Ionic Conditions Research Articles

Orai1 Boosts SK3 Channel Activation.

Simple SummaryBreast, colon, and prostate cancer account for about a third of cancer cases and a fifth of cancer deaths. At the molecular level, one reason for the development of cancer is the dysfunction or altered co-regulation of cellular proteins. In this study, we focused on the co-regulation of ion channels, specifically the prominent Ca2+ ion channel Orai1 and the Ca2+ activated K+ ion channel SK3. It has recently been reported that their interplay promotes the growth of breast and colon cancer cells, but the molecular determinants for their co-regulation have remained elusive. In this study, we set out to characterize their interplay and the crucial regions therefore required. Moreover, we found that the function of prostate cancer cells is also controlled by the interplay of Ca2+ and the Ca2+ sensitive K+ channels. Our findings provide a better understanding of the co-regulation of these ion channels, which could be used in the future for the development of novel therapeutics.The interplay of SK3, a Ca2+ sensitive K+ ion channel, with Orai1, a Ca2+ ion channel, has been reported to increase cytosolic Ca2+ levels, thereby triggering proliferation of breast and colon cancer cells, although a molecular mechanism has remained elusive to date. We show in the current study, via heterologous protein expression, that Orai1 can enhance SK3 K+ currents, in addition to constitutively bound calmodulin (CaM). At low cytosolic Ca2+ levels that decrease SK3 K+ permeation, co-expressed Orai1 potentiates SK3 currents. This positive feedback mechanism of SK3 and Orai1 is enabled by their close co-localization. Remarkably, we discovered that loss of SK3 channel activity due to overexpressed CaM mutants could be restored by Orai1, likely via its interplay with the SK3–CaM binding site. Mapping for interaction sites within Orai1, we identified that the cytosolic strands and pore residues are critical for a functional communication with SK3. Moreover, STIM1 has a bimodal role in SK3–Orai1 regulation. Under physiological ionic conditions, STIM1 is able to impede SK3–Orai1 interplay by significantly decreasing their co-localization. Forced STIM1–Orai1 activity and associated Ca2+ influx promote SK3 K+ currents. The dynamic regulation of Orai1 to boost endogenous SK3 channels was also determined in the human prostate cancer cell line LNCaP.

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.

In Situ EXAFS Study of Sr Adsorption on TiO2(110) under High Ionic Strength Wastewater Conditions

In order to provide important details concerning the adsorption reactions of Sr, batch reactions and a set of both ex situ and in situ Grazing Incidence X-ray Absorption Fine Structure (GIXAFS) adsorption experiments were completed on powdered TiO2 and on rutile(110), both reacted with either SrCl2 or SrCO3 solutions. TiO2 sorption capacity for strontium (Sr) ranges from 550 ppm (SrCl2 solutions, second order kinetics) to 1400 ppm (SrCO3 solutions, first order kinetics), respectively, and is rapid. Sr adsorption decreased as a function of chloride concentration but significantly increased as carbonate concentrations increased. In the presence of carbonate, the ability of TiO2 to remove Sr from the solution increases by a factor of ~4 due to rapid epitaxial surface precipitation of an SrCO3 thin film, which registers itself on the rutile(110) surface as a strontianite-like phase (d-spacing 2.8 Å). Extended X-ray Absorption Fine Structure (EXAFS) results suggest the initial attachment is via tetradental inner-sphere Sr adsorption. Moreover, adsorbates from concentrated SrCl2 solutions contain carbonate and hydroxyl species, which results in both inner- and outer-sphere adsorbates and explains the reduced Sr adsorption in these systems. These results not only provide new insights into Sr kinetics and adsorption on TiO2 but also provide valuable information concerning potential improvements in effluent water treatment models and are pertinent in developing treatment methods for rutile-coated structural materials within nuclear power plants.

