Spread Films of Human Serum Albumin at the Air-Water Interface: Optimization, Morphology, and Durability

Exploring proteins at soft interfaces and in thin liquid films - From classical methods to advanced applications of reflectometry

The history of the topic of proteins at soft interfaces dates back to the 19th century, and until the present day, it has continuously attracted great scientific interest. A multitude of experimental methods and theoretical approaches have been developed to serve the research progress in this large domain of colloid and interface science, including the area of soft colloids such as foams and emulsions. From classical methods like surface tension adsorption isotherms, surface pressure-area measurements for spread layers, and surface rheology probing the dynamics of adsorption, nowadays, advanced surface-sensitive techniques based on spectroscopy, microscopy, and the reflection of light, X-rays and neutrons at liquid/fluid interfaces offers important complementary sources of information. Apart from the fundamental characteristics of protein adsorption layers, i.e., surface tension and surface excess, the nanoscale structure of such layers and the interfacial protein conformations and morphologies are of pivotal importance for extending the depth of understanding on the topic. In this review article, we provide an extensive overview of the application of three methods, namely, ellipsometry, X-ray reflectometry and neutron reflectometry, for adsorption and structural studies on proteins at water/air and water/oil interfaces. The main attention is placed on the development of experimental approaches and on a discussion of the relevant achievements in terms of notable experimental results. We have attempted to cover the whole history of protein studies with these techniques, and thus, we believe the review should serve as a valuable reference to fuel ideas for a wide spectrum of researchers in different scientific fields where proteins at soft interface may be of relevance.

Effects of Aqueous Isotopic Substitution on the Adsorption Dynamics and Dilational Rheology of β-Lactoglobulin Layers at the Water/Air Interface

The effect of the degree of isotopic substitution of the aqueous medium on the adsorption kinetics and the surface dilational rheological behavior at the water/air interface of the globular protein β-lactoglobulin was investigated. Aqueous solutions with fixed concentrations of 1 μM protein and 10 mM hydrogenous buffer with controlled pH 7 were prepared in H2O, D2O, and an isotopic mixture of 8.1% v/v D2O in H2O (called air contrast matched water, ACMW). Using a bubble shape analysis tensiometer, we obtained various experimental dependencies of the dilational viscoelasticity modulus E as a function of the dynamic surface pressure and of the frequency and amplitude of bubble surface area oscillations, either in the course of adsorption or after having reached a steady state. In general, the results revealed virtually no effect from substituting H2O by ACMW but distinct albeit relatively weak effects for intermediate adsorption times for D2O as the aqueous phase. In the final stage of adsorption, established after around 10 h, the equilibrium adsorption and the dilational rheological behavior of all protein layers under investigation are only very weakly affected by the presence of D2O. The obtained results help to design experimental protocols for protein adsorption studies, for example, by neutron reflectivity.

Applications of serum albumins in delivery systems: Differences in interfacial behaviour and interacting abilities with polysaccharides

One of the major applications of Serum Albumins is their use as delivery systems for lipophilic compounds in biomedicine. Their biomedical application is based on the similarity with Human Serum Albumin (HSA), as a fully biocompatible protein. In general, Bovine Serum Albumin (BSA) is treated as comparable to its human homologue and used as a model protein for fundamental studies since it is available in high amounts and well understood. This protein can act as a carrier for lipophilic compounds or as protective shell in an emulsion-based vehicle. Polysaccharides are generally included in these formulations in order to increase the stability and/or applicability of the carrier. In this review, the main biomedical applications of Albumins as drug delivery systems are first presented. Secondly, the differences between BSA and HSA are highlighted, exploring the similarities and differences between these proteins and their interaction with polysaccharides, both in solution and adsorbed at interfaces. Finally, the use of Albumins as emulsifiers for emulsion-based delivery systems, concretely as Liquid Lipid Nanocapsules (LLNs), is revised and discussed in terms of the differences encountered in the molecular structure and in the interfacial properties. The specific case of Hyaluronic Acid is considered as a promising additive with important applications in biomedicine. The literature works are thoroughly discussed highlighting similarities and differences between BSA and HSA and their interaction with polysaccharides encountered at different structural levels, hence providing routes to control the optimal design of delivery systems.

