Effect of surfactant agents and lipids on optical resolution of amino acid by ultrafiltration membranes containing bovine serum albumin

Ultrafiltration experiments for the optical resolution of racemic phenylalanine were performed in a solution system containing bovine serum albumin (BSA) and surfactant agents (Triton X-100, Tween 20, sodium dodecyl sulfate), lipid (phosphaticylcholine) and fatty acid (palmitic acid sodium salt). It was found that -phenylalanine preferentially existed in the permeate at pH 7.0 due to the binding of BSA to -phenylalanine in the feed and that the separation factors (=concentration ratio of -isomer to -isomer in the permeate) increased with a decrease in the BSA solution containing no additives and in the BSA solution containing Triton X-100 or Tween 20. The unusual tendency that the separation factors were less than unity was observed and the separation factors decreased with a decrease in the feed concentration of phenylalanine during the ultrafiltration containing the palmitic acid sodium salt or the phosphatidylcholine. This is caused by the fact that the binding constants of -phenylalanine to BSA are higher than those of -phenylalanine in the BSA solution containing the palmitic acid sodium salt or phosphatidylcholine. Since there were found conformational changes of BSA in the presence of palmitic acid sodium salt based on circular dichroism measurements of BSA solution, the conformational changes of BSA were attributed to the higher affinity of -phenylalanine to BSA than that of -phenylalanine in the BSA solution containing the palmitic acid sodium salt or phosphatidylcholine.     

Mixed monolayers of bovine serum albumin with a nonionic surfactant (Tween 80)

Mixed monolayers of bovine serum albumin (BSA) with a nonionic surfactant (Tween 80) are obtained by spreading solutions containing both components over the surface of a subphase (water with pH 6) over a wide range of solution compositions. According to compression-expansion isotherms, the mixed monolayers are of the condensed type when the BSA concentrations in the solution are far higher than or equal to the surfactant concentration. Such monolayers mostly consist of BSA-Tween 80 (1: 1) complexes. In contrast, a BSA monolayer is of the expanded type. When Tween 80 in the solution prevails over Tween 80, the monolayers become unstable. The results of this work pertain to the monitoring of the properties of protein-surfactant mixtures and design of Langmuir-Blodgett (LB) films.     

Activity of membrane-bound NADH-methemoglobin reductase and physical state of lipids in erythrocyte membranes

The activity of membrane-bound NADH-methemoglobin reductase in erythrocytes and the physical state of lipids in erythrocyte membranes under oxidative stress in cells were studied. A decrease of the activity of membrane-bound NADH-methemoglobin reductase and a change of physical state of the lipid bilayer of membranes under oxidative stress were found in erythrocytes in vivo and in vitro.     

Interfacial behaviour and mechanical properties of spread lung surfactant protein/lipid layers

The surface behaviour of spread dipalmitoyl phosphatidyl choline (DPPC), lung surfactant protein C (SP-C), and their mixtures were characterised using a captive bubble surfactometer. The surface tension was determined by using axisymmetric bubble shape analysis. Surface dilatational rheological behaviour was characterised by sinusoidal oscillation of the bubble volume and at frequencies 0.006-0.025 Hz. The pi/A isotherms of DPPC, SP-C, and their mixtures were described with a generalised equation of state. Monolayer cycling of mixed DPPC/SP-C layers yields isotherms with a plateau in the range of 50-53 mN/m. When the surface pressure becomes higher SP-C is squeezed out of the film, but it re-enters the film upon expansion. Surface dilatational elasticities of DPPC films had a maximum at about 30 mN/m. At higher surface pressures, the films became brittle and the elasticity decreased. A slightly pronounced maximum was found at a surface pressure exceeding 55 mN/m. The dilatational viscosity had two distinct maxima, corresponding with those in the elasticity curves, i.e. one before the minimum area demand, and one in the range of over-compression. This was explained by the formation of a second ordered complex structure in the range of film over-compression. SP-C films show continuously increasing dilatational elasticities and viscosities with a maximum at f approximately 0.02 Hz. Mixed monolayers, DPPC+2 mol% SP-C, had dilatational elasticities increasing with surface pressure. In contrast to DPPC alone, an elasticity maximum appeared in the range of the squeeze out plateau. The dilatational viscosity had two distinct maxima as observed for DPPC, whereas the maximum before the squeeze out plateau is very broad like that of SP-C. The viscosity decreased for frequencies higher 0.02 Hz favouring elastic properties of the film. Our data provide experimental evidence that SP-C mixed with DPPC yield higher elasticities and viscosities as compared with films formed by the single components. This behaviour is likely to support breathing cycles, especially for the turn from inspiration to expiration and vice versa.     

