Langmuir-Blodgett films formed by continuously varying surface pressure. Characterization by IR spectroscopy and epifluorescence microscopy

Continuous protein-density gradients: A new approach to correlate physical cues with cell response

To assess cellular behavior within heterogeneous tissues, such as bone, skin, and nerves, scaffolds with biophysical gradients are required to adequately replicate the in vivo interaction between cells and their native microenvironment. In this study, we introduce a strategy for depositing ultrathin films comprised of laminin-111 with precisely controlled biophysical gradients onto planar substrates using the Langmuir-Blodgett (LB) technique. The gradient is created by controlled desynchronization of the barrier compression and substrate withdrawal speed during the LB deposition process. Characterization of the films was performed using techniques such as atomic force microscopy and confocal fluorescence microscopy, enabling the comprehensive analysis of biophysical parameters along the gradient direction. Furthermore, human adipose-derived stem cells were seeded onto the gradient films to investigate the influence of protein density on cell attachment, showing that the distribution of the cells can be modulated by the arrangement of the laminin at the air-water interface. The presented approach not only allowed us to gain insights into the intricate interplay between biophysical cues and cell behavior within complex tissue environments, but it is also suited as a screening approach to determine optimal protein concentrations to achieve a target cellular output.

Compositional, structural and functional properties of discrete coexisting complexes within bronchoalveolar pulmonary surfactant

Lung surfactant (LS) stabilizes the respiratory surface by forming a film at the alveolar air-liquid interface that reduces surface tension and minimizes the work of breathing. Typically, this surface-active agent has been isolated from animal lungs both for research and biomedical applications. However, these materials are constituted by complex membranous architectures including surface-active and inactive lipid/protein assemblies. In this work, we describe the composition, structure and surface activity of discrete membranous entities that are part of a LS preparation isolated from bronchoalveolar lavages of porcine lungs. Seven different fractions could be resolved from whole surfactant subjected to sucrose density gradient centrifugation. Detailed compositional characterization revealed differences in protein and cholesterol content but no distinct saturated:unsaturated phosphatidylcholine ratios. Moreover, no significant differences were detected regarding apparent hydration at the headgroup region of membranes, as reported by the probe Laurdan, and lipid chain mobility analysed by electron spin resonance (ESR) in spite of the variety of membranous assemblies observed by transmission electron microscopy. In addition, six of the seven separated LS subfractions formed similar, essentially disordered-like, interfacial films and performed efficient surface activity, under physiologically relevant conditions. Altogether, our work show that a LS isolated from porcine lungs is comprised by a heterogenous population of membranous assemblies lacking freshly secreted unused LS complexes sustaining highly dehydrated and ordered membranous assemblies as previously reported. We propose that surfactant subfractions may illustrate intermediates in sequential structural steps within the structural transformations occurring along the respiratory compression-expansion cycles.

Polyhydroxyalkanoate Nanoparticles for Pulmonary Drug Delivery: Interaction with Lung Surfactant

Polyhydroxyalkanoates (PHA) are polyesters produced intracellularly by many bacterial species as energy storage materials, which are used in biomedical applications, including drug delivery systems, due to their biocompatibility and biodegradability. In this study, we evaluated the potential application of this nanomaterial as a basis of inhaled drug delivery systems. To that end, we assessed the possible interaction between PHA nanoparticles (NPs) and pulmonary surfactant using dynamic light scattering, Langmuir balances, and epifluorescence microscopy. Our results demonstrate that NPs deposited onto preformed monolayers of DPPC or DPPC/POPG bind these surfactant lipids. This interaction facilitated the translocation of the nanomaterial towards the aqueous subphase, with the subsequent loss of lipid from the interface. NPs that remained at the interface associated with liquid expanded (LE)/tilted condensed (TC) phase boundaries, decreasing the size of condensed domains and promoting the intermixing of TC and LE phases at submicroscopic scale. This provided the stability necessary for attaining high surface pressures upon compression, countering the destabilization induced by lipid loss. These effects were observed only for high NP loads, suggesting a limit for the use of these NPs in pulmonary drug delivery.

