Dioleoylphosphatidylcholine Monolayer Research Articles

Thermodynamic Analysis of Myelin Basic Protein Adsorbed on Liquid Crystalline Dioleoylphosphatidylcholine Monolayer.

To investigate the stability and dynamic characteristics of monolayer adsorbed on unsaturated lipid dioleoylphosphatidylcholine (DOPC) with varying concentrations of myelin basic protein (MBP), the system is studied by applying Langmuir technique and making atomic force microscope (AFM) observation, which is based on the mass conservation equation analysis method referred to in the thermodynamics theory. As indicated by surface pressure-mean molecular area (π − A) and surface pressure-adsorption time (π − T) isotherms, the physical properties of monolayer derived from the interaction of varying concentrations of MBP with liquid crystalline unsaturated lipid DOPC molecules were qualitatively studied. As revealed by surface morphology analysis with AFM, the micro region was expanded as the concentration of MBP in the subphase was on the increase, suggesting that hydrophobic interactions led to the MBP insertion, thus causing accumulation of the MBP on the surface of the monolayer. Experimental results have demonstrated that the partition coefficient of the interaction between MBP and unsaturated phospholipid DOPC and the molecular area of MBP adsorbed on the monolayer film was calculated using the mass conservation equation. In addition, not only does the varying concentration of MBP in the subphase exerts significant effects on the arrangement and conformation of DOPC monolayer, it also has certain guiding significance to exploring the structural changes to biofilm supramolecular aggregates as well as the pathogenesis and treatment of related diseases.

Au25(SCnH2n+1)18 Clusters in Biomimetic Membranes: Role of Size, Charge, and Transmembrane Potential in Direct Membrane Permeation

Gold nanoclusters and nanoparticles are promising materials for applications in nanomedicine, and therefore, understanding their interaction with cell membranes is of particular importance. A series of neutral and anionic Au25(SCnH2n+1)18 monolayer protected clusters (MPCs) (briefly, Cn0 and Cn– clusters), was embedded into two types of biomimetic membranes supported by mercury electrodes. The first was a dioleoylphosphatidylcholine (DOPC) self-assembled monolayer (SAM), whereas the second was a tethered bilayer lipid membrane (tBLM) obtained by first anchoring a thiolipid monolayer to the mercury surface and then self-assembling a DOPC monolayer on top of it. The diameter of these clusters, from 1.7 to 2.7 nm depending on the thiolate ligand, is smaller than the thickness of biomembranes and biomimetic membranes. Both neutral and anionic Au25(SCnH2n+1)18 MPCs can penetrate the lipid bilayer moiety of the tBLM, without disrupting it; in particular, anionic Au25 clusters require positive transmembrane pote...

Impact of polyphenol-rich green tea extracts on the protection of DOPC monolayer against damage caused by ozone induced lipid oxidation.

Effectiveness of green tea (compared to two single polyphenols) in removing ozone derived reactive oxygen species acting on dioleoylphosphatidylcholine monolayers was determined. Lipid oxidation was followed by changes in mechanical properties of the layer spread on the aqueous subphase containing various amounts of ozone in the presence and absence of polyphenolics. It was shown that the tea extract (containing 8.5 x 10-4 mg/cm3 polyphenols) is capable of inactivating 0.4 ppm ozone. The DPPH radical scavenging test set polyphenols in the order of increasing activity, consistent with their protective effect in relation to lipid oxidation, showing the highest efficacy of EGCG.

