Hofmeister challenges: ion binding and charge of the BSA protein as explicit examples

Specific buffer effects on the formation of BSA protein corona around amino-functionalized mesoporous silica nanoparticles

The effect of buffer species on biomolecules and biomolecule-nanoparticle interactions is a phenomenon that has been either neglected, or not understood. Here, we study the formation of a BSA protein corona (PC) around amino-functionalized mesoporous silica nanoparticles (MSN-NH2) in the presence of different buffers (Tris, BES, cacodylate, phosphate, and citrate) at the same pH (7.15) and different concentrations (10, 50, and 100 mM). We find that BSA adsorption is buffer specific, with the adsorbed amount of BSA being 4.4 times higher in the presence of 100 mM Tris (184 ± 3 mg/g) than for 100 mM citrate (42 ± 2 mg/g). That is a considerable difference that cannot be explained by conventional theories. The results become clearer if the interaction energies between BSA and MSN-NH2, considering the electric double layer (EEDL) and the van der Waals (EvdW) terms, are evaluated. The buffer specific PC derives from buffer specific zeta potentials that, for MSN-NH2, are positive with Tris and negative with citrate buffers. A reversed sign of zeta potentials can be obtained by considering polarizability-dependent dispersion forces acting together with electrostatics to give the buffer specific outcome. These results are relevant not only to our understanding of the formation of the PC but may also apply to other bio- and nanosystems in biological media.

Specific anion effects on urease activity: A Hofmeister study

The effects of a range of electrolytes on the hydrolysis of urea by the enzyme urease is explored. The autocatalytic behavior of urease in unbuffered solutions and its pH clock reactions are studied. The concentration dependence of the experimental variables is analyzed in terms of specific ion-enzyme interactions and hydration. The results offer insights into the molecular mechanisms of the enzyme, and on the nature of its interactions with the electrolytes. We found that urease can tolerate mild electrolytes in its environment, while it is strongly inhibited by both strong kosmotropic and strong chaotropic anions. This study may cast light on an alternative therapy for Helicobacter pylori infections and contribute to the design of innovative materials and provide new approaches for the modulation of the enzymatic activity.

Molecular interactions of acids and salts with polyampholytes

The Hofmeister series characterizes the ability of salt anions to precipitate polyampholytes/proteins. However, the variation of protein size in the bulk solution of acids and the effect of salts on the same have not been studied well. In this article, the four acids (CH3COOH, HNO3, H2SO4, and HCl) and their effects on the hydrodynamic radius (RH) of gelatin in the bulk solution are investigated. The effects of Na salt with the same anions are also considered to draw a comparison between the interactions of acids and salts with polyampholytes. It is suggested that the interactions of polyampholytes with acids are different from those of salts. The interaction series of polyampholytes with acids with respect to the RH of the polyampholyte is CH3COO->NO3->Cl->SO42- whereas the interaction series with salts is SO42->CH3COO->Cl->NO3-. These different interactions are due to equilibration between acid dissociation and protonation of polyampholytes. Another important factor contributing to the interactions in weak acids is the fact that undissociated acid hinders the movement of dissociated acid. Experiments and simulations were performed to understand these interactions, and the results were identical in terms of the trend in RH (from the experiments) and the radius of gyration (Rg) (from the simulations). It is concluded that the valence of ions and dissociation affect the interaction in the case of acids. However, the interactions are influenced by the kosmotropic and chaotropic effect, hydration, and mobility in the case of salts.

Bioinspired engineered proteins enable universal anchoring strategy for surface functionalization

HYPOTHESIS

Conventional coating strategies and materials for bio-applications with protective, diagnostic, and therapeutic functions are commonly limited by their arduous preparation processes and lack of on-demand functionalities. Herein, inspired by the 'root-leaf' structure of grass, a series of novel polyacrylate-conjugated proteins can be engineered with sticky bovine serum albumin (BSA) protein as a 'root' anchoring layer and a multifunctional polyacrylate as a 'leaf' functional layer for the facile coating procedure and versatile surface functionalities.

EXPERIMENTS

The engineered proteins were synthesized based on click chemistry, where the 'root' layer can universally anchor onto both organic and inorganic substrates through a facile dip/spraying method with excellent stability in harsh solution conditions, thanks to its multiple adaptive molecular interactions with substrates that further elucidated by molecular force measurements between the 'root' BSA protein and substrates. The 'leaf' conjugated-polyacrylates imparted coatings with versatile on-demand functionalities, such as resistance to over 99% biofouling in complex biofluids, pH-responsive performance, and robust adhesion with various nanomaterials.

FINDINGS

By synergistically leveraging the universal anchoring capabilities of BSA with the versatile physicochemical properties of polyacrylates, this study introduces a promising and facile strategy for imparting novel functionalities to a myriad of surfaces through engineering natural proteins and biomaterials for biotechnical and nanotechnical applications.

