (multilayers in lipid/water dispersions) model systems. The 2D membrane models have been characterized by means of pressure-area isotherms and grazing incidence X-ray diffraction (GIXD) measurements. Differential scanning calorimetry (DSC) and simultaneous small- and wide-angle X-ray diffraction have been applied to lipid aqueous dispersions. All NSAIDs studied altered the main transition temperature of the gel to liquid-crystalline phase transition, with the arylacetic acid derivatives (acemetacin and indomethacin) showing the largest effects. A comparison of the results reveals distinct structural features of the membrane models after interaction with the NSAID. All drugs induced perturbations in the lipid liquid-crystalline phase, suggesting a major change in the hydration behavior of DPPC. Again, the largest effects on the structural parameters are found for the arylacetic acid derivatives. The results obtained in the different model systems give indications of the role of the membrane/NSAID interactions that might also be important for NSAID gastric injury.

Chiral discrimination in a monolayer of a triple-chain phosphatidylcholine.

Monolayers of an enantiomeric and a racemic triple-chain phosphatidylcholine (PC) at the air/water interface are studied by film balance measurements and x-ray diffraction. Although the area per three tails exceeds that per head, we observe tail ordering dependent on headgroup chirality and chain tilt. This indicates lateral headgroup interactions. The influence of the chiral carbon is suppressed at higher lateral pressures, and a centered-rectangular unit cell with tails tilted into the nearest neighbor (NN) direction is observed for both the enantiomer and the racemate. The distortion of the lattice changes at medium pressures from NN to NNN (next-nearest neighbor direction) with decreasing temperature. The phase behavior of the racemate at 15 degrees C is compared with that of a triple-chain PC with a branched chain of reduced length. Whereas the PC with the longer branched chain exhibits only a NN tilted phase at all pressures, the PC with the shorter branched chain has a rich polymorphism (NNN-NN-upright hexagonal packing) under increased lateral pressure.

Generic phase behavior of branched-chain phospholipid monolayers

Monolayers of chemically modified triple-chain phospholipids have been investigated at the air/water interface using pressure-area isotherms. The condensed phases of the lipids were characterized by grazing incidence X-ray diffraction (GIXD). Increasing chain length corresponds to a temperature effect, which was quantified for different lipids, depending on the head group structure using isotherm (two-dimensional systems) and DSC (three-dimensional systems) measurements. The combination of structure investigations revealed generic phase diagrams, which describe the phase behavior of multiple-chain lipids in two dimensions. For the 1-acyl-2-O-alkyl phospholipids, the generic phase diagram exhibits only L(2d), LS and LE phases while the exchange of the position of the branched acyl and the non-branched alkyl chains at the glycerol backbone leads to a much richer polymorphism (L(2h), L(2d), Ov, LS, S, tau, LE). Here we present the first experimental evidence of the unusual tau phase for multiple-chain lipid monolayers. This phase exhibits an undistorted in-plane lattice despite of tilted chains.

Penetration of the antimicrobial peptide dicynthaurin into phospholipid monolayers at the liquid-air interface

This work focuses on the adsorption kinetics of dicynthaurin with lipid monolayers, the effect of peptide adsorption on the structure of the lipid condensed chain lattice, peptide orientation and secondary structure in the adsorbed state. The studies with DPPG as model system revealed strong adsorption and massive incorporation of the peptide into the monolayer. Infrared reflection absorption spectroscopy (IRRAS) experiments showed that the secondary structure of the peptide is maintained upon adsorption. Specular X‐ray reflectivity showed the destabilization of the condensed phase of the pure lipid monolayer and revealed a tilted orientation of the long axis of the peptide helix of about 40° from the surface normal. Incorporation of the peptide was found to be pressure dependent, and at high pressure a “squeeze‐out” was observed; however, the peptide remained localized to the interface, as suggested by infrared data. These findings were supported by optical fluorescence microscopy measurements which showed the squeeze‐out of the peptide on water, but not under physiological conditions. The results suggest that dicynthaurin is able to adsorb to the phosphatidylglycerol‐rich inner cytoplasmic membrane of bacteria and alter membrane integrity. To identify and interact with membrane motifs that are characteristic of microbes, but which are absent in eukaryotic cells, might be an intrinsic ability of peptide antibiotics.

