Dipalmitoylphosphatidylethanolamine Monolayers Research Articles

Doxorubicin Interaction with Lipid Monolayers Leads to Decreased Membrane Stiffness when Experiencing Compression-Expansion Dynamics.

Physical membrane models permit to study and quantify the interactions of many external molecules with monitored and simplified systems. In this work, we have constructed artificial Langmuir single-lipid monolayers with dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidylserine (DPPS), or sphingomyelin to resemble the main lipid components of the mammalian cell membranes. We determined the collapse pressure, minimum area per molecule, and maximum compression modulus (Cs-1) from surface pressure measurements in a Langmuir trough. Also, from compression/expansion isotherms, we estimated the viscoelastic properties of the monolayers. With this model, we explored the membrane molecular mechanism of toxicity of the well-known anticancer drug doxorubicin, with particular emphasis in cardiotoxicity. The results showed that doxorubicin intercalates mainly between DPPS and sphingomyelin, and less between DPPE, inducing a change in the Cs-1 of up to 34% for DPPS. The isotherm experiments suggested that doxorubicin had little effect on DPPC, partially solubilized DPPS lipids toward the bulk of the subphase, and caused a slight or large expansion in the DPPE and sphingomyelin monolayers, respectively. Furthermore, the dynamic viscoelasticity of the DPPE and DPPS membranes was greatly reduced (by 43 and 23%, respectively), while the reduction amounted only to 12% for sphingomyelin and DPPC models. In conclusion, doxorubicin intercalates into the DPPS, DPPE, and sphingomyelin, but not into the DPPC, membrane lipids, inducing a structural distortion that leads to decreased membrane stiffness and reduced compressibility modulus. These alterations may constitute a novel, early step in explaining the doxorubicin mechanism of action in mammalian cancer cells or its toxicity in non-cancer cells, with relevance to explain its cardiotoxicity.

Ethanolamine phospholipids at the air-water interface as cell membranes models of microorganisms to study the nanotoxicology of sakuranetin

Sakuranetin, a natural flavonoid isolated as the main compound from roots of the Brazilian plant Baccharis retusa (Asteraceae), was incorporated in lipid membranes as Langmuir monolayers. For that, the mammalian lipid dipalmitoyl phosphatidylethanolamine (DPPE) was chosen. Sakuranetin condensed the monolayers at high surface pressures, decreased the surface compression modulus and reduced the molecular order of the acyl chains (reduction of all-trans/gauche conformers ratio), and increased the heterogeneity of the interface, forming aggregates. These combined results lead to the conclusion that this lipophilic compound may interact with the lipidic layers, preferentially at the lipid-air interface, to minimize the free energy, and reaches such conformation disturbing the thermodynamic, structural, mechanical, rheological, and morphological properties of the well-packed DPPE monolayer.

Effect of partial O-methylation in dehydrodieugenol on its antitrypanosomal activity - correlation with the toxicity using cell membrane models

Biseugenol (1), a neolignan with antiprotozoal activity against Trypanosoma cruzi, was partially methylated, and the compound obtained – methyl biseugenol (2) – had its activity evaluated against the extracellular (trypomastigotes) and intracellular (amastigotes) forms of T. cruzi. It was observed that both compounds 1 and 2 exhibited similar effects against trypomastigotes (IC50 of 11.7 and 16.2 μM, respectively), whereas compound 2 displayed higher activity against amastigotes (IC50 = 8.2 μM) in comparison with biseugenol (IC50 = 15.4 μM). Additionally, reduced toxicity against NCTC cells for compound 2 was observed (CC50 > 200 μM), differently from compound 1 with CC50 = 58.0 μM. Aiming to understand better the molecular mechanism of the biological action of compound 2, the prodrug was incorporated into cellular membrane models constituted of Langmuir monolayers of the lipids dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), dipalmitoylphosphatidylserine (DPPS), and dipalmitoylphosphatidylglycerol (DPPG). The lipid-drug interaction was inferred through tensiometry, surface potential, infrared spectroscopy (PM-IRRAS), and Brewster angle microscopy (BAM). The prodrug expanded DPPC and DPPG monolayers and condensed DPPE ones, as well as presented characteristic behaviors regarding the chemical structure of the lipid considering expansion-compression curves, surface potential-area isotherms, and stability of previously compressed monolayers to relevant-biological surface pressures. PM-IRRAS indicated a molecular disorder for DPPC and DPPS alkyl chains in the presence of the drug. BAM revealed the presence of domains in the DPPG and DPPE monolayers, which was probably induced by the prodrug. These data suggest, in general, that the lipid composition modulates the interaction of compound 2, whose results are expected to correlate to its trypanocidal activity, which involves the plasma membrane of T. cruzi as the primary target, i.e., the first barrier that the compound should encounter to interact with the microorganism.

