Moderate Ionic Strength Research Articles

Low adsorption affinity of athabasca oil sands naphthenic acid fraction compounds to a peat-mineral mixture

Much of the toxicity in oil sands process-affected water in Athabasca oil sands tailings has been attributed to naphthenic acids (NAs) and associated naphthenic acid fraction compounds (NAFCs). Previous work has characterized the environmental behaviour and fate of these compounds, particularly in the context of constructed treatment wetlands. There is evidence that wetlands can attenuate NAFCs in natural and engineered contexts, but relative contributions of chemical, biotic, and physical adsorption with sequestration require deconvolution. In this work, the objective was to evaluate the extent to which prospective wetland substrate material may adsorb NAFCs using a peat-mineral mix (PMM) sourced from the Athabasca Oil Sands Region (AOSR). The PMM and NAFCs were first mixed and then equilibrated across a range of NAFC concentrations (5–500 mg/L) with moderate ionic strength and hardness (∼200 ppm combined Ca2+ and Mg2+) that approximate wetland water chemistry. Under these experimental conditions, low sorption of NAFCs to PMM was observed, where sorbed concentrations of NAFCs were approximately zero mg/kg at equilibrium. When NAFCs and PMM were mixed and equilibrated together at environmentally relevant concentrations, formula diversity increased more than could be explained by combining constituent spectra. The TOC present in this PMM was largely cellulose-derived, with low levels of thermally recalcitrant carbon (e.g., lignin, black carbon). The apparent enhancement of the concentration and diversity of components in PMM/NAFCs mixtures are likely related to aqueous solubility of some PMM-derived organic materials, as post-hoc combination of dissolved components from PMM and NAFCs cannot replicate enhanced complexity observed when the two components are agitated and equilibrated together.

Hydraulic conductivity of two geosynthetic clay liners with different bentonite granule sizes

Experiments were conducted to evaluate the hydraulic conductivity of two commercially available geosynthetic clay liners (GCLs) with similar properties but different bentonite granule sizes (D50=1.0 mm or 0.3 mm). A synthetic municipal solid waste (MSW) leachate prepared in the laboratory and four actual coal combustion product (CCP) leachates collected from the field were used as permeant solutions to represent different permeant chemical conditions for waste containment applications. The two GCLs had comparable hydraulic conductivity to dilute or moderate ionic strength (I) leachates (< 0.1 M). With more concentrated leachates (I > 0.1 M), hydraulic conductivity of the GCL containing finer bentonite granules (FG) was 10 to 500 times lower than the hydraulic conductivity of the GCL containing coarser bentonite granules (CG) under the same equilibrium conditions. These results suggest that GCLs containing finer bentonite granules may be less vulnerable to permeant chemistry.

The Metabolism of 2-arachidonoylglycerol in Rod Outer Segments Is Modulated by Proteins Involved in the Phototransduction Process.

We previously reported that 2-arachidonoylglycerol (2-AG) synthesis by diacylglycerol lipase (DAGL) and lysophosphatidate phosphohydrolase (LPAP) and hydrolysis by monoacylglycerol lipase (MAGL) in rod outer segments (ROS) from bovine retina were differently modified by light applied to the retina. Based on these findings, the aim of the present research was to evaluate whether 2-AG metabolism could be modulated by proteins involved in the visual process. To this end, ROS kept in darkness (DROS) or obtained in darkness and then subjected to light (BROS) were treated with GTPγS and GDPβS, or with low and moderate ionic strength buffers for detaching soluble and peripheral proteins, or soluble proteins, respectively. Only DAGL activity was stimulated by the application of light to the ROS. GTPγS-stimulated DAGL activity in DROS reached similar values to that observed in BROS. The studies using different ionic strength show that (1) the highest decrease in DROS DAGL activity was observed when both phosphodiesterase (PDE) and transducin α (Tα) are totally membrane-associated; (2) the decrease in BROS DAGL activity does not depend on PDE association to membrane, and that (3) MAGL activity decreases, both in DROS and BROS, when PDE is not associated to the membrane. Our results indicate that the bioavailability of 2-AG under light conditions is favored by G protein-stimulated increase in DAGL activity and hindered principally by Tα/PDE association with the ROS membrane, which decreases DAGL activity.