Whey protein isolate-gum Acacia Maillard conjugates as emulsifiers for nutraceutical emulsions: Impact of glycation methods on physicochemical stability and in vitro bioaccessibility of β-carotene emulsions

The physicochemical stability and in vitro bioaccessibility of β-carotene emulsions stabilized by the whey protein isolate (WPI)-gum Acacia (GA) conjugates prepared by dry-heating (Con WPI (D)) and ultrasound (Con WPI (U)) Maillard reaction were investigated in this study. The conjugate-stabilized emulsions have a larger mean size than the WPI-stabilized emulsion. The stability of emulsions under freeze–thaw treatment, high ionic strength conditions, thermal treatment and pH conditions near the isoelectric point of WPI were improved by glycation. The chemical stability of β-carotene in emulsions was also enhanced by WPI-GA conjugates. The WPI-GA conjugate-stabilized emulsions have better stability for droplet aggregation during in vitro simulated gastrointestinal digestion, which increased β-carotene bioaccessibility. However, the Con WPI (U)-stabilized emulsion shows poor thermal stability and lower β-carotene bioaccessibility than the Con WPI (D)-stabilized emulsion, which may be attributed to ultrasound-induced structural changes during the process of glycation.

Adsorption and interaction forces of commercial Poly(naphthalene sulfonate) (PNS) and Poly(carboxylate ether) (PCE) polyelectrolytes with negatively charged surfaces in monovalent and divalent electrolytes

Adsorption of polyelectrolytes on colloidal particles alters the interaction forces and rheological properties of colloidal suspensions. Poly(naphthalene sulfonate) (PNS) and poly(carboxylate ether) (PCE) polyelectrolytes are commonly added to improve the flowability of hydrated cement paste in low-water conditions. The interactions of commercial PNS and PCE admixtures with negatively charged mica in aqueous electrolytes were directly studied with a surface forces apparatus in this work. In 0.1 M K2SO4, robust adsorption of PNS induced a strong steric repulsion. A slightly thicker adsorption layer formed in low ionic strength 5 mM Ca(NO3)2 exhibiting long-range electrostatic repulsion and short-range steric repulsion. Increasing the Ca(NO3)2 concentration to 0.1 M doubled the adsorption layer thickness and strongly screened any electrostatic repulsion. Contact of the adsorbed PNS layers caused weak physical cohesion under both low and high calcium concentrations presumably due to Ca2+ ion bridging of the opposing PNS layers. These findings indicate that (i) divalent Ca2+ possesses a much stronger ability than monovalent K+ to assemble PNS molecules resulting in significantly thicker adsorption and (ii) the interaction force between PNS adsorption layers in high ionic strength conditions is dominated by steric repulsion. In comparison, PCE did not appreciably adsorb from 0.1 M Ca(NO3)2. This finding challenges the well-accepted divalent ion bridging adsorption mechanism of PCE onto negatively charged surfaces and warrants further study.

Ex situ and in situ Magnetic Phase Synthesised Magneto-Driven Alginate Beads

Biocompatible magnetic hydrogels provide a great source of synthetic materials, which facilitate remote stimuli, enabling safer biological and environmental applications. Prominently, the ex situ and in situ magnetic phase integration is used to fabricate magneto-driven hydrogels, exhibiting varied behaviours in aqueous media. Therefore, it is essential to understand their physicochemical properties to target the best material for each application. In this investigation, three different types of magnetic alginate beads were synthesised. First, by direct, ex situ, calcium chloride gelation of a mixture of Fe3O4 nanoparticles with an alginate solution. Second, by in situ synthesis of Fe3O4 nanoparticles inside of the alginate beads and third, by adding an extra protection alginate layer on the in situ synthesised Fe3O4 nanoparticles alginate beads. The three types of magnetic beads were chemically and magnetically characterised. It was found that they exhibited particular stability to different pH and ionic strength conditions in aqueous solution. These are essential properties to be controlled when used for magneto-driven applications such as targeted drug delivery and water purification. Therefore, this fundamental study will direct the path to the selection of the best magnetic bead synthesis protocol according to the defined magneto-driven application.

The Ionic Selectivity of Lysenin Channels in Open and Sub-Conducting States.