Sodium Dynamics in the Cellular Environment

Sodium ions are essential for the functions of biological cells, and they are maintained at the balance between intra- and extracellular environments. The quantitative assessment of intra- and extracellular sodium as well as its dynamics can provide crucial physiological information on a living system. ²³Na nuclear magnetic resonance (NMR) is a powerful and noninvasive technique to probe the local environment and dynamics of sodium ions. However, due to the complex relaxation behavior of the quadrupolar nucleus in the intermediate-motion regime and because of the heterogeneous compartments and diverse molecular interactions in the cellular environment, the understanding of the ²³Na NMR signal in biological systems is still at the early stage. In this work, we characterize the relaxation and diffusion of sodium ions in the solutions of proteins and polysaccharides, as well as in the in vitro samples of living cells. The multi-exponential behavior of ²³Na transverse relaxation has been analyzed according to the relaxation theory to derive the crucial information related to the ionic dynamics and molecular binding in the solutions. The bi-compartment model of transverse relaxation and diffusion measurements can corroborate each other to quantify the fractions of intra- and extracellular sodium. We show that ²³Na relaxation and diffusion can be used to monitor the viability of human cells, which offers versatile NMR metrics for in vivo studies.

Bionic Engineered Protein Coating Boosting Anti-Biofouling in Complex Biological Fluids

Implantable medical devices have been widely applied in diagnostics, therapeutics, organ restoration, and other biomedical areas, but often suffer from dysfunction and infections due to irreversible biofouling. Inspired by the self-defensive "vine-thorn" structure of climbing thorny plants, a zwitterion-conjugated protein is engineered via grafting sulfobetaine methacrylate (SBMA) segments on native bovine serum albumin (BSA) protein molecules for surface coating and antifouling applications in complex biological fluids. Unlike traditional synthetic polymers of which the coating operation requires arduous surface pretreatments, the engineered protein BSA@PSBMA (PolySBMA conjugated BSA) can achieve facile and surface-independent coating on various substrates through a simple dipping/spraying method. Interfacial molecular force measurements and adsorption tests demonstrate that the substrate-foulant attraction is significantly suppressed due to strong interfacial hydration and steric repulsion of the bionic structure of BSA@PSBMA, enabling coating surfaces to exhibit superior resistance to biofouling for a broad spectrum of species including proteins, metabolites, cells, and biofluids under various biological conditions. This work provides an innovative paradigm of using native proteins to generate engineered proteins with extraordinary antifouling capability and desired surface properties for bioengineering applications.

BSA fragmentation specifically induced by added electrolytes: An electrospray ionization mass spectrometry investigation

Biointerfaces are significantly affected by electrolytes according to the Hofmeister series. This work reports a systematic investigation on the effect of different metal chlorides, sodium and potassium bromides, iodides and thiocyanates, on the ESI/MS spectra of bovine serum albumin (BSA) in aqueous solution at pH = 2.7. The concentration of each salt was varied to maximize the quality of the ESI/MS spectrum, in terms of peak intensity and bell-shaped profile. The ESI/MS spectra of BSA in the absence and in the presence of salts showed a main protein pattern characterized by the expected mass of 66.5 kDa, except the case of BSA/RbCl (mass 65.3 kDa). In all systems we observed an additional pattern, characterized by at least three peaks with low intensity, whose deconvolution led to suggest the formation of a BSA fragment with a mass of 19.2 kDa. Only NaCl increased the intensity of the peaks of the main BSA pattern, while minimizing that of the fragment. NaCl addition seems to play a crucial role in stabilizing the BSA ionized interface against hydrolysis of peptide bonds, through different synergistic mechanisms. To quantify the observed specific electrolyte effects, two "Hofmeister" parameters (H_s and P_s) are proposed. They are obtained using the ratio of (BSA-Salt)/BSA peak intensities for both the BSA main pattern and for its fragment. SYNOPSIS: NaCl stabilizes BSA ion and almost prevents fragmentation due to denaturing pH.

Modulation of global stability, ligand binding and catalytic properties of trypsin by anions

Specific salts effect is well-known on stability and solubility of proteins, however, relatively limited knowledge is known regarding the effect on catalytic properties of enzymes. Here, we examined the effect of four sodium anions on thermal stability and catalytic properties of trypsin and binding of the fluorescent probe, p-aminobenzamidine (PAB), to the enzyme. We show that the specific anions effect on trypsin properties agrees with the localization of the anions in the Hofmeister series. Thermal stability of trypsin, T_m, the affinity of the fluorescent probe to the binding site, K_d, and the rate constant, k_cat, of trypsin-catalyzed hydrolysis of the substrate N-benzoyl-L-arginine ethyl ester (BAEE) increase with increasing kosmotropic character of anions in the order: perchlorate<bromide<chloride<sulfate, while the value of Michaelis constant, K_M, decreases. Correlations between the values of T_m, K_d for PAB, k_cat, and K_M for BAEE in the presence of 1 M studied salts suggest interrelation among these parameters of the enzyme. Global stabilization as well as increased rigidity of trypsin is accompanied by strengthening of interaction with fluorescent probe PAB and in accordance with decreasing values of K_M for the substrate BAEE. Strong correlations between parameters characterizing the trypsin properties with the charge densities of anions clearly indicate direct electrostatic interaction as a basis of the specific anion effect on the conformational and functional properties of the enzyme.

Consequences of variability in α-synuclein fibril structure on strain biology

Synucleinopathies are a group of clinically and neuropathologically distinct protein misfolding diseases caused by unique α-synuclein conformations, or strains. While multiple atomic resolution cryo-electron microscopy structures of α-synuclein fibrils are now deposited in Protein Data Bank, significant gaps in the biological consequences arising from each conformation have yet to be unraveled. Mutations in the α-synuclein gene (SNCA), cofactors, and the solvation environment contribute to the formation and maintenance of each disease-causing strain. This review highlights the impact of each of these factors on α-synuclein misfolding and discusses the implications of the resulting structural variability on therapeutic development.