Adsorption of surfactant molecules onto the surface of colloidal particles: Case of like-charged species

Mixed systems of nanoparticles and surfactants have a broad range of applications from consumer products and medicine to inkjet printing and oil recovery. The interparticle interactions can be tuned in presence of surfactants and are dependent on surface charge of both species, particle’s wettability, surfactant solubility, and solution conditions such as electrolyte concentration and pH. The case of oppositely charged particles and surfactants has been extensively examined in the literature, primarily to tune the wettability of particles. In contrast, the behavior in systems of like-charged species such as negatively charged particles and anionic surfactants remains poorly understood, with conflicting findings reported in previous studies on the adsorption of surfactants. By conducting a comprehensive investigation, in this study we shed light on the factors that influence the adsorption of surfactant onto the particle surface, both promoting and preventing it, and unravel the underlying mechanisms governing such behavior. Silica nanoparticles were used as the negatively charged particle, sodium dodecyl sulfate (SDS) was used as the surfactant and potassium nitrate was used as the salt, while the pH was adjusted by potassium hydroxide and nitric acid, to investigate the effect of surfactants on the surface characteristics of the silica nanoparticles under various operating conditions. The zeta potential of particles along with the solution conductivity were obtained via mobility measurements. Using this information, the solution’s Debye length and the particle’s surface charge density were estimated. It was found that interpreting the outcome solely based on the zeta potential data might not reveal the adsorption of SDS onto the particle surface as no supercharging effect could be detected in some cases. However, shifting the perspective to charge density shows that SDS increased the particles charge density for pH values in the range of 2–5, corresponding to the pH conditions at which vicinal and geminal silanol groups are dissociated. This effect was more pronounced at moderate total ionic strengths between 1 and 10 mM, where SDS activity was found to be higher and the Debye length was sufficiently short. The increase in the particle’s charge density was attributed to the tail-down adsorption of SDS onto the particle surface via entropically-driven interactions. These findings offer valuable insights into like-charged mixed particle/surfactant systems and bring clarity to the scientific community regarding this complex and previously inconclusive topic.

Quantification of the effect of gas–water–equilibria on carbonate precipitation

The expanding geothermal energy sector still faces performance issues due to scalings in pipes and surface level installations, which require elevated operation pressure levels and costly maintenance. For facilities in the North Alpine Foreland Basin, the precipitation of {hbox {CaCO}}_{3} is the main problem which is a consequence of the disruption of the lime-carbonic acid equilibrium during production. The formation of gas bubbles plays a key role in the scaling process. This work presents experiments in a bubble column to quantify the effects of gas stripping on carbonate precipitation and an extension of PhreeqC to include kinetic exchange between a gas phase and water for the simulation of the experimental results. With the same hybrid model not only precipitation of {hbox {CaCO}}_{3} but also the dissolution of scalings by the injection of {hbox {CO}}_{2} could be quantified. The bubble column was filled with tap water and brine. By varying the ionic strength of the solution, a wider range of geothermal waters was covered. Air and {hbox {CO}}_{2} were introduced at the bottom. The precipitates built on the column wall were analyzed with Raman spectroscopy: injecting air into tap water at low ionic strength led to the formation of aragonite with 59.8% of the precipitates remaining at the column wall and the rest as particles in dispersion. At moderate ionic strength the dominant polymorph was calcite and 81.5% of the crystals were attached to the wall. At high ionic strength precipitation was inhibited. The presence of crystallization nuclei reduced the time for precipitation, but not the amount of scalings formed. Injecting {hbox {CO}}_{2} into the solution completely removed the scalings from the column wall. The model and its experimental backup lay the foundation for a process-based prediction of the scales (not only) in geothermal systems.

Interplay of electrolyte concentration and molecular weight of polyDADMAC on cellulose surface adsorption

Cationic polyelectrolytes (PEs) are commonly used additives in manufacturing of cellulose based products such as regenerated fibers and paper to tailor their product properties. Here we are studying the adsorption of poly(diallyldimethylammonium chloride), PD, on cellulose, using in situ surface plasmon resonance spectroscopy (SPR) measurements. We employ model surfaces from regenerated cellulose xanthate (CX) and trimethylsilyl cellulose (TMSC), mimicking industrially relevant regenerated cellulose substrates. The effects of the PDs molecular weight were strongly depending on the ionic strength and type of electrolyte (NaCl vs CaCl2). Without electrolytes, the adsorption was monolayer-type, i.e. independent of molecular weight. At moderate ionic strength, adsorption increased due to more pronounced PE coiling, while at high ionic strength electrostatic shielding strongly reduced adsorption of PDs. Results exhibited pronounced differences for the chosen substrates (cellulose regenerated from xanthate (CXreg) vs. regenerated from trimethylsilyl cellulose, TMSCreg). Consistently higher adsorbed amounts of the PD were determined on CXreg surfaces compared TMSC. This can be attributed to a more negative zeta potential, a higher AFM roughness and a higher degree of swelling (investigated by QCM-D) of the CXreg substrates.