Structure, morphology and reversible hysteresis nature of human serum albumin (HSA) monolayer on water surface

Compression-decompression surface pressure (π)-specific molecular area (A) isotherm cycle of human serum albumin (HSA) monolayer is performed on water surface at four different subphase pH conditions, i.e., below and above the isoelectric point (pI ≈ 4.7) of HSA molecule. For all pH conditions, the decompression curve nearly follows the compression curve, however, at pH ≈ 5.0, hysteresis is observed at higher surface pressure. Out-of-plane structures and in-plane morphologies obtained from the X-ray reflectivity and AFM studies show that only the film thickness variation takes place with the change in surface pressure, which is also evidenced from the BAM images. With increase in surface pressure, the oblate-shaped HSA molecules start tilting making an angle with the water surface and as the monolayer is decompressed the molecules regain their initial untilted monomolecular configuration. Depending upon the subphase pH and local surface charge of the specific protein molecule, electrostatic repulsive interaction dominates over the van der Waals attraction and as a result decompression curve follows the compression curve as the molecules repel each other, however, closer to the isoelectric point as strength of the interactions reverses, a hysteresis is obtained at higher surface pressure and accordingly monolayer behaviour modifies on the water surface.

β-Lactoglobulin Adsorption Layers at the Water/Air Surface: 3. Neutron Reflectometry Study on the Effect of pH

Several characteristics of β-lactoglobulin (BLG) layers adsorbed at the air/water interface exhibit a strong pH dependence, but our knowledge on the underlying structure-property relations is still fragmental. Here, we therefore extend our recent studies by neutron reflectometry (NR) and provide a comprehensive overview through direct measurements of the surface excess Γ and the layers' molecular structure. This enables comparison with available literature data to draw general conclusions. The NR experiments were performed at various pH values and within a wide range of protein concentrations, C_BLG. Adsorption kinetics measurements in air-contrast-matched-water and over a narrow Q_z range enabled direct quantification of the dynamic surface excess Γ(t) and are found to be consistent with ellipsometry data. Near the isoelectric point, pI, the rates of adsorption and Γ are maximal but only at sufficiently high C_BLG. NR data collected over a wider Q_z range and in two aqueous isotopic contrasts revealed the structure of adsorbed BLG layers at a steady state close to equilibrium. Independent of the pH, BLG was found to form dense monolayers with average thicknesses of 1.1 nm, suggesting flattening of the BLG globules upon adsorption as compared with their bulk dimensions (≈3.5 nm). Near pI and at sufficiently high C_BLG, a thick (≈5.5 nm) but looser secondary sublayer is additionally formed adjacent to the dense primary monolayer. The thickness of this sublayer can be interpreted in terms of disordered BLG dimers. The results obtained and notably the specific interfacial structuring of BLG near pI complement previous observations relating the impact of solution pH and C_BLG on other interfacial characteristics such as surface pressure and surface dilational viscoelasticity modulus.

Irremovable Blood Stain in Lung: Air-to-Interface Transport of Albumin and Its Mechanical Response to Biaxial Compression/Expansion

Serum proteins are believed to trigger a sudden failure of lung function, but to date the mechanism remains elusive. Most studies have focused on the transport of the proteins from the subphase to the lung surfactant interface, although the opposite direction of transport, i.e., from air-to-interface, could be equally important. Here, we report that physiological concentrations of serum droplets can rapidly form a film upon exposure to air, and the entire film can be transferred to the lung surfactant interface upon coalescence, displacing it. This film was mechanically stable and remains intact even for multiple biaxial compression/expansion cycles. Our findings provide a mechanism of lung surfactant replacement by serum proteins that is fundamentally different from the subphase-to-interface transport and demonstrate that it is nearly impossible to remove the film from the interface where the lung surfactant should be, thus impairing the lung in a permanent way.

Adsorption of Denaturated Lysozyme at the Air-Water Interface: Structure and Morphology

The application of protein deuteration and high flux neutron reflectometry has allowed a comparison of the adsorption properties of lysozyme at the air-water interface from dilute solutions in the absence and presence of high concentrations of two strong denaturants: urea and guanidine hydrochloride (GuHCl). The surface excess and adsorption layer thickness were resolved and complemented by images of the mesoscopic lateral morphology from Brewster angle microscopy. It was revealed that the thickness of the adsorption layer in the absence of added denaturants is less than the short axial length of the lysozyme molecule, which indicates deformation of the globules at the interface. Two-dimensional elongated aggregates in the surface layer merge over time to form an extensive network at the approach to steady state. Addition of denaturants in the bulk results in an acceleration of adsorption and an increase of the adsorption layer thickness. These results are attributed to incomplete collapse of the globules in the bulk from the effects of the denaturants as a result of interactions between remote amino acid residues. Both effects may be connected to an increase of the effective total volume of macromolecules due to the changes of their tertiary structure, that is, the formation of molten globules under the influence of urea and the partial unfolding of globules under the influence of GuHCl. In the former case, the increase of globule hydrophobicity leads to cooperative aggregation in the surface layer during adsorption. Unlike in the case of solutions without denaturants, the surface aggregates are short and wormlike, their size does not change with time, and they do not merge to form an extensive network at the approach to steady state. To the best of our knowledge, these are the first observations of cooperative aggregation in lysozyme adsorption layers.