The electrochemical gradients established across cell membranes are paramount for the execution of biological functions. Besides ion channels, other transporters, such as exogenous pore-forming toxins, may present ionic selectivity upon reconstitution in natural and artificial lipid membranes and contribute to the electrochemical gradients. In this context, we utilized electrophysiology approaches to assess the ionic selectivity of the pore-forming toxin lysenin reconstituted in planar bilayer lipid membranes. The membrane voltages were determined from the reversal potentials recorded upon channel exposure to asymmetrical ionic conditions, and the permeability ratios were calculated from the fit with the Goldman–Hodgkin–Katz equation. Our work shows that lysenin channels are ion-selective and the determined permeability coefficients are cation and anion-species dependent. We also exploited the unique property of lysenin channels to transition to a stable sub-conducting state upon exposure to calcium ions and assessed their subsequent change in ionic selectivity. The observed loss of selectivity was implemented in an electrical model describing the dependency of reversal potentials on calcium concentration. In conclusion, our work demonstrates that this pore-forming toxin presents ionic selectivity but this is adjusted by the particular conduction state of the channels.

Real-time monitoring the interfacial dynamic processes at model cell membranes: Taking cell penetrating peptide TAT as an example

A real-time and molecule-level monitoring of the interfacial dynamic interactions between molecules and a cell membrane is of vital importance. Herein, taking TAT, one of the most representative cell penetrating peptides, as an example, a photo-voltage transient technique and a dynamic giant bistratal vesicle (GBV) leakage method were combined with the traditional giant unilamellar vesicle (GUV) leakage assays, to provide a molecule-level understanding of the dynamic membrane interaction process performed in a low ionic strength and neutral pH condition. The photo-voltage test based on supported phospholipid bilayers showed a quick disturbance (<1 min) followed by a continuous reconstruction of the membrane by peptides, leading to a slight destruction (at TAT concentrations lower than 1 μg mL−1, i.e., 0.64 μM) or strong damage (e.g. at 10 μg mL−1, i.e., 6.4 μM) of the bilayer structure. The GUV/GBV leakage assays further demonstrated the TAT-induced membrane deformation and transmembrane diffusion of dyes, which occurred in an immediate, linear, and TAT-concentration dependent manner. Moreover, the flux of dye across the substrate-immobilized membranes was approximately three times of that across the substrate-free ones. This work gives information on time and molecular mechanism of the TAT-membrane interactions, demonstrates the different permeabilizing effects of TAT on immobilized and free membranes. Overall, it provides useful strategies to investigate the nano-bio interfacial interactions in a simple, global and real-time way.

Pre-steady-state Kinetic Analysis of Amino Acid Transporter SLC6A14 Reveals Rapid Turnover Rate and Substrate Translocation.

SLC6A14 (solute carrier family 6 member 14) is an amino acid transporter, driven by Na+ and Cl− co-transport, whose structure, function, and molecular and kinetic mechanism have not been well characterized. Its broad substrate selectivity, including neutral and cationic amino acids, differentiates it from other SLC6 family members, and its proposed involvement in nutrient transport in several cancers suggest that it could become an important drug target. In the present study, we investigated SLC6A14 function and its kinetic mechanism after expression in human embryonic kidney (HEK293) cells, including substrate specificity and voltage dependence under various ionic conditions. We applied rapid solution exchange, voltage jumps, and laser photolysis of caged alanine, allowing sub-millisecond temporal resolution, to study SLC6A14 steady state and pre-steady state kinetics. The results highlight the broad substrate specificity and suggest that extracellular chloride enhances substrate transport but is not required for transport. As in other SLC6 family members, Na+ binding to the substrate-free transporter (or conformational changes associated with it) is electrogenic and is likely rate limiting for transporter turnover. Transient current decaying with a time constant of <1ms is also observed after rapid amino acid application, both in forward transport and homoexchange modes, indicating a slightly electrogenic, but fast and not rate-limiting substrate translocation step. Our results, which are consistent with kinetic modeling, suggest rapid transporter turnover rate and substrate translocation with faster kinetics compared with other SLC6 family members. Together, these results provided novel information on the SLC6A14 transport cycle and mechanism, expanding our understanding of SLC6A14 function.

