DC and AC Conductivity,Biosolubility and Thermal Properties of Mg-Doped Na2O–CaO–P2O5 Glasses

Bioactive glasses have recently been extensively used to replace, regenerate, and repair hard tissues in the human body because of their ability to bond with living tissue. In this work, the effects of replacing Na₂O with MgO on the electrical, biosolubility, and thermal properties of the target glass 10Na₂O–60P₂O₅–30CaO (in mol%) were investigated. The electrical properties of the glasses were studied with the impedance spectroscopy technique. At 473 K, DC conductivity values decreased from 4.21 × 10⁻¹¹ to 4.21 × 10⁻¹² S cm⁻¹ after complete substitution of MgO for Na₂O. All samples had a similar activation energy of the DC conduction process ~1.27 eV. Conduction mechanisms were found to be due to hop of ions: Na⁺, Mg²⁺_, and probable H⁺. FTIR analysis showed that, as the Mg content increased, the Q² unit (PO₂⁻) shifted towards higher wavenumbers. The proportion of Q³ unit (P₂O₅) decreased in the glass structure. This confirmed that the replacement of Na⁺ by Mg²⁺ was accompanied by concurrent polymerization of the calcium–phosphate glass network. The biosolubility test in the phosphate-buffered saline solution showed that the magnesium addition enhanced the biosolubility properties of Na₂O–CaO–P₂O₅ glasses by increasing their dissolution rate and supporting forming CaP-rich layers on the surface. The glass transition temperature increased, and thermal stability decreased substantially upon substitution of Na₂O by MgO.     

Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates the electrogenic Na/HCO3 cotransporter NBCe1-A expressed in Xenopus oocytes

Bicarbonate transporters are regulated by signaling molecules/ions such as protein kinases, ATP, and Ca²⁺. While phospholipids such as PIP₂ can stimulate Na-H exchanger activity, little is known about phospholipid regulation of bicarbonate transporters. We used the patch-clamp technique to study the function and regulation of heterologously expressed rat NBCe1-A in excised macropatches from Xenopus laevis oocytes. Exposing the cytosolic side of inside-out macropatches to a 5% CO₂/33 mM HCO₃⁻ solution elicited a mean inward current of 14 pA in 74% of macropatches attached to pipettes (−V_p = −60 mV) containing a low-Na⁺, nominally HCO₃⁻-free solution. The current was 80–90% smaller in the absence of Na⁺, approximately 75% smaller in the presence of 200 μM DIDS, and absent in macropatches from H₂O-injected oocytes. NBCe1-A currents exhibited time-dependent rundown that was inhibited by removing Mg²⁺ in the presence or absence of vanadate and F⁻ to reduce general phosphatase activity. Applying 5 or 10 μM PIP₂ (diC8) in the presence of HCO₃⁻ induced an inward current in 54% of macropatches from NBC-expressing, but not H₂O-injected oocytes. PIP₂-induced currents were HCO₃⁻-dependent and somewhat larger following more NBCe1-A rundown, 62% smaller in the absence of Na⁺, and 90% smaller in the presence of 200 μM DIDS. The polycation neomycin (250–500 μM) reduced the PIP₂-induced inward current by 69%; spermine (100 μM) reduced the current by 97%. Spermine, poly-D-lysine, and neomycin all reduced the baseline HCO₃⁻-induced inward currents by as much as 85%. In summary, PIP₂ stimulates NBCe1-A activity, and phosphoinositides are regulators of bicarbonate transporters.     

Structures and characterization of digoxin- and bufalin-bound Na+,K+-ATPase compared with the ouabain-bound complex

