In-Plane Structures of Synthetic Oligolactose Lipid Monolayers-Impact of Saccharide Chain Length

Oxidation of lipid membrane cholesterol by cholesterol oxidase and its effects on raft model membrane structure

The effects of a peripheral protein - cholesterol oxidase (3β-hydroxysteroid oxidase, ChOx) on the characteristics of model lipid membranes composed of cholesterol, cholesterol:sphingomyelin (1:1), and the raft model composed of DOPC:Chol:SM (1:1:1) were investigated using two membrane model systems: the flat monolayer prepared by the Langmuir technique and the curved model consisting of liposome of the same lipids. The planar monolayers and liposomes were employed to follow membrane cholesterol oxidation to cholestenone catalyzed by ChOx and changes in the lipid membrane structure accompanying this reaction. Changes in the structure of liposomes in the presence of the enzyme were reflected in the changes of hydrodynamic diameter and fluorescence microscopy images, while changes of surface properties of planar membranes were evaluated by grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy. UV-Vis absorbance measurements confirmed the activity of the enzyme in the tested systems. A better understanding of the interactions between the enzyme and the cell membrane may help in finding alternative ways to decrease excessive cholesterol levels than the common approach of treating hypercholesterolemia with statins, which are not free from undesirable side effects, repeatedly reported in the literature and observed by the patients.

Statin Action Targets Lipid Rafts of Cell Membranes: GIXD/PM-IRRAS Investigation of Langmuir Monolayers

Lipid rafts are condensed regions of cell membranes rich in cholesterol and sphingomyelin, which constitute the target for anticholesterolemic drugs - statins. In this work, we use for the first time a combined grazing-incidence X-ray diffraction (GIXD)/polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS)/Brewster angle microscopy (BAM) approach to show the statin effect on model lipid rafts and its components assembled in Langmuir monolayers at the air-water interface. Two representatives of these drugs, fluvastatin (FLU) and cerivastatin (CER), of different hydrophobicity were chosen, while cholesterol (Chol) and sphingomyelin (SM), and their 1:1 mixture were selected to form condensed monolayers of lipid rafts. The effect of statins on the single components of lipid rafts indicated that both the hydrophobicity of the drugs and the organization of the layer determined the drug-lipid interaction. For cholesterol monolayers, only the most hydrophobic CER was effectively changing the film structure, while for the less organized sphingomyelin, the biggest effect was observed for FLU. This drug affected both the polar headgroup region as shown by PM-IRRAS results and the 2D crystalline structure of the SM monolayer as evidenced by GIXD. Measurements performed for Chol/SM 1:1 models proved also that the statin effect depends on the presence of Chol-SM complexes. In this case, the less hydrophobic FLU was not able to penetrate the binary layer at all, while exposure to the hydrophobic CER resulted in the phase separation and formation of ordered assemblies. The changes in the membrane properties were visualized by BAM images and GIXD patterns and confirmed by thermodynamic parameters of hysteresis in the Langmuir monolayer compression-decompression experiments.

Interplay of Trans- and Cis-Interactions of Glycolipids in Membrane Adhesion

Glycolipids mediate stable membrane adhesion of potential biological relevance. In this article, we investigate the trans- and cis-interactions of glycolipids in molecular dynamics simulations and relate these interactions to the glycolipid-induced average separations of membranes obtained from neutron scattering experiments. We find that the cis-interactions between glycolipids in the same membrane leaflet tend to strengthen the trans-interactions between glycolipids in apposing leaflets. The trans-interactions of the glycolipids in our simulations require local membrane separations that are significantly smaller than the average membrane separations in the neutron scattering experiments, which indicates an important role of membrane shape fluctuations in glycolipid trans-binding. Simulations at the experimentally measured average membrane separations provide a molecular picture of the interplay between glycolipid attraction and steric repulsion of the fluctuating membranes probed in the experiments.