Probing the roles of pH and ionic strength on electrostatic binding of tetracycline by dissolved organic matters: Reevaluation of modified fitting model

The binding performance of dissolved organic matters (DOM) plays a critical role in the migration, diffusion and removal of various residual pollutants in the natural water environment. In the current study, four typical DOMs (including bovine serum proteins BSA (proteins), sodium alginate SAA (polysaccharides), humic acid HA and fulvic acid FA (humus)) are selected to investigate the binding roles in zwitterionic tetracycline (TET) antibiotic under various ionic strength (IS = 0.001-0.1 M) and pH (5.0-9.0). The dialysis equilibration technique was employed to determine the binding concentrations of TET, and the influence of IS and pH on binding performance was evaluated via UV-vis spectroscopy, total organic carbon (TOC), and Excitation-Emission-Matrix spectra (EEM), zeta potentials and molecule size distribution analysis. Our results suggested that carboxyl and phenolic hydroxyl were identified as the main contributors to TET binding based on the fourier transform infrared spectroscopy (FTIR) analysis, and the binding capability of four DOMs followed as HA > FA » BSA > SAA. The biggest binding concentrations of TET by 10 mg C/L HA, FA, BSA and SAA were 0.863 μM, 0.487 μM, 0.084 μM and 0.086 μM, respectively. The higher binding capability of HA and FA is mainly attributed to their richer functional groups, lower zeta potential (HA/FA = -15.92/-13.54 mV) and the bigger molecular size (HA/FA = 24668/27750 nm). IS significantly inhibits the binding interaction by compressing the molecular structure and the surface electric double layer, while pH had a weak effect. By combining the Donnan model and the multiple linear regression analysis, a modified Karickhoff model was established to effectively predict the binding performance of DOM under different IS (0.001-0.1 M) and pH (5.0-9.0) conditions, and the R² of linear fitting between experiment-measured logK_DOC and model-calculated logK_OC were 0.94 for HA and 0.91 for FA. This finding provides a theoretical basis for characterizing and predicting the binding performance of various DOMs to residual micropollutants in the natural water environment.

Salt-dependent passive adsorption of IgG1κ-type monoclonal antibodies on hydrophobic microparticles

Understanding how antibodies adsorb on solid surfaces is essential for developing effective approaches to control this process. In this study, passive adsorptions on the hydrophobic solid surface of a polystyrene microparticle (MP) of two highly similar IgG1 κ-type monoclonal antibodies (mAbs), rituximab, and trastuzumab, were examined in the presence of Hofmeister salts. Except of kosmotropic salts, the screening of electrostatic interactions using salts reduces the passive adsorption of mAbs on MP. To better understand the ion-specific adsorption process, salt-dependent Langmuir isotherm parameters were obtained and correlated for two mAbs. We find that while their maximum adsorption capacities to MPs are highly correlated (r > 0.9), the salt-dependent profiles of adsorption binding constants, Kobs, differ substantially. For rituximab, Kobs increases >10-fold in an ion-specific manner; for trastuzumab, Kobs remains constant. We conclude that even minor sequence variations among the mAbs can affect the adsorption, as well as the molecular forces attracting proteins to a solid surface. This difference might originate from the heterogeneous orientation of the adsorbed mAbs.

Liquid-to-Solid Phase Transitions of Imidazolium-Based Zwitterionic Polymers Induced by Hofmeister Anions

In this study, we compared the responses of two different types of zwitterionic polymers (ZPs), polyvinylimidzole sulfobetaine (poly(SBVI)) and polymethacrylate sulfobetaine (poly(SBMA)) to Hofmeister anions. Although the anions of the two ZPs were the same as the sulfonate anions and only the types of their cations were different from each other, the aggregation behavior of each in the salt aqueous solution was remarkably different. Consequently, poly(SBVI) exhibited both salting-in and salting-out effects depending on the type and concentration of salt, while poly(SBMA) only exhibited the anti-polyelectrolyte effect. The results of this study provide a deeper understanding of the behavior of zwitterionic polymers in salt solutions and will greatly expand their applications.

Specific Anion Effects on Charged-Neutral Random Copolymers: Interplay between Different Anion-Polymer Interactions

The study of ion specificities of charged-neutral random copolymers is of great importance for understanding specific ion effects on natural macromolecules. In the present work, we have investigated the specific anion effects on the thermoresponsive behavior of poly([2-(methacryloyloxy)ethyl trimethylammonium chloride]-co-N-isopropylacrylamide) [P(METAC-co-NIPAM)] random copolymers. Our study demonstrates that the anion specificities of the P(METAC-co-NIPAM) copolymers are dependent on their chemical compositions. The specific anion effects on the copolymers with high mole fractions of poly(N-isopropylacrylamide) (PNIPAM) are similar to those on the PNIPAM homopolymer. As the mole fraction of PNIPAM decreases to a certain value, a V-shaped anion series can be observed in terms of the anion-specific cloud point temperature of the copolymer, as induced by the interplay between different anion-polymer interactions. Our study also suggests that both the direct and the indirect anion-polymer interactions contribute to the anion specificities of the copolymers. This work would improve our understanding of the relationship between the ion specificities and the ion-macromolecule interactions for naturally occurring macromolecules.