Pulmonary surfactant and drug delivery: Vehiculization, release and targeting of surfactant/tacrolimus formulations

This work explores the potential for strategizing pulmonary surfactant (PS) for drug delivery over the respiratory air-liquid interface: the interfacial delivery. The efficacy of PS- and interface-assisted drug vehiculization was determined both in vitro and in vivo using a native purified porcine PS combined with the hydrophobic anti-inflammatory drug Tacrolimus (TAC), a calcineurin inhibitor. In vitro assays were conducted in a novel double surface balance setup designed to emulate compression-expansion dynamics applied to interfacially connected drug donor and recipient compartments. In this setup, PS transported TAC efficiently over air-liquid interfaces, with compression/expansion breathing-like dynamics enhancing rapid interface-assisted diffusion and drug release. The efficacy of PS-assisted TAC vehiculization was also evaluated in vivo in a mouse model of lipopolysaccharide (LPS)-induced acute lung injury (ALI). In anesthetized mice, TAC combined with PS was intra-nasally (i.n) instilled prior administering i.n. LPS. PS/TAC pre-treatment caused greater TAC internalization into a higher number of lung cells obtained from bronchoalveolar lavages (BAL) than TAC pre-treatment alone. Additionally, the PS/TAC combination but not TAC or PS alone attenuated the LPS-induced pro-inflammatory effects reducing cells and proteins in BAL fluid. These findings indicated that PS-mediated increase in TAC uptake blunted the pro-injurious effects of LPS, suggesting a synergistic anti-inflammatory effect of PS/drug formulations. These in vitro and in vivo results establish the potential utility of PS to open novel effective delivery strategies for inhaled drugs.

Molecular and biophysical basis for the disruption of lung surfactant function by chemicals

With the intention to move away from animal testing for the toxicological evaluation of chemicals comes the need to develop new approach methodologies which are mechanism-anchored and target relevant key events leading to an adverse outcome. To date, no validated alternative methods are available for studying the acute inhalation toxicity potential of airborne chemicals but the constrained drop surfactometer measuring the surface tension of a drop of lung surfactant presents as a promising candidate. Indeed, the correlation of the increase in minimum surface tension of lung surfactant in vitro with changes in the breathing patterns of mice after inhalation of test compounds has been shown in multiple studies. However, the causal factors leading to lung surfactant inactivation remain speculative. This paper combines molecular and biophysical methods (constrained drop and captive bubble surfactometers, Langmuir-Blodgett balance, epifluorescence microscopy, cryogenic transmission electron microscopy, and differential scanning calorimetry) applied to purified porcine lung surfactant and dipalmitoylphosphatidylcholine interfacial films to gain insights into the disruption of lung surfactant function by three chemicals known to show acute inhalation toxicity (trimethoxyoctylsilane, methyl 3-oxo-2-pentylcyclopentaneacetate, and diisopentyl ether). The results of this study suggest that the test chemicals intercalate between the phospholipids at the air-liquid interface, reduce the stability of the films, and decrease the cohesivity of interface-associated multilayered structures thereby perturbing the lung surfactant surface activity. These findings contribute to a better understanding of chemically-induced lung surfactant function disruption.

Probing Interfacial Structure and Dynamics of Model and Natural Asphaltenes at Fluid-Fluid Interfaces

Asphaltenes are largely responsible for crude oil interfacial behavior. Due to their complex molecular nature, studying connections between interfacial properties and molecular structure is challenging, and these connections remain unclear. Several groups have reported on the interfacial behavior of asphaltenes, but a unified picture of both interfacial dynamics and thermodynamics is still missing. We seek to establish connections between asphaltene interfacial morphology and interfacial dynamics by combining interfacial dilatational deformation with microscopic structural imaging analysis. Understanding the behavior of natural asphaltene samples is made difficult by the inherent molecular variability. Therefore, we have also studied the behavior of an asphaltene model compound to draw fundamental structure-property relationships. This work contains simultaneous interfacial deformation and microscopy in systems of natural and model asphaltenes at air-water and decane-water interfaces. How the dynamics of natural asphaltenes influences the morphological and thermodynamic state of the air-water and decane-water interfaces is discussed based on the deviations observed between isotropic and anisotropic deformations. Areas where model asphaltenes can help us to understand the behavior of natural asphaltenes are identified such as its high surface pressure activity and aggregation character. An aggregation mechanism for model and natural asphaltenes is proposed based on an observed relationship between microscopic and millimetric aggregates.