On the interaction of the highly charged peptides casocidins with biomimetic membranes

Casocidin I and II (CI and CII) are structurally related antimicrobial peptides made of 39 and 31 amino acids, respectively, which derive from natural proteolytic processing of αs2-casein and adopt an ordered α-helical structure in biomimetic membranes. Their putative membrane-permeabilizing activity was investigated at Hg-supported self-assembled monolayers (SAMs) and at tethered bilayer lipid membranes (tBLMs); the latter consisted of a monolayer of 2,3,di-O-phytanyl-sn-glycerol-1-tetraethylene-glycol-d,l-α lipoic acid ester thiolipid (DPTL), with a dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) monolayer on top of it. Interaction of CI and CII with these biomimetic membranes was studied by four electrochemical techniques at pH 3, 5.4 and 6.8. Peptide incorporation in tBLMs was attempted via scans of electrochemical impedance spectra. Experiments demonstrated that CI and CII penetrate SAMs as well as the distal DOPC monolayer of DPTL/DOPC tBLMs, but not the proximal phytanyl monolayer, with the only exception of CII at pH 5.4. Conversely, CII permeabilized DPTL/DOPS tBLMs to a moderate extent at all investigated pH values by forming holes across the membrane, but not ion channels. Structural distribution of charged residues seemed to prevent CII from having a hydrophobic side of the α-helix capable of stabilizing a regular ion channel in the lipid matrix.

Substituents modulate biphenyl penetration into lipid membranes

Electrochemical impedance techniques and fluorescence spectroscopic methods have been applied to the study of the interaction of ortho (o)-, meta (m)- and para (p)-Cl-, o-, m- and p-HO-, p-H3CO-, p-H3C-, p-NC- and p-O3−S- substituted biphenyls (BPs) with Hg supported dioleoyl phosphatidylcholine (DOPC) monolayers and DOPC vesicles. Non-planar o-substituted BPs exhibit the weakest interactions whereas planar p-substituted BPs interact to the greatest extent with the DOPC layers. The substituted BP/DOPC monolayer and bilayer interaction depends on the effect of the substituent on the aromatic electron density, which is related to the substituents' mesomeric Hammetts constants. Substituted BPs with increased ring electron density do not increase the DOPC monolayer thickness on Hg and penetrate the DOPC vesicle membranes to the greatest extent. Substituted BPs with lower ring electron density can cause an increase in the monolayer's thickness on Hg depending on their location and they remain in the interfacial and superficial layer of the free standing DOPC membranes. Quantum mechanical calculations correlate the binding energy between the substituted BP rings and methyl acetate, as a model for the –CH2-(CO)O-CH2- fragment of a DOPC molecule, with the location of BPs within the DOPC monolayer.

Role of electrolyte in the occurrence of the voltage induced phase transitions in a dioleoyl phosphatidylcholine monolayer on Hg

The present study investigates the effect of electrolyte ions on the occurrence of potential induced phase transitions at ca. -0.93V and -0.99V vs Ag/AgCl respectively of dioleoyl phosphatidylcholine (DOPC) monolayers on the mercury (Hg) surface. The interaction of both inorganic and organic electrolytes with DOPC was studied and it was shown that the alterations in the nature of the capacitance peaks representing the successive phase transitions depend on the ionic strength and the interfacial properties of the electrolyte ions. Higher ionic strength and cations with stronger binding affinity to DOPC give rise to steeper electric fields across the phospholipid layer shifting the apparent potential characterising capacitance peak-1 to more positive values. Conversely the anion, I−, which binds to DOPC, lessens the field gradient across the lipid and shifts the potential characterising capacitance peak-1 to more negative potentials. At more negative potentials to those characterising capacitance peak-1, electrolyte penetrates the DOPC. As a consequence a higher ionic strength and smaller hydrated cations with higher charge density enhance the screening of Hg, leading to stronger electric fields at the Hg surface. This effect shortens the onset and increases the rate dependence on the potential of the adsorption of bilayer patches to Hg by a nucleation and growth (N & G) process.