Interactions of l-arginine with Langmuir monolayers of common phospholipids at the air-water interface

The interactions of l-arginine (l-arg) with Langmuir monolayers of three most common phospholipids, which are sodium salt of dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE), have been investigated at the air-water interface. The surface pressure-area (π-A) isotherms of these monolayers have been measured with a film balance and monolayer morphology has been observed by a Brewster angle microscopy (BAM). The DPPG monolayers on pure water do not show any phase transition but show irregular shaped condensed phases formed just after evaporation of the solvent at 20 °C. However, this monolayer on l-arg solution subphase indicates a first-order phase transition from liquid expanded to liquid condensed (LE-LC) phases and forms LC domains at the same temperature. With an increase in the l-arg concentration in the subphase up to 5.0 × 10−4 M, the π-A shows an overall increasingly greater expansion in the molecular area. All of the π-A isotherms recorded on ≥5.0 × 10−4 M l-arg solution subphases almost coincide with each other. These changes in the phase behavior have been explained by the fact that l-arg having guanidinium cationic group undergoes strong hydrogen bonding interaction with the anionic phosphatidylglycerol (PG-) head group. The bonding between two molecules is further strengthened by electrostatic attraction between cationic l-arg and anionic PG- ions. The BAM observation of the monolayer morphology supports this explanation. On the other hand, a very negligible interaction has been observed between l-arg and DPPC or DPPE monolayers. The π-A isotherms in the presence of l-arg for both the amphiphiles show a very little expansion only in the LE phase region, but coincide in the so called solid phase region. The monolayer morphology of both the monolayers also supports these results. This little effect of expansion in the LE region may be explained by the ion-pair formation between cationic l-arg and anionic head groups in the monolayers at lower pressures. However, due to compression at high pressure, the l-arg molecules are squeezed out from the amphiphile head groups.

X-Ray Reflectivity and Diffraction Studies of Doxorubicin Binding to Model Lipid Membranes

The effect of anticancer antibiotic doxorubicin on structural organization of anionic lipid monolayers has been studied. X-ray reflectivity and grazing incidence diffraction techniques were applied to monitor the changes in 2D structure and electron density distribution of Langmuir monolayer composed of negatively charged dipalmitoylphosphatidylglycerol (DPPG) and dioleoylphosphatidylserine (DOPS). For comparison, monolayer of zwitterionic dipalmitoylphosphatidylethanolamine (DPPE) also was investigated. The presented experimental results suggest that doxorubicin interaction with anionic lipid monolayers (DPPG and DOPS) proceeds preferentially via electrostatic attraction—positively charged amino groups of doxorubicin bind to negatively charged head groups of phospholipid molecules. Based on the obtained data, the penetration of doxorubicin into the hydrophobic part of anionic lipid monolayers does not occur. X-ray measurements on DPPE monolayer indicated that doxorubicin did not cause any significant alterations of molecular packing in condensed monolayer of zwitterionic DPPE molecules.

Structure Dependence of Pyridine and Benzene Derivatives on Interactions with Model Membranes.