In Vitro Functional and Structural Characterization of A Synthetic Clinical Pulmonary Surfactant with Enhanced Resistance to Inhibition

CHF5633 is a novel synthetic clinical pulmonary surfactant preparation composed by two phospholipid species, dipalmitoyl phosphatidylcholine (DPPC) and palmitoyloleoyl phosphatidylglycerol (POPG), and synthetic analogues of the hydrophobic surfactant proteins SP-B and SP-C. In this study, the interfacial properties of CHF5633 in the absence and in the presence of inhibitory serum proteins have been assessed in comparison with a native surfactant purified from porcine lungs and with poractant alpha, a widely used clinical surfactant preparation. The study of the spreading properties of CHF5633 in a Wilhelmy balance, its ability to adsorb and accumulate at air-liquid interfaces as revealed by a multiwell fluorescence assay, and its dynamic behavior under breathing-like compression-expansion cycling in a Captive Bubble Surfactometer (CBS), all revealed that CHF5633 exhibits a good behavior to reduce and sustain surface tensions to values below 5 mN/m. CHF5633 shows somehow slower initial interfacial adsorption than native surfactant or poractant alpha, but a better resistance to inhibition by serum proteins than the animal-derived clinical surfactant, comparable to that of the full native surfactant complex. Interfacial CHF5633 films formed in a Langmuir-Blodgett balance coupled with epifluorescence microscopy revealed similar propensity to segregate condensed lipid domains under compression than films made by native porcine surfactant or poractant alpha. This ability of CHF5633 to segregate condensed lipid phases can be related with a marked thermotropic transition from ordered to disordered membrane phases as exhibited by differential scanning calorimetry (DSC) of CHF5633 suspensions, occurring at similar temperatures but with higher associated enthalpy than that shown by poractant alpha. The good interfacial behavior of CHF5633 tested under physiologically meaningful conditions in vitro and its higher resistance to inactivation by serum proteins, together with its standardized and well-defined composition, makes it a particularly useful therapeutic preparation to be applied in situations associated with lung inflammation and edema, alone or in combined strategies to exploit surfactant-facilitated drug delivery.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Surface Activity as a Crucial Factor of the Biological Actions of Ole e 1, the Main Aeroallergen of Olive Tree (Olea europaea) Pollen

Aeroallergens are airborne substances-mainly proteins-capable of triggering Th2-immune responses in respiratory allergies. They enter into the body through the upper airways, reaching the mucosa afterward. Mucosae lining at the luminal side consists of an epithelial barrier completely covered by mucus and pulmonary surfactant. Both pulmonary surfactant and plasma membrane of the epithelial cells represent two physiological phospholipid-based barriers where allergens first impact before triggering their biological effects. The interaction of allergens with lipids at relevant physiological surfaces could promote structural changes on the molecule, resulting on a potential modification of its allergenic properties. In this work, we have first described the surface and phospholipid interaction capabilities of the clinically relevant aeroallergen Ole e 1, the main allergen of olive tree pollen. By using epifluorescence microscopy of Langmuir transferred films, we observed that lipid-packed ordered domains may function as a preferential location for allergen to accumulate at the air-liquid interface, an effect that is abolished in the presence of cholestenone. The possible implications of phospholipid-interfacial effects in the modification of allergen structural and functional properties will be discussed.

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

Electromodified Langmuir–Blodgett films of a cobalt-based metallosurfactant support water oxidation at turnover numbers of up to 54 000 h⁻¹.

Surface behavior of peptides from E1 GBV-C protein: Interaction with anionic model membranes and importance in HIV-1 FP inhibition