Ionic Strength-Dependent Attachment of Pseudomonas aeruginosa PAO1 on Graphene Oxide Surfaces.

Graphene oxide (GO) is a widely used antimicrobial and antibiofouling material in surface modification. Although the antibacterial mechanisms of GO have been thoroughly elucidated, the dynamics of bacterial attachment on GO surfaces under environmentally relevant conditions remain largely unknown. In this study, quartz crystal microbalance with dissipation monitoring (QCM-D) was used to examine the dynamic attachment processes of a model organism Pseudomonas aeruginosa PAO1 onto GO surface under different ionic strengths (1-600 mM NaCl). Our results show the highest bacterial attachment at moderate ionic strengths (200-400 mM). The quantitative model of QCM-D reveals that the enhanced bacterial attachment is attributed to the higher contact area between bacterial cells and GO surface. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM) analysis were employed to reveal the mechanisms of the bacteria-GO interactions under different ionic strengths. The strong electrostatic and steric repulsion at low ionic strengths (1-100 mM) was found to hinder the bacteria-GO interaction, while the limited polymer bridging caused by the collapse of biopolymer layers reduced cell attachment at a high ionic strength (600 mM). These findings advance our understanding of the ionic strength-dependent bacteria-GO interaction and provide implications to further improve the antibiofouling performance of GO-modified surfaces.

Application of a Simple Short-Range Attraction and Long-Range Repulsion Colloidal Model toward Predicting the Viscosity of Protein Solutions.

Some hard-sphere colloidal models have been criticized for inaccurately predicting the solution viscosity of complex biological molecules like proteins. Competing short-range attractions and long-range repulsions, also known as short-range attraction and long-range repulsion (SALR) interactions, have been thought to affect the microstructure of a protein solution at low to moderate ionic strength. However, such interactions have been implicated primarily in causing phase transition, protein gelation, or reversible cluster formation, and their effect on protein solution viscosity change is not fully understood. In this work, we show the application of a hard-sphere colloidal model with SALR interactions toward predicting the viscosity of dilute to semi-dilute protein solutions. The comparison is performed for a globular-shaped albumin and Y-shaped therapeutic monoclonal antibody that are not explained by previous colloidal models. The model predictions show that it is the coupling between attractions and repulsions that gives rise to the observed experimental trends in solution viscosity as a function of pH, concentration, and ionic strength. The parameters of the model are obtained from measurements of the second virial coefficient and net surface charge/zeta-potential, without additional fitting of the viscosity.

On-Chip Electrokinetic Micropumping for Nanoparticle Impact Electrochemistry.

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid-solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

Chemical thermodynamics of ternary M-An(VI)-CO3 system (M = Mg, Ca, Sr, and Ba)

Abstract This review provides an overview of the chemical thermodynamics on ternary earth-alkaline metal-actinyl-tricarbonate systems (i.e., M-AnO2-CO3, M = Mg, Ca, Sr, and Ba) and discusses the aqueous complexation and dissolution/precipitation equilibrium for these ternary aqueous systems. The aqueous ternary U(VI) carbonate species are remarkably predominant in the U(VI) speciation under natural environmental conditions at ambient temperature and moderate ionic strength condition, while the omnipresence, according to recent studies, would be hindered by an increase in temperature and ionic strength. With respect to the ternary solid U(VI) carbonate phases, most of the previously reported data have been focused on physical properties and thus a notable lack of available data on chemical thermodynamic properties, i.e., solubility product constant, has been identified. Nevertheless, substantial influences of these ternary M-AnO2-CO3 systems on the aqueous speciation and the solubility limiting phase under the natural environmental condition are taken into account according to the thermodynamic calculation. The authors point out that the completeness of the chemical thermodynamic model for predicting the chemical behavior of actinides in nature can be further improved on the basis of a sufficient understanding of ternary M-AnO2-CO3 systems.

Cognate DNA Recognition by Engrailed Homeodomain Involves a Conformational Change Controlled via an Electrostatic-Spring-Loaded Latch.