Elastic compliance and adsorption profiles of Bovine serum albumin at fluid/solid interface in the presence of electrolytes

Non-trivial topology of proteins under shear suggests that even small structural changes in proteins result in dramatic variations in the mechanical properties and stability. In this study, we have analysed the elastic compliance of solvated bovine serum albumin (BSA) with NaCl,MgCl2, FeCl3 of concentration-ranging from 50 mM to 250 mM using Quartz crystal microbalance with dissipation. The compliance shows a reverse Hofmeister trend (Na +  Collapse

Key Words

• BSA
• Elastic compliance
• FRET
• Intra molecular interaction
• Ion cloud

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MESH Headings

• Serum Albumin, Bovine/chemistry
• Animals
• Cattle
• Adsorption
• Electrolytes/chemistry
• Elasticity
• Magnesium Chloride/chemistry
• Quartz Crystal Microbalance Techniques
• Sodium Chloride/chemistry
• Scattering, Small Angle
• Ferric Compounds/chemistry
• Chlorides/chemistry
• X-Ray Diffraction

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Grants Collapse

Affiliation(s)

A Bruntha Advanced Materials Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
R Radhipriya Advanced Materials Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India
Thanikaivelan Palanisamy Advanced Materials Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
A Dhathathreyan Advanced Materials Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India.

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Improved accuracy and precision in electrophoretic NMR experiments. Current control and sample cell design

Electrophoretic NMR has the capacity to provide unique physico-chemical information but is limited by a variety of experimental artifacts, such as thermal convection and electrolytic products in the sample. Here we present some simple modifications to the experimental hardware and protocol that, in a significant number of cases, can much improve experimental accuracy and precision. We show that one can strongly reduce artifacts in a symmetric sample cell with an appropriate feeding of current and with a porous plug suitably inserted. This latter feature requires that the electric field pulses across the sensitive volume are implemented as current-controlled pulses applied to the sample. Measurements with current-controlled pulses have the additional advantage of not requiring calibration with samples of known electrophoretic mobility.

Determination of Protein Charge in Aqueous Solution Using Electrophoretic Light Scattering: A Critical Investigation of the Theoretical Fundamentals and Experimental Methodologies

Studies are reported of the measurement of electrophoretic mobilities of bovine serum albumin (BSA) in aqueous potassium chloride solutions as a function of ionic strength and pH using electrophoretic light scattering (ELS). It is demonstrated that the use of palladium or platinum electrodes should be avoided and that platinized platinum electrodes are necessary to avoid interference from unwanted electrochemical phenomena at the electrode-liquid interface. Potentiometric acid titration was performed to quantify the amount of protonic charge per protein molecule at the same pH values as the electrophoretic mobility measurements. It is shown that appropriate selection of an electrokinetic model yields excellent agreement between predicted and experimental electrophoretic mobilities across the ranges of pH and ionic strength studied in accordance with the protonic charge values obtained by titration. The experimental results are explained in terms of protonation, chloride counterion binding, and protein molecule permeability. This work highlights specific requirements of using ELS for confident analysis of proteins in aqueous solutions.

FTIR Spectroscopy Study of the Secondary Structure Changes in Human Serum Albumin and Trypsin under Neutral Salts

The effect of neutral salts on protein conformation was first analyzed by Hofmeister in 1888, however, even today this phenomenon is not completely understood. To clarify this effect, we studied changes in the secondary structure of two proteins: human serum albumin with predominantly α-helical structure and porcine pancreas β-trypsin with the typical β-structural arrangement in aqueous solutions of neutral salts (KSCN, KCl, (NH₄)₂SO₄). The changes in the secondary structure were studied at 23 °C and 80 °C by using the second derivative deconvolution method of the IR spectra. Our results demonstrated that the ability of the salts to stabilize/destabilize these two proteins correlates with the Hofmeister series of ions. At the same time, some exceptions were also observed. The destabilization of the native structures of both α-helical albumin and β-structural trypsin upon interaction with neutral salts leads to the formation of intermolecular β-sheets typical for amyloid fibrils or amorphous aggregates. Thus, our quantitative FTIR-spectroscopy analysis allowed us to further clarify the mechanisms and complexity of the neutral salt actions on protein structures which may lead to strategies preventing unwelcome misfolding of proteins.

Hofmeister Series: Insights of Ion Specificity from Amphiphilic Assembly and Interface Property

Hofmeister series (HS), ion specific effect, or lyotropic sequence acts as a pivotal part in a number of biological and physicochemical phenomena, e.g., changing the solubility of hydrophobic solutes, the cloud points of polymers and nonionic surfactants, the activities of various enzymes, the action of ions on an ion-channel, and the surface tension of electrolyte solutions, etc. This review focused on how ion specificity influences the critical micelle concentration (CMC) and how the thermoresponsive behavior of surfactants, and the dynamic transition of the aggregate, controls the aggregate transition and gel formation and tunes the properties of air/water interfaces (Langmuir monolayer and interfacial free energy). Recent progress of the ion specific effect in bulk phase and at interfaces in amphiphilic systems and gels is summarized. Applications and a molecular level theoretical explanation of HS are discussed comprehensively. This review is aimed to supply a fresh and comprehensive understanding of Hofmiester phenomena in surfactants, polymers, colloids, and interface science and to provide a guideline to design the microstructures and templates for preparation of nanomaterials.