Structural Changes in Films of Pulmonary Surfactant Induced by Surfactant Vesicles

When compressed by the shrinking alveolar surface area during exhalation, films of pulmonary surfactant in situ reduce surface tension to levels at which surfactant monolayers collapse from the surface in vitro. Vesicles of pulmonary surfactant added below these monolayers slow collapse. X-ray scattering here determined the structural changes induced by the added vesicles. Grazing incidence X-ray diffraction on monolayers of extracted calf surfactant detected an ordered phase. Mixtures of dipalmitoyl phosphatidylcholine and cholesterol, but not the phospholipid alone, mimic that structure. At concentrations that stabilize the monolayers, vesicles in the subphase had no effect on the unit cell, and X-ray reflection showed that the film remained monomolecular. The added vesicles, however, produced a concentration-dependent increase in the diffracted intensity. These results suggest that the enhanced resistance to collapse results from enlargement by the additional material of the ordered phase.

Headgroup-Ordered Monolayers of Uncharged Glycolipids Exhibit Selective Interactions with Ions

Selective interactions of ions with charge-neutral saccharides can have far-reaching consequences in biological and wet-technological contexts but have so far been observed only indirectly. Here, we directly quantify by total-reflection X-ray fluorescence the preferential accumulation of ions near uncharged saccharide surfaces in the form of glycolipid Langmuir monolayers at air/water interfaces exhibiting different levels of structural ordering. Selective interactions with ions from the aqueous subphase are observed for monolayers featuring crystalline ordering of the saccharide headgroups, as determined by grazing-incidence X-ray diffraction. The attracted ion species depend on the structural motifs displayed by the ordered saccharide layer. Our results may constitute a basis to understand the salt-specific swelling of wood materials and various phenomena in membrane biophysics.

Bacterial lipopolysaccharides form physically cross-linked, two-dimensional gels in the presence of divalent cations

We established a bacterial membrane model with monolayers of bacterial lipopolysaccharides (LPS Re and LPS Ra) and quantified their viscoelastic properties by using an interfacial stress rheometer coupled to a Langmuir film balance. LPS Re monolayers exhibited purely viscous behaviour in the absence of calcium ions, while the same monolayers underwent a viscous-to-elastic transition upon compression in the presence of Ca(2+). Our results demonstrated for the first time that LPSs in bacterial outer membranes can form two-dimensional elastic networks in the presence of Ca(2+). Different from LPS Re monolayers, the LPS Ra monolayers showed a very similar rheological transition both in the presence and absence of Ca(2+), suggesting that longer saccharide chains can form 2D physical gels even in the absence of Ca(2+). By exposure of the monolayers to the antimicrobial peptide protamine, we could directly monitor the differences in resistance of bacterial membranes according to the presence of calcium.

Grazing incidence X-ray diffraction studies of condensed double-chain phospholipid monolayers formed at the soft air/water interface

The use of highly brilliant synchrotron light sources in the middle of the 1980s for X-ray diffraction has revolutionized the research of condensed monolayers. Since then, monolayers gained popularity as convenient quasi two-dimensional model systems widely used in biophysics and material science. This review focuses on structures observed in one-component phospholipid monolayers used as simplified two-dimensional models of biological membranes. In a monolayer system the phase transitions can be easily triggered at constant temperature by increasing the packing density of the lipids by compression. Simultaneously the monolayer structure changes are followed in situ by grazing incidence X-ray diffraction. Competing interactions between the different parts of the molecule are responsible for the different monolayer structures. These forces can be modified by chemical variations of the hydrophobic chain region, of the hydrophilic head group region or of the interfacial region between chains and head groups. Modifications of monolayer structures triggered by changes of the chemical structure of double-chain phospholipids are highlighted in this paper.

Synthesis of Bioinspired Carbohydrate Amphiphiles that Promote and Inhibit Biofilms

The synthesis and characterization of a new class of bioinspired carbohydrate amphiphiles that modulate Pseudomonas aeruginosa biofilm formation are reported. The carbohydrate head is an enantiopure poly-amido-saccharide (PAS) prepared by a controlled anionic polymerization of β-lactam monomers derived from either glucose or galactose. The supramolecular assemblies formed by PAS amphiphiles are investigated in solution using fluorescence assays and dynamic light scattering. Dried samples are investigated using X-ray, infrared spectroscopy, and transmission electron microscopy. Additionally, the amphiphiles are evaluated for their ability to modulate biofilm formation by the Gram-negative bacterium Pseudomonas aeruginosa. Remarkably, from a library of eight amphiphiles, we identify a structure that promotes biofilm formation and two structures that inhibit biofilm formation. Using biological assays and electron microscopy, we relate the chemical structure of the amphiphiles to the observed activity. Materials that modulate the formation of biofilms by bacteria are important both as research tools for microbiologists to study the process of biofilm formation and for their potential to provide new drug candidates for treating biofilm-associated infections.

