Structural and topographical characteristics of adsorbed WPI and monoglyceride mixed monolayers at the air-water interface

In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.     

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.     

Structural characteristics of dipalmitoylphosphatidylcholine and hydrolysates from sunflower protein isolate mixed monolayers spread at the air-water interface

In this work, surface film balance and Brewster angle microscopy techniques have been used to analyze the structural characteristics (structure, topography, reflectivity, thickness, miscibility, and interactions) of hydrolysates from sunflower protein isolate (SPI) and dipalmitoylphosphatidylcholine (DPPC) mixed monolayers spread on the air-water interface. The degree of hydrolysis (DH) of SPI, low (5.62%), medium (23.5%), and high (46.3%), and the protein/DPPC mass fraction were analyzed as variables. The structural characteristics of the mixed monolayers deduced from the surface pressure (pi)-area (A) isotherms depend on the interfacial composition and degree of hydrolysis. At surface pressures lower than the equilibrium surface pressure of SPI hydrolysate (pi(e)(SPI hydrolysate)), both DPPC and protein are present in the mixed monolayer. At higher surface pressures (at pi > pi(e)(SPI hydrolysate)), collapsed protein residues may be displaced from the interface by DPPC molecules. The differences observed between pure SPI hydrolysates and DPPC in reflectivity (I) and monolayer thickness during monolayer compression have been used to analyze the topographical characteristics of SPI hydrolysates and DPPC mixed monolayers at the air-water interface. The topography, reflectivity, and thickness of mixed monolayers confirm at microscopic and nanoscopic levels the structural characteristics deduced from the pi-A isotherms.     

Structural analysis of PEO-PBO copolymer monolayers at the air-water interface

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethylene-b-oxybutylene (E(23)B(8)) copolymer film at the air-water interface. The XR data were fitted using both a one- and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.     

Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface

Surface shear viscosity of food emulsifiers may contribute appreciably to the long-term stability of food dispersions (emulsions and foams). In this work we have analyzed the structural, topographical, and shear characteristics of a whey protein isolate (WPI) and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface at pH 7 and at 20 degrees C. The surface shear viscosity (etas) depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity varies greatly with the surface pressure. In general, the greater the surface pressure, the greater are the values of etas. The values of etas for the mixed WPI-monoolein monolayer were more than one order of magnitude lower than those for a WPI-monopalmitin mixed film, especially at the higher surface pressures. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the shear characteristics of the mixed films. A shear-induced change in the topography and a segregation between domains of the film forming components were also observed. The displacement of the WPI by the monoglycerides is facilitates under shear conditions, especially for WPI-monoolein mixed films.     

Structural properties and Raman spectroscopy of lipid Langmuir monolayers at the air-water interface

Spectra of octadecylamine (ODA) Langmuir monolayers and egg phosphatidylcholine (PC)/ODA-mixed monolayers at the air-water interface have been acquired. The organization of the monolayers has been characterized by surface pressure-area isotherms. Application of polarized optical microscopy provides further insight in the domain structures and interactions of the film components. Surface-enhanced Raman scattering (SERS) data indicate that enhancement in Raman spectra can be obtained by strong interaction between headgroups of the surfactants and silver particles in subphase. By mixing ODA with phospholipid molecules and spreading the mixture at the air-water interface, we acquired vibrational information of phospholipid molecules with surfactant-aided SERS effect.     

Hydrogen-bonded monolayers and interdigitated multilayers at the air-water interface