Dermcidin, an anionic antimicrobial peptide: influence of lipid charge, pH and Zn2+on its interaction with a biomimetic membrane

The mechanism of membrane permeabilization by dermcidin (DCD-1L), an antimicrobial peptide present in human sweat, was investigated at a mercury-supported monolayer of dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) and at a mercury-supported tethered bilayer lipid membrane (tBLM) consisting of a thiolipid (DPTL) with a DOPC or DOPS monolayer self-assembled on top of it. In an unbuffered solution of pH 5.4, DCD-1L is almost neutral and permeabilizes a DPTL/DOPS tBLM at transmembrane potentials, ϕtrans, which are physiological. In a pH 7 buffer solution DCD-1L bears two negative charges and has no effect on a DPTL/DOPC tBLM, whereas it permeabilizes a DPTL/DOPS tBLM only outside the physiological ϕtrans range; however, the presence of zinc ion induces DCD-1L to permeabilize the DPTL/DOPS tBLM at physiological ϕtrans values. The effect of zinc ions suggests a DCD-1L conformation with its positive N-terminus embedded in the lipid bilayer and the negative C terminus floating on the membrane surface. This conformation can be stabilized by a zinc ion bridge between the His(38) residue of the C terminus and the carboxyl group of DOPS. Chronocoulometric potential jumps from ϕtrans ≅ +160 mV to sufficiently negative values yield charge transients exhibiting a sigmoidal shape preceded by a relatively long "foot". This behavior is indicative of ion-channel formation characterized by disruption of DCD-1L clusters adsorbed on top of the lipid bilayer, incorporation of the resulting monomers and their aggregation into hydrophilic pores by a mechanism of nucleation and growth.

Interaction of Imidazolium-Based Room-Temperature Ionic Liquids with DOPC Phospholipid Monolayers: Electrochemical Study

To test the biocompatible character of room-temperature ionic liquids (ILs), the interaction of various ILs with biological membrane (biomembrane) models was studied in this work. Dioleoyl phosphatidylcholine (DOPC) adsorbed on a mercury (Hg) electrode forms an impermeable defect-free monolayer which is a well established biomembrane model, prone to be studied by electrochemical techniques. We have monitored the modifications of the Hg supported monolayer caused by ILs using rapid cyclic voltammetry (RCV), alternating current voltammetry (ACV), and electrochemical impedance spectroscopy (EIS). A series of imidazolium-based ILs were investigated whose interaction highlighted the role of anion and lateral side chain of cation during the interaction with DOPC monolayers. It was shown that the hydrophobic and lipophilic character of the IL cations is a primary factor responsible for this interaction. Hg-supported monolayers provide an accurate analysis of the behavior of ILs at the interface of a biomembrane leading to a comprehensive understanding of the interaction mechanisms involved. At the same time, these experiments show that the Hg-phospholipid model is an effective toxicity sensing technique as shown by the correlation between literature in vivo toxicity data and the data from this study.

ZnO nanoparticle interactions with phospholipid monolayers

Aqueous ZnO nanoparticle dispersions interaction with a dioleoyl phosphatidylcholine (DOPC) monolayer is reported in this paper. ZnO-DOPC interactions were investigated using rapid cyclic voltammetry (RCV) by focusing on the effect of the interactions on the characteristics of the capacitance current peaks representing two potential induced phase transitions. Results showed: – (1) The order of interaction of common commercially sourced nanoparticles with DOPC coated Hg electrodes was NanoTek>NanoShield>metals basis. This extent of interaction was inversely related to the ZnO particle size where the metals basis nanoparticles were strongly aggregated. The contribution of the non-ionic dispersant added by manufacturer to the NanoTek and NanoShield interaction was uncertain. (2) Freshly prepared aqueous Nanosun ZnO nanoparticle (∼25nm) dispersions interacted with and penetrated DOPC coated Hg electrodes. Aggregation of the nanoparticles, coating of the ZnO with phosphate and coating of the ZnO with fulvic acid minimised ZnO–DOPC interaction. (3) In-house synthesised ZnO nanoparticles of lower primary particle size (∼6nm) than Nanosun ZnO nanoparticles interacted strongly with DOPC coated Hg electrodes with no evidence of penetration of the nanoparticle in the DOPC monolayer. Even after considerable aggregation of the particle to between 1 and 10μm, a strong interaction of the in-house synthesised ZnO with DOPC was observed.