Molecular organization of bacterial membrane lipids in mixed systems--A comprehensive monolayer study combined with Grazing Incidence X-ray Diffraction and Brewster Angle Microscopy experiments

To properly design and investigate new antibacterial drugs a detailed description of the organization of bacterial membrane is highly important. Therefore in this work we performed a comprehensive characteristic of the Langmuir monolayers composed of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) mixed in a wide range of composition and treated as an artificial cytoplasmic layer of bacterial membrane. To obtain detailed information on the properties of these films we combined the analysis of the surface pressure-area curves with the surface potential measurements, Brewster Angle Microscopy studies and Grazing Incidence X-ray Diffraction experiments. It was found that the investigated phospholipids mix nonideally in the monolayers and that the most favorable packing of molecules occurs at their equimolar proportion. This is directly connected with the formation of hydrogen bonds between both types of molecules in the system. All the collected experimental data evidenced that dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidylglycerol (DPPG) form highly ordered associates of fixed (DPPE:DPPG 1:1) stoichiometry. The obtained results allow one to conclude a nonuniform distribution of lipids in bacterial membranes and the existence of domains composed of the investigated phospholipids. The latter seems to be of great importance in the perspective of further studies on the mechanism of action of antibacterial agents.

Properties of β-sitostanol/DPPC monolayers studied with Grazing Incidence X-ray Diffraction (GIXD) and Brewster Angle Microscopy

Although the influence of structurally modified sterols on artificial membranes has been intensively investigated, studies on the properties of stanols, which are saturated analogs of sterols, are very rare. Therefore, we have performed Grazing Incidence X-ray Diffraction (GIXD) experiments aimed at studying in-plane organization of a plant stanol-β-sitostanol monolayer and its mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine - DPPC at the air/water interface. The collected GIXD data, resulting in-plane parameters and BAM images provide information on molecular organization and in-plane ordering of the investigated films. It was found that the lateral organization of β-sitostanol/DPPC monolayers depends on their composition. The oblique structure of the in-plane lattice of tilted hydrophobic region of molecules, found for DPPC film, is maintained at 10 mol% of stanol in the system. However, at 30 and 90 mol% of stanol in the mixture, the arrangement of molecules is hexagonal and they are oriented perpendicularly to the interface. With the addition of stanol the extend of the in-plane order of the monolayers decreases. Moreover, in mixtures the ordered domains consist of both monolayer's components. Additionally, β-sitostanol film is of similar in-plane organization as the corresponding sterol monolayer (β-sitosterol) and stanol induces condensing effect on DPPC.

Enantioselective recognition between polydiacetylene nucleolipid monolayers and complementary oligonucleotides

A two-dimensional bio/synthetic hybrid system at the air-solution interface made of a polymerized diacetylene Langmuir film with nucleobase modified headgroups is presented. The polymerized film presents a crystalline array of nucleobases, capable of specific binding of complementary mononucleoside or oligonucleotide sequences. Mixed monolayers of the linear polyconjugated polydiacetylene (PDA) films derivatized with cytosine (10,12-pentacosadiyne-cytidyl, PDC) monomers and alcohol-terminated diacetylene lipid (10,12-pentacosadiynol, PDOH) at a 3:1 ratio (PDC 75%) were compressed and polymerized at the air-water interface with circular polarized light (CPL) or nonpolarized UV light. Here we report a grazing incidence X-ray diffraction (GIXD) investigation of PDC films polymerized to different chirality and hybridized with complementary ssDNA strands. We have demonstrated enantioselective interactions on synthetic structured interfaces produced by Langmuir surface compression followed by polymerization with circular polarized UV light (CPL). The left- and right-CPL polymerized light exhibit the same well-defined crystalline structure. The observed difference between left- and right-CPL polymerized PDC 75% Langmuir films compressed over the complementary mononucleotide guanosine or hybridized with fully complementary ssG(12)T(5) oligonucleotide in the subphase suggests that they are indeed enantiomeric structures, capable of enantioselective binding of their natural ligand, guanosine, solely as a result of surface induced asymmetry in "left" but not in "right" form. This observation may also be related to the intriguing question of chiral selection during the early period of "Origin of Life". We show that achiral compounds, as a result of irradiation with circular polarized light, can organize in chiral surface structures capable of amplification of biopolymer binding of particular handedness.