Specific Ion Effects on the Enzymatic Degradation of Polymeric Marine Antibiofouling Materials

It is expected that the widely dispersed ions in seawater would have strong influence on the performance of polymeric marine antibiofouling materials through the modulation of enzymatic degradation of the materials. In this work, poly(ε-caprolactone)-based polyurethane and poly(triisopropylsilyl methacrylate-co-2-methylene-1,3-dioxepane) have been employed as model systems to explore the specific ion effects on the enzymatic degradation of polymeric marine antibiofouling materials. Our study demonstrates that the specific ion effects on the enzymatic degradation of the polymer films are closely correlated with the ion-specific enzymatic hydrolysis of the ester. In the presence of different cations, the effectiveness of the enzyme to degrade the polymer films is dominated by the direct specific interactions between the cations and the negatively charged enzyme molecules. In the presence of different anions, the kosmotropic anions give rise to a high enzyme activity in the degradation of polymer films induced by the salting-out effect, whereas the chaotropic anions lead to a low enzyme activity in the degradation of the polymer films owing to the salting-in effect. This work highlights the opportunities available for the use of specific ion effects to modulate the enzymatic degradation of polymeric antibiofouling materials in the marine environment.

Ion Specificity and Nonmonotonic Protein Solubility from Salt Entropy

The addition of salt to protein solutions can either increase or decrease the protein solubility, and the magnitude of this effect depends on the salt used. We show that these effects can be captured using a theory that includes attractive and repulsive electrostatic interactions, nonelectrostatic protein-ion interactions, and ion-solvent interactions via an effective solvated ion radius. We find that the ion radius has significant effects on the translational entropy of the salt, which leads to salt specificity in the protein solubility. At low salt, the dominant effect comes from the entropic cost of confining ions within the aggregate, whereas at high concentrations, the salt drives a depletion attraction that favors aggregation. Our theory explains the reversal in the Hofmeister series observed in lysozyme cloud point measurements and semi-quantitatively describes the solubility of lysozyme and chymosin crystals. We present a comparison of the contributions to the free energy and give guidelines for when salting in or salting out should be expected.

Synergism between Specific Halide Anions and pH Effects during Nanosecond Laser Fragmentation of Ligand-Free Gold Nanoparticles

Gold nanoclusters (AuNCs) with diameters smaller than 3 nm are an emerging field of research because they possess interesting optical properties, such as photoluminescence. However, to date, it is still difficult to distinguish whether these properties originate from the cores of the nanoparticles or from the adsorbates on their surfaces. Hence, there is a high demand for ligand-free, ultra-small particles because they make it possible to study ligand and core effects separately. Pulsed laser fragmentation in liquids (LFL) is a convenient route for the synthesis of ligand-free AuNCs. The influence of physical parameters, such as melting and evaporation, on the LFL process is well understood both theoretically and experimentally. However, the impact of the chemical composition of the medium during LFL, which critically affects the particle formation process, has been less well examined. Therefore, in this work, we elucidate the extent to which the ionic strength, the pH value, and the nature of the halide anion that is present, that is, F^-, Cl^-, Br^-, or I^-, influence the particle size distribution of the LFL product and the mean yield of small particles (<3 nm) of the product. We showed that the yield of small particles can be enhanced by the synergism between pH and specific ion effects, which probably is attributable to the adsorption of specific anions. In addition, our findings indicated that anion-based stabilization depends critically on the type of anion. A direct Hofmeister effect was observed for anions in the neutral pH regime, whereas an indirect Hofmeister series was reported in alkaline solution, which probably was due to the more hydrophilic surfaces of the AuNCs that were formed.

Equilibrium Protein Adsorption on Nanometric Vegetable Oil Hybrid Film/Water Interface Using Neutron Reflectometry

Nanofilms of about 2 nm thickness have been formed at the air-water interface using functionalized castor oil (ICO) with cross-linkable silylated groups. These hybrid films represent excellent candidates for replacing conventional polymeric materials in biomedical applications, but they need to be optimized in terms of biocompatibility, which is highly related to protein adsorption. Neutron reflectivity has been used to study the adsorption of two model proteins, bovine serum albumin and lysozyme, at the silylated oil (ICO)-water interface in the absence and presence of salt at physiologic ionic strength and pH and at different protein concentrations. These measurements are compared to adsorption at the air-water interface. While salt enhances adsorption by a similar degree at the air-water and oil-water interfaces, the impact of the oil film is significant with adsorption at the oil-water interface 3-4-fold higher compared to that at the air-water interface. Under these conditions, the concentration profiles of the adsorbed layers for both proteins indicate multilayer adsorption. The thickness of the outer layer (oil side) is close to the dimension of the minor axis of the protein molecule, ∼30 Å, suggesting a sideway orientation with the long axis parallel to the interface. The inner layer extends to 55-60 Å. Interestingly, in all cases, the composition of the oil film remains intact without significant protein penetration into the film. The optimal adsorption on these nanofilms, 1.7-2.0 mg·m^-2, is comparable to the results obtained recently on thick solid cross-linked films using a quartz crystal microbalance and atomic force microscopy, showing in particular that adsorption at these ICO film interfaces under standard physiological conditions is nonspecific. These results furnish useful information toward the elaboration of vegetable oil-based nanofilms in direct nanoscale applications or as precursor films in the fabrication of thicker macroscopic films for biomedical applications.