Pyridine-based small-molecule drugs, vitamins, and cofactors are vital for many cellular processes, but little is known about their interactions with membrane interfaces. These specific membrane interactions of these small molecules or ions can assist in diffusion across membranes or reach a membrane-bound target. This study explores how minor differences in small molecules (isoniazid, benzhydrazide, isonicotinamide, nicotinamide, picolinamide, and benzamide) can affect their interactions with model membranes. Langmuir monolayer studies of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylethanolamine (DPPE), in the presence of the molecules listed, show that isoniazid and isonicotinamide affect the DPPE monolayer at lower concentrations than the DPPC monolayer, demonstrating a preference for one phospholipid over the other. The Langmuir monolayer studies also suggest that nitrogen content and stereochemistry of the small molecule can affect the phospholipid monolayers differently. To determine the molecular interactions of the simple N-containing aromatic pyridines with a membrane-like interface, 1H one-dimensional NMR and 1H-1H two-dimensional NMR techniques were utilized to obtain information about the position and orientation of the molecules of interest within aerosol-OT (AOT) reverse micelles. These studies show that all six of the molecules reside near the AOT sulfonate headgroups and ester linkages in similar positions, but nicotinamide and picolinamide tilt at the water-AOT interface to varying degrees. Combined, these studies demonstrate that small structural changes of small N-containing molecules can affect their specific interactions with membrane-like interfaces and specificity toward different membrane components.

Biophysical characterization of monofilm model systems composed of selected tear film phospholipids

The tear film protects the eye from foreign particles and pathogens, prevents excess evaporation, provides lubrication, and maintains a high quality optical surface necessary for vision. The anterior layer of tear film consists of polar and non-polar lipid layers. The polar lipids form a monolayer on the aqueous subphase, acting as surfactants for the non-polar lipid multilayer. A tear film polar lipid biomimetic consisting of dipalmitoyl phosphatidylcholine (DPPC), dipalmitoyl phosphatidylethanolamine (DPPE), palmitoyl glucosylceramide (PGC), and palmitoyl sphingomyelin (PSM) was characterized using Langmuir monolayers and Brewster angle microscopy (BAM). Lipid combinations formed very stable monolayers, especially those containing DPPC or PSM. Surface experiments and elasticity analyses revealed that PGC resulted in more condensed and rigid mixed monolayers. DPPE provided resistance to large changes in lipid ordering over a wide surface pressure range. Ternary mixtures containing DPPE and PGC with either DPPC or PSM experienced the greatest lipid ordering within the natural tear film surface pressure range suggesting that these lipids are important to maintain tear film integrity during the inter-blink period. Finally, BAM images revealed unique structures within monolayers of DPPC, DPPE, and PGC at the natural tear film surface pressure. 3D analysis of these domains suggested either the formation of multilayers or outward protrusions at surface pressures far below the point of irreversible collapse as seen on the isotherm. This entails that the polar lipids of tear film may be capable of multilayer formation or outward folding as a mechanism to prevent rupture of the tear film during a blink.

Effect of myelin basic protein on membrane lipid monolayers composed of DPPC and DPPE molecules

By analyzing the Langmuir monolayer surface pressure-mean molecular area ( π -A) curves, the interaction between myelin basic protein (MBP) and dipalmitoylphosphatidyl-choline cell membranes (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE), which are the different head group of lipid molecules, has been systematically studied, respectively. The results showed that: (1) with the MBP concentration in subphase increasing, the isotherm of DPPC and DPPE monolayer moved to lager average molecular area direction when the number of the lipid on the interface was constant; (2) when the surface pressure of the monolayer reached to 10 mN/m, one MBP molecule could combined with (140 ± 3) DPPC and (100 ± 3) DPPE molecules, respectively; as the surface pressure increased, the number of proteins interacted with two kinds of phospholipid molecules inserting the interface decreased; (3) with the increase of protein concentration, the monolayer film formed by the lipid molecules becomes more loose and MBP molecules are easy to insert into DPPE with smaller head; (4) the π-T curves indicated that the presence of protein could make DPPC monolayer surface pressure decreased, and the higher the concentration of protein was, the more for decreasing values of the surface pressure. Then the DPPC was brought into the MBP subphase; (5) for DPPE monolayer, the protein combined with DPPE inserted the membrane interface, causing the increase of surface pressure, and the higher the protein concentration was, the greater change the pressure appeared.