Triblock-Copolymer-Assisted Mixed-Micelle Formation Results in the Refolding of Unfolded Protein

The present work reports a new strategy for triblock-copolymer-assisted refolding of sodium dodecyl sulfate (SDS)-induced unfolded serum protein human serum albumin (HSA) by mixed-micelle formation of SDS with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer EO₂₀PO₆₈EO₂₀ (P123) under physiological conditions. The steady-state and time-resolve fluorescence results show that the unfolding of HSA induced by SDS occurs in a stepwise manner through three different phases of binding of SDS, which is followed by a saturation of interaction. Interestingly, the addition of polymeric surfactant P123 to the unfolded protein results in the recovery of ∼87% of its α-helical structure, which was lost during SDS-induced unfolding. This is further corroborated by the return of the steady-state and time-resolved fluorescence decay parameters of the intrinsic tryptophan (Trp214) residue of HSA to the initial nativelike condition. The isothermal titration calorimetry (ITC) data also substantiates that there is almost no interaction between P123 and the native state of the protein. However, the mixed-micelle formation, accompanied by substantial binding affinities, removes the bound SDS molecules from the scaffolds of the unfolded state of the protein. On the basis of our experiments, we conclude that the formation of mixed micelles between SDS and P123 plays a pivotal role in refolding the protein back to its nativelike state.

Evaluating polymeric biomaterial-environment interfaces by Langmuir monolayer techniques

Polymeric biomaterials are of specific relevance in medical and pharmaceutical applications due to their wide range of tailorable properties and functionalities. The knowledge about interactions of biomaterials with their biological environment is of crucial importance for developing highly sophisticated medical devices. To achieve optimal in vivo performance, a description at the molecular level is required to gain better understanding about the surface of synthetic materials for tailoring their properties. This is still challenging and requires the comprehensive characterization of morphological structures, polymer chain arrangements and degradation behaviour. The review discusses selected aspects for evaluating polymeric biomaterial-environment interfaces by Langmuir monolayer methods as powerful techniques for studying interfacial properties, such as morphological and degradation processes. The combination of spectroscopic, microscopic and scattering methods with the Langmuir techniques adapted to polymers can substantially improve the understanding of their in vivo behaviour.

Complex Behavior of Aqueous α-Cyclodextrin Solutions. Interfacial Morphologies Resulting from Bulk Aggregation

The spontaneous aggregation of α-cyclodextrin (α-CD) molecules in the bulk aqueous solution and the interactions of the resulting aggregates at the liquid/air interface have been studied at 283 K using a battery of techniques: transmission electron microscopy, dynamic light scattering, dynamic surface tensiometry, Brewster angle microscopy, neutron reflectometry, and ellipsometry. We show that α-CD molecules spontaneously form aggregates in the bulk that grow in size with time. These aggregates adsorb to the liquid/air interface with their size in the bulk determining the adsorption rate. The material that reaches the interface coalesces laterally to form two-dimensional domains on the micrometer scale with a layer thickness on the nanometer scale. These processes are affected by the ages of both the bulk and the interface. The interfacial layer formed is not in fast dynamic equilibrium with the subphase as the resulting morphology is locked in a kinetically trapped state. These results reveal a surprising complexity of the parallel physical processes taking place in the bulk and at the interface of what might have seemed initially like a simple system.

Formation of a Rigid Hydrophobin Film and Disruption by an Anionic Surfactant at an Air/Water Interface

Hydrophobins are amphiphilic proteins produced by fungi. Cerato-ulmin (CU) is a hydrophobin that has been associated with Dutch elm disease. Like other hydrophobins, CU stabilizes air bubbles and oil droplets through the formation of a persistent protein film at the interface. The behavior of hydrophobins at surfaces has raised interest in their potential applications, including use in surface coatings, food foams, and emulsions and as dispersants. The practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, alone and in the presence of added surfactants. In this study, the adsorption behavior of CU at air/water interfaces is characterized by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to adsorb irreversibly at air/water interfaces. The magnitude of the dilatational modulus increases with adsorption time and surface pressure until CU eventually forms a rigid film. The persistence of this film is tested through the sequential addition of strong surfactant sodium dodecyl sulfate (SDS) to the bulk liquid adjacent to the interface. SDS is found to coadsorb to interfaces precoated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU by SDS. Sequential adsorption results in mixed layers with properties not observed in interfaces generated from complexes formed in the bulk. These results lend insight to the complex interfacial interactions between hydrophobins and surfactants.