Effect of bovine serum albumin on tartrate-modified manganese ferrite nano hollow spheres: spectroscopic and toxicity study

The emerging category of magneto-fluorescent tartrate-modified MnFe₂O₄ nano hollow spheres (T-MnFe₂O₄ NHSs) can be considered as promising candidates for biomedical applications. The interaction of bovine serum albumin (BSA) with T-MnFe₂O₄ NHSs has been studied using several spectroscopic techniques, which suggest that the interaction occurs by an electrostatic mechanism. Furthermore, BSA enhances the charge transfer transition from the tartrate ligand to the metal ions along with the d-d transition of Fe³⁺ ions on NHSs surfaces at different pH. Very strong salt bridge formation occurs between the lysine of the BSA surface and the tartrate in basic medium (pH 10), followed by the acidic (pH 3) and neutral medium (pH 7), respectively. Systematic fluorescence microscopic analysis reveals that BSA significantly enhances the contrast of T-MnFe₂O₄ NHSs in UV and blue light excitation because of the extended charge transfer from BSA to T-MnFe₂O₄ NHSs. Our report demonstrates great potential in the field of nanotechnology and biomedical applications. In vitro toxicity analysis using RAW 264.7 celline and in vivo studies on Wister rats revealed that the T-MnFe₂O₄ NHSs are benign. Furthermore, T-MnFe₂O₄ NHSs also appear to be an antimicrobial agent. Therefore, T-MnFe₂O₄ NHSs can be explored for future therapeutic applications.

Hydrodynamic and Electrophoretic Properties of Trastuzumab/HER2 Extracellular Domain Complexes as Revealed by Experimental Techniques and Computational Simulations

The combination of hydrodynamic and electrophoretic experiments and computer simulations is a powerful approach to study the interaction between proteins. In this work, we present hydrodynamic and electrophoretic experiments in an aqueous solution along with molecular dynamics and hydrodynamic modeling to monitor and compute biophysical properties of the interactions between the extracellular domain of the HER2 protein (eHER2) and the monoclonal antibody trastuzumab (TZM). The importance of this system relies on the fact that the overexpression of HER2 protein is related with the poor prognosis breast cancers (HER2++ positives), while the TZM is a monoclonal antibody for the treatment of this cancer. We have found and characterized two different complexes between the TZM and eHER2 proteins (1:1 and 1:2 TZM:eHER2 complexes). The conformational features of these complexes regulate their hydrodynamic and electrostatic properties. Thus, the results indicate a high degree of molecular flexibility in the systems that ultimately leads to higher values of the intrinsic viscosity, as well as lower values of diffusion coefficient than those expected for simple globular proteins. A highly asymmetric charge distribution is detected for the monovalent complex (1:1 complex), which has strong implications in correlations between the experimental electrophoretic mobility and the modeled net charge. In order to understand the dynamics of these systems and the role of the specific domains involved, it is essential to find biophysical correlations between dynamics, macroscopic transport and electrostatic properties. The results should be of general interest for researchers working in this area.

Solution Properties of Amphoteric Random Copolymers Bearing Pendant Sulfonate and Quaternary Ammonium Groups with Controlled Structures

Amphoteric random copolymers P(AMPS/APTAC50) _x, where x = 41, 89, and 117, composed of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) and 3-acrylamidopropyltrimethylammonium chloride (APTAC) were prepared via reversible addition-fragmentation chain transfer radical polymerization. P(AMPS/APTAC50) _x can dissolve in pure water to form small interpolymer aggregates. In aqueous solutions of NaCl, P(AMPS/APTAC50) _x can dissolve in the unimer state. Amphoteric random copolymer P(AMPS/APTAC50)_c with high molecular weight was prepared via conventional free-radical polymerization. Although P(AMPS/APTAC50)_c cannot dissolve in pure water, it can dissolve in aqueous solutions of NaCl. In amphoteric random copolymers with high molecular weight, the possibility of continuous sequences of monomers with the same charge may increase, which may cause strong interactions between polymer chains. When fetal bovine serum (FBS) and polyelectrolytes were mixed in phosphate-buffered saline, the hydrodynamic radius and light-scattering intensity increased. There was no interaction between P(AMPS/APTAC50) _x and FBS because corresponding increases could not be observed.

Ion-specificity and surface water dynamics in protein solutions

Ion-specific effects at the protein surface are investigated here in light of the changes they infer to surface water dynamics, as observed by 1H NMR relaxation (at 20 MHz). Two well-known proteins, hen egg-white lysozyme (LZM) and bovine serum albumin (BSA), show qualitatively opposite trends in the transverse relaxation rate, R2(1H), along a series of different monovalent salt anions in the solution. Presence of salt ions increases R2(1H) in the case of lysozyme and diminishes it in the case of BSA. The effect magnifies for larger and more polarizable ions. The same contrasting effect between the two proteins is observed for protein-solvent proton exchange. This hints at subtle effects ion-binding might have on the accessibility of water surface sites on the protein. We suggest that the combination of the density of surface charge residues and surface roughness, at the atomic scale, dictates the response to the presence of salt ions and is proper to each protein. Further, a dramatic increase in R2(1H) is found to correlate closely with the formation of protein aggregates. The same ordering of salts in their ability to aggregate lysozyme, as seen previously by cloud point measurements, is reproduced here by R2(1H). 1H NMR relaxation data is supplemented by 35Cl and 14N NMR relaxation for selected salt ions to probe the ion-binding itself.