Electrochemical Modeling of the Silica Nanoparticle–Biomembrane Interaction

The interaction of amorphous colloidal silica (SiO(2)) nanoparticles of well-defined sizes with a dioleoyl phosphatidylcholine (DOPC) monolayer on a mercury (Hg) film electrode has been investigated. It was shown using electrochemical methods and microcalorimetry that particles interact with the monolayer, and the electrochemical data shows that the extent of interaction is inversely proportional to the particle size. Scanning electron microscopy (SEM) images of the electrode-supported monolayers following exposure to the particles shows that the nanoparticles bind to the DOPC monolayer irrespective of their size, forming a particle monolayer on the DOPC surface. A one-parameter model was developed to describe the electrochemical results where the fitted parameter is an interfacial layer thickness (3.2 nm). The model is based on the adsorptive interactions operating within this interfacial layer that are independent of the solution pH and solution ionic strength. The evidence implies that the most significant forces determining the interactions are van der Waals in character.

Interaction of poly(N-isopropylacrylamide) (pNIPAM) based nanoparticles and their linear polymer precursor with phospholipid membrane models

Poly(N-isopropylacrylamide) (pNIPAM) is a thermoresponsive polymer which has promising applications in nanomedicine for drug delivery. The cross-linking of pNIPAM based copolymer using the chain collapse method leads to the synthesis of pNIPAM based polymer nanoparticles. This study looks at the interaction of pNIPAM polymers and pNIPAM nanoparticles with biomembrane models of, (i) a dioleoyl phosphatidylcholine (DOPC) monolayer on a mercury (Hg) electrode and (ii) DOPC and dimyristoyl phosphatidylcholine (DMPC) vesicles. The following techniques were used to follow the interactions: Dynamic light scattering (DLS), differential scanning calorimetry (DSC), rapid cyclic voltammetry (RCV) and electrochemical impedance spectroscopy (EIS). Results showed that the polymers interacted more extensively than the nanoparticles with the phospholipid. The interaction of the polymer was more rapid and led to a polymer-phospholipid conjugate whereas the nanoparticle adsorbed on the phospholipid monolayer surface and penetrated the monolayer at longer contact times. The association of the linear polymer with the phospholipid can be related to the larger molecular area available with the pendant −Cl groups and the inherent polymeric flexibility compared to the nanoparticle structure. The apparent dissociation constant for nanoparticles–DOPC complex was Kd,app=1.67∗10−5±1.2∗10−6moldm−3. The apparent kinetic constant of nanoparticle penetration through the DOPC monolayer was k2,app=1.054∗10−2±9.1∗10−4s−1. It can be concluded therefore that the pNIPAM nanoparticle because of its lower affinity for phospholipids is more appropriate for medical applications.

Properties of oxidized phospholipid monolayers: An atomistic molecular dynamics study

Non-saturated lipids in the lung surfactant are prone to oxidation by oxidative species present in air. In this Letter, oxidized monolayers of dioleoylphosphatidylcholine (DOPC) were studied by means of both molecular dynamics and potential of mean force calculations. Structural changes of the monolayer following oxidation were revealed, including orientational reversal of oxidized chains. Accumulation of oxidized lipids in the monolayer is observed for moderate oxidation ratios, whereas removal of the short-chain oxidation product is predicted at long timescales. Massive oxidation leads to the loss of the well-ordered monolayer structure and partial solubilization of the oxidized lipids in the aqueous subphase.