Transcription factors must scan genomic DNA, recognize the cognate sequence of their control element(s), and bind tightly to them. The DNA recognition process is primarily carried out by their DNA binding domains (DBD), which interact with the cognate site with high affinity and more weakly with any other DNA sequence. DBDs are generally thought to bind to their cognate DNA without changing conformation (lock-and-key). Here, we used nuclear magnetic resonance and circular dichroism to investigate the interplay between DNA recognition and DBD conformation in the engrailed homeodomain (enHD), as a model case for the homeodomain family of eukaryotic DBDs. We found that the conformational ensemble of enHD is rather flexible and becomes gradually more disordered as ionic strength decreases following a Debye–Hückel’s dependence. Our analysis indicates that enHD’s response to ionic strength is mediated by a built-in electrostatic spring-loaded latch that operates as a conformational transducer. We also found that, at moderate ionic strengths, enHD changes conformation upon binding to cognate DNA. This change is of larger amplitude and somewhat orthogonal to the response to ionic strength. As a consequence, very high ionic strengths (e.g., 700 mM) block the electrostatic-spring-loaded latch and binding to cognate DNA becomes lock-and-key. However, the interplay between enHD conformation and cognate DNA binding is robust across a range of ionic strengths (i.e., 45 to 300 mM) that covers the physiologically-relevant conditions. Therefore, our results demonstrate the presence of a mechanism for the conformational control of cognate DNA recognition on a eukaryotic DBD. This mechanism can function as a signal transducer that locks the DBD in place upon encountering the cognate site during active DNA scanning. The electrostatic-spring-loaded latch of enHD can also enable the fine control of DNA recognition in response to transient changes in local ionic strength induced by variate physiological processes.

Particle/wall electroviscous effects at the micron scale: comparison between experiments, analytical and numerical models

We report a experimental study of the motion of 1 μm single particles interacting with functionalized walls at low and moderate ionic strengths conditions. The 3D particle’s trajectories were obtained by analyzing the diffracted particle images (point spread function). The studied particle/wall systems include negatively charged particles interacting with bare glass, glass covered with polyelectrolytes and glass covered with a lipid monolayer. In the low salt regime (pure water) we observed a retardation effect of the short-time diffusion coefficients when the particle interacts with a negatively charged wall; this effect is more severe in the perpendicular than in the lateral component. The decrease of the diffusion as a function of the particle–wall distance h was similar regardless the origin of the negative charge at the wall. When surface charge was screened or salt was added to the medium (10 mM), the diffusivity curves recover the classical hydrodynamic behavior. Electroviscous theory based on the thin electrical double layer (EDL) approximation reproduces the experimental data except for small h. On the other hand, 2D numerical solutions of the electrokinetic equations showed good qualitative agreement with experiments. The numerical model also showed that the hydrodynamic and Maxwellian part of the electroviscous total drag tend to zero as h → 0 and how this is linked with the merging of both EDL’s at close proximity.

Enhancing the population of the encounter complex affects protein complex formation efficiency

Optimal charge distribution is considered to be important for efficient formation of protein complexes. Electrostatic interactions guide encounter complex formation that precedes the formation of an active protein complex. However, disturbing the optimized distribution by introduction of extra charged patches on cytochrome c peroxidase does not lead to a reduction in productive encounters with its partner cytochrome c. To test whether a complex with a high population of encounter complex is more easily affected by suboptimal charge distribution, the interactions of cytochrome c mutant R13A with wild‐type cytochrome c peroxidase and a variant with an additional negative patch were studied. The complex of the peroxidase and cytochrome c R13A was reported to have an encounter state population of 80%, compared to 30% for the wild‐type cytochrome c. NMR analysis confirms the dynamic nature of the interaction and demonstrates that the mutant cytochrome c samples the introduced negative patch. Kinetic experiments show that productive complex formation is fivefold to sevenfold slower at moderate and high ionic strength values for cytochrome c R13A but the association rate is not affected by the additional negative patch on cytochrome c peroxidase, showing that the total charge on the protein surface can compensate for less optimal charge distribution. At low ionic strength (44 mm), the association with the mutant cytochrome c reaches the same high rates as found for wild‐type cytochrome c, approaching the diffusion limit.