Calorimetric studies of interactions between low molecular weight salts and bovine serum albumin in water at pH values below and above the isoionic point

Isothermal titration calorimetry was used to determine the temperature and salt concentration dependence of the enthalpy of mixing, Δ_mix H, of bovine serum albumin (BSA) in aqueous buffer solutions with several low molecular weight salts. Three buffers were used: acetate (pH = 4.0), MOPS (7.2), and borate (9.2). Since the isoionic point of BSA is at pI ≈ 4.7, the net charge of BSA in acetate buffer was positive (≈ +20), while in the other two buffer solutions it was negative (≈ -15 in MOPS and ≈ -25 in borate). The majority of the recorded heat effects were exothermic, while only at pH = 9.2 a weak endothermic effect upon mixing BSA with LiCl, NaCl, and KCl was observed. For all buffer solutions the absolute values of Δ_mix H of sodium salts followed the order: NaCl < NaBr < NaNO₃ < NaI < NaSCN, which is the reverse Hofmeister series for anions. The magnitude of the effects was the largest in acetate buffer and decreased with an increasing pH value of the solution. While the effect of varying the anion of the added salts was strongly pronounced at all pH values, the effect of the cation (LiCl, NaCl, KCl, RbCl and CsCl salts) was weak. The most interesting feature of the results obtained for pH > pI was the fact that Δ_mix H were considerably more sensitive to the anion (co-ion to the net BSA charge) than to the cation species. This indicated that anions interacted quite strongly with the BSA even at pH values where the net charge of the protein was negative. We showed that Δ_mix H at high addition of salts correlated well with the enthalpy of hydration of the corresponding salt anion. This finding suggested, consistently with some previous studies, that a part of the exothermic contribution to Δ_mix H originated from the hydration changes upon the protein-salt interaction. Theoretical analysis, based on the primitive model of highly asymmetric electrolyte solutions solved within the mean spherical approximation, was used to estimate Coulomb effects upon mixing.

Interrogating the Osmotic Pressure of Self-Crowded Bovine Serum Albumin Solutions: Implications of Specific Monovalent Anion Effects Relative to the Hofmeister Series

The free-solvent-based (FSB) model and osmotic pressure were used to probe the ion binding and protein hydration for self-crowded bovine serum albumin in 0.15 M NaF, NaCl, NaI, and NaSCN solutions. All experiments were conducted with solutions at pH 7.4. The regressed results of the FSB model behavior to the measured osmotic pressure were excellent, albeit, the osmotic pressure data for NaSCN were noisy. The resulting ion binding and hydration were realistic values and the covariance of the two parameters was exceptionally low, providing substantial credibility to the FSB model. The results showed that the kosmotropic F^- and neutral Cl^- solutions generated significantly higher ion binding and protein hydration than the chaotropic solutions of I^- and SCN^-. Further, the ionic strength ratio and resulting hydration implied that the chaotropic solutions had substantially higher aggregation than the other salts investigated. Overall, the FSB model provides an additional, complementary tool to contribute to the analysis of crowded protein solutions relative to anions in the Hofmeister series as it can interrogate crowded solutions directly; something that is not possible with many measurement techniques.

Development of constant-pH simulation methods in implicit solvent and applications in biomolecular systems

pH is a critical parameter for biological and technological systems directly related with electrical charges. It can give rise to peculiar electrostatic phenomena, which also makes them more challenging. Due to the quantum nature of the process, involving the forming and breaking of chemical bonds, quantum methods should ideally by employed. Nevertheless, due to the very large number of ionizable sites, different macromolecular conformations, salt conditions, and all other charged species, the CPU time cost simply becomes prohibitive for computer simulations, making this a quite complex problem. Simplified methods based on Monte Carlo sampling have been devised and will be reviewed here, highlighting the updated state-of-the-art of this field, advantages, and limitations of different theoretical protocols for biomolecular systems (proteins and nucleic acids). Following a historical perspective, the discussion will be associated with the applications to protein interactions with other proteins, polyelectrolytes, and nanoparticles.

Thermodynamic Study of the Interaction of Bovine Serum Albumin and Amino Acids with Cellulose Nanocrystals

The interaction of bovine serum albumin (BSA) with sulfated, carboxylated, and pyridinium-grafted cellulose nanocrystals (CNCs) was studied as a function of the degree of substitution by determining the adsorption isotherm and by directly measuring the thermodynamics of interaction. The adsorption of BSA onto positively charged pyridinium-grafted cellulose nanocrystals followed Langmuirian adsorption with the maximum amount of adsorbed protein increasing linearly with increasing degree of substitution. The binding mechanism between the positively charged pyridinum-grafted cellulose nanocrystals and BSA was found to be endothermic and based on charge neutralization. A positive entropy of adsorption associated with an increase of the degree of disorder upon addition of BSA compensated for the unfavorable endothermic enthalpy and enabled formation of pyridinium-g-CNC-BSA complexes. The endothermic enthalpy of adsorption was further found to decrease as a function of increasing degree of substitution. Negatively charged cellulose nanocrystals bearing sulfate and/or carboxylic functionalities were found to not interact significantly with the BSA protein. To investigate in more detail the role of single amino acids in the adsorption of proteins onto cellulose nanocrystals, we also studied the interaction of different types of amino acids with CNCs, i.e., charged (lysine, aspartic acid), aromatic (tryptophan, tyrosine), and polar (serine) amino acids. We found that none of the single amino acids bound with CNCs irrespective of surface charge and that therefore the binding of proteins with CNCs appears to require larger amino acid sequences that induce a greater entropic contribution to stabilize binding. Single amino acids are thus not adsorbed onto cellulose nanocrystals.

