Dynamics of β-Lactoglobulin Penetration into Langmuir Monolayers of 2D Condensating Phospholipid

Enzymatic Degradation of Oligo(ε-caprolactone)s End-Capped with Phenylboronic Acid Derivatives at the Air-Water Interface

The influence of terminal functionalization of oligo(ε-caprolactone)s (OCL) with phenylboronic acid pinacol ester or phenylboronic acid on the enzymatic degradation behavior at the air-water interface is investigated by the Langmuir monolayer degradation technique. While the unsubstituted OCL immediately degrades after injection of the enzyme lipase from Pseudomonas cepacia, enzyme molecules are incorporated into the films based on end-capped OCL before degradation. This incorporation of enzymes does not inhibit or suppress the film degradation, but retards it significantly. A specific binding of lipase to the polymer monolayer allows studying the enzymatic activity of bound proteins and the influence on the degradation process. The functionalization of a macromolecule with phenyl boronic acid groups is an approach to investigate their interactions with diol-containing biomolecules like sugars and to monitor their specified impact on the enzymatic degradation behavior at the air-water interface.

The interaction between the outer layer of a mixed ion pair amphiphile/double-chained cationic surfactant vesicle and DNA: a Langmuir monolayer study

The charge density of vesicular bilayers plays an important role in the structure characteristic of the vesicle-DNA complex for gene delivery. In this work, the charge density effect of catanionic vesicle surfaces on the association behavior of the vesicle with DNA was explored with the model Langmuir monolayer approach. The interaction of negatively charged DNA with positively charged Langmuir monolayers composed of catanionic vesicle-forming materials, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and dihexadecyldimethylammonium bromide (DHDAB), was investigated with surface pressure-area isotherms, area-time relaxation curves and Brewster angle microscope images. The results showed that the adsorption of DNA molecules onto the monolayers was enhanced with an increased DHDAB molar fraction (XDHDAB), which was apparently related to the increased charge density of the monolayers. With XDHDAB being increased up to 0.5, the mixed monolayers with a higher XDHDAB, or higher charge density, possessed a more stable characteristic at high surface pressures, at which the molecular status was close to that in a corresponding vesicular bilayer, due to the DHDAB-improved molecular packing/interaction. It was found that the composition of the mixed HTMA-DS-DHDAB monolayers at high surface pressures would be affected by the adsorbed DNA with the extent depending on XDHDAB. For the formation of stable HTMA-DS-DHDAB monolayer-DNA complexes, a strong electrostatic interaction of DNA with a monolayer of high charge density and a high monolayer stability characteristic resulting from DHDAB-improved molecular packing/interaction were thus required. The finding has an implication for the formulation of catanionic vesicles composed of an ion pair amphiphile, HTMA-DS, with DHDAB in gene delivery applications.

Penetration of milk-derived antimicrobial peptides into phospholipid monolayers as model biomembranes

Three antimicrobial peptides derived from bovine milk proteins were examined with regard to penetration into insoluble monolayers formed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG). Effects on surface pressure (Π) and electric surface potential (ΔV) were measured, Π with a platinum Wilhelmy plate and ΔV with a vibrating plate. The penetration measurements were performed under stationary diffusion conditions and upon the compression of the monolayers. The two type measurements showed greatly different effects of the peptide-lipid interactions. Results of the stationary penetration show that the peptide interactions with DPPC monolayer are weak, repulsive, and nonspecific while the interactions with DPPG monolayer are significant, attractive, and specific. These results are in accord with the fact that antimicrobial peptides disrupt bacteria membranes (negative) while no significant effect on the host membranes (neutral) is observed. No such discrimination was revealed from the compression isotherms. The latter indicate that squeezing the penetrant out of the monolayer upon compression does not allow for establishing the penetration equilibrium, so the monolayer remains supersaturated with the penetrant and shows an under-equilibrium orientation within the entire compression range, practically.

Competition between DPPC and SDS at the air-aqueous interface

Vibrational sum frequency generation spectroscopy is used to study the interactions of the charged soluble organic surfactant sodium dodecyl sulfate (SDS) with an insoluble 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer at the air-aqueous interface. Results indicate that the surfactant species compete for surface sites in the mixed system, with a lower monolayer number density of DPPC molecules being observed in the presence of dodecyl sulfate anions at the interface. Spectroscopic results also indicate that fewer dodecyl sulfate chains reside at the interface when the insoluble DPPC film is present. Increased conformational ordering of the acyl chains of both the DPPC molecules and the interfacial dodecyl sulfate anions is observed in the mixed system. Additionally, charged surfactant SDS promotes the alignment of the interfacial water molecules even in the presence of a DPPC monolayer.

Penetration of surfactin into phospholipid monolayers: nanoscale interfacial organization

Atomic force microscopy (AFM) combined with surface pressure-area isotherms were used to probe the interfacial behavior of phospholipid monolayers following penetration of surfactin, a cyclic lipopeptide produced by Bacillus subtilis strains. Prior to penetration experiments, interfacial behavior of different surfactin molecules (cyclic surfactins with three different aliphatic chain lengths--S13, S14, and S15--and a linear surfactin obtained by chemical cleavage of the cycle of the surfactin S15) has been investigated. A more hydrophobic aliphatic chain induces greater surface-active properties of the lipopeptide. The opening of the peptide ring reduces the surface activity. The effect of phospholipid acyl chain length (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine- (DPPC), and distearoylphosphatidylcholine) and phospholipid polar head (DPPC, dipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylserine) on monolayer penetration properties of the surfactin S15 has been explored. Results showed that while the lipid monolayer thickness and the presence of electrostatic repulsions from the interfacial film do not significantly influence surfactin insertion, these parameters strongly modulate the ability of the surfactin to alter the nanoscale organization of the lipid films. We also probed the effect of surfactin structure (influence of the aliphatic chain length and of the cyclic structure of the peptide ring) on the behavior of DPPC monolayers. AFM images and isotherms showed that surfactin penetration is promoted by longer lipopeptide chain length and a cyclic polar head. This indicates that hydrophobic interactions are of main importance for the penetration power of surfactin molecules.

