X-Ray Diffraction and Reflectivity Validation of the Depletion Attraction in the Competitive Adsorption of Lung Surfactant and Albumin

Lung surfactant (LS) and albumin compete for the air-water interface when both are present in solution. Equilibrium favors LS because it has a lower equilibrium surface pressure, but the smaller albumin is kinetically favored by faster diffusion. Albumin at the interface creates an energy barrier to subsequent LS adsorption that can be overcome by the depletion attraction induced by polyethylene glycol (PEG) in solution. A combination of grazing incidence x-ray diffraction (GIXD), x-ray reflectivity (XR), and pressure-area isotherms provides molecular-resolution information on the location and configuration of LS, albumin, and polymer. XR shows an average electron density similar to that of albumin at low surface pressures, whereas GIXD shows a heterogeneous interface with coexisting LS and albumin domains at higher surface pressures. Albumin induces a slightly larger lattice spacing and greater molecular tilt, similar in effect to a small decrease in the surface pressure. XR shows that adding PEG to the LS-albumin subphase restores the characteristic LS electron density profile at the interface, and confirms that PEG is depleted near the interface. GIXD shows the same LS Bragg peaks and Bragg rods as on a pristine interface, but with a more compact lattice corresponding to a small increase in the surface pressure. These results confirm that albumin adsorption creates a physical barrier that inhibits LS adsorption, and that PEG in the subphase generates a depletion attraction between the LS aggregates and the interface that enhances LS adsorption without substantially altering the structure or properties of the LS monolayer.     

Molecular weight dependence of the depletion attraction and its effects on the competitive adsorption of lung surfactant

Albumin competes with lung surfactant for the air-water interface, resulting in decreased surfactant adsorption and increased surface tension. Polyethylene glycol (PEG) and other hydrophilic polymers restore the normal rate of surfactant adsorption to the interface, which re-establishes low surface tensions on compression. PEG does so by generating an entropic depletion attraction between the surfactant aggregates and interface, reducing the energy barrier to adsorption imposed by the albumin. For a fixed composition of 10 g/L (1% wt.), surfactant adsorption increases with the 0.1 power of PEG molecular weight from 6 kDa-35 kDa as predicted by simple excluded volume models of the depletion attraction. The range of the depletion attraction for PEG with a molecular weight below 6 kDa is less than the dimensions of albumin and there is no effect on surfactant adsorption. PEG greater than 35 kDa reaches the overlap concentration at 1% wt. resulting in both decreased depletion attraction and decreased surfactant adsorption. Fluorescence images reveal that the depletion attraction causes the surfactant to break through the albumin film at the air-water interface to spread as a monolayer. During this transition, there is a coexistence of immiscible albumin and surfactant domains. Surface pressures well above the normal equilibrium surface pressure of albumin are necessary to force the albumin from the interface during film compression.     

Enhanced surfactant adsorption via polymer depletion forces: a simple model for reversing surfactant inhibition in acute respiratory distress syndrome

Lung surfactant adsorption to an air-water interface is strongly inhibited by an energy barrier imposed by the competitive adsorption of albumin and other surface-active serum proteins that are present in the lung during acute respiratory distress syndrome. This reduction in surfactant adsorption results in an increased surface tension in the lung and an increase in the work of breathing. The reduction in surfactant adsorption is quantitatively described using a variation of the classical Smolukowski analysis of colloid stability. Albumin adsorbed to the interface induces an energy barrier to surfactant diffusion of order 5 k(B)T, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of 100. Adding hydrophilic, nonadsorbing polymers such as polyethylene glycol to the subphase provides a depletion attraction between the surfactant aggregates and the interface that eliminates the energy barrier. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Depletion forces can likely be used to overcome barriers to adsorption at a variety of liquid-vapor and solid-liquid interfaces.     

Role of the surface excess of palmitoyl coenzyme A in the 1-acylglycerol-3-phosphate acyltransferase reaction catalyzed by microsomes.