New insights into lung surfactant monolayers using vibrational sum frequency generation spectroscopy

At the air-water interface, interfacial molecular structure, intermolecular interactions, film relaxation and film respreading of model lung surfactant monolayers were studied using vibrational sum frequency generation (VSFG) spectroscopy combined with a Langmuir film balance. Chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoyl-phosphatidylglycerol (POPG), palmitic acid (PA) and tripalmitin were investigated. In the DPPC-d62-PA binary monolayer, PA showed a condensing effect on the DPPC chains. On the contrary, in the DPPC-d62-POPG binary monolayer, POPG showed a fluidizing effect on the DPPC chains. In the ternary monolayer system of DPPC-d62-POPG-PA, the balance between the fluidizing and the condensing effect was also observed. In addition, the film relaxation behavior of DPPC-d62 and the enhanced film stability of DPPC-d62 caused by the addition of tripalmitin were observed. Real-time VSFG was also employed to study the respreading properties of a complex lung surfactant mixture containing DPPC-d62, POPG, PA and KL4 (a mimic of SP-B) peptide, which revealed DPPC enrichment after film compression.     

The model of the action mechanism of SP-C in the lung surfactant monolayers

The action mechanism of surfactant protein C (SP-C) in the lung surfactant monolayers is studied. On the basis of the SP-C molecular structure, a detailed interaction model is developed to describe the interaction of phospholipids/SP-C in the lung surfactant monolayers. It is supposed that: (1) in an alveolus monolayer, SP-C molecules are surrounded by phosphatidylglycerol (PG). When the monolayer is compressed, SP-C molecules can promote PG molecules to be squeezed out; (2) during compressing of the monolayer, unsaturated-PG molecules form a collapse pit firstly when liquid-expanded state (LE) components achieve the collapse pressure. Then, SP-C's alpha-helix is attracted by the collapse pit and both alpha-helix and PG molecules are squeezed out speedily. Finally, the squeezed-out matters can form a lipid-protein aggregation in the subphase. The lipid-protein aggregation, in the centre of which, there is the hydrophobic alpha-helix section surrounded by PG molecules; (3) during the monolayer expanding, because of the increasing of the monolayer's surface tension, the structure of the lipid-protein aggregation is disturbed and reinserts into the surface of the monolayer rapidly. On the basis of analyzing the energies change of the squeeze-out process, a mathematical model is obtained to calculate the squeezed-out number of DPPG molecules when a SP-C molecule squeezes out in a monolayer. According to the model, it is concluded that SP-C has the capability to promote the squeeze-out and the reinsertion of most of PG component in an alveolus monolayer, the prediction data agree well with the experimental data.     

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.     

The structure of zwitterionic phosphocholine surfactant monolayers

The structure of a zwitterionic phosphocholine (PC) surfactant monolayer adsorbed on the surface of water has been determined using neutron reflectivity in combination with H/D isotopic substitution. The most significant results of this study are the level of hydration of the PC headgroup and the lack of dehydration with increasing temperature and salt addition. The fraction of the alkyl chain (f(c)) immersed in water for all three chain isomers studied was found to be around 0.15, suggesting that the PC headgroup geometries influenced not only the headgroup hydration but also the degree of immersion of the alkyl chain in water. At the critical micelle concentration (CMC), the number of water molecules associated with the PC headgroup in C(m)PC (m = 12, 14, 16) was on order of 15. This value was significantly greater than that obtained for nonionic and ionic surfactants with similar limiting area per molecule at the CMC (A(cmc)). However, the fraction of the chain immersed in water for the ionic and nonionic surfactants was much greater. This suggests that the unique surface biocompatibility of PC surfactants arises from their strong affinity for water, and the relatively low fraction of mixing with the alkyl chain arises from the higher structural order within the PC monolayer. As surface coverage decreased, the number of water molecules associated with each PC headgroup increased, but f(c) remained constant for all the surfactants. This observation was consistent with the small variation in the thickness of the headgroup region, and the entire layer changed little with surfactant concentration. This is attributed to the role of PC headgroup geometries to maintain the conformational order within the layer as packing density varies. Further structural analysis based on a kinematic approach showed that, as the chain length was increased from C12 to C14 to C16 at the CMC, the angle of tilt for the alkyl chain increased from 40 degrees to 48 degrees to 53 degrees , respectively, whereas the thickness of the whole layer and that of the PC head region was largely constant. The almost vertical projection of the PC headgroup from these single alkyl chain surfactants is in sharp contrast to its strongly tilted conformation, as reported for dichain phospholipids such as dipalmitoyl glycerol phosphocholine (DPPC).     