Modulation of intermembrane interaction and bending rigidity of biomembrane models via carbohydrates investigated by specular and off-specular neutron scattering

We designed artificial models of biological membranes by deposition of synthetic glycolipid membrane multilayers on planar silicon substrates. In contrast to commonly used phospholipid membranes, this offers the unique possibility to study the influence of membrane-bound saccharide chains (cell glycocalix) on the membrane mechanics. Taking advantage of the planar sample geometry, we carried out specular and off-specular neutron scattering experiments to identify out-of-plane and in-plane scattering vector components. By considering the effects of finite sample sizes, we were able to simulate the measured two-dimensional reciprocal space maps within the framework of smectic liquid-crystal theory. The results obtained both at controlled humidity and in bulk water clearly indicate that a subtle change in the molecular chemistry of the saccharides strongly influences intermembrane interactions and membrane bending rigidities.

Phospholipase D activity is regulated by product segregation and the structure formation of phosphatidic acid within model membranes

Phospholipase D from Streptomyces chromofuscus (scPLD) hydrolyzes phosphatidylcholines (PC) to produce choline and phosphatidic acid (PA), a lipid messenger molecule within biological membranes. To scrutinize the influence of membrane structure on scPLD activity, three different substrate-containing monolayers are used as model systems: pure dipalmitoylphosphatidylcholine (DPPC) as well as equimolar mixtures of DPPC/n-hexadecanol (C(16)OH) and DPPC/dipalmitoylglycerol (DPG). The activity of scPLD toward these monolayers is tested by infrared reflection-absorption spectroscopy and exhibits different dependencies on surface pressure. For pure DPPC, the catalytic turnover drastically drops above 20 mN/m. On addition of C(16)OH, this strong decrease starts at 5 mN/m. For the DPPC/DPG system, the reaction yield linearly decreases between 5 and 25 mN/m. The difference in scPLD activity is correlated to the phase state of the monolayers as examined by x-ray diffraction, Brewster angle microscopy, and atomic force microscopy. Because the additives C(16)OH and DPG mediate the miscibility of PC and PA, only a basal activity of scPLD is observed toward the mixed systems at higher surface pressures. At pure DPPC monolayers, scPLD is activated after the segregation of initially formed PA. Furthermore, scPLD is inhibited when the lipids in the PA-rich domains adopt an upright orientation. This phenomenon offers a self-regulating mechanism for the concentration of the second messenger PA within biological membranes.

Modifying dipalmitoylphosphatidylcholine monolayers by n-hexadecanol and dipalmitoylglycerol

The monolayer structure of pure dipalmitoylphosphatidylcholine (DPPC) and equimolar mixtures of DPPC/n-hexadecanol (C(16)OH) and DPPC/dipalmitoylglycerol (DPG) are studied by the film balance technique and grazing incidence X-ray diffraction measurements. At 20 degrees C, the binary systems exhibit complete miscibility. In contrast to pure DPPC monolayers, a condensing effect is observed in the presence of both non-phospholipid additives; but the phase transition behavior differs. The tilt angle of the hydrocarbon chains in the DPPC/C(16)OH mixture is significantly smaller than in pure DPPC monolayers. The tilt of the chains is even further reduced in the mixed monolayer of DPPC/DPG. A comparison of the three systems reveals distinct structural features such as phase state, chain tilt, and molecular area over a wide range of surface pressures. Therefore, these monolayers provide a highly suitable model to investigate the influence of structural parameters on biological processes occurring at the membrane surface, e.g. enzymatic reactions and adsorption events.