Crystalline monolayers of octadecylsulfonate amphiphiles (C18S) separated by hydrophilic guanidinium (G) spacer molecules were formed at the air-water interface at a surface coverage that was consistent with that expected for a fully condensed monolayer self-assembled by hydrogen bonding between the G ions and the sulfonate groups. The surface pressure-area isotherms reflected reinforcement of this monolayer by hydrogen bonding between the G ions and the sulfonate groups, and grazing incidence X-ray diffraction (GIXD) measurements, performed in-situ at the air-water interface, revealed substantial tilt of the alkyl hydrophobes (t = 49 degrees with respect to the surface normal), which allowed the close packing of the C18 chains needed for a stable crystalline monolayer. This property contrasts with behavior observed previously for monolayers of hexadecylbiphenylsulfonate (C16BPS) and G, which only formed crystallites upon compression, accompanied by ejection of the G ions from the air-water interface. Upon compression to higher surface pressures, GIXD revealed that the highly tilted (G)C18S monolayer crystallites transformed to a self-interdigitated (G)C18S crystalline multilayer accompanied by a new crystalline monolayer phase with slightly tilted alkyl chains and disordered sulfonate headgroups. This transformation was dependent on the rate of compression, suggesting kinetic limitations for the "zipper-like" transformation from the crystalline monolayer to the self-interdigitated (G)C18S crystalline multilayer.     

The determination of work of compression of protein monolayers at the air-water interface

Summary In the present study, monolayers of various proteins were investigated at the air-water interface. The work of compression,W _c (Helmholtz free energy) has been determined from the surface pressure-area compression isotherms. A linear relationship was found betweenW _c and the amount of protein present at the surface. Further, it is shown that this relation holds both for completely unfolded (Bovine-serum-albumin, Ovalbumin) and for incompletely unfolded (Transferrin, Myoglobin) proteins.β-Lipoprotein isotherms also gave a similar dependence. It is further shown that the amount of protein lost into the subphase can be determined from a plot ofW _c versus protein added at the interface. The results are discussed in relation to the constitution of protein molecules at the surface.     

Protein displacement by monoglyceride at the air-water interface evaluated by surface shear rheology combined with Brewster angle microscopy

In this work we have used different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial shear rheology), to analyze the static (structure, topography, reflectivity, miscibility, and interactions) and flow characteristics (surface shear characteristics) of milk protein (beta-casein, caseinate, and beta-lactoglobulin) and monoglyceride (monopalmitin and monoolein) mixed films spread and adsorbed on the air-water interface. The structural, topographical, and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity (eta(s)) varies greatly with the surface pressure (pi). In general, the greater the pi values, the greater were the values of eta(s). Moreover, the eta(s) value is also sensitive to the miscibility and/or displacement of film-forming components at the interface. At surface pressures lower than that for protein collapse, protein and monoglyceride coexist at the air-water interface. At surface pressures higher than that for the protein collapse, a squeezing of collapsed protein domains by monoglycerides was deduced. Near to the collapse point, the mixed film is dominated by the presence of the monoglyceride. Different proteins and monoglycerides show different interfacial structure, topography, and shear viscosity values, confirming the importance of protein and monoglyceride structure in determining the interfacial characteristics (interactions) of mixed films. The values of eta(s) are lower for disordered (beta-casein or caseinate) than for globular (beta-lactoglobulin) proteins and for unsaturated (monoolein) than for saturated (monopalmitin) monoglycerides in the mixed film. The displacement of the protein by the monoglycerides is facilitated under shear conditions.     

Mixed monolayers of poly-alanine and lipids at the air-water interface

Summary The mixed monolayers of poly-alanine + stearyl alcohol and poly-alanine + cholesterol were studied at the air-water interface. In the mixed monolayers the surface pressure-area isotherms showed three collapse states. The first and the third collapse pressures were identical in magnitude with the collapse pressures of pure components. The intermediate collapse pressure in the poly-alanine + stearyl alcohol was found to be ca. 5 dyne/cm higher than that was observed in the poly-alanine + cholesterol system. Further, the mixed films in both systems were found to show no deviation from the ideality rule. The magnitude o f the intermediate collapse state is shown to be related to the van der Waals forces present in the lipid films.With 6 figures     

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air-water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression-expansion cycle of the monolayer is not reversible and the second pi-A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.     