Potassium Glutamate and Glycine Betaine Induce Self-Assembly of the PCNA and β-Sliding Clamps

Sliding clamps are oligomeric ring-shaped proteins that increase the efficiency of DNA replication. The stability of the Escherichia coli β-clamp, a homodimer, is particularly remarkable. The dissociation equilibrium constant of the β-clamp is of the order of 10 pM in buffers of moderate ionic strength. Coulombic electrostatic interactions have been shown to contribute to this remarkable stability. Increasing NaCl concentration in the assay buffer results in decreased dimer stability and faster subunit dissociation kinetics in a way consistent with simple charge-screening models. Here, we examine non-Coulombic ionic effects on the oligomerization properties of sliding clamps. We determined relative diffusion coefficients of two sliding clamps using fluorescence correlation spectroscopy. Replacing NaCl by KGlu, the primary cytoplasmic salt in E. coli, results in a decrease of the diffusion coefficient of these proteins consistent with the formation of protein assemblies. The UV-vis spectrum of the β-clamp labeled with tetramethylrhodamine shows the characteristic absorption band of dimers of rhodamine when KGlu is present in the buffer. This suggests that KGlu induces the formation of assemblies that involve two or more rings stacked face-to-face. Results can be quantitatively explained on the basis of unfavorable interactions between KGlu and the functional groups on the protein surface, which drive biomolecular processes that bury exposed surface. Similar results were obtained with the Saccharomyces cerevisiae PCNA sliding clamp, suggesting that KGlu effects are not specific to the β-clamp. Clamp association is also promoted by glycine betaine, a zwitterionic compound that accumulates intracellularly when E. coli is exposed to high concentrations of extracellular solute. Possible biological implications are discussed.

Effect of Ionic Strength on the Aggregation Propensity of Aβ1-42 Peptide: An In-silico Study

Background: Aggregation of misfolded proteins under stress conditions in the cell might lead to several neurodegenerative disorders. Amyloid-beta (Aβ1-42) peptide, the causative agent of Alzheimer’s disease, has the propensity to fold into β-sheets under stress, forming aggregated amyloid plaques. This is influenced by factors such as pH, temperature, metal ions, mutation of residues, and ionic strength of the solution. There are several studies that have highlighted the importance of ionic strength in affecting the folding and aggregation propensity of Aβ1-42 peptide. Objective: To understand the effect of ionic strength of the solution on the aggregation propensity of Aβ1-42 peptide, using computational approaches. Materials and Methods: In this study, Molecular Dynamics (MD) simulations were performed on Aβ1-42 peptide monomer placed in (i) 0 M, (ii) 0.15 M, and (iii) 0.30 M concentration of NaCl solution. To prepare the input files for the MD simulations, we have used the Amberff99SB force field. The conformational dynamics of Aβ1-42 peptide monomer in different ionic strengths of the solutions were illustrated from the analysis of the corresponding MD trajectory using the CPPtraj tool. Results: From the MD trajectory analysis, we observe that with an increase in the ionic strength of the solution, Aβ1-42 peptide monomer shows a lesser tendency to undergo aggregation. From RMSD and SASA analysis, we noticed that Aβ1-42 peptide monomer undergoes a rapid change in conformation with an increase in the ionic strength of the solution. In addition, from the radius of gyration (Rg) analysis, we observed Aβ1-42 peptide monomer to be more compact at moderate ionic strength of the solution. Aβ1-42 peptide was also found to hold its helical secondary structure at moderate and higher ionic strengths of the solution. The diffusion coefficient of Aβ1-42 peptide monomer was also found to vary with the ionic strength of the solution. We observed a relatively higher diffusion coefficient value for Aβ1-42 peptide at moderate ionic strength of the solution. Conclusion: Our findings from this computational study highlight the marked effect of ionic strength of the solution on the conformational dynamics and aggregation propensity of Aβ1-42 peptide monomer.

Competitive inhibition of protein adsorption to silica surfaces by their coating with high density charge polyelectrolytes

The adsorption of proteins to silica surface is a common process mainly governed by the electrostatic attractive interaction between the pH-dependent negatively silica surface and the positive charges of the biomolecule. This process often reduces the conformational stability of the adsorbed protein and may reduce its biological functionality mostly due to multimolecular processes such as aggregation and fibrillation. Here we show that high-density charge cationic polyelectrolytes may successfully compete with the protein for the silica surface containing deprotonated-silanol groups. Therefore, the coating of silica surfaces with these cationic polyelectrolytes precludes the adsorption of the protein to the solid surface. Intensive water washing of the polyelectrolyte-coated silica surfaces had does not result in polyelectrolyte release (even at moderate ionic strength) maintaining the solid surface protected from protein adsorption.

Contact and macroscopic ageing in colloidal suspensions.