The effect of the injection of alkanols and alkanediols beneath dipalmitoyl phosphatidylethanol amine monolayers

The energies of activation of the interaction of alkanols and alkanediols with dipalmitoyl phosphatidylethanol amine monolayers spread at the air/water interface were estimated from the increase in the surface pressure with the concentration of the injected alcohol. The energies of activation are linear functions of the chain length of the injected alkanol for the one-, two-, three-, six-, and eight-carbon chains and of the alkanediols for the two-, three- and six-carbon chains. The plots of the data that exist in the literature of the reflection coefficients of these alkanols and alkanediols against the chain length obtained in biological membranes present similar linearities.

Integration of Ganglioside GT 1b Receptor into DPPE and DPPC Phospholipid Monolayers: An X-Ray Reflectivity and Grazing-Incidence Diffraction Study

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivity, the in-plane and out-of-plane structures of mixed-ganglioside GT 1b-phospholipid monolayers were investigated at the air-liquid interface and compared with monolayers of the pure components. The receptor GT 1b is involved in the binding of lectins and toxins, including botulinum neurotoxin, to cell membranes. Monolayers composed of 20 mol % ganglioside GT 1b, the phospholipid dipalmitoyl phosphatidylethanolamine (DPPE), and the phospholipid dipalmitoyl phosphatidylcholine (DPPC) were studied in the gel phase at 23°C and at surface pressures of 20 and 40 mN/m, and at pH 7.4 and 5. Under these conditions, the two components did not phase-separate, and no evidence of domain formation was observed. The x-ray scattering measurements revealed that GT 1b was intercalated within the host DPPE/DPPC monolayers, and slightly expanded DPPE but condensed the DPPC matrix. The oligosaccharide headgroups extended normally from the monolayer surfaces into the subphase. This study demonstrated that these monolayers can serve as platforms for investigating toxin membrane binding and penetration.

Molecular modeling and simulation of Mycobacterium tuberculosis cell wall permeability.

The low permeability of the mycobacterial cell wall is thought to contribute to the intrinsic drug resistance of mycobacteria. In this study, the permeability of the Mycobacterium tuberculosis cell wall is studied by computer simulation. Thirteen known drugs with diverse chemical structures were modeled as solutes undergoing transport across a model for the M. tuberculosis cell wall. The properties of the solute-membrane complexes were investigated by means of molecular dynamics simulation, especially the diffusion coefficients of the solute molecules inside the cell wall. The molecular shape of the solute was found to be an important factor for permeation through the M. tuberculosis cell wall. Predominant lateral diffusion within, as opposed to transverse diffusion across, the membrane/cell wall system was observed for some solutes. The extent of lateral diffusion relative to transverse diffusion of a solute within a biological cell membrane may be an important finding with respect to absorption distribution, metabolism, elimination, and toxicity properties of drug candidates. Molecular similarity measures among the solutes were computed, and the results suggest that compounds having high molecular similarity will display similar transport behavior in a common membrane/cell wall environment. In addition, the diffusion coefficients of the solute molecules across the M. tuberculosis cell wall model were compared to those across the monolayers of dipalmitoylphosphatidylethanolamine and dimyristoylphosphatidylcholine, are two common phospholipids in bacterial and animal membranes. The differences among these three groups of diffusion coefficients were observed and analyzed.

Lateral and Vertical Nanophase Separation in Langmuir−Blodgett Films of Phospholipids and Semifluorinated Alkanes

It has recently been found that monodisperse surface micelles (hemimicelles) were formed in Langmuir monolayers of the semifluorinated alkane C8F17C16H33 (F8H16) after transfer onto silicon wafers. Grazing incidence X-ray diffraction studies have demonstrated that compression of mixed Langmuir monolayers made from combinations of dipalmitoyl phosphatidylethanolamine (DPPE) and diblock F8H16 in various molar ratios resulted in the complete expulsion of the diblock molecule at high surface pressure. F8H16 then formed a second layer on top of a DPPE-only monolayer, demonstrating a novel type of reversible, pressure-induced, vertical phase separation. Using atomic force microscopy and X-ray reflectivity, we show now that mixed DPPE/F8H16 (1:1.3) Langmuir-Blodgett films transferred onto silicon wafers below 10 mN m(-1) are laterally phase separated and consist of domains of F8H16 surface micelles in coexistence with a monolayer of DPPE. The density of the network of F8H16 surface micelles increases when the surface pressure of transfer increases. Around 10 mN m(-1), the F8H16 surface micelles start to glide on the DPPE monolayer, progressively overlying it, until total coverage is achieved.