Interactions of Pluronics with phospholipid monolayers at the air-water interface

Pluronics are triblock copolymers which are extensively applied excipients shown to interact with cell membranes. The aim of our study was to apply monolayer techniques and epifluorescence microscopy to investigate the interaction behavior between selected Pluronics and phospholipid monolayers which serve as a model of cell membranes. The results showed that Pluronic L61 with hydrophobic proportions much larger than those of F68 demonstrated condensed film-like surface behavior while F68 exhibited more expanded behavior. The increments of surface pressure and the changes of image were more obvious in adding Pluronic L61 than F68 to the subphase of dipalmitoylphosphatidylcholine (DPPC) monolayers, which indicated that the interaction may be related to van der Waals forces and hydrophobic interaction. Pluronics selected with higher hydrophobicities demonstrated larger surface activities and penetration abilities while being added to the subphase of DPPC and dimyristoylphosphatidylcholine (DMPC) monolayers. Pluronic P85 and F68 were found to be squeezed to subphase at higher surface pressures, which may be attributed to their relatively higher hydrophilicities.     

Close-packed monolayers of charged Janus-type nanoparticles at the air-water interface

Two DNA-block copolymers, poly(caprolactone)-DNA and poly(methyl metacrylate)-DNA, were synthesized by conjugation of a short single strand of DNA (12 or 22 mer) to a single reactive group at one end of the synthetic polymer. These polymers self-assemble in water, without the need of any cosolvent, forming micelle-like aggregates that were imaged by TEM. The solution behavior of the bioconjugated polymers was investigated by surface tension measurements. In the direction of dilution, the surface tension was measured using a down-scaled Wilhelmy plate method. To proceed in the reverse direction (concentration), we measured the surface tension of a sessile drop during its evaporation. This latter method was firstly validated using ionic and non-ionic surfactants, including polymeric surfactants. It was then applied to investigate the unimer to micelles transition of the DNA-block copolymers. In all cases, a reversible transition was observed demonstrating the existence of a critical micellar concentration, close to 0.01 mmol L⁻¹ for all the conjugates. The CMC was only slightly influenced by the length of the hydrophilic DNA block.     

Adsorption of DNA to octadecylamine monolayers at the air-water interface

In this work, surface properties of octadecylamine (ODA) monolayers in the presence of different concentrations of calf thymus DNA in the aqueous subphase covering a range of 2-8μM have been investigated. The increase of DNA concentration is accompanied by a marked increment in the expansion of the corresponding isotherms. In addition, there is a change in the profile of the isotherms ranging from an abrupt liquid-solid transition for the lipid monolayer on pure water to a slow condensation of the monolayer in a liquid state when DNA is added to the subphase, demonstrating the effective adsorption of the polynucleotide to the long chain amine monolayer. Additional phase transitions appear in the isotherms upon addition of sufficient amount of DNA, revealing the existence of specific processes such as folding or squeezing out of the DNA. This system is, however, highly reversible during compression-expansion cycles due to the strong interaction between the two components. These results are also supported by Brewster Angle Microscopy (BAM) images showing significant changes in the morphology of the film. Integral reflectivity of the BAM microscope has been used to study both isotherms themselves and the kinetic process of DNA inclusion into the lipid-like ODA monolayer. This parameter has been proven to be very effective for quantification of the monolayer processes showing high consistency with the compressibility and kinetics results.     

Interfacial orientation of DOPC liposomes spread at the air-water interface

The kinetics of surface film formation from DOPC small unilamellar vesicles spread at constant surface is studied, by measuring the time evolution of the surface pressure and of the surface potential. The thermodynamical approach describing the interfacial orientation process shows the importance of the electrical affinity of orientation and of the electrical surface pressure.     

Albumin binding and insertion into PS-b-PEO monolayers at air-water interface

Interaction of human serum albumin with poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO) monolayer at air/solution interface was studied by measuring surface pressure. The density of PEO chains in the monolayer was controlled using Langmuir trough barriers. The thickness of PS-b-PEO monolayer prior to and after albumin adsorption was computed from in situ surface plasmon resonance (SPR) measurements. Depending on the initial PEO surface density the surface pressure kinetics of albumin insertion displayed two different regimes: below the PEO “pancake-brush” transition albumin binding was initially very rapid and itself induced the “pancake-brush” transition in the monolayer, and above the “pancake-brush” transition where some albumin penetration into the free PS-b-PEO monolayer still occurred into the PEO “brush”. In the case of SPR-immobilized monolayer, more than 0.1 PEO chain/nm² was required to inhibit albumin or ferritin adsorption. A half-reduction of albumin adsorption required approx. three-fold higher PEO surface density than the half-reduction of ferritin adsorption.     

