Stability investigation of the mixed DPPC/protein monolayer at the air–water interface

Studies of crab digestive phospholipase acting on phospholipid monolayers: Activation by temperature

Secreted phospholipases A2 (sPLA2) are water-soluble lipolytic enzymes that act at the interface of organized lipid substrates, where the catalytic step is coupled to various interfacial phenomena as enzyme penetration, solubilisation of reaction products, lateral packing and loss of mechanical stability of organized assemblies of phospholipid molecule, among others. Using the monomolecular film technique, we compared the interfacial properties of crab digestive sPLA₂ (CDPL) with those of the porcine pancreatic one (PPPL). A kinetic study on the surface pressure dependency of the two sPLA₂ was performed using monomolecular films of three different substrates: di C₁₂-PC (1.2-dilauroyl-sn-glycerol-3-phosphocholine); di C₁₂-PG (1.2-dilauroyl-sn-glycerol-3-phosphoglycerol) and di C₁₂-PE (1.2-dilauroyl-sn-glycerol-3-phosphoethanolamine). The use of a substrate in monolayer state, during the catalytic reactions, allows us to monitor the effect of several physicochemical parameters by altering the "quality of interface". The effect of temperature on the hydrolysis rate of these substrates was also checked. Our results show that activities of both phospholipases were affected by the variation of the subphase temperature. CDPL was irreversibly inactivated by p-bromo-phenacyl bromide, the specific inhibitor of sPLA2. The hyperbolic catalytic behaviour observed was coherent with hopping mode of action, one of the two characteristic mechanisms of interfacial catalysis of sPLA2.

Behaviour of protein (BSA)-lipid (DMPA) mixed monolayer on the spreading order of the individual component

Surface pressure (π) - mean molecular area (A) isotherms of protein (BSA) - lipid (DMPA) mixed films are examined by varying their ratio and altering the spreading order of BSA and DMPA on the water surface to study the protein-lipid interactions and the corresponding structures and patterns at different interfacial conditions. π-A isotherms and compression-decompression isotherm cycles of protein-lipid mixed monolayers below and above of the isoelectric point of BSA (pI ≈ 4.8) are also examined. Below the isoelectric point of BSA (pH ≈ 4.0), i.e., when BSA is weakly hydrophobic and has net positive charge shows low hysteresis irrespective of the spreading order of the molecules. However, at pH ≈ 7.0, i.e., when the overall charge of BSA is negative and is strongly hydrophobic the protein-lipid mixed films display higher hysteresis value. Besides the properties of the isotherms, the surface morphology and secondary conformations of protein inside the mixed films are obtained from X-ray reflectivity, atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy respectively after depositing the mixed films on solid substrates. Nearly similar information is obtained after altering the spreading order of BSA and DMPA, which indicates that the spreading of molecules on the water surface is one of the better ways of forming the lipid-protein mixed film at the air-water interface.

Influence of PPH dendrimers' surface functions on the activation of human monocytes: a study of their interactions with pure lipid model systems

The influence of surface functions on the interactions between Poly(PhosphorHydrazone) PPH dendrimers and human monocytes is discussed on the basis of complementary biological and physicochemical studies on membrane models (monolayers and multi-lamellar vesicles). The studies were performed on both an active and non-toxic phosphonic acid capped dendrimer and a non-active but toxic carboxylic acid capped one. On the one hand, comparative studies of the behaviour of DPPC monolayers in the presence or absence of PPH dendrimers in the subphase showed differences in the phase transitions, highlighting interactions between both dendrimers and phospholipid monolayers, with a larger incidence for the carboxylic acid capped dendrimer (negative control), validating its cellular toxicity. On the other hand, comparative biological studies (activation of human monocytes and binding of fluorescent dendrimers on human monocytes) show the pre-eminence of phosphonic acid capped dendrimers towards specific binding and subsequent activation of human monocytes.

Serum albumin in 2D: a Langmuir monolayer approach

Understanding of protein interaction at the molecular level raises certain difficulties which is the reason a model membrane system such as the Langmuir monolayer technique was developed. Ubiquitous proteins such as serum albumin comprise 50% of human blood plasma protein content and are involved in many biological functions. The important nature of this class of protein demands that it be studied in detail while modifying the experimental conditions in two dimensions to observe it in all types of environments. While different from bulk colloidal solution work, the two dimensional approach allows for the observation of the interaction between molecules and subphase at the air-water interface. Compiled in this review are studies which highlight the characterization of this protein using various surroundings and also observing the types of interactions it would have when at the biomembrane interface. Free-energy changes between molecules, packing status of the bulk analyte at the interface as well as phase transitions as the monolayer forms a more organized or aggregated state are just some of the characteristics which are observed through the Langmuir technique. This unique methodology demonstrates the chemical behavior and physical behavior of this protein at the phase boundary throughout the compression of the monolayer.