The ageing behaviour of dense suspensions or pastes at rest is almost exclusively attributed to structural dynamics. Here, we identify another ageing process, contact-controlled ageing, consisting of the progressive stiffening of solid-solid contacts of an arrested colloidal suspension. By combining rheometry, confocal microscopy and particle-scale mechanical tests using laser tweezers, we demonstrate that this process governs the shear-modulus ageing of dense aqueous silica and polymer latex suspensions at moderate ionic strengths. We further show that contact-controlled ageing becomes relevant as soon as Coulombic interactions are sufficiently screened out that the formation of solid-solid contacts is not limited by activation barriers. Given that this condition only requires moderate ion concentrations, contact-controlled ageing should be generic in a wide class of materials, such as cements, soils or three-dimensional inks, thus questioning our understanding of ageing dynamics in these systems.

Determination of Protein-Protein Interactions in a Mixture of Two Monoclonal Antibodies.

The coformulation of monoclonal antibody (mAb) mixtures provides an attractive route to achieving therapeutic efficacy where the targeting of multiple epitopes is necessary. Controlling and predicting the behavior of such mixtures requires elucidating the molecular basis for the self- and cross-protein-protein interactions and how they depend on solution variables. While self-interactions are now beginning to be well understood, systematic studies of cross-interactions between mAbs in solution do not exist. Here, we have used static light scattering to measure the set of self- and cross-osmotic second virial coefficients in a solution containing a mixture of two mAbs, mAbA and mAbB, as a function of ionic strength and pH. mAbB exhibits strong association at a low ionic strength, which is attributed to an electrostatic attraction that is enhanced by the presence of a strong short-ranged attraction of nonelectrostatic origin. Under all solution conditions, the measured cross-interactions are intermediate self-interactions and follow similar patterns of behavior. There is a strong electrostatic attraction at higher pH values, reflecting the behavior of mAbB. Protein-protein interactions become more attractive with an increasing pH due to reducing the overall protein net charges, an effect that is attenuated with an increasing ionic strength due to the screening of electrostatic interactions. Under moderate ionic strength conditions, the reduced cross-virial coefficient, which reflects only the energetic contribution to protein-protein interactions, is given by a geometric average of the corresponding self-coefficients. We show the relationship can be rationalized using a patchy sphere model, where the interaction energy between sites i and j is given by the arithmetic mean of the i-i and j-j interactions. The geometric mean does not necessarily apply to all mAb mixtures and is expected to break down at a lower ionic strength due to the nonadditivity of electrostatic interactions.

Interaction between indium tin oxide nanoparticles and ferricytochrome c: Conformation, redox state, and adsorption scheme.

The conformations and redox states of ferricytochrome c, before and after adsorption onto the surface of the indium tin oxide (ITO) nanoparticles, are studied to reveal the interaction nature between the cytochrome c and the conducting metal oxide surface. The characterizations with resonance Raman scattering and UV-Vis absorption reveal that the change of pH at moderate ionic strength induces transitions of conformations and redox-states, which suggests that there is intramolecular electron transfer. The conformations of the cytochrome c species are maintained after adsorption onto or collision with the ITO surface, but the redox states change significantly, and the change depends on the surface structure of the ITO nanoparticle. The adsorption or collision processes are governed by the pH-dependent electrostatic interaction between the proteins and the buffer anions bound to the ITO surface. This adsorption scenario differs from the conventional ones.

Spectrophotometric study of the uranyl monobenzoate complex at moderate ionic strength

The protonation of benzoate and the complexation of uranyl monobenzoate in NaClO4 solutions ranging from 0.1 to 0.5 M were quantitatively investigated by means of ultraviolet–visible (UV–Vis) spectrophotometry. The spectrophotometric study revealed that the uranyl monobenzoate species has a significantly enhanced absorption than that of free uranyl, especially in the UV region. The non-linear regression analysis of the UV–Vis absorption data was the first to calculate the protonation constant of benzoate and the stability constant of uranyl monobenzoate at moderate ionic strength. Based on the SIT (specific ion interaction theory), the protonation constants were extrapolated to the zero ionic strength as logβP° = 4.21 ± 0.01, and the ion interaction coefficient of the benzoate anion with the sodium cation was determined as ε(Ben-,Na+) = 0.17 ± 0.05 kg·mol−1. In the same manner, the stability constant of uranyl monobenzoate at zero ionic strength and the ion interaction coefficient of the uranyl monobenzoate cation with perchlorate anion were determined to be logβU° = 2.91 ± 0.01 and ε(UO2Ben+,ClO4-) = 0.22 ± 0.07 kg·mol−1, respectively.