Removal of impurities from bismuth pickling solution using solvent extraction with TBP

This work proposes an extraction process for thorough removal of various impurities in crude bismuth. In the process, crude bismuth was pickled with nitric acid, and then the bismuth in the filtrate was extracted into the organic phase by TBP. After scrubbing with a saturated NaNO3 solution, stripping was carried out by NaOH solution, and a rod-shaped α-Bi2O3 precipitate was obtained. Increasing the concentration of NO3− and TBP and the ratio of Vo/Va could greatly increase the extraction efficiency of bismuth or the separation factor of impurities. Bismuth is extracted in the form of Bi(NO3)3·TBP, and the results of quantum chemical calculations indicated that both intermolecular forces and chemical bonds exist between the Bi(NO3)3 and TBP molecules. Thermodynamic analysis based on the Debye-Hückel equation under large ionic strength conditions showed that the extraction reaction is exothermic and proceeds spontaneously. Under the optimized extraction conditions, the extraction efficiency, elution efficiency, and stripping efficiency of bismuth were 98.5%, 0.55%, and 96.2%, respectively. All of the impurity elements were thoroughly removed after extraction. In particular, the separation factor βBi/M of M = Te, As, Cu, and Sb, which are prone to codeposition with bismuth during electrodeposition, were 160, 183, 982, and 359, respectively. The purity of α-Bi2O3 obtained by this method was as high as 99.95%

Size and roughness dependent temperature effects on surface charge of silica nanoparticles

Silica nanoparticles (SNP) with different sizes and surface areas are used in numerous micro/nanofluidic applications, while their surface charge properties play a major role in their function. In many of these applications, SNPs also undergo temperature variation. We present that an increase in temperature yields a substantial increase in SNP surface charge depending on nanoparticle size and surface roughness, which cannot be estimated by existing theory. As a continuation of our earlier work characterizing the deviation of SNP surface charging from theoretical predictions due to curvature and EDL overlap effects, this study presents the differentiation from the theory in temperature dependence under various conditions. As we calculate surface chemistry as a function of local ionic conditions (Charge Regulation), temperature variation changed the equilibrium constants of protonation/deprotonation reactions of the SNP surface, in addition to changes occurring in relative permittivity and ionic mobilities. Results show that variation of SNP surface charge by temperature decreases by decreasing particle size and/or increasing roughness size, compare to theoretical flat plate calculations considering similar temperature-dependent properties and charge regulation on the surface. We characterized these deviations by obtaining an “electrokinetic similarity” between different systems of various size and roughness at various ionic conditions based on the non-dimensional groups of λ/D P and λ/D R . Based on these, we devised a phenomenological model as an extension to the flat plate theory to successfully predict the surface charge of SNPs as a function of the particle size, roughness size, and temperature. The current findings are important for the characterization of SNPs through temperature variations and can also be used to adjust the surface charge of SNPs by tuning the temperature. • Nanoparticle surface charge increases with increasing temperature. • Temperature effects on surface charge change with particle size and surface condition. • Temperature dependent ionic diffusion and surface charge regulation were considered. • Temperature dependent solution of PNP with charge regulation is presented. • A model for nanoparticle charging is extended from analytical solution.

Asymmetric Interplay Between K+ and Blocker and Atomistic Parameters From Physiological Experiments Quantify K+ Channel Blocker Release.

Modulating the activity of ion channels by blockers yields information on both the mode of drug action and on the biophysics of ion transport. Here we investigate the interplay between ions in the selectivity filter (SF) of K+ channels and the release kinetics of the blocker tetrapropylammonium in the model channel KcvNTS. A quantitative expression calculates blocker release rate constants directly from voltage-dependent ion occupation probabilities in the SF. The latter are obtained by a kinetic model of single-channel currents recorded in the absence of the blocker. The resulting model contains only two adjustable parameters of ion-blocker interaction and holds for both symmetric and asymmetric ionic conditions. This data-derived model is corroborated by 3D reference interaction site model (3D RISM) calculations on several model systems, which show that the K+ occupation probability is unaffected by the blocker, a direct consequence of the strength of the ion-carbonyl attraction in the SF, independent of the specific protein background. Hence, KcvNTS channel blocker release kinetics can be reduced to a small number of system-specific parameters. The pore-independent asymmetric interplay between K+ and blocker ions potentially allows for generalizing these results to similar potassium channels.

