Experimental techniques to study protein-surfactant interactions: New insights into competitive adsorptions via drop subphase and interface exchange

Protein surfactant (PS) interactions is an essential topic for many fundamental and technological applications such as life science, nanobiotechnology processes, food industry, biodiesel production and drug delivery systems. Several experimental techniques and data analysis approaches have been developed to characterize PS interactions in bulk and at interfaces. However, to evaluate the mechanisms and the level of interactions quantitatively, e.g., PS ratio in complexes, their stability in bulk, and reversibility of their interfacial adsorption, new experimental techniques and protocols are still needed, especially with relevance for in-situ biological conditions. The available standard techniques can provide us with the basic understanding of interactions mainly under static conditions and far from physiological criteria. However, detailed measurements at complex interfaces can be formidable due to the sophisticated tools required to carefully probe nanometric phenomena at interfaces without disturbing the adsorbed layer. Tensiometry-based techniques such as drop profile analysis tensiometry (PAT) have been among the most powerful methods for characterizing protein's and surfactant's adsorption layers at interfaces via measuring equilibrium and dynamic interfacial tension and dilational rheology analysis. PAT provides us with insightful data such as kinetics and isotherms of adsorption and related surface activity parameters. However, the data analysis and interpretation can be challenging for mixed protein-surfactant solutions via standard PAT experimental protocols. The combination of a coaxial double capillary (micro flow exchange system) with drop profile analysis tensiometry (CDC-PAT) is a promising tool to provide valuable results under different competitive adsorption/desorption conditions via novel experimental protocols. CDC-PAT provides unique experimental protocols to exchange the droplet subphase in a continuous dynamic mode during the in-situ analysis of the corresponding interfacial adsorbed layer. The contribution of diffusion/convection mechanisms on the kinetics of the adsorption/desorption processes can also be investigated using CDC-PAT. Here, firstly, we review the commonly available techniques for characterizing protein-surfactant interactions in the bulk phase and at interfaces. Secondly, we give an overview for applications of the coaxial double capillary PAT setup for investigations of mixed protein-surfactant adsorbed layers and address recently developed protocols and analysis procedures. Exploring the competitive sequential adsorption of proteins and surfactants and the reversibility of pre-adsorbed layers via the subphase exchange are the particular experiments we can perform using CDC-PAT. Also the sequential and simultaneous competitive adsorption/desorption processes of some ionic and nonionic surfactants (SDS, CTAB, DTAB, and Triton) and proteins (bovine serum albumin (BSA), lysozyme, and lipase) using CDC-PAT are discussed. Last but not least, the fabrication of micro-nanocomposite layers and membranes are additional applications of CDC-PAT discussed in this work.

Recent Advances in Studying Interfacial Adsorption of Bioengineered Monoclonal Antibodies

Monoclonal antibodies (mAbs) are an important class of biotherapeutics; as of 2020, dozens are commercialized medicines, over a hundred are in clinical trials, and many more are in preclinical developmental stages. Therapeutic mAbs are sequence modified from the wild type IgG isoforms to varying extents and can have different intrinsic structural stability. For chronic treatments in particular, high concentration (≥ 100 mg/mL) aqueous formulations are often preferred for at-home administration with a syringe-based device. MAbs, like any globular protein, are amphiphilic and readily adsorb to interfaces, potentially causing structural deformation and even unfolding. Desorption of structurally perturbed mAbs is often hypothesized to promote aggregation, potentially leading to the formation of subvisible particles and visible precipitates. Since mAbs are exposed to numerous interfaces during biomanufacturing, storage and administration, many studies have examined mAb adsorption to different interfaces under various mitigation strategies. This review examines recent published literature focusing on adsorption of bioengineered mAbs under well-defined solution and surface conditions. The focus of this review is on understanding adsorption features driven by distinct antibody domains and on recent advances in establishing model interfaces suitable for high resolution surface measurements. Our summary highlights the need to further understand the relationship between mAb interfacial adsorption and desorption, solution aggregation, and product instability during fill-finish, transport, storage and administration.