Surface excess values for palmitoyl-coenzyme A have been determined at the air-water interface. In the bulk concentration range of 0.23 to 3.7 muM, the surface concentration of palmitoyl-CoA ranges from 0.7 to 1.4 times 10- minus 10 mol/cm-2. The molecules of palmitoyl-CoA in the surface layer behave as if they were in a monolayer with each molecule occupying a limiting molecular area of 79 A-2. The distribution of palmitoyl-CoA between bulk and surface phases can be described by a Langmuir adsorption isotherm with an equilibrium constant of 0.33 muM. This constant is identical to the apparent Km for palmitoyl-CoA in the 1-acylglycerol-3-phosphate acyltransferase reaction catalyzed by microsomes. The results of this study, together with those from earlier work, suggest that the observed saturation behavior of the enzymatic reaction reflects the formation of a positive surface excess of palmitoyl-CoA in the vicinity of the catalytic site.     

Organophosphorus hydrolase at the air-water interface: secondary structure and interaction with paraoxon

The secondary structure of organophosphorus hydrolase (OPH) at the air-water interface was studied using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The shape and position of the amide I and amide II bands were used to estimate the surface conformation and orientation of OPH. The PM-IRRAS results indicated that the enzyme did not unfold for the range of surface pressure used (0-30 mN/m). At low surface pressures, the signal of amide I was very weak and the intensity was almost the same as amide II. Upon further compression, the PM-IRRAS signal and the ratio of the intensity of amide I and amide II both increase, implying an increased interfacial concentration of the enzyme. From the amide I/amide II ratio and the band position, it was deduced that the enzyme adopts a conformation which gives a higher occupied surface at low surface pressure and rotates to a more vertical orientation at high surface pressures. The compression and decompression of the OPH monolayer indicated that the fingerprint of the secondary structure at the air-water interface was reversible. PM-IRRAS was also used to investigate the pH effect of the subphase on the secondary structure of OPH. The secondary structure of OPH at the air-water interface was well defined when the pH of the subphase was near its isoelectric point (IP, pH 7.6). However, it adopted a different orientation when the subphase pH values were higher or lower than the IP with formation of random coil structure. The hydrolysis of organophosphorus compound paraoxon by OPH was also studied at the air-water interface by PM-IRRAS. The pH effect and the interaction with paraoxon both seem to orientate the enzyme more in the plane of the interface and to produce random coil structure.     

Thermodynamic and dynamic characteristics of hydroxypropylmethylcellulose adsorbed films at the air-water interface

Surface pressure isotherms and structural and surface dilatational properties of three hydroxypropylmethycelluloses (HPMCs, called E4M, E50LV, and F4M) adsorbed films at the air-water interface were determined. In this work we present evidence that HPMC molecules are able to diffuse and saturate the air-water interface at very low concentrations in the bulk phase. As bulk concentration increased, structural changes at a molecular level occurred at the interface. These changes corresponded to transition from an expanded structure (structure I) to a condensed one (structure II). When the surface concentration of HPMC was high enough, the collapse of the monolayer was observed. The three HPMCs formed very elastic films at the air-water interface, even at low surface pressures. E4M showed features that make it unique. For instance it showed the highest surface activity, mainly at low bulk concentrations (<10(-4) wt %). The differences observed in surface activity may be attributed to differences in the hydroxypropyl molar substitution and molecular weight of HPMC. All three HPMCs formed films of similar viscoelasticity and elastic dilatational modulus, which can be accounted for by their similar degree of methyl substitution.     

Pulmonary surfactant proteins SP-B and SP-C in spread monolayers at the air-water interface: II. Monolayers of pulmonary surfactant protein SP-C and phospholipids.

The interaction of the hydrophobic pulmonary surfactant protein SP-C with dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and DPPC:DPPG (7:3, mol:mol) in spread monolayers at the air-water interface has been studied. At low concentrations of SP-C (about 0.5 mol% or 3 weight%protein) the protein-lipid films collapsed at surface pressures of about 70 mN.m-1, comparable to those of the lipids alone. At initial protein concentrations higher than 0.8 mol%, or 4 weight%, the isotherms displayed kinks at surface pressures of about 50 mN.m-1 in addition to the collapse plateaux at the higher pressures. The presence of less than 6 mol%, or 27 weight%, of SP-C in the protein-lipid monolayers gave a positive deviation from ideal behavior of the mean areas in the films. Analyses of the mean areas in the protein-lipid films as functions of the monolayer composition and surface pressure showed that SP-C, associated with some phospholipid (about 8-10 lipid molecules per molecule of SP-C), was squeezed out from the monolayers at surface pressures of about 55 mN.m-1. The results suggest a potential role for SP-C to modify the composition of the monolayer at the air-water interface in the alveoli.     

