Salting Up of Proteins at the Air/Water Interface

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.

pH jump kinetics in colliding microdroplets: accelerated synthesis of azamonardine from dopamine and resorcinol

Recent studies report the dramatic acceleration of chemical reactions in micron-sized compartments. In the majority of these studies the exact acceleration mechanism is unknown but the droplet interface is thought to play a significant role. Dopamine reacts with resorcinol to form a fluorescent product azamonardine and is used as a model system to examine how droplet interfaces accelerate reaction kinetics. The reaction is initiated by colliding two droplets levitated in a branched quadrupole trap, which allows the reaction to be observed in individual droplets where the size, concentration, and charge are carefully controlled. The collision of two droplets produces a pH jump and the reaction kinetics are quantified optically and in situ by measuring the formation of azamonardine. The reaction was observed to occur 1.5 to 7.4 times faster in 9-35 micron droplets compared to the same reaction conducted in a macroscale container. A kinetic model of the experimental results suggests that the acceleration mechanism arises from both the more rapid diffusion of oxygen into the droplet, as well as increased reagent concentrations at the air-water interface.

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.

Electronic Structure and Solvation Effects from Core and Valence Photoelectron Spectroscopy of Serum Albumin

X-ray photoelectron spectroscopy of bovine serum albumin (BSA) in a liquid jet is used to investigate the electronic structure of a solvated protein, yielding insight into charge transfer mechanisms in biological systems in their natural environment. No structural damage was observed in BSA following X-ray photoelectron spectroscopy in a liquid jet sample environment. Carbon and nitrogen atoms in different chemical environments were resolved in the X-ray photoelectron spectra of both solid and solvated BSA. The calculations of charge distributions demonstrate the difficulty of assigning chemical contributions in complex systems in an aqueous environment. The high-resolution X-ray core electron spectra recorded are unchanged upon solvation. A comparison of the valence bands of BSA in both phases is also presented. These bands display a higher sensitivity to solvation effects. The ionization energy of the solvated BSA is determined at 5.7 ± 0.3 eV. Experimental results are compared with theoretical calculations to distinguish the contributions of various molecular components to the electronic structure. This comparison points towards the role of water in hole delocalization in proteins.

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.

Revealing the Molecular Physics of Lattice Self-Assembly by Vibrational Hyperspectral Imaging

Lattice self-assemblies (LSAs), which mimic protein assemblies, were studied using a new nonlinear vibrational imaging technique called vibrational sum-frequency generation (VSFG) microscopy. This technique successfully mapped out the mesoscopic morphology, microscopic geometry, symmetry, and ultrafast dynamics of an LSA formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). The spatial imaging also revealed correlations between these different physical properties. Such knowledge shed light on the functions and mechanical properties of LSAs. In this Feature Article, we briefly introduce the fundamental principles of the VSFG microscope and then discuss the in-depth molecular physics of the LSAs revealed by this imaging technique. The application of the VSFG microscope to the artificial LSAs also paved the way for an alternative approach to studying the structure-dynamic-function relationships of protein assemblies, which were essential for life and difficult to study because of their various and complicated interactions. We expect that the hyperspectral VSFG microscope could be broadly applied to many noncentrosymmetric soft materials.

Recovery of Fatty Acid Monolayers by Salts Investigated by Sum-Frequency Generation Spectroscopy

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C₁₄H₂₉COOH (pentadecanoic acid), C₁₅H₃₁COOH (palmitic acid), and C₁₆H₃₃COOH (heptadecanoic acid)) are stable at a neutral pH (pH ≈ 6) but become unstable at a high pH (pH ≈ 11). Further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CH_x and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl₂ to recover the monolayer at a high pH. A much lower concentration of Mg²⁺ as compared to Ca²⁺ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.

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

The surface adsorption of bovine serum albumin in pure water and salted aqueous solutions was studied by neutron reflection. With the contrast match technique, the surface excess in null reflecting water as a function of the protein concentration was revealed. It is found that, in a concentration range from 1 ppm (parts per million, mg/L) to 1000 ppm, without salts, the surface excess shows a profound peak at around 20 ppm; with salts, the surface excess increases steadily with the protein concentration. When the surface excess at a specific protein concentration is viewed, the introduction of sodium chloride causes either a salting down effect (surface adsorption decline) or a salting up effect (surface adsorption increase), depending upon the protein concentration. The salting up effect is observed at the low (∼1 ppm) and high (∼1000 ppm) concentrations, and the salting down effect dominates the intermediate concentration range. The change in solution pH relative to the isoelectric point (PI) can act as a simple indicator for the salting up or salting down behavior. When the solution pH is shifted toward the PI by adding salts, surface adsorption enhances; when the solution pH is shifted away from the PI by adding salts, surface adsorption declines.

Unsaturated Lipid Accelerates Formation of Oligomeric β-Sheet Structure of GP41 Fusion Peptide in Model Cell Membrane

Membrane fusion of the viral and host cell membranes is the initial step of virus infection and is catalyzed by fusion peptides. Although the β-sheet structure of fusion peptides has been proposed to be the most important fusion-active conformation, it is still very challenging to experimentally identify different types of β-sheet structures at the cell membrane surface in situ and in real time. In this work, we demonstrate that the interface-sensitive amide II spectral signals of protein backbones, generated by the sum frequency generation vibrational spectroscopy, provide a sensitive probe for directly capturing the formation of oligomeric β-sheet structure of fusion peptides. Using human immunodeficiency virus (HIV) glycoprotein GP41 fusing peptide (FP23) as the model, we find that formation speed of oligomeric β-sheet structure depends on lipid unsaturation. The unsaturated lipid such as POPG can accelerate formation of oligomeric β-sheet structure of FP23. The β-sheet structure is more deeply inserted into the hydrophobic region of the POPG bilayer than the α-helical segment. This work will pave the way for future researches on capturing intermediate structures during membrane fusion processes and revealing the fusion mechanism.

Acidic Environment Significantly Alters Aggregation Pathway of Human Islet Amyloid Polypeptide at Negative Lipid Membrane

The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) at cell membrane has a close relationship with the development of type 2 diabetes (T2DM). This aggregation process is susceptible to various physiologically related factors, and systematic studies on condition-mediated hIAPP aggregation are therefore essential for a thorough understanding of the pathology of T2DM. In this study, we combined surface-sensitive amide I and amide II spectral signals from the protein backbone, generated simultaneously in a highly sensitive femtosecond broad-band sum frequency generation vibrational spectroscopy system, to examine the effect of environmental pH on the dynamical structural changes of hIAPP at membrane surface in situ and in real time. Such a combination can directly discriminate the formation of β-hairpin-like monomer and oligomer/fibril at the membrane surface. It is evident that, in an acidic milieu, hIAPP slows down its conformational evolution and alters its aggregation pathway, leading to the formation of off-pathway oligomers. When matured hIAPP aggregates are exposed to basic subphase, partial conversion from β-sheet oligomers into ordered β-sheet fibrillar structures is observed. When exposed to acidic environment, however, hIAPP fibrils partially converse into more loosely patterned β-sheet oligomeric structures.