Adsorbed and spread beta-casein monolayers at oil-water interfaces

A previous study (Langmuir 2003, 19, 8436) used a Langmuir type pendant drop film balance to form beta-casein monolayers at the air-water interface. The present paper reports the application of that technique to the formation of protein monolayers at liquid interfaces. This technique allows a direct comparison between spread and adsorbed beta-casein interfacial behaviors that is presented in terms of their pi-A isotherms and static elasticity moduli. Pi-A isotherms of adsorbed and spread protein have been compared and found to be fairly similar in shape, stability, and also hysteresis phenomena. Examination of the elasticity moduli of both layers shows a similar analogy although slight differences arise and are interpreted in terms of the protein unfolding extent attained by both procedures at the oil interface.     

Langmuir monolayers of a photoisomerizable macrocycle surfactant

An amphiphilic photoisomerizable macrocycle has been prepared that forms stable Langmuir monolayers at the air-water interface. The hydrophilic core of the molecule switches between closed and open isomers upon irradiation by the appropriate wavelengths of light. Isotherm measurements, Brewster angle microscope images, and atomic force micrographs (of transferred Langmuir-Blodgett films) suggest a phase transition between a face-on to a tilted edge-on molecular orientation as a function of surface concentration. In the face-on phase, in situ photoisomerization results in a reversible increase in surface pressure due to greater molecular crowding in the open configuration.     

Interfacial behavior of lipid nanocapsules spread on model membrane monolayers

The lipid nanocapsules (LNCs) spread at the air–water interface (A/W) undergo destabilization and disaggregation leading to formation of a triglyceride (TG) surface film. The kinetics of reorganization and formation of TG surface film were followed by measuring either the change of surface pressure at constant area or the surface area at constant surface pressure. From the obtained experimental data were determined the effectiveness of TG spreading and the rate of LNC disaggregation at A/W interface covered with preformed model membrane monolayers of DPPC, Curosurf®, and mucus. Partial LNC stabilization due to their interaction with the model membrane monolayers was observed and characterized by atomic force microscopy (AFM). The obtained results demonstrated that the LNCs spread on mucus surface layer, which models the epithelial surface were more stable than if they were spread either on DPPC or Curosurf® surface layers, which emulate the alveolar surface.     

Self-organization of a wedge-shaped surfactant in monolayers and multilayers

The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at approximately 10 degrees C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are approximately 0.5 nm thinner than the LC regions. AFM images were also obtained of self-assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self-assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.     

Adsorption of oil into surfactant monolayers and structure of mixed surfactant+oil films

We describe a tensiometric method for determining the adsorption isotherm of an oil on a surfactant monolayer adsorbed at the air–water surface. The method involves measuring the surface pressure of oil, π_oil, as a function of its activity, a_oil, varied by changing the relative vapour pressure. We compare the isotherm of dodecane adsorption onto a C₁₂E₅ monolayer determined in this way with that measured directly using neutron reflectivity. The agreement between the two allows us to conclude that, at least for this system, addition of oil does not result in a change in the chemical potential of the surfactant. Structural analysis of the dodecane+C₁₂E₅ mixed film has been performed with neutron reflectivity using two contrasts. In one, only the surfactant chain region is highlighted, whilst in the other only the oil film is visible. We document, for the first time, changes in thickness and packing density of both the oil and surfactant chains in the mixed surfactant+oil layer upon increasing oil content. For a diverse range of other oil+surfactant systems, we have determined the initial (S_i) and equilibrium (S_e) spreading coefficients of oil by measuring π_oil following addition of liquid oil (at unit activity) to surfactant solution surfaces. Those systems in which S_e is close to zero display a repulsive van der Waals component of the disjoining pressure-oil film thickness isotherm, whilst with toluene as oil the calculated isotherm is attractive, consistent with non-spreading observed for this oil.     

Cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers spread on silicic acid substrates

Mixed cholesterol-dipalmitoyl cephalin and cholesterol-dilauroyl cephalin monolayers are slightly more expanded on silicic acid substrates than on silica-free substrates. Plotting the mean molecular area of the mixed monolayers against the mole fraction of cephalin shows that cholesterol produces condensation of the cephalin monolayer whether or not the substrate contains silicic acid, and the more expanded the pure phospholipid film, the greater is the condensation produced. These phenomena have been tentatively interpreted in terms of hydrophobic interactions between hydrocarbon chains and electrostatic interactions between the horizontally oriented polar groups of the cephalin molecules.     

Adsorption of CETP on monolayers formed from HDL extracted lipids

To obtain information on the interactions between CETP and HDL3 lipoproteins, we have studied (by surface tension measurements) the adsorption of the CETP at the air–water interface and at the interface between the water and monolayers formed by spreading of lipids extracted from HDL3. We have compared the interfacial behavior of CETP and ApoA-1 (the constitutive protein of HDL3); and the influence of monolayers composition and pressure on the kinetics of the CETP adsorption. The results obtained show that CETP was more expanded than the ApoA-1 which adsorbed more strongly at the air–water interface. CETP adsorbs more and quickly at the lipid interface that at the air–interface, specially for 20% fraction of cholesterol in the monolayer. Our results show that the adsorption of the CETP at the HDL3 surface lipids are strongly dependent of the composition of the monolayer and that the exclusion pressure of CETP varied from 31 to 33.7 mN m⁻¹ with the addition of cholesterol. Finally, the kinetics of the adsorption at water–lipid interface exhibited two steps (quick increase followed by slow decrease of the excess surface pressure) which should indicate a penetration into monolayer followed by a partial desorption of phospholipids with or without cholesterol corresponding to a proteolipid association.     

Simulating the effect of surfactant structure on bending moduli of monolayers

We have used dissipative particle dynamics to simulate amphiphilic monolayers on the interface between oil and water. An ultralow interfacial tension is imposed by means of Monte Carlo to resemble the amphiphilic films that separate oil and water regions in microemulsions. We calculate the bending modulus by analyzing the undulation spectrum. By varying the surfactant chain length and topology we investigate the effect of surfactant structure and composition of the monolayer on the bending moduli. We find that increasing the thickness has a larger effect than increasing the density of the layer. This follows from the observations that at a given interfacial tension, the bending modulus increases with chain length and is larger for linear than branched surfactants. The increase with chain length is approximately linear, which is slower than the theoretical predictions at a fixed area. We also investigated a binary mixture of short and long surfactants compared to pure layers of the same average chain length. We find a roughly linear decrease in bending modulus with mole fraction of short surfactants. Furthermore, the mixed film has a lower bending modulus than the corresponding pure film for all mole fractions. Linking the bending moduli to the structure of the surfactants is an important step in predicting the stability of microemulsions.     

Molecular interactions of cord factor with dipalmitoylphosphatidylcholine monolayers: implications for lung surfactant dysfunction in pulmonary tuberculosis

Molecular interactions between mycobacterial cell wall lipid, cord factor (CF) and the abundant surfactant lipid, dipalmitoylphosphatidylcholine (DPPC) were investigated using Langmuir monolayers at physiological temperatures (37 degrees C). Surface topography of the films was visualized by atomic force microscopy (AFM). Thermodynamic behavior of the mixed monolayers was evaluated by investigating the molecular area excess, excess Gibbs free energy of mixing and maximum compressibility modulus (SCM(max)). Cord factor formed immiscible and thermodynamically unstable monolayers with DPPC. Monolayer presence of cord factor altered the physical state of DPPC monolayers from liquid condensed to liquid expanded with the lowering of SCM(max) from 160 to 40 mN/m, respectively. AFM imaging exhibited smooth homogenous surface topography of DPPC films which in the presence of cord factor was markedly altered with the appearance of aggregates and increased surface roughness. The results highlight the capacity of cord factor to disturb DPPC monolayer organization and structure. Interfacial presence of cord factor results in DPPC monolayer fluidization. Lung surfactant function is attributed to its ability to form well packed low compressibility films. Such molecular interactions suggest a dysfunction of lung surfactant in pulmonary tuberculosis due to surfactant monolayer fluidization.