Coadsorption of a Monoclonal Antibody and Nonionic Surfactant at the SiO2/Water Interface

During the formulation of therapeutic monoclonal antibodies (mAbs), nonionic surfactants are commonly added to attenuate structural rearrangement caused by adsorption/desorption at interfaces during processing, shipping, and storage. We examined the adsorption of a mAb (COE-3) at the SiO₂/water interface in the presence of pentaethylene glycol monododecyl ether (C₁₂E₅), polysorbate 80 (PS80-20EO), and a polysorbate 80 analogue with seven ethoxylates (PS80-7EO). Spectroscopic ellipsometry was used to follow COE-3 dynamic adsorption, and neutron reflection was used to determine interfacial structure and composition. Neither PS80-20EO nor C₁₂E₅ had a notable affinity for COE-3 or the interface under the conditions studied and thus did not prevent COE-3 adsorption. In contrast, PS80-7EO did coadsorb but did not influence the dynamic process or the equilibrated amount of absorbed COE-3. Near equilibration, COE-3 underwent structural rearrangement and PS80-7EO started to bind the COE-3 interfacial layer and subsequently formed a well-defined surfactant bilayer via self-assembly. The resultant interfacial layer comprised an inner mAb layer of about 70 Å thickness and an outer surfactant layer of a further 70 Å, with distinct transitional regions across the mAb-surfactant and surfactant-bulk water boundaries. Once formed, such interfacial layers were very robust and worked to prevent further mAb adsorption, desorption, and structural rearrangement. Such robust interfacial layers could be anticipated to exist for formulated mAbs stored in type II glass vials; further research is required to understand the behavior of these layers for siliconized glass syringes.

Surface Tension and Adsorption Studies by Drop Profile Analysis Tensiometry

Surface tension and dilational viscoelasticity of solutions of various surfactants measured with bubble and drop profile analysis tensiometry are discussed. The study also includes experiments on the co-adsorption of surfactant molecules from a solution drop and alkane molecules from saturated alkane vapor phase. Using experimental data for 12 surfactants with different surface activities, it is shown that depletion due to adsorption of surfactant from the drop bulk can be significant. An algorithm is proposed quantitatively to take into consideration the depletion effect which is required for a correct description of the co-adsorption of alkanes on the solution drop surface and the correct analysis of experimental dynamic surface tension data to determine the adsorption mechanism. Bubble and drop profile analysis tensiometry is also the method of choice for measuring the dilational viscoelasticity of the adsorbed interfacial layer. The same elasticity moduli are obtained with the bubble and drop method only when the equilibrium surface pressures are sufficiently small (Π < 15 mN m-1). When the surface pressure for a surfactant solution is larger than this value, the viscoelasticity moduli determined from drop profile experiments become significantly larger than those obtained from bubble profile measurements.

Formation of protein/surfactant adsorption layer as studied by dilational surface rheology

The review discusses the mechanism of formation of protein/surfactant adsorption layers at the liquid - gas interface. The complexes of globular proteins usually preserve their compact structure a low surfactant concentrations. Therefore a simple kinetic model of the adsorption of charged compact nanoparticles is discussed first and compared with experimental data. The increase of surfactant concentrations results in various conformational transitions in the surface layer. One can obtain information on the changes of the adsorption layer structure using the dilational surface rheology. The kinetic dependencies of the dynamic surface elasticity are strongly different for the adsorption of unfolded macromolecules and compact globules, and have local maxima in the former case corresponding to different steps of the adsorption. These distinctions allow tracing the changes of the tertiary structure of protein/surfactant complexes in the surface layer. The adsorption from mixed solutions of ionic surfactants with β-casein, β-lactoglobulin, bovine serum albumin and myoglobin is discussed with some details.

