Salting Up and Salting Down of Bovine Serum Albumin Layers at the Air-Water Interface

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.

Influence of pH and ionic strength on the bulk and interfacial rheology and technofunctional properties of hazelnut meal protein isolate

The functional, bulk, and interfacial shear rheological properties of hazelnut protein isolate were studied at different pH values between 3.0 and 8.0 and ionic strength levels between 0.0 and 1.0 M. The results showed that pH significantly affected protein solubility, emulsion properties, water and oil holding capacities, foam stability, surface hydrophobicity, and free -SH groups. The highest surface hydrophobicity, free -SH groups, and better functional properties were observed at pH 8.0. Protein solubility also increased with increasing ionic strength up to 0.6 M. The emulsion and foam stability of hazelnut protein isolate showed similar changes with protein solubility. The flow behavior of hazelnut protein suspensions was found to be shear thinning with the highest consistency index at pH 3.0 and the lowest at pH 6.0, however, the ionic strength did not significantly affect the consistency coefficient but did cause a significant change in the flow behavior index, with the lowest value observed at 0.6 M. The best gel structure in hazelnut proteins was observed at pH 3.0 and 4.0. The addition of ions at 0.4 and 0.6 M concentrations resulted in an improved viscoelastic character. The hazelnut protein isolate was also found to form solid-like viscoelastic layers at both air-water and oil-water interfaces, with the interfacial adsorption behavior affected by both pH and ionic strength. Overall, these results suggest that pH and ionic strength have significant effects on the functional and rheological properties of hazelnut protein isolate, which may have the potential as an auxiliary substance in food systems.

New Insights into Cation- and Temperature-Driven Protein Adsorption to the Air-Water Interface through Infrared Reflection Studies of Bovine Serum Albumin

The chemistry and structure of the air-ocean interface modulate biogeochemical processes between the ocean and atmosphere and therefore impact sea spray aerosol properties, cloud and ice nucleation, and climate. Protein macromolecules are enriched in the sea surface microlayer and have complex adsorption properties due to the unique molecular balance of hydrophobicity and hydrophilicity. Additionally, interfacial adsorption properties of proteins are of interest as important inputs for ocean climate modeling. Bovine serum albumin is used here as a model protein to investigate the dynamic surface behavior of proteins under several variable conditions including solution ionic strength, temperature, and the presence of a stearic acid (C₁₇COOH) monolayer at the air-water interface. Key vibrational modes of bovine serum albumin are examined via infrared reflectance-absorbance spectroscopy, a specular reflection method that ratios out the solution phase and highlights the aqueous surface to determine, at a molecular level, the surface structural changes and factors affecting adsorption to the solution surface. Amide band reflection absorption intensities reveal the extent of protein adsorption under each set of conditions. Studies reveal the nuanced behavior of protein adsorption impacted by ocean-relevant sodium concentrations. Moreover, protein adsorption is most strongly affected by the synergistic effects of divalent cations and increased temperature.

Protein-Nanoparticle Interactions Govern the Interfacial Behavior of Polymeric Nanogels: Study of Protein Corona Formation at the Air/Water Interface

Biomedical applications of nanoparticles require a fundamental understanding of their interactions and behavior with biological interfaces. Protein corona formation can alter the morphology and properties of nanomaterials, and knowledge of the interfacial behavior of the complexes, using in situ analytical techniques, will impact the development of nanocarriers to maximize uptake and permeability at cellular interfaces. In this study we evaluate the interactions of acrylamide-based nanogels, with neutral, positive, and negative charges, with serum-abundant proteins albumin, fibrinogen, and immunoglobulin G. The formation of a protein corona complex between positively charged nanoparticles and albumin is characterized by dynamic light scattering, circular dichroism, and surface tensiometry; we use neutron reflectometry to resolve the complex structure at the air/water interface and demonstrate the effect of increased protein concentration on the interface. Surface tensiometry data suggest that the structure of the proteins can impact the interfacial properties of the complex formed. These results contribute to the understanding of the factors that influence the bio-nano interface, which will help to design nanomaterials with improved properties for applications in drug delivery.

Effects of the Temperature and Salt Concentration on the Structural Characteristics of the Protein (PDB Code 1BBL)

The effect of the temperature and salt solution on the structural characteristics of the protein 1BBL was investigated by molecular dynamics simulations. The paper presents simulation results regarding the non-bonded energy and the structural stability of the protein immersed in salt solutions with different concentrations and temperatures. Our work demonstrates that the electrostatic potential energy and van der Waals energy of the system show the opposite changes with the influence of the external environment. Since the electrostatic potential energy changes more obviously, it is dominated in the non-bonding interactions. The structural parameters, such as the root mean square deviation and the radius of gyration, increased initially and decreased afterward with the increase of the salt concentration. The protein presented the loose structure with a relative low stability when it was immersed in a monovalent solution with a salt concentration of 0.8 mol/L. The salt concentration corresponding to the maximum value of structural parameters in the monovalent salt solution was double that in the divalent salt solution. It was also concluded that the protein presented a compact and stable structure when immersed in salt solutions with a high concentration of 2.3 mol/L. The analysis of the root mean square deviation and root mean square fluctuation of the protein sample also exhibited that the structural stability and chain flexibility are strongly guided by the effect of the temperature. These conclusions help us to understand the structural characteristics of the protein immersed in the salt solutions with different concentrations and temperatures.

Amino Acids Are Driven to the Interface by Salts and Acidic Environments

Amino acids (AAs), the building blocks of proteins, are enriched by several orders of magnitude in sea spray aerosols compared to ocean waters. This suggests that AAs may reside at the air-water interface and be highly surface active. Using surface tension measurements, infrared reflection-absorption spectroscopy, and molecular dynamics simulations, we show that AAs are surface active and that salts and low-pH environments are drivers of surface activity. At typical sea spray salt concentrations and pH values, we determine that the surface coverage of hydrophobic AAs increases by approximately 1 order of magnitude. Additionally, divalent cations such as Ca²⁺ and Mg²⁺ can further increase AA surface propensity, particularly at neutral pH. Overall, these results indicate that AAs are likely to be found at increased concentrations at the surface of sea spray aerosols, where they can impact the cloud activation properties of the aerosol and enhance peptide formation under certain conditions.

Dimethyl Sulfoxide-Free and Water-Soluble Fluorescent Probe for Detection of Bovine Serum Albumin Prepared by Ionic Co-assembly of Amphiphiles

Detection of bovine serum albumin (BSA) is an important issue in the sense of medical applications and enzymatic reactions; however, the recently developed fluorescent probes require the involvement of dimethyl sulfoxide (DMSO), which may be detrimental to proteins. In this study, we demonstrated a DMSO-free and water-soluble fluorescent probe prepared by ionic co-assembly of amphiphiles. The cationic amphiphile is a newly designed molecule (denoted by DPP-12) bearing a conjugated diketopyrrolopyrrole (DPP) and two tetraphenylethylene groups. It turns out that the fluorescence emission of DPP-12 depends on the amount of anionic amphiphilic sodium dodecyl benzene sulfonate (SDBS). The fluorescence intensity first increases and then decreases with the concentration of SDBS, and each branch presents a linear relationship. BSA consumes SDBS by the formation of complexes, thus leading to an increase of fluorescence intensity of the mixed solution of DPP-12 and SDBS. Therefore, the mixed solution of DPP-12 and SDBS was applied as a fluorescent probe to detect the low concentration of BSA by back-titration. This fluorescent probe does not require DMSO and has good tolerance to metal ions in blood and good photostability. The limit of detection is as low as 940 nM, almost 3 orders of magnitude lower than the content in organisms.