Structural, topographical, and rheological characteristics of β-casein–dioleoyl phosphatidylcholine (DOPC) mixed monolayers

The surface pressure ( π)–area ( A) isotherms and Brewster angle microscopy (BAM) images of β-casein–dioleoyl phosphatidylcholine (DOPC) mixed films spread on buffered water at pH 7 and 9 and at 20 °C were determined as a function of the mass fraction of DOPC in the mixture ( X DOPC). The structural characteristics, miscibility, and topography (morphology and reflectivity) of DOPC–β-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in DOPC–β-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the DOPC concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area and elasticity ( E) it was deduced that DOPC and β-casein form a practically immiscible monolayer at the air–water interface. The BAM images and the evolution with the surface pressure of the reflectivity of BAM images give complementary information on the interactions and structural characteristics of DOPC–β-casein mixed monolayers, which corroborate the conclusions derived from the π– A isotherm. After the spreading or just after the expansion at π ≈ 0 we have observed the presence of 2D-foams, typical topography of DOPC monolayers at low π. The 2D-foams disappear after the compression of the monolayer and the topography is homogenous and isotropic. From the reflectivity of BAM images it is possible to distinguish between the coexistence of DOPC and β-casein or β-casein displacement by DOPC, depending on the surface pressure. The surface dilatational properties of the mixed films corroborate the coexistence of DOPC and β-casein at π lower than the equilibrium spreading pressure ( π e) of β-casein and β-casein displacement by DOPC at π > π e of β-casein. The phenomena observed appear to be generic for protein and polar (monoglycerides) and ionizable (phospholipid) mixed monolayers.

The effect of pH on surface dilatational and shear properties of phospholipid monolayers

We have studied the surface dilatational (surface dilatational modulus, E, its elastic, E d, and viscous, E v, components, and loss angle tangent, tan θ) and surface shear (elastic, G′, and viscous components, η) characteristics of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers, at 20 °C and at pH 5, 7, and 9. The stress response to compression–expansion sinusoidal deformation of the interface was measured in a modified Wilhelmy-type trough with two oscillating barriers and in a ring trough (with an isotropic area deformation), as a function of deformation amplitude and frequency. Measurements of surface shear rheology were carried out by using an automatic Du Noüy ring oscillatory apparatus. The monolayer structure and, especially, the conditions at which the monolayer collapses, determine the viscoelastic behaviour under dilatational and shear deformations of the monolayer and the linear response of the stress to area deformation. The non-linear viscoelastic behaviour of the interface has been associated with the phospholipid monolayer collapse. It was found that the dilatational modulus is not only determined by the interactions between spread molecules (which depend on the surface pressure or surface density), but that the structure of the spread monolayer also plays an important role. Surface shear rheology is more sensible to the effect of pH on surface rheological parameters of phospholipid monolayers than surface dilatational rheology. The surface shear characteristics for DPPC and DOPC monolayers were higher at pH 5, where attractive van der Waals interactions between phospholipid molecules take place, and were lower at pH 9, where electrostatic repulsions between phospholipid molecules weakened the monolayer structure. The surface dilatational and shear moduli were also higher for DPPC than for DOPC at every pH.

Effect of Hydrocarbon Chain and pH on Structural and Topographical Characteristics of Phospholipid Monolayers

Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.

A method for characterization of sea surface microlayer based on monolayer properties in presence and absence of phospholipids

Electrochemical impedance spectroscopy, ac voltammetry and fractal analysis were used to characterize model compounds, compound mixtures and extracted samples of sea surface microlayer (ssm) und underlying water (ulw). The reasons for carrying out this work were to use the scientific basis of these characterizations in future on-line analytical procedures of ssm. The mercury (Hg) drop electrode uncoated and coated with a monolayer of dioleoyl phosphatidylcholine (DOPC) was used as an experimental basis for investigation of the major sea surface film forming material. Firstly, the interaction of the uncoated and DOPC coated Hg electrode with model water insoluble compounds of increasing polarity was investigated. The compounds studied in order of increasing polarity were: nonadecane, stearic acid, cholesterol and cardiolipin. Subsequently the electrochemical response of the system to different ssm extracts was compared to signals observed with model compounds to demonstrate method selectivity. From the electrochemical results, it is observed that both the molecular structure and polarity of the investigated compounds have a role in their interaction with the uncoated and DOPC coated electrode. In the fractal analysis the increase above 2 of fractal dimension D imparted to the DOPC layer is related to the degree of apolarity of the additive model compound. Consistent with this, the more apolar hexane extracted ssm 2 imparts a fractal dimension D value of 2.45 when incorporated in DOPC layers. The electrochemical response to the ssm and ulw follows that characteristic of sterol compounds.