Effects of surface pressure on the structure of distearoylphosphatidylcholine monolayers formed at the air/water interface

The structure of the monolayer formed at an air/water interface by the phospholipid distearoylphosphatidylcholine (DSPC) has been determined as a function of the monolayer surface pressure (pi) using Brewster angle microscopy and neutron reflectivity. The microscopy studies demonstrate that the DSPC molecules form an extremely homogeneous monolayer on the water surface with no evidence of any domain formation. The neutron reflectivity measurements provide information on the thickness of the DSPC alkyl chains, head groups, and associated solvent distributions, along with the separations between these distributions and the interfacial area per molecule. Partial structure factor analyses of the reflectivity data show that the area occupied by each DSPC molecule decreases from 49 A2 at pi = 20 mN/m to 44 A2 at pi = 50 mN/m. There are concomitant increases in the widths of the lipids' alkyl chains and headgroup distributions (modeled as Gaussians), with the former rising from 18 A (at pi = 20 mN/m) to 20 A (at pi = 50 mN/m) and the latter rising from 14 A (at pi = 20 mN/m) to 18 A (at pi = 50 mN/m). The compression of the monolayer is also shown to give rise to an increased surface roughness, the principal component of which is found to be the thermal roughness caused by capillary waves. At all surface pressures studied (covering the range from 20 to 50 mN/m), the alkyl chains and head groups of the DSPC are found to have roughly the same orientations, with the alkyl chains tilted with respect to the surface normal by about 34 degrees and the head groups lying parallel to the interface normal, projecting vertically down into the aqueous subphase. Given the various trends noted on how the structure of the DSPC monolayer changes as a function of pi, we extrapolate to consider the structure of the monolayer immediately before its collapse.

Dynamic adsorption and characterization of phospholipid and mixed phospholipid/protein layers at liquid/liquid interfaces

Drop profile analysis tensiometry is applied to study the adsorption dynamics of phospholipids, proteins and phospholipid/protein mixtures at liquid/liquid interfaces. Measurements of the dynamic interfacial tension of phospholipid layers give information on the adsorption mechanism and the structure of the adsorption layer. The equilibrium and dynamic adsorption of pure protein solutions, i.e. human serum album (HSA), beta-lactoglobulin (beta-LG), beta-casein (beta-CA), can be explained well by the thermodynamic model of Frumkin and the diffusion-controlled adsorption theory. The adsorption behavior from mixed phospholipid/protein solutions was also investigated in terms of dynamic interfacial tensions. Interestingly, a "skin-like" folded film of pure protein or phospholipid/protein complex layers can be observed at curved surfaces at the water/oil interfaces. The addition of phospholipids accelerates the formation of the folded structure at the drop surface through co-adsorption of proteins.

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.

Hydrolysis characterization of phospholipid monolayers catalyzed by different phospholipases at the air-water interface

Combination of some newly developed microscopic and spectroscopic techniques with conventional Langmuir monolayer method can provide more quantitative information with the molecular orientation and reorganization process of spread amphiphilic molecules at the air/water interface. These techniques are extended to investigate the hydrolysis process of spreading lipid monolayer catalyzed by different enzymes, phospholipases A2, C and D, respectively. Synchrotron X-ray diffraction and infrared reflection absorption spectroscopy are able directly to give the structural information of the assembled monolayer, interfacial activity of amphiphiles and their components at the interface. Microscopic technique such as Brewster angle microscopy (BAM), fluorescence microscopy (FM) can be used to trace the morphological changes dynamically as the spreading lipid monolayer is hydrolyzed at the air/water interface. We summary here some latest progress in this filed and give a brief review over the hydrolysis features of phospholipid monolayer catalyzed by different enzymes. It is attempted to establish a model of membrane hydrolysis process in order to better understand the mechanism of membrane metabolism and signal transduction in a living system.

Induced removal of dipalmitoyl phosphatidylcholine by the exclusion of fibrinogen from compressed monolayers at air/liquid interfaces

The induced removal of dipalmitoyl phosphatidylcholine (DPPC) by the exclusion of fibrinogen from mixed DPPC/fibrinogen monolayers at compressed air/liquid interfaces was analyzed. The surface pressure-area hysteresis curves of the monolayers at interfaces were obtained by a Langmuir trough. The hysteresis curves of equilibrium fibrinogen adsorption layers suggest that fibrinogen desorption during the area compression stage became significant at a higher bulk concentration of 1000 ppm. For mixed monolayers of DPPC with fibrinogen, the fibrinogen molecules were expelled from the interface upon compression due to the presence of insoluble DPPC molecules. The squeeze-out of fibrinogen molecules evidently removed a significant number of DPPC molecules from the interface, with the extent depending on fibrinogen surface concentration. During the subsequent area expansion stage, fibrinogen molecules entered the interface and participated in the rise of surface pressure. The induced loss of free DPPC molecules at the interface by the expelled fibrinogen molecules during the area compression stage was then evaluated from the hysteresis curves.

In situ measurement of the displacement of protein films from the air/water interface by surfactant

The displacement of spread protein films from the air/water interface by surfactant was followed using Brewster angle microscopy (BAM) and interfacial rheology. The displacement of beta-lactoglobulin and beta-casein by a nonionic surfactant was monitored as a function of both surface pressure and time. In both cases, protein displacement occurred over the same surface pressure range that had been observed previously by atomic force microscopy (AFM). In the case of the beta-lactoglobulin, surfactant domains grew large enough in the protein film to be visible in the BAM images. The shapes of the domains were very similar to those seen previously by AFM in the late stages of displacement. The results from both proteins confirm the results published previously while highlighting some implications for the application of the "orogenic" model of displacement for large protein films. The surface rheological data showed that the beta-lactoglobulin/surfactant mixed film retained much of its elasticity until the latter stages of displacement. This indicates that at least in the early stages of displacement, the mixed film was dominated by the behavior of the protein in the film.