Thermodynamics, interfacial pressure isotherms and dilational rheology of mixed protein-surfactant adsorption layers

Proteins and their mixtures with surfactants are widely used in many applications. The knowledge of their solution bulk behavior and its impact on the properties of interfacial layers made great progress in the recent years. Different mechanisms apply to the formation process of protein/surfactant complexes for ionic and non-ionic surfactants, which are governed mainly by electrostatic and hydrophobic interactions. The surface activity of these complexes is often remarkably different from that of the individual protein and has to be considered in respective theoretical models. At very low protein concentration, small amounts of added surfactants can change the surface activity of proteins remarkably, even though no strongly interfacial active complexes are observed. Also small added amounts of non-ionic surfactants change the surface activity of proteins in the range of small bulk concentrations or surface coverages. The modeling of the equilibrium adsorption behavior of proteins and their mixtures with surfactants has reached a rather high level. These models are suitable also to describe the high frequency limits of the dilational viscoelasticity of the interfacial layers. Depending on the nature of the protein/surfactant interactions and the changes in the interfacial layer composition rather complex dilational viscoelasticities can be observed and described by the available models. The differences in the interfacial behavior, often observed in literature for studies using different experimental methods, are at least partially explained by a depletion of proteins, surfactants and their complexes in the range of low concentrations. A correction of these depletion effects typically provides good agreement between the data obtained with different methods, such as drop and bubble profile tensiometry.

Adsorption of proteins at the solution/air interface influenced by added nonionic surfactants at very low concentrations for both components. 2. Effect of different surfactants and theoretical model

The influence of the addition of the nonionic surfactants dodecyl dimethyl phosphine oxide (C12DMPO), tetradecyl dimethyl phosphine oxide (C14DMPO), decyl alcohol (C10OH), and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the surface tension is studied. It is shown that a significant decrease of the water/air surface tension occurs for all the surfactants studied at very low concentrations (10(-5)-10(-3) mmol/L). All measurements were performed with the buoyant bubble profile method. The dynamics of the surface tension was simulated using the Fick and Ward-Tordai equations. The calculation results agree well with the experimental data, indicating that the equilibration times in the system studied do not exceed 30 000 s, while the time required to attain the equilibrium on a plane surface is by one order of magnitude higher. To achieve agreement between theory and experiment for the mixtures, a supposition was made about the influence of the concentration of nonionic surfactant on the adsorption activity of the protein. The adsorption isotherm equation of the protein was modified accordingly, and this corrected model agrees well with all experimental data.

Adsorption behavior of hydrophobin and hydrophobin/surfactant mixtures at the air-water interface

The adsorption of the surface-active protein hydrophobin, HFBII, and the competitive adsorption of HFBII with the cationic, anionic, and nonionic surfactants hexadecyltrimethylammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), has been studied using neutron reflectivity, NR. HFBII adsorbs strongly at the air-water interface to form a dense monolayer ∼30 Å thick, with a mean area per molecule of ∼400 Å(2) and a volume fraction of ∼0.7, for concentrations greater than 0.01 g/L, and the adsorption is independent of the solution pH. In competition with the conventional surfactants CTAB, SDS, and C(12)E(6) at pH 7, the HFBII adsorption totally dominates the surface for surfactant concentrations less than the critical micellar concentration, cmc. Above the cmc of the conventional surfactants, HFBII is displaced by the surfactant (CTAB, SDS, or C(12)E(6)). For C(12)E(6) this displacement is only partial, and some HFBII remains at the surface for concentrations greater than the C(12)E(6) cmc. At low pH (pH 3) the patterns of adsorption for HFBII/SDS and HFBII/C(12)E(6) are different. At concentrations just below the surfactant cmc there is now mixed HFBII/surfactant adsorption for both SDS and C(12)E(6). For the HFBII/SDS mixture the structure of the adsorbed layer is more complex in the region immediately below the SDS cmc, resulting from the HFBII/SDS complex formation at the interface.

Interfacial assembly of proteins and peptides: recent examples studied by neutron reflection

Through reviewing a number of recent neutron reflection studies of interfacial adsorption of peptides and proteins, this paper aims to demonstrate the significance of this technique in studying interfacial biomolecular processes by illustrating the typical structural details that can be derived. The review will start with the introduction of relevant theoretical background, followed by an outline of representative biomolecular systems that have recently been studied to indicate the technical strengths of neutron reflection.