Monoglycerides and beta-lactoglobulin adsorbed films at the air-water interface. structure, microscopic imaging, and shear characteristics

In this work we have analyzed the structural, topographical, and shear characteristics of mixed monolayers formed by adsorbed beta-lactoglobulin (beta-lg) and spread monoglyceride (monopalmitin or monoolein) on a previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm, Brewster angle microscopy (BAM), and surface shear characteristics were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The pi-A isotherm and BAM images deduced for adsorbed beta-lactoglobulin-monoglyceride mixed films at pi lower than the equilibrium surface pressure of beta-lactoglobulin (pi(e)(beta-lg)) indicate that beta-lactoglobulin and monoglyceride coexist at the interface. However, the interactions between protein and monoglyceride are somewhat weak. At higher surface pressures (at pi > or = pi(e)(beta-lg)) a protein displacement by the monoglyceride from the interface takes place. The surface shear viscosity (eta(s)) of mixed films is very sensitive to protein-monoglyceride interactions and displacement as a function of monolayer composition (protein/monoglyceride fraction) and surface pressure. Shear can induce change in the morphology of monoglyceride and beta-lactoglobulin domains, on the one hand, and segregation between domains of the film-forming components on the other hand. In addition, the displacement of beta-lactoglobulin by the monoglycerides is facilitated under shear conditions.     

Interactions of anthracyclines with zwitterionic phospholipid monolayers at the air-water interface

The present note describes the use of surface pressure measurements (Langmuir monolayer technique) for the analysis of interactions of two different anthracyclines (adriamycin and daunorubicin) with a non-ionic, zwitterionic phospholipid monolayer, at the air-water interface. Because the surface membrane of the cell is the first barrier encountered by the anthracyclines in the treatment of cancer, drug-membrane interactions studied in model (monolayers or bilayers) and natural systems play an important role in the understanding of the bioactivity properties of these molecules. We report here the rate constants of the adsorption process of adriamycin and daunorubicin in the presence of a zwitterionic phospholipid monolayer at the air-water interface. Because interactions with the lipid monolayer strongly depend on the molecular packing of the lipid, we investigated this process at a relatively low surface pressure (7 mN/m), the interactions being favoured by the gaseous and liquid expanded structure of the lipid monolayer. The apparent molecular area of these molecules during the insertion into the lipid film and their interactions with the phospholipid polar head groups was evaluated and the estimated percentage of anthracyclines at the interface after adsorption into the lipid monolayer is briefly discussed. The rate constants for the adsorption and desorption process at the water-monolayer interface have been calculated on the basis of a single-exponential model. The observed difference of these parameters for daunorubicin and adriamycin suggests a different interaction of these anthracyclines during the adsorption to and/or penetration across the phospholipid monolayer.     

Miscibility of binary monolayers at the air-water interface and interaction of protein with immobilized monolayers by surface plasmon resonance technique

The miscibility and stability of the binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA) at the air-water interface and the interaction of ferritin with the immobilized monolayers have been studied in detail using surface pressure-area isotherms and surface plasmon resonance technique, respectively. The surface pressure-area isotherms indicated that the binary monolayers of DPPC and DOMA at the air-water interface were miscible and more stable than the monolayers of the two individual components. The surface plasmon resonance studies indicated that ferritin binding to the immobilized monolayers was primarily driven by the electrostatic interaction and that the amount of adsorbed protein at saturation was closely related not only to the number of positive charges in the monolayers but also to the pattern of positive charges at a given mole fraction of DOMA. The protein adsorption kinetics was determined by the properties of the monolayers (i.e., the protein-monolayer interaction) and the structure of preadsorbed protein molecules (i.e., the protein-protein interaction).