Intermolecular interaction of phosphatidylinositol with the lipid raft molecules sphingomyelin and cholesterol.

Diacylphosphatidylinositol (PI) is the starting reactant in the process of phosphatidylinositide-related signal transduction mediated through the lipid raft domain. We investigated intermolecular interactions of PI with major raft components, sphingomyelin (SM) and cholesterol (Chol), using surface pressure–molecular area (π–A) isotherm measurements. The classical mean molecular area versus composition plot showed that the measured mean molecular areas are smaller in PI/Chol mixed monolayers and larger in PI/SM mixed monolayers than those calculated on the basis of the ideal additivity. These results indicate that PI interacts attractively with Chol and repulsively with SM. In addition, we energetically evaluated the interaction of PI with SM/Chol mixtures and found that the mixing energy of PI/SM/Chol ternary monolayers decreased as the molar ratio of Chol to SM increased. In order to quantitatively analyze the distribution of PI we calculated the chemical potentials of mixing of PI into the SM/Chol mixed monolayer and into the dioleoylphosphatidylcholine (DOPC) monolayer, which was used as a model for the fluid matrix, on the basis of partial molecular area analysis. Analysis using the chemical potential of mixing of PI suggested that partition of PI molecules between these two monolayers can be changed by a factor of about 1.7 in response to change in Chol molar fraction in the SM/Chol mixed monolayer from 0.3 to 0.6 when the concentration of PI in the DOPC monolayer is kept constant at 7 mol%.

An Electrochemical Study into the Interaction between Complement-Derived Peptides and DOPC Mono- and Bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.

Interaction of Bispyridinium Compounds with Phospholipid Layers in Presence and Absence of Electric Field

AbstractThe nonspecific biological activity of compounds with cell membranes is of great importance when considering them for pharmaceutical applications. The interactions of a group of therapeutic bispyridinium compounds with phospholipid monolayers of dioleoyl phosphatidylcholine (DOPC) on a mercury electrode (HMDE) are reported in this paper. Impedance versus frequency plots and capacitance‐potential curves were recorded for the model membrane system. The penetration of the compounds into the monolayer is shown to be dependent on the applied electric field. Both electron withdrawing and electron donating substituents of the bispyridinium compounds promote interaction with the compounds at applied negative potential.

Some implications of nanoscience in food dispersion formulations containing phospholipids as emulsifiers

This contribution is concerned with phospholipid films in relation to food dispersions such as emulsions and foams. Structural, morphological and surface rheological characteristics of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air–water interface at 20 °C and at pH 5, 7, and 9, by means of surface pressure ( π)–area ( A) isotherms coupled with Brewster angle microscopy (BAM), atomic force microscopy (AFM) and surface dilatational and shear rheometry. From the π– A isotherms it was deduced that DPPC monolayers show structural polymorphism at the air–water interface. DOPC monolayers formed a liquid-expanded (LE) structure under all experimental conditions, a consequence of the weak molecular interactions due to the double bond in the hydrocarbon chain. Electrostatic interactions between film-forming components influence the monolayer structure. BAM and AFM images corroborate, at a microscopic and at nanoscopic level respectively, the structural polymorphism deduced from the π– A isotherm for DPPC monolayers as a function of surface pressure and the pH of the aqueous phase. However, the homogeneous morphology of DOPC monolayers at a microscopic level, as observed by BAM, shows structural heterogeneity at a nanoscopic level when observed by AFM. The relative monolayer thickness increases with surface pressure and is a maximum at the collapse point, especially for DPPC monolayers. The results confirm that the interfacial rheological characteristic measured under dilatation and shear conditions are very dependent on the structural characteristics and morphology of the phospholipids (DPPC and DOPC) monolayers.