Overcoming rapid inactivation of lung surfactant: Analogies between competitive adsorption and colloid stability

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air-liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air-water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.     

Atomic tritium as an instrument for study of protein behavior at the air-water interface

Atomic tritium was successfully applied as an instrument for study of protein behavior at the air-water interface. Samples of lysozyme solution in 20 mM phosphate buffer (pH 7.0) with concentration of 2 mg/ml incubated at the room temperature for 1 h were exposed to bombardment with tritium atoms generated on hot tungsten wire in special vacuum device. This procedure resulted in substitution of hydrogen atoms by radioactive tritium in the thin surface layer of studied preparations. Analysis of experimental data on intramolecular radioactivity distribution in lysozyme and computer simulation of tritium bombardment allowed us to suggest two equally probable opposite orientations of lysozyme molecule in the adsorption layer at the air-water interface.     

Mechanisms of polyelectrolyte enhanced surfactant adsorption at the air-water interface

Chitosan, a naturally occurring cationic polyelectrolyte, restores the adsorption of the clinical lung surfactant Survanta to the air-water interface in the presence of albumin at much lower concentrations than uncharged polymers such as polyethylene glycol. This is consistent with the positively charged chitosan forming ion pairs with negative charges on the albumin and lung surfactant particles, reducing the net charge in the double-layer, and decreasing the electrostatic energy barrier to adsorption to the air-water interface. However, chitosan, like other polyelectrolytes, cannot perfectly match the charge distribution on the surfactant, which leads to patches of positive and negative charge at net neutrality. Increasing the chitosan concentration further leads to a reduction in the rate of surfactant adsorption consistent with an over-compensation of the negative charge on the surfactant and albumin surfaces, which creates a new repulsive electrostatic potential between the now cationic surfaces. This charge neutralization followed by charge inversion explains the window of polyelectrolyte concentration that enhances surfactant adsorption; the same physical mechanism is observed in flocculation and re-stabilization of anionic colloids by chitosan and in alternate layer deposition of anionic and cationic polyelectrolytes on charged colloids.     

Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

A novel method for the acquisition of surface enhanced Raman (SER) spectra of model membranes of dipalmitoylphosphatidic acid (DPPA) in Langmuir layers at the air-water interface is reported. The approach is based on the electrochemical formation of a buoyant thin layer of coalesced silver colloids in the vicinity of the phosphatidic acid head groups at the interface. This Ag layer is an excellent platform for SER scattering, which shows the spectral features from all parts of the molecule and water between the Ag surface and the DPPA layer. The observation of the spectral response from the phosphatidic acid head groups is of particular significance, allowing insight into their chemical state and orientation at the air-water interface.     

Biosurfactant production by Pseudomonas fluorescens 378: growth and product characteristics

Summary An isolate of Pseudomonas fluorescens, strain 378 was shown to produce a novel surface active compound (code name AP-6). The compound is unique in being a high molecular weight compound but has, in some aspects, properties of a low molecular weight surfactant. The product is extracellular and its formation appeared to be partly growth-associated. Using a semisynthetic medium, fermentor cultivations were performed in the pH range 6.8–8.4. The product yield was optimal at pH 8.0 and gave a final concentration of 210 times critical micelle dilution. At higher pH, specific growth rate, final biomass and product concentration decreased. It consists mainly of carbohydrates and protein, the molecular weight is 1×10⁶ and the isoelectric point is pH 9.1.The surface tension of an aqueous solution reached 27 mN/m which is a very low value even compared to other surfactants of considerably lower size and the critical micelle concentration was less than 10 mg/l in 0.9% (w/v) NaCl. The kinetics of the adsorption process at the air-water interface was studied using the drop volume technique, and the reaction was found to be rapid, considering the size of the molecule. A concentration as low as 0.025 g/l reached a surface tension of 30 mN/m within 70 s.     