Interaction of antimicrobial peptide protegrin with biomembranes.

The antimicrobial peptide protegrin-1 (PG-1) interacts with membranes in a manner that strongly depends on membrane lipid composition. In this research we use an approach representing the outer layers of bacterial and red blood cell membranes with lipid monolayers and using a combination of insertion assay, epifluorescence microscopy, and surface x-ray scattering to gain a better understanding of antimicrobial peptide's mechanism of action. We find that PG-1 inserts readily into anionic dipalmitoyl-phosphatidylglycerol, palmitoyl-oleoyl-phosphatidylglycerol, and lipid A films, but significantly less so into zwitterionic dipalmitoyl-phosphatidylcholine, palmitoyl-oleoyl-phosphatidylcholine, and dipalmitoyl-phosphatidylethanolamine monolayers under similar experimental conditions. Epifluorescence microscopy shows that the insertion of PG-1 into the lipid layer results in the disordering of lipid packing; this disordering effect is corroborated by grazing incidence x-ray diffraction data. X-ray reflectivity measurements further point to the location of the peptide in the lipid matrix. In a pathologically relevant example we show that PG-1 completely destabilizes monolayer composed of lipid A, the major component in the outer membrane of Gram-negative bacteria, which is likely to be the mechanism by which PG-1 disrupts the outer membrane, thus allowing it to reach the target inner membrane.

Structural Reorganization of Phospholipid Headgroups upon Recrystallization of an S-Layer Lattice

Structural details of the coupling of bacterial surface (S)-layers to the phospholipid, dipalmitoylphosphatidylethanolamine (DPPE), have been characterized using X-ray and neutron reflectometry. We studied the binding andrecrystallization of S-protein isolated from B. sphaericus CCM2177 at DPPE monolayers on aqueous surfaces. Particular emphasis has been put on investigations of the lipid/protein interface in a joint refinement of X-ray and neutron data which reveals alterations of the molecular-level organization of the lipid headgroups upon protein binding and recrystallization: Peptide material interpenetrates the phospholipid headgroups almost in its entire depth but does not affect the hydrophobic lipid acyl chains. Consistent with FTIR results, we find that the headgroup hydration is reduced by ∼40% upon peptide interpenetration. On average, the equivalent of ∼65 electrons associated with the peptide, i.e., less than one peptide side group, interacts directly with one DPPE headgroup within the surface film. This suggests that the protein attaches to specific molecular moieties within the lipid monolayer which may form a lateral pattern within the film area that reflects the properties of the monomolecular protein crystal sheet.

Packing of Ganglioside-Phospholipid Monolayers: An X-Ray Diffraction and Reflectivity Study

Using synchrotron grazing-incidence x-ray diffraction (GIXD) and reflectivity, the in-plane and out-of-plane structure of mixed ganglioside-phospholipid monolayers was investigated at the air-water interface. Mixed monolayers of 0, 5, 10, 20, and 100 mol% ganglioside GM 1 and the phospholipid dipalmitoylphosphatidylethanolamine (DPPE) were studied in the solid phase at 23°C and a surface pressure of 45 mN/m. At these concentrations and conditions the two components do not phase-separate and no evidence for domain formation was observed. X-ray scattering measurements reveal that GM 1 is accommodated within the host DPPE monolayer and does not distort the hexagonal in-plane unit cell or out-of-plane two-dimensional (2-D) packing compared with a pure DPPE monolayer. The oligosaccharide headgroups were found to extend normally from the monolayer surface, and the incorporation of these glycolipids into DPPE monolayers did not affect hydrocarbon tail packing (fluidization or condensation of the hydrocarbon region). This is in contrast to previous investigations of lipopolymer-lipid mixtures, where the packing structure of phospholipid monolayers was greatly altered by the inclusion of lipids bearing hydrophilic polymer groups. Indeed, the lack of packing disruptions by the oligosaccharide groups indicates that protein-GM 1 interactions, including binding, insertion, chain fluidization, and domain formation (lipid rafts), can be studied in 2-D monolayers using scattering techniques.