INFLUENCE OF SIDE-CHAIN LENGTH ON PHOSPHOLIPID ORDERING IN TWO DIMENSIONS

Abstract Phosphoethanolamines (PE) with a 2-branched hexadecanoyl chain (ethyl, butyl and tetradecyl) at the sn-1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn-2 position have been synthesised. They form stable monolayers at the air/water interface. The thermodynamic properties are characterised using the classical film balance technique. The structure of the monolayer phase has been characterised by grazing incidence X-ray diffraction. The introduction of an ethyl side-chain changes the tilt direction in a rectangular unit cell from nearest neighbour (NN) to next-nearest neighbour (NNN). The elongation of the side-chain to butyl disturbs the monolayer packing and a phase with an undefined tilt direction is formed. At low lateral pressures the triple-chain PE forms slightly tilted phases. At higher pressures a hexagonal packing of upright oriented chains can be observed. Initial X-ray experiments of aqueous dispersions of the triple-chain PE point to an interdigitation of head groups from adjacent bilayers.

Ordering in Langmuir monolayers of branched chain phospholipids

Abstract The ordering of branched chain glycerophosphoethanolamines (PE) with different side chain length ( n =1, 2, 5 and 14) is systematically studied and compared with that of the corresponding double chain dipalmitoyl-phosphatidylethanolamine (DPPE) monolayers. The PEs have a 2-branched (methyl, ethyl, pentyl and tetradecyl) hexadecanoyl chain at the sn -1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn -2 position. Thermodynamic data obtained by π – A isotherm measurements at different temperatures are coupled with two-dimensional (2D) texture and 2D structure information obtained by Brewster angle microscopy (BAM) and X-ray diffraction at grazing incidence (GIXD), respectively. With increasing side chain length up to n =5, the ordering of the long alkyl chains is increasingly disturbed so that n -16PE monolayers are highly disordered and cannot form a condensed phase. Already the ethyl side chain reduces the ordering of 2-16PE monolayer so strongly that not only the polar tilt of the alkyl chains increases, but also in a stretched lattice a varying tilt azimuth of the alkyl chains is indicated. A side chain elongation nearly to the length of the two main chains enhances the ordering to that of a triple chain phospholipid with the alkyl chains oriented perpendicularly in a hexagonal lattice. Texture and shape of the dendritic or fractal-like condensed phase domains are similar to that known from DPPE monolayers. However, the individual differences in the inner textures found for the different side chain lengths, corroborate the GIXD results. At short side chains, domain regions of different tilt direction occur, whereas at long side chain the domains show no differences in the inner texture.

Phase behavior of methyl branched 1-acyl-2-alkyl-glycerophosphoethanolamines.

Abstract Phase transitions both in mono- and multilayers have been studied for homologous methyl-branched chain 1-acyl-2-alkyl-phosphatidylethanolamines (PE) differing in acyl chain lengths. In bulk systems two phase transitions have been observed. While with increasing chain length the main transition temperature increases, the second transition observed at higher temperatures shifts to lower values. In monolayers at the air–water interface both T 0 and T c increase with increasing chain length. These characteristic temperatures were obtained from the linear extrapolation of transition pressure ( π c ) and area involved in the transition as function of temperature towards zero values, respectively. The structures of condensed monolayers have been characterized using grazing incidence X-ray diffraction (GIXD) measurements. The introduction of a methyl-branching in double chain PEs drastically increases the tilt angles of the hydrocarbon chains. The transition towards a hexagonal untilted phase is completely suppressed. Compared with the symmetrical non-branched dipalmitoylglycerophosphoethanolamine (DPPE), 1(2C 1 -18:0)-2H-PE shows a change in the tilt direction towards next nearest neighbor (NNN) and 1(2C 1 -15:0)-2H-PE displays a superposition of multiple lattices of constant tilt but different tilt azimuths.

Phase behaviour of a methyl branched phosphatidylethanolamine in two and three-dimensional systems

Abstract The influence of a methyl branching on the phase behaviour of a glycerophosphoethanolamine (PE) both in monolayers at the air-water interface and in multilayers in the lipid-water dispersions has been studied using grazing incidence X-ray diffraction (GIXD) and simultaneous small and wide-angle X-ray diffraction measurements, respectively. The methyl branch in 1-(α-methylstearoyl)-2-O-hexadecyl-PE ( 1 ) induces in centred rectangular phases of monolayers a change in the tilt direction towards NNN and suppresses the transition to a hexagonal phase with upright oriented chains, known from unbranched DPPE. In the lipid/water dispersions a phase sequence Lβ′→Lα→HII was observed with increasing temperature. The introduction of the methyl branch results in the stabilisation of non-lamellar phases. Our investigations have shown that slight chemical changes in the hydrophobic region of PEs affect the phase behaviour drastically both in mono- and multilayers.