Insertion of tear proteins into a meibomian lipids film

The eyelid meibomian gland secretions form the outer layer of the tear film. That layer functions as a lubricant during a blink, and as a barrier against intrusion of foreign bodies. The lipid film is also exposed to proteins present in the aqueous phase that may adsorb there, and thus form an integral part of the surface of the tear film, or possibly, cause disruption to the outermost layer. Therefore, the adsorption of tear proteins to the meibomian lipid layer was object of the present investigation. A model tear was set up coating a pendant drop of saline with a film of meibomian lipids and measuring variations of the interfacial pressure after the injection of tear proteins into the aqueous subphase at their physiological concentration. All tear proteins adsorbed at the interface causing the initial surface pressure to increase. For each protein, a limiting surface pressure at which a given protein was no longer able to insert into the lipid layer was found. Among the proteins tested, lipocalin was the most surface active one and inserted into the lipid layer in the whole range of surface pressure exerted by the meibomian lipid mixture. Lactoferrin, lysozyme and IgA also interacted with the lipids whereas albumin interacted more weakly. The timescale of the protein insertion into the lipid layer was of the order of 10(2) s. It was hypothesized that protein adsorption at the interface could be associated with structural changes. Indeed, the enzymatic activity of lysozyme was maintained in the presence of an outermost meibomian lipid layer that prevented its denaturation while exposure at the air/aqueous interface induced significant lysozime degradation. meibomian lipid composition is therefore functional to maintain tear proteins activity.

Simple model for prediction of surface tension of mixed surfactant solutions

A rigorous theoretical model is presented which describes the equilibrium behaviour of a surfactant mixtures at liquid/fluid interfaces. The theory describes mixtures of surfactants with different molar areas and accounts for the non-ideality of the surface layer. The theoretical results are in good agreement with experimental data and support the idea of additivity of the interaction parameters in the surface layer. The rigorous equation of state is transformed into simple relationships for the description of the adsorption behaviour of mixed surfactant systems. The model requires surface tensions of the single surfactant systems or the adsorption isotherms to construct the isotherm of the mixture while no extra interaction parameters between the different compounds are assumed. The model is tested with a number of literature data, such as mixed sodium alkyl sulfates, mixtures of betaine homologues BHB12 with BHB16, non-ionic surfactant mixtures, and anionic-nonionic mixtures (1-butanol with BHB12, and oxethylated decanol (C10EO5) with sodium dodecyl sulfate). The agreement between experimental data and the theoretical calculations is excellent. This approach can be especially important for practical applications of surfactant mixtures for which experimental data are scarce.

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.

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.

Penetration of dissolved amphiphiles into two-dimensional aggregating lipid monolayers

The review demonstrates the recent theoretical and experimental progress in the understanding of penetration systems at the air-water interface in which a dissolved amphiphile (surfactant, protein) penetrates into a Langmuir monolayer. The critical review of the existing theoretical models which describe the thermodynamics of the penetration are critically reviewed. Although a rigorous thermodynamic analysis of penetration systems is unavailable due to their complexity, some model assumptions, e.g. the invariability of the activity coefficient of the insoluble component of the monolayer during the penetration of the soluble component results in reasonable solutions. New theoretical models describing the equilibrium behaviour of the insoluble monolayers which undergo the 2D aggregation in the monolayer, and the equations of state and adsorption isotherms which assume the existence of multiple states (conformations) of a protein molecule within the monolayer and the non-ideality of the adsorbed monolayers are now available. The theories which describe the penetration of a soluble surfactant into the main phases of Langmuir monolayers were presented first for the case of the mixture of the molecules possessing equal partial molar surfaces (the mixture of homologues), with further extension of the models to include the interesting process of the protein penetration into the monolayer of 2D aggregating phospholipid. This extension was based on a concept which subdivides the protein molecules into independent fragments with areas equal to those of the phospholipid molecule. Various mechanisms for the effect of the soluble surfactant on the aggregation of the insoluble component were considered in the theoretical models: (i) no effect on the aggregate formation process; (ii) formation of mixed aggregates; and (iii) the influence on the aggregating process via the change of aggregation constant, but without any formation of mixed aggregates. Accordingly depending on the mechanism, different forms of the equations of state of the monolayer and of the adsorption isotherms of soluble surfactant are predicted. Based on the shape of the experimental pi-A isotherms, interesting conclusions can be drawn on the real mechanism. First experimental evidence has been provided that the penetration of different proteins and surfactants into a DPPC monolayer in a fluid-like state induces a first order main phase transition of pure DPPC. The phase transition is indicated by a break point in the pi(t) penetration kinetics curves and the domain formation by BAM. Mixed aggregates of protein with phospholipid are not formed. These results agree satisfactorily with the predictions of the theoretical models. New information on phase transition and phase properties of Langmuir monolayers penetrated by soluble amphiphiles are obtained by coupling of the pi(t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of beta-lactoglobulin into DPPC monolayers have shown that protein penetration occurs without any specific interactions with the DPPC molecules and the condensed phase consists only of DPPC.