Development of pH-Sensitive Chitosan-g-poly(acrylamide-co-acrylic acid) Hydrogel for Controlled Drug Delivery of Tenofovir Disoproxil Fumarate.

The present study aimed to develop a pH-sensitive chitosan-based hydrogel for controlled delivery of an anti-hepatitis B drug, tenofovir disoproxil fumarate (TDF). Free radical polymerization was utilized to graft acrylamide and acrylic acid using N,N-methylene bisacrylamide as the crosslinker. Physicochemical characterization confirmed the synthesis of thermally stable chitosan-g-poly(acrylamide-co-acrylic acid) hydrogels with well-defined pores within a fibrous surface. The prepared hydrogels exhibited pH and ionic strength sensitivity, with the swelling significantly lower under acidic and strong ionic strength conditions but higher in neutral and basic solutions. In addition, cytotoxicity studies on HeLa cell lines proved the cytocompatibility of the drug delivery material and its readiness for physiological applications. The encapsulation of TDF in the hydrogels was optimized and an encapsulation efficiency and a drug loading percentage of 96% and 10% were achieved, respectively. More interestingly, in vitro release studies demonstrated a pH-dependent release of TDF from hydrogels. The release at pH 7.4 was found to be up to five times higher than at pH 1.2 within 96 h. This further suggested that the newly developed hydrogel-loaded TDF could be proposed as a smart delivery system for oral delivery of anti-hepatitis B drugs.

Changes in H+, K+, and Ca2+ Concentrations, as Observed in Seizures, Induce Action Potential Signaling in Cortical Neurons by a Mechanism That Depends Partially on Acid-Sensing Ion Channels.

Acid-sensing ion channels (ASICs) are activated by extracellular acidification. Because ASIC currents are transient, these channels appear to be ideal sensors for detecting the onset of rapid pH changes. ASICs are involved in neuronal death after ischemic stroke, and in the sensation of inflammatory pain. Ischemia and inflammation are associated with a slowly developing, long-lasting acidification. Recent studies indicate however that ASICs are unable to induce an electrical signaling activity under standard experimental conditions if pH changes are slow. In situations associated with slow and sustained pH drops such as high neuronal signaling activity and ischemia, the extracellular K+ concentration increases, and the Ca2+ concentration decreases. We hypothesized that the concomitant changes in H+, K+, and Ca2+ concentrations may allow a long-lasting ASIC-dependent induction of action potential (AP) signaling. We show that for acidification from pH7.4 to pH7.0 or 6.8 on cultured cortical neurons, the number of action potentials and the firing time increased strongly if the acidification was accompanied by a change to higher K+ and lower Ca2+ concentrations. Under these conditions, APs were also induced in neurons from ASIC1a–/– mice, in which a pH of ≤ 5.0 would be required to activate ASICs, indicating that ASIC activation was not required for the AP induction. Comparison between neurons of different ASIC genotypes indicated that the ASICs modulate the AP induction under such changed ionic conditions. Voltage-clamp measurements of the Na+ and K+ currents in cultured cortical neurons showed that the lowering of the pH inhibited Na+ and K+ currents. In contrast, the lowering of the Ca2+ together with the increase in the K+ concentration led to a hyperpolarizing shift of the activation voltage dependence of voltage-gated Na+ channels. We conclude that the ionic changes observed during high neuronal activity mediate a sustained AP induction caused by the potentiation of Na+ currents, a membrane depolarization due to the changed K+ reversal potential, the activation of ASICs, and possibly effects on other ion channels. Our study describes therefore conditions under which slow pH changes induce neuronal signaling by a mechanism involving ASICs.