The interaction between a peptide sequence from GB virus C E1 protein (E1P8) and its structural analogs (E1P8-12), (E1P8-13), and (E1P8-21) with anionic lipid membranes (POPG vesicles and POPG, DPPG or DPPC/DPPG (2:1) monolayers) and their association with HIV-1 fusion peptide (HIV-1 FP) inhibition at the membrane level were studied using biophysical methods. All peptides showed surface activity but leakage experiments in vesicles as well as insertion kinetics in monolayers and lipid/peptide miscibility indicated a low level of interaction: neither E1P8 nor its analogs induced the release of vesicular content and the exclusion pressure values (πe) were clearly lower than the biological membrane pressure (24-30 mN m(-1)) and the HIV-1 FP (35 mN m(-1)). Miscibility was elucidated in terms of the additivity rule and excess free energy of mixing (GE). E1P8, E1P8-12 and E1P8-21 (but not E1P8-13) induced expansion of the POPG monolayer. The mixing process is not thermodynamically favored as the positive GE values indicate. To determine how E1 peptides interfere in the action of HIV-1 FP at the membrane level, mixed monolayers of HIV-1 FP/E1 peptides (2:1) and POPG were obtained. E1P8 and its derivative E1P8-21 showed the greatest HIV-1 FP inhibition. The LC-LE phase lipid behavior was morphologically examined via fluorescence microscopy (FM) and atomic force microscopy (AFM). Images revealed that the E1 peptides modify HIV-1 FP-lipid interaction. This fact may be attributed to a peptide/peptide interaction as indicated by AFM results. Finally, hemolysis assay demonstrated that E1 peptides inhibit HIV-1 FP activity.

Composition, structure and mechanical properties define performance of pulmonary surfactant membranes and films

The respiratory surface in the mammalian lung is stabilized by pulmonary surfactant, a membrane-based system composed of multiple lipids and specific proteins, the primary function of which is to minimize the surface tension at the alveolar air-liquid interface, optimizing the mechanics of breathing and avoiding alveolar collapse, especially at the end of expiration. The goal of the present review is to summarize current knowledge regarding the structure, lipid-protein interactions and mechanical features of surfactant membranes and films and how these properties correlate with surfactant biological function inside the lungs. Surfactant mechanical properties can be severely compromised by different agents, which lead to surfactant inhibition and ultimately contributes to the development of pulmonary disorders and pathologies in newborns, children and adults. A detailed comprehension of the unique mechanical and rheological properties of surfactant layers is crucial for the diagnostics and treatment of lung diseases, either by analyzing the contribution of surfactant impairment to the pathophysiology or by improving the formulations in surfactant replacement therapies. Finally, a short review is also included on the most relevant experimental techniques currently employed to evaluate lung surfactant mechanics, rheology, and inhibition and reactivation processes.

Adaptations to hibernation in lung surfactant composition of 13-lined ground squirrels influence surfactant lipid phase segregation properties

Pulmonary surfactant lines the entire alveolar surface, serving primarily to reduce the surface tension at the air-liquid interface. Surfactant films adsorb as a monolayer interspersed with multilayers with surfactant lipids segregating into different phases or domains. Temperature variation, which influences lipid physical properties, affects both the lipid phase segregation and the surface activity of surfactants. In hibernating animals, such as 13-lined ground squirrels, which vary their body temperature, surfactant must be functional over a wide range of temperatures. We hypothesised that surfactant from the 13-lined ground squirrel, Ictidomys tridecemlineatus, would undergo appropriate lipid structural re-arrangements at air-water interfaces to generate phase separation, sufficient to attain the low surface tensions required to remain stable at both low and high body temperatures. Here, we examined pressure-area isotherms at 10, 25 and 37°C and found that surfactant films from both hibernating and summer-active squirrels reached their highest surface pressure on the Wilhelmy-Langmuir balance at 10°C. Epifluorescence microscopy demonstrated that films of hibernating squirrel surfactant display different lipid micro-domain organisation characteristics than surfactant from summer-active squirrels. These differences were also reflected at the nanoscale as determined by atomic force microscopy. Such re-arrangement of lipid domains in the relatively more fluid surfactant films of hibernating squirrels may contribute to overcoming collapse pressures and support low surface tension during the normal breathing cycle at low body temperatures.

Phase-field model for the morphology of monolayer lipid domains

Phase-separated domains exist in multicomponent lipid monolayers and bilayers. We present here a phase-field model that takes into account the competition between lipid dipole-dipole interactions and line tension to define the domain morphology. A dynamic equation for the phase-field is solved numerically showing stationary non-circular shapes like starfish shapes. This phase-field model could be applied to study the dynamic properties of complex problems like phase segregation in pulmonary surfactant membranes and films.