Atomic force microscopy studies of functional and dysfunctional pulmonary surfactant films, II: albumin-inhibited pulmonary surfactant films and the effect of SP-A

Pulmonary surfactant (PS) dysfunction because of the leakage of serum proteins into the alveolar space could be an operative pathogenesis in acute respiratory distress syndrome. Albumin-inhibited PS is a commonly used in vitro model for studying surfactant abnormality in acute respiratory distress syndrome. However, the mechanism by which PS is inhibited by albumin remains controversial. This study investigated the film organization of albumin-inhibited bovine lipid extract surfactant (BLES) with and without surfactant protein A (SP-A), using atomic force microscopy. The BLES and albumin (1:4 w/w) were cospread at an air-water interface from aqueous media. Cospreading minimized the adsorption barrier for phospholipid vesicles imposed by preadsorbed albumin molecules, i.e., inhibition because of competitive adsorption. Atomic force microscopy revealed distinct variations in film organization, persisting up to 40 mN/m, compared with pure BLES monolayers. Fluorescence confocal microscopy confirmed that albumin remained within the liquid-expanded phase of the monolayer at surface pressures higher than the equilibrium surface pressure of albumin. The remaining albumin mixed with the BLES monolayer so as to increase film compressibility. Such an inhibitory effect could not be relieved by repeated compression-expansion cycles or by adding surfactant protein A. These experimental data indicate a new mechanism of surfactant inhibition by serum proteins, complementing the traditional competitive adsorption mechanism.     

Surface characterization of human serum albumin and sodium perfluorooctanoate mixed solutions by pendant drop tensiometry and circular dichroism

The interfacial behavior of mixed human serum albumin (HSA)/sodium perfluorooctanoate (C8FONa) solutions is examined by using two experimental techniques, pendant drop tensiometry and circular dichroism spectroscopy. Through the analysis of the surface tension of the mixed solutions, surface competitive adsorption at the air-water interface between C8FONa and HSA is detected. The dynamic adsorption curves exhibit the distinct regimes in their time-dependent surface tension. The nature of these regimes is further analyzed in terms of the variation of the molecules surface areas. As a consequence, a compact and dense structure was formed where protein molecules were interconnected and overlapped. Thus, a reduction of the area occupied per molecule from 100 to 0.2 nm(2) is interpreted as a gel-like structure at the surface. The presence of the surfactant seems to favor the formation of this interfacial structure. Finally, measurements of circular dichroism suggests a compaction of the protein due to the association with the surfactant given by an increase of alpha-helix structure in the complexes as compared to that of pure protein.     

Interaction of chitosan with cell membrane models at the air-water interface

In this paper we employed phospholipid Langmuir monolayers as membrane models to probe interactions with chitosan. Using a combination of surface pressure--area and surface potential--area isotherms and rheological measurements with the pendent drop technique, we observed that chitosan interacts with phospholipid molecules at the air-water interface. We propose a model in which chitosan interacts with the phospholipids mainly through electrostatic interactions, but also including H-bonding and hydrophobic forces, depending on the phospholipid packing density. At large areas per molecule, chitosan in the subphase adsorbs onto the monolayer, expanding it. At small areas per molecule, chitosan is located in the subsurface. Indeed, a mixed chitosan-phospholipid monolayer can be transferred onto solid supports, even at high surface pressures. The effects of chitosan on the viscoelastic properties of phospholipid monolayers may be taken as evidence for the ability of chitosan to disrupt cell membranes.     