Electrostatics analysis of the mutational and pH effects of the N-terminal domain self-association of the major ampullate spidroin

Spider silk is a fascinating material combining mechanical properties such as maximum strength and high toughness comparable or better than man-made materials, with biocompatible degradability characteristics. Experimental measurements have shown that pH triggers the dimer formation of the N-terminal domain (NTD) of the major ampullate spidroin 1 (MaSp 1). A coarse-grained model accounting for electrostatics, van der Waals and pH-dependent charge-fluctuation interactions, by means of Monte Carlo simulations, gave us a more comprehensive view of the NTD dimerization process. A detailed analysis of the electrostatic properties and free energy derivatives for the NTD homoassociation was carried out at different pH values and salt concentrations for the protein wild type and for several mutants. We observed an enhancement of dipole-dipole interactions at pH 6 due to the ionization of key amino acids, a process identified as the main driving force for dimerization. Analytical estimates based on the DVLO theory framework corroborate our findings. Molecular dynamics simulations using the OPEP coarse-grained force field for proteins show that the mutant E17Q is subject to larger structural fluctuations when compared to the wild type. Estimates of the association rate constants for this mutant were evaluated by the Debye-Smoluchowski theory and are in agreement with the experimental data when thermally relaxed structures are used instead of the crystallographic data. Our results can contribute to the design of new mutants with specific association properties.

Charge Regulation in the Electrical Double Layer: Ion Adsorption and Surface Interactions

Charge regulation in the electrical double layer has important implications for ion adsorption, interparticle forces, colloidal stability, and deposition phenomena. Although charge regulation generally receives little attention, its consequences can be major, especially when considering interactions between unequally charged surfaces. The present article discusses common approaches to quantify such phenomena, especially within classical Poisson-Boltzmann theory, and pinpoints numerous situations where a consideration of charge regulation is essential. For the interpretation of interaction energy profiles, we advocate the use of the constant regulation approximation, which summarizes the surface properties in terms of two quantities, namely, the diffuse layer potential and the regulation parameter. This description also captures some pronounced regulation effects observed in the presence of multivalent ions.

Interactions between Polyelectrolyte Brushes and Hofmeister Ions: Chaotropes versus Kosmotropes

We have investigated the interactions between the positively charged poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes and the Hofmeister anions and the interactions between the negatively charged poly(3-sulfopropyl methacrylate potassium) (PSPMA) brushes and the Hofmeister cations using a combination of quartz crystal microbalance with dissipation and spectroscopic ellipsometry. A V-shaped anion series is observed in terms of the ion-specific interactions between the PMETAC brushes and the Hofmeister anions. We have found that the chaotropic and kosmotropic anions interact with the PMETAC brushes in different manners. The ion-specific interactions between the PMETAC brushes and the chaotropic anions are dominated by the direct interactions between the anions and the positively charged quaternary ammonium group via ion pairing mediated by ionic hydration strength or polarizability, whereas the ion-specific interactions between the PMETAC brushes and the kosmotropic anions are dominated by the competition for water molecules between the anions and the brushes. The ion-specific interactions between the PMETAC brushes and the anions have significant influences on both the hydration and the conformation of the brushes. The cations exhibit weaker specific ion effects on the PSPMA brushes in comparison with the specific anion effects on the PMETAC brushes.

Specific ion effects on particle aggregation induced by monovalent salts within the Hofmeister series

Ion specific effects of monovalent salts on charging and aggregation for two types of polystyrene latex particles were investigated by electrophoresis and time-resolved light scattering. The chemical composition of the electrolytes was systematically varied in the experiments. Accordingly, NaH2PO4, NaF, NaCl, NaBr, NaNO3, and NaSCN were used to vary the anions and N(CH3)4Cl, NH4Cl, CsCl, KCl, NaCl, and LiCl for the cations. The salt concentration dependence of the electrophoretic mobilities indicates that the surface charge was screened by the counterions when their concentrations increased. For the SCN(-) ions, adsorption on positively charged particles leads to charge reversal. The aggregation rates are small at low electrolyte concentrations indicating stable dispersions under these conditions, and they increase with the salt concentration. When viscosity corrections are taken into account, no ion specific effects in the fast aggregation regime can be established. The slow and fast aggregation regimes are separated by the critical coagulation concentration (CCC). Within the experimental error, the CCCs are the same in systems containing different co-ions but the same counterions, with the exception of ammonium salts. However, the variation of counterions leads to different CCC values due to specific interaction of the counterions with the surface. These values follow the Hofmeister series for negatively charged sulfate latex particles, while the reversed order was observed for positively charged amidine latex. Comparison between experimental CCCs and those calculated by the theory of Derjaguin, Landau, Verwey, and Owerbeek reveals that variations in the surface charge due to ionic adsorption are mainly responsible for the ion specific effects in the aggregation process.