Thermodynamics, adsorption kinetics and rheology of mixed protein-surfactant interfacial layers

Depending on the bulk composition, adsorption layers formed from mixed protein/surfactant solutions contain different amounts of protein. Clearly, increasing amounts of surfactant should decrease the amount of adsorbed proteins successively. However, due to the much larger adsorption energy, proteins are rather strongly bound to the interface and via competitive adsorption surfactants cannot easily displace proteins. A thermodynamic theory was developed recently which describes the composition of mixed protein/surfactant adsorption layers. This theory is based on models for the single compounds and allows a prognosis of the resulting mixed layers by using the characteristic parameters of the involved components. This thermodynamic theory serves also as the respective boundary condition for the dynamics of adsorption layers formed from mixed solutions and their dilational rheological behaviour. Based on experimental studies with milk proteins (beta-casein and beta-lactoglobulin) mixed with non-ionic (decyl and dodecyl dimethyl phosphine oxide) and ionic (sodium dodecyl sulphate and dodecyl trimethyl ammonium bromide) surfactants at the water/air and water/hexane interfaces, the potential of the theoretical tools is demonstrated. The displacement of pre-adsorbed proteins by subsequently added surfactant can be successfully studied by a special experimental technique based on a drop volume exchange. In this way the drop profile analysis can provide tensiometry and dilational rheology data (via drop oscillation experiments) for two adsorption routes--sequential adsorption of the single compounds in addition to the traditional simultaneous adsorption from a mixed solution. Complementary measurements of the surface shear rheology and the adsorption layer thickness via ellipsometry are added in order to support the proposed mechanisms drawn from tensiometry and dilational rheology, i.e. to show that the formation of mixed adsorption layer is based on a modification of the protein molecules via electrostatic (ionic) and/or hydrophobic interactions by the surfactant molecules and a competitive adsorption of the resulting complexes with the free, unbound surfactant. Under certain conditions, the properties of the sequentially formed layers differ from those formed simultaneously, which can be explained by the different locations of complex formation.

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.

Aggregation behaviors of gelatin with cationic gemini surfactant at air/water interface

The dilational rheological properties of gelatin with cationic gemini surfactant 1,2-ethane bis(dimethyl dodecyl ammonium bromide) (C(12)C(2)C(12)) at air/water interface were investigated using oscillating barriers method at low frequency (0.005-0.1 Hz), which was compared with single-chain surfactant dodecyltrimethyl ammonium bromide (DTAB). The results indicate that the maximum dilational modulus and the film stability of gelatin-C(12)C(2)C(12) are higher than those of gelatin-DTAB. At high concentration of C(12)C(2)C(12) or DTAB, the dilational modulus of gelatin-surfactant system becomes close to that corresponding to pure surfactant, suggesting gelatin at interface is replaced by surfactant. This replacement is also observed by surface tension measurement. However, it is found that gelatin-C(12)C(2)C(12) system has two obvious breaks but gelatin-DTAB has not in surface tension isotherms. These phenomena are ascribed to the double charges and strong hydrophobicity of C(12)C(2)C(12). Based on these experimental results, a mechanism of gelatin-surfactant interaction at air/water interface is proposed.

Proteins at fluid interfaces: adsorption layers and thin liquid films

A review in which many original published results of the authors as well as many other papers are discussed. The structure and some properties of the globular proteins are shortly presented, special accent being put on the alpha-chymotrypsin (alpha-ChT), lysozyme (LZ), human serum albumin (HSA), and bovine serum albumin (BSA) which have been used in the experiments with thin liquid films. The behaviour of protein adsorption layers (PAL) is extensively discussed. The dynamics of PAL formation, including the kinetics of adsorption as well as the time evolution of the surface tension of protein aqueous solutions, are considered. A considerable place is devoted to the surface tension and adsorption isotherms of the globular protein solutions, the simulation of PAL by interacting hard spheres, the experimental surface tension isotherms of the above mentioned proteins, and the interfacial tension isotherms for the protein aqueous solution/oil interface. The rheological properties of PAL at fluid interfaces are shortly reviewed. After a brief information about the experimental methods for investigation of protein thin liquid (foam or emulsion) films, the properties of the protein black foam films are extensively discussed: the conditions for their formation, the influence of the electrolytes and pH on the film type and stability, the thermodynamic properties of the black foam films, the contact angles film/bulk and their dynamic hysteresis. The next center of attention concerns some properties of the protein emulsion films: the conditions for formation of emulsion black films, the formation and development of a dimpling in microscopic, circular films. The protein-phospholipid mixed foam films are also briefly considered.