A real-time and in-situ monitoring of the molecular interactions between drug carrier polymers and a phospholipid membrane

The dynamic interactions between drug carrier molecules and a cell membrane can not be ignored in their clinical use. Here a simple, label-free and non-invasive approach, photo-voltage transient method, combined with the atomic force microscopy, dynamic giant unilamellar vesicle leakage assay and cytotoxicity method, was employed for a real-time monitoring of the interaction process. Two representative polymer molecules, polyoxyethylene (35) lauryl ether (Brij35) and polyvinylpyrrolidone (PVPk30), were taken as examples to interact with a phospholipid bilayer membrane in a low ionic strength and neutral pH condition. Brij35 demonstrated an adsorption-accumulation-permeabilization dominated process under the modulation of polymer concentration in the solution. In contrast, PVPk30 performed a dynamic balance between adsorption-desorption of the molecules and/or permeabilization-resealing of the membrane. Such difference explains the high and low cytotoxicity of them, respectively, in the living cell tests. Briefly, through combining the photo-voltage approach with conventional fluorescent microscopy method, this work demonstrates new ideas on the time and membrane actions of polymer surfactants which should be taken into account for their biomedical applications.

The Chloride Conductance Inhibitor NS3623 Enhances the Activity of a Non-selective Cation Channel in Hyperpolarizing Conditions

Handbooks of physiology state that the strategy adopted by red blood cells (RBCs) to preserve cell volume is to maintain membrane permeability for cations at its minimum. However, enhanced cation permeability can be measured and observed in specific physiological and pathophysiological situations such as in vivo senescence, storage at low temperature, sickle cell anemia and many other genetic defects affecting transporters, membrane or cytoskeletal proteins. Among cation pathways, cation channels are able to dissipate rapidly the gradients that are built and maintained by the sodium and calcium pumps. These situations are very well-documented but a mechanistic understanding of complex electrophysiological events underlying ion transports is still lacking. In addition, non-selective cation (NSC) channels present in the RBC membrane have proven difficult to molecular identification and functional characterization. For instance, NSC channel activity can be elicited by Low Ionic Strength conditions (LIS): the associated change in membrane potential triggers its opening in a voltage dependent manner. But, whereas this depolarizing media produces a spectacular activation of NSC channel, Gárdos channel-evoked hyperpolarization's have been shown to induce sodium entry through a pathway thought to be conductive and termed Pcat. Using the CCCP method, which allows to follow fast changes in membrane potential, we show here (i) that hyperpolarization elicited by Gárdos channel activation triggers sodium entry through a conductive pathway, (ii) that chloride conductance inhibition unveils such conductive cationic conductance, (iii) that the use of the specific chloride conductance inhibitor NS3623 (a derivative of Neurosearch compound NS1652), at concentrations above what is needed for full anion channel block, potentiates the non-selective cation conductance. These results indicate that a non-selective cation channel is likely activated by the changes in the driving force for cations rather than a voltage dependence mechanism per se.

Determining folding and binding properties of the C-terminal SH2 domain of SHP2.

SH2 domains are a class of protein–protein interaction modules with the function to recognize and bind sequences characterized by the presence of a phosphorylated tyrosine. SHP2 is a protein phosphatase involved in the Ras‐ERK1/2 signaling pathway that possess two SH2 domains, namely, N‐SH2 and C‐SH2, that mediate the interaction of SHP2 with various partners and determine the regulation of its catalytic activity. One of the main interactors of the SH2 domains of SHP2 is Gab2, a scaffolding protein with critical role in determining cell differentiation. Despite their key biological role and the importance of a correct native fold to ensure it, the mechanism of binding of SH2 domains with their ligands and the determinants of their stability have been poorly characterized. In this article, we present a comprehensive kinetic study of the folding of the C‐SH2 domain and the binding mechanism with a peptide mimicking a region of Gab2. Our data, obtained at different pH and ionic strength conditions and supported by site‐directed mutagenesis, highlight the role of electrostatic interactions in the early events of recognition. Interestingly, our results suggest a key role of a highly conserved histidine residue among SH2 family in the interaction with negative charges carried by the phosphotyrosine of Gab2. Moreover, the analysis of the equilibrium and kinetic folding data of C‐SH2 describes a complex mechanism implying a change in rate‐limiting step at high denaturant concentrations. Our data are discussed under the light of previous works on N‐SH2 domain of SHP2 and other SH2 domains.