Ordering in Langmuir monolayers of branched chain phospholipids

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.

Bacterial S-Layer Protein Coupling to Lipids: X-Ray Reflectivity and Grazing Incidence Diffraction Studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38–66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness l z ≈ 90 Å of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to ∼20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of ∼131 Å.

Investigation of the Molecular Organization in Langmuir−Blodgett Films Using Polarized Infrared Spectra: Comparison of Two Methods

We compare two methods for obtaining structural information on Langmuir-Blodgett films from polarized infrared spectroscopy. For sufficiently uniform films, we have already shown (refs 11 and 12) that all polarized spectroscopic properties could be characterized by a single quantity that we call the electrical surface susceptibility tensor, y. The imaginary parts of the susceptibility tensor could be readily obtained from reflectance measurements with the electric field parallel (Im(γ τ )) and perpendicular (Im(γ n )) to the plane of the film, independently of any specific properties of the film. This, in turn, could be related to the characteristics of individual molecules comprising the film, and their geometric disposition if the molecules are assumed to be interacting dipoles, which corrects, to a large extent, for the local field effects. We report the results ofan infrared study performed on a dipalmitoylphosphatidylethanolamine monolayer. Attenuated total reflection spectra for both s and p polarization were recorded, and the spectra of Im( γt ) and Im(γ n ), as well as those ofthe imaginary part of the refractive index (k t and k n ), are presented. Finally, the information deduced from the Im(γ x ) and k x (x = s, p) are compared, and the molecular orientation is discussed both in terms of Im(γ x ) and k x . We show that by using this approach one obtains more truthworthy measurements of the disposition of oriented molecules at surfaces than those obtained from absorption coefficients.

Grafted poly-(ethylene glycol) on lipid surfaces inhibits protein adsorption and cell adhesion

Monolayers of dipalmitoyl-phosphatidylethanolamine (DPPE) mixing with various mole percentages of distearoyl-phosphatidylethanolamine (DSPE)-conjugated poly-(ethylene glycol) (PEG m.w. 750–5000) were deposited on DPPE-coated glass surfaces by the Langmuir-Blodgett method. Increasing percentages of grafted PEG in these supported lipid surfaces increasingly inhibit the adsorption of bovine serum albumin (BSA), laminin, and fibronectin. Increasing percentages of grafted PEG also inhibit the adhesion of erythrocytes, lymphocytes, and macrophages to these supported lipid surfaces. The adsorption of proteins on lipid coated glass surfaces were assayed by the fluorescence of FITC-labelled proteins. Cell adhesion was measured mainly by microscopic counting. The concentration of PEG-grafted lipids required for the inhibition of erythrocyte adhesion decreases with increasing molecular weight of the grafted PEG. The inhibitory effects are strongly dependent on the graft density of PEG at low concentrations, but weakly dependent on graft density at higher concentrations. For DSPE-PEG5000, the change of graft density dependency occurs approximately at the complete coverage of the lipid surface by the grafted polymer in the mushroom conformation (0.7 mol%), and the transition to partial brush conformation. The change-overs become less distinctive for grafted PEG of lower molecular weights, probably due to the failure of strictly mushroom and brush models of the polymer. The relative inhibitory efficiency is protein or cell dependent. The implication on the function of stealth liposomes is discussed.

Solid-Phase Synthesis of the Exposed Capsid Peptide VP2(96-107) of the Hepatitis A Virus. Study of Its Physicochemical Interactions with Phospholipids

The synthesis on a solid support of a dodecapeptide containing a HAV-VP2 exposed sequence is described. The surface activity of this peptide as well as its miscibility and interactions with dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), and phosphatidylinositol (PI) were determined. Interaction energies calculated applying the Gibbs equation gave very small values, thus suggesting either ideal miscibility or weak interactions. In all cases the electrical charge in DPPE and PI monolayers seems to create a high electrostatic potential, decreasing its interaction with any external molecule. This peptide was able to accommodate a certain amount of anilinonaphthalenesulfonate molecules, showing an intermediate hydrophobic character. Moreover, after incubation with saturated liposomes, the peptide showed a clear rigidifying effect at the level of polar heads, especially at temperatures under the transition temperature.