Keratin-rhamnolipids and keratin-sodium dodecyl sulfate interactions at the air/water interface

Keratin-surfactant interactions at the air/solution interface may be indicative for skin irritation by surfactants. In this study pure keratin was extracted from chicken feathers as described by Schrooyen at al. and the adsorption properties of biologically produced rhamnolipids (RL) and sodium dodecyl sulfate (SDS) mixed with keratin molecules were investigated by surface tension measurements. Keratin was added in three specific amounts to RL and SDS aqueous solutions of varying concentrations, respectively, and keratin-RL and keratin-SDS interactions were investigated at pH 6.2 and 5.0. The surface tension measurements showed that surfactant-keratin complexes were formed in the solutions at pH 6.2. At low RL concentrations the added amount of keratin decreased the surface tension as a result of the hydrophobic keratin-RL complexes formed, which have covered the air/solution interface. Beginning from moderate RL concentrations, less surface active keratin-RL complexes were formed due to hydrophilization of the former complexes and they were removed from the interface to the bulk phase as a result of the competitive adsorption mechanism and pure RL molecules covered the interface gradually. At low SDS concentrations surface tension decrease was also observed by the covering of the air/solution interface with keratin-SDS complexes. At moderate SDS concentrations the keratin-SDS complexes exhibited a more surface active feature. Unlike the keratin-RL complexes, they have remained at the interface until the higher SDS concentrations and keratin ratios. However, at the minimum surface tension value of the pure SDS at its CMC value, the surface tension of the keratin-SDS mixture reached the value of pure SDS solution and the air/solution interface has been covered again only with pure SDS monomers. The keratin-surfactant solutions prepared at pH 5 showed lower surface tension values compared to that measured at pH 6.2 and the surface tension behavior of RL and SDS resembled each other. Above a certain surfactant concentration the addition of keratin to surfactant solutions rendered the surface less surface active compared to their pure surfactant solutions at pH 5.0. The consideration of the surface tension behavior of two anionic surfactants has shown that the RL-keratin interactions are milder than SDS-keratin interactions at pH 6.2 and 5.0.

Adsorption of frog foam nest proteins at the air-water interface

The surfactant properties of aqueous protein mixtures (ranaspumins) from the foam nests of the tropical frog Physalaemus pustulosus have been investigated by surface tension, two-photon excitation fluorescence microscopy, specular neutron reflection, and related biophysical techniques. Ranaspumins lower the surface tension of water more rapidly and more effectively than standard globular proteins under similar conditions. Two-photon excitation fluorescence microscopy of nest foams treated with fluorescent marker (anilinonaphthalene sulfonic acid) shows partitioning of hydrophobic proteins into the air-water interface and allows imaging of the foam structure. The surface excess of the adsorbed protein layers, determined from measurements of neutron reflection from the surface of water utilizing H(2)O/D(2)O mixtures, shows a persistent increase of surface excess and layer thickness with bulk concentration. At the highest concentration studied (0.5 mg ml(-1)), the adsorbed layer is characterized by three distinct regions: a protruding top layer of approximately 20 angstroms, a middle layer of approximately 30 angstroms, and a more diffuse submerged layer projecting some 25 angstroms into bulk solution. This suggests a model involving self-assembly of protein aggregates at the air-water interface in which initial foam formation is facilitated by specific surfactant proteins in the mixture, further stabilized by subsequent aggregation and cross-linking into a multilayer surface complex.

NEUTRON REFLECTION FROM LIQUID INTERFACES

Recent applications of neutron reflectometry to the study of wet interfaces are described. An outline is given of the basic principles that allow the techniques to determine composition and structure in a variety of situations. These are the adsorption of surfactant molecules at air/liquid and solid/liquid interfaces, the shape of the segment-density profiles of different types of polymer, including block copolymers and polyelectrolytes, adsorption in mixed surfactant and polymer/surfactant systems, and interfacial systems of biophysical interest.