Nonmonotonic Coupled Dissolution‐Precipitation Reactions at the Mineral–Water Interface

AbstractDissolution is inherent to fluid‐mineral systems. Yet its impact on minerals reacting with electrolytes is overlooked. Here, a novel nonmonotonic behavior for the surface interactions of carbonates (calcite and Mg‐calcite) with organic acids is reported. Applying a bioinspired approach, Mg‐calcite sensors via amorphous precursors, avoiding any preconditioning with functional groups are synthesized. A quartz crystal microbalance is used to study the mass changes of the mineral on contact with organic acids under varying ionic conditions, temperatures, and flow velocities. Supported by confocal Raman microscopy and potentiometric titrations, nonmonotonous mass developments are found as a function of Ca2+concentration and flowrate, and attributed to three coupled chemical reactions: i) carbonate dissolution via Ca2+ion complexation with organic molecules, and the formation of organo‐calcium compounds as ii) a surface phase at the mineral–water interface, and iii) particles in the bulk fluid. These processes depend on local ion contents and the precipitation onset (i.e., saturation index) of organo‐calcium salts, both of which substantially differ in the bulk fluid and in the fluid boundary layer at mineral interfaces. This continuum between dissolution and precipitation provides a conceptual framework to address reactions at mineral interfacial across disciplines including biomineralization, ocean acidification and reservoir geochemistry.

Salt-stress adaptation of yeast as a simple method to improve high-gravity fermentation in an industrial medium.

While Saccharomyces cerevisiae is a popular organism to produce ethanol, its fermentation performance is affected at high sugar concentrations due to osmotic stress. We hypothesized that adaptation under ionic stress conditions will improve the fermentation performance at high sugar concentrations due to cross-stress adaptation. We, therefore, adapted a high-performance yeast strain, S. cerevisiae CEN.PK 122, to increasing salt concentrations in an industrial medium. Control cells were adapted in the medium without added salt. The cells adapted to 3.5% (w/v) salt concentration demonstrated a superior performance when fermenting 10-30% (w/v) glucose. When fermenting 30% (w/v) glucose, the ethanol yields of the adapted cells (0.49 ± 0.01gg-1) were about 30% higher than the control cells (0.37 ± 0.01gg-1) and are comparable with the best reported to date for any medium employed. Similar improvements were also observed when fermenting 10% (w/v) sucrose. However, little improvement in fermentation was observed at the higher temperature tested (40°C), even though the growth of the adapted cells was greater when tested in YPD medium. The improvements in fermentation at 30°C were primarily related to the faster growth of the adapted cells and not to an increase in specific intake rates. Additionally, a significantly reduced lag phase was also observed when fermenting 30% (w/v) glucose. Thus, our work shows the application of a simple strategy to significantly improve high-gravity fermentation (HGF) performance through adaptation. KEY POINTS: • Cell adapted on 3.5% NaCl made 28% more ethanol when fermenting 30% glucose. • The adapted cells had reduced lag phase, grew faster, and produced less glycerol. • The improvements were not related to increased specific rates of production.

Direct visualization of the effect of DNA structure and ionic conditions on HU\u2013DNA interactions

Architectural DNA–binding proteins are involved in many important DNA transactions by virtue of their ability to change DNA conformation. Histone-like protein from E. coli strain U93, HU, is one of the most studied bacterial architectural DNA–binding proteins. Nevertheless, there is still a limited understanding of how the interactions between HU and DNA are affected by ionic conditions and the structure of DNA. Here, using optical tweezers in combination with fluorescent confocal imaging, we investigated how ionic conditions affect the interaction between HU and DNA. We directly visualized the binding and the diffusion of fluorescently labelled HU dimers on DNA. HU binds with high affinity and exhibits low mobility on the DNA in the absence of Mg2+; it moves 30-times faster and stays shorter on the DNA with 8 mM Mg2+ in solution. Additionally, we investigated the effect of DNA tension on HU–DNA complexes. On the one hand, our studies show that binding of HU enhances DNA helix stability. On the other hand, we note that the binding affinity of HU for DNA in the presence of Mg2+ increases at tensions above 50 pN, which we attribute to force-induced structural changes in the DNA. The observation that HU diffuses faster along DNA in presence of Mg2+ compared to without Mg2+ suggests that the free energy barrier for rotational diffusion along DNA is reduced, which can be interpreted in terms of reduced electrostatic interaction between HU and DNA, possibly coinciding with reduced DNA bending.