Interfacial behavior of recombinant forms of human pulmonary surfactant protein SP-C

The behavior at air-liquid interfaces of two recombinant versions of human surfactant protein SP-C has been characterized in comparison with that of native palmitoylated SP-C purified from porcine lungs. Both native and recombinant proteins promoted interfacial adsorption of dipalmitoylphosphatidylcholine bilayers to a limited extent, but catalyzed very rapid formation of films from different lipid mixtures containing both zwitterionic and anionic phospholipids. Once at the interface, the recombinant variants exhibited compression-driven structural transitions, consistent with changes in the orientation of the deacylated N-terminal segment, which were not observed in the native protein. Compression isotherms of lipid/protein films suggest that the recombinant SP-C forms promote expulsion at high pressures of a higher number of lipid molecules per mole of protein than does native SP-C. A more dynamic conformation of the N-terminal segment in recombinant SP-C forms is likely also responsible for facilitating compression-driven condensation of domains in anionic phospholipid films as observed by epifluorescence microscopy. Finally, both native palmitoylated SP-C and the phenylalanine-containing recombinant versions facilitate similarly the repetitive compression-expansion dynamics of lipid/protein films, which were able to reach maximal surface pressures with practically no hysteresis along multiple quasi-static or dynamic cycles.

Meconium impairs pulmonary surfactant by a combined action of cholesterol and bile acids

Mechanisms for meconium-induced inactivation of pulmonary surfactant as part of the meconium aspiration syndrome in newborn infants, to our knowledge, are not clearly understood. Here we have studied the biophysical mechanisms of how meconium affects surface activity of pulmonary surfactant and whether the membrane-perturbing effects of meconium can be mimicked by exposure of surfactant to a mixture of bile acids and cholesterol. Surface activity of pulmonary surfactant complexes purified from animal lungs was analyzed in the absence and in the presence of meconium in standard surface balances and in a captive bubble surfactometer. We have also evaluated accumulation of surfactant at the air-liquid interface by what we believe to be a novel microtiter plate fluorescent assay, and the effect of meconium components on surfactant membrane fluidity using Laurdan fluorescence thermotropic profiles and differential scanning calorimetry thermograms. Rapid interfacial adsorption, low surface tension upon film compression, efficient film replenishment upon expansion, and thermotropic properties of surfactant complexes are all adversely affected by meconium, and, in a similar manner, they are affected by cholesterol/taurocholate mixtures but not by taurocholate alone. We conclude that inhibition of surfactant by meconium can be mimicked by a bile salt-promoted incorporation of excess cholesterol into surfactant complexes. These results highlight the potential pathogenic role of cholesterol-mobilizing agents as a crucial factor resulting in cholesterol induced alterations of structure and dynamics of surfactant membranes and films.

Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.

Structural characterization of the monolayer-multilayer transition in a pulmonary surfactant model: IR studies of films transferred at continuously varying surface pressures

The four-component system acyl chain perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC)/1,2-dipalmitoylphosphatidylglycerol/ (DPPG)/pulmonary surfactant protein SP-C/cholesterol provides a useful model for in vitro biophysical studies of the reversible monolayer to multilayer transition that occurs during compression <--> expansion cycles in the lung. Monolayer films of this mixture (with chain perdeuterated DPPC-d62) at the air/water interface have been transferred to solid substrates under conditions of continuously varying surface pressure, an approach termed COVASP (continuously varying surface pressures) (Langmuir 2007, 23, 4958). The thermodynamic properties of the Langmuir films have been examined with pressure-area isotherms, while the molecular properties of the film constituents in the transferred films in the monolayer and multilayer phases have been examined with IR spectroscopy. Quantitative intensity measurements of the DPPC-d62, DPPG, and SP-C components in each phase reveal that the DPPG and SP-C constituents are relatively enriched in the multilayer compared with the DPPC-d62, although all three species are present in both phases. Some molecular structure information is available from the surface-pressure-induced variation in IR parameters. The DPPC-d62 exhibits slightly increased conformational order in the multilayer phase as detected from decreases in the CD2 stretching frequencies upon compression, while the lipid phosphate residues become dehydrated, as deduced from increases in the 1245 cm-1 symmetric PO2- stretching frequency. A small increase is observed in the protein amide I frequency; possible interpretations of these changes are presented. The current observations are compared with ideas contained in the "squeeze-out hypothesis" (Handbook of Physiology, The Respiratory System; American Physiological Society Press: Bethesda, MD, 1986; Vol. III, p 247) and in the "liquid crystalline collapse" model (Biophys. J. 2003, 84, 3792). Within the limitation of the current procedures, the data contain elements from both these descriptions of the monolayer transformation. Extensions and possible limitations of the COVASP-IR method are discussed.