Effect of Interfaces on Small, Starved Marine Bacteria

The copiotrophic marine Vibrio sp. strain DW1, shown previously in batch culture to increase in numbers at the onset of starvation and then to form viable small cells with low endogenous respiration, appears to have a survival advantage at interfaces. Vibrio sp. strain DW1 behaved differently at interfaces compared with the aqueous phase under starvation conditions: (i) small cells were observed at an air-water interface without nutrients, (ii) nutrients added to the air-water interface quickly produced larger cells at the surface, (iii) motility persisted many hours longer at the solid-water interface of a dialysis membrane in a microchamber at the onset of starvation, and (iv) regrowth and division at the solid-liquid interface occurred quickly and at nutrient concentrations too low to permit growth in the aqueous phase. It was concluded that, if small starved cells from copiotrophic bacteria can reach an interface, additional survival mechanisms become available to them: (i) interfaces constitute areas of favorable nutrient conditions, and (ii) interfaces lacking a sufficient amount of nutrient, nevertheless, trigger cells to become smaller, thus increasing their surface/volume ratio and the packing density.     

Interactions of the antifungal mycosubtilin with ergosterol-containing interfacial monolayers

Mycosubtilin, an antimicrobial lipopeptide produced by Bacillus subtilis, is characterized by strong antifungal activities. The molecular mechanisms of its biological activities on the membranes of the sensitive yeasts or fungi have not yet been clearly elucidated. Our purpose was to mimic the mycosubtilin interactions with these membranes using various Langmuir monolayers. Since the major sterol of yeasts or fungi is ergosterol, the interactions of mycosubtilin with monolayers constituted by ergosterol, DPPC/ergosterol or DPPC/sphingomyelin/ergosterol were examined at different initial surface pressures (Πi). Plotting the mycosubtilin-induced surface pressure increases versus Πi allowed to determine that the exclusion pressures of mycosubtilin from these different monolayers is higher than the surface prevailing within the biological membranes. However, this behavior was lost when mycosubtilin was interacting with ergosteryl acetate-containing monolayers. This suggests the involvement of the sterol alcohol group in the mycosubtilin interactions within membranes. Furthermore, the behavior of mycosubtilin with stigmasterol, similar to that observed with ergosterol, differs from that previously observed with cholesterol, suggesting a role of the alkyl side chain of the sterols. The adsorption of mycosubtilin to ergosterol monolayers induced changes in the lipopeptide orientation at the air-water interface as revealed by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Moreover, imaging the air-water interface by Brewster angle microscopy (BAM) indicates that mycosubtilin induced changes in the organization and morphology of monolayers containing pure ergosterol with the appearance of small condensed dots, suggesting again that the target of mycosubtilin might be the ergosterol present in the membranes of the sensitive yeasts or fungi.     

The Effect of monoglycerides on structural and topographical characteristics of adsorbed beta-casein films at the air-water interface

The effect of monoglycerides (monopalmitin and monoolein) on the structural and topographical characteristics of beta-casein adsorbed film at the air-water interface has been analyzed by means of surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM). At surface pressures lower than that for the beta-casein collapse (pi(c)(beta-casein)), attractive interactions between beta-casein and monoglycerides were observed. At higher surface pressures, the collapsed beta-casein is partially displaced from the interface by monoglycerides. However, beta-casein displacement by monoglycerides is not quantitative at the monoglyceride concentrations studied in this work. From the results derived from these experiments, we have concluded that interactions, miscibility, and displacement of proteins by monoglycerides in adsorbed mixed monolayers at the air-water interface depend on the particular protein-monoglyceride system, the interactions between film-forming components being higher for adsorbed than for spread films. The adsorbed films are more segregated than spread films, and both collapsed protein domains and monoglyceride domains in adsorbed films are smaller than for spread films.     

Changes in the orientation of mouse myeloma immunoglobulin G in monolayers after treatment with sodium deoxycholate

Molecules of normal mouse IgG are oriented horizontally in monolayers at air-water interface unlike the molecules of mouse IgG1 kappa secreted by MOPC-21 myeloma which have vertical orientation. Sodium desoxycholate processing of both preparations at concentrations below the critical micelle concentration resulted in abnormal IgG1 kappa preservation and normal IgG acquisition of vertical orientation in monolayers. When sodium desoxycholate was used for IgG modification at concentration higher than the critical micelle concentration both normal and abnormal IgG had horizontal orientation in monolayers.