Cardiotonic steroids (CTSs) are specific and potent inhibitors of the Na⁺,K⁺-ATPase, with highest affinity to the phosphoenzyme (E2P) forms. CTSs are comprised of a steroid core, which can be glycosylated, and a varying number of substituents, including a five- or six-membered lactone. These functionalities have specific influence on the binding properties. We report crystal structures of the Na⁺,K⁺-ATPase in the E2P form in complex with bufalin (a nonglycosylated CTS with a six-membered lactone) and digoxin (a trisaccharide-conjugated CTS with a five-membered lactone) and compare their characteristics and binding kinetics with the previously described E2P–ouabain complex to derive specific details and the general mechanism of CTS binding and inhibition. CTSs block the extracellular cation exchange pathway, and cation-binding sites I and II are differently occupied: A single Mg²⁺ is bound in site II of the digoxin and ouabain complexes, whereas both sites are occupied by K⁺ in the E2P–bufalin complex. In all complexes, αM4 adopts a wound form, characteristic for the E2P state and favorable for high-affinity CTS binding. We conclude that the occupants of the cation-binding site and the type of the lactone substituent determine the arrangement of αM4 and hypothesize that winding/unwinding of αM4 represents a trigger for high-affinity CTS binding. We find that the level of glycosylation affects the depth of CTS binding and that the steroid core substituents fine tune the configuration of transmembrane helices αM1–2.Cardiotonic steroids (CTSs) induce diverse physiological effects on, for example, heart muscle and blood pressure regulation, but the underlying mechanisms remain unknown, despite a long history of therapeutic applications and model studies. It is widely recognized that they target Na⁺,K⁺-ATPase, and a direct consequence of their binding is an inhibition of the enzyme. Their positive inotropic effect in cardiomyocytes has been related to coupling between Na⁺,K⁺-ATPase and Na⁺/Ca²⁺-exchanger through the intracellular Na⁺ concentration, whereas numerous other outcomes observed on the cellular level have led to hypotheses of the existence of signaling cascade mechanisms with Na⁺,K⁺-ATPase acting as a receptor. The minimal functional unit of the enzyme is an αβ-complex, and because there exist four α- and three β-isoforms of the Na⁺,K⁺-ATPase, the variations in the heterodimer composition and a vast number of CTSs differing in apparent isoform specificities (1) add to the complexity and multiplicity of reported physiological responses.The conserved structural core shared by all CTSs includes a cis-trans-cis ring-fused steroid core with two methyl substituents at steroid positions C10_β and C13_β, two hydroxyl groups (OH3_β and OH14_β), and an unsaturated lactone ring at the C17_β position, among which the lactone and OH14_β are critical for binding to Na⁺,K⁺-ATPase (2, 3). The type of lactone at the C17_β position divides natural CTSs into cardenolides (five-membered lactone rings) and bufadienolides (six-membered lactone rings). Finally, many CTSs are glycosylated by one to four carbohydrate residues at OH3_β (Fig. S1). It has been shown that glycosylation improves CTS affinity toward the Na⁺,K⁺-ATPase and contributes (at least in the case of digoxin and digitoxin) to their Na⁺,K⁺-ATPase isoform selectivity, with up to fourfold preference for α2/α3 over α1 (1).The recently published crystal structure of the Na⁺,K⁺-ATPase phosphoenzyme (E2P) in complex with the widely studied CTS ouabain (4, 5) showed that the high-affinity CTS-binding site is constituted by the transmembrane helices αM1–6 of the catalytic α-subunit, forming a pocket exposed to the extracellular side and overlapping with the extracellular ion exchange pathway (6). The E2P–ouabain structure also revealed details on protein–ligand interactions facilitating high-affinity CTS binding compared with a low-affinity ouabain complex (7). Among the important features brought to view by the high-affinity complex structure were (i) a Mg²⁺ ion occupying cation-binding site II, (ii) the rearrangement of αM4, forming the structural basis for the well-known antagonistic effect of K⁺ on ouabain binding, and (iii) an E2P-specific configuration of αM1–2 on the cytoplasmic side, whereas the extracellular end of this helix pair closes in on the CTS-binding site (4). Biochemical experiments showing competitive interactions between K⁺ and Mg²⁺ suggested that the nature of the cation in site II is a determinant for ouabain affinity. In addition, long-range interactions between the unsaturated, polarized five-membered lactone ring of ouabain and the Mg²⁺ ion were suggested as a factor for CTS recognition and differentiation. Despite previous reports showing that glycosylated CTSs have higher Na⁺,K⁺-ATPase affinity than their aglycones, no specific interactions were observed between the sugar moiety of ouabain and the protein to explain that effect.To gain a better understanding of the structure–activity relationship of the CTSs, we have crystallized the E2P form of the pig kidney Na⁺,K⁺-ATPase (α₁β₁γ) in complex with two CTSs: bufalin (a nonglycosylated bufadienolide) and digoxin (a trisaccharide-conjugated cardenolide) (Fig. 1A), which also are pharmacological agents. We further performed experiments on CTS binding to Na⁺,K⁺-ATPase, including the aglycones digitoxigenin and ouabagenin (Fig. S1). The data revealed notable qualitative differences in kinetics of the enzyme interactions with the glycosylated vs. nonglycosylated CTSs as well as a remarkable insensitivity of bufalin binding to K⁺. The time course of Na⁺,K⁺-ATPase inhibition under steady-state conditions, mimicking the interactions with CTSs in vivo, revealed that binding occurs in two steps. The impact of separate structural components, such as sugar and lactone moieties, on the individual steps of CTS binding is discussed on the basis of our structural and biochemical data.Open in a separate windowFig. 1.Structural comparison of the crystal structures of the high-affinity Na⁺,K⁺-ATPase α₁β₁γ E2P–CTS complexes. The phosphoenzyme stabilized by bufalin, digoxin, and ouabain (5) is depicted in blue, green, and gray cartoons, respectively, and the bufalin, digoxin, and ouabain molecules are represented by magenta, orange, and dark gray sticks, respectively. The K⁺ and Mg²⁺ ions are represented by purple and yellow spheres, respectively. (A) Structural representation of the CTSs digoxin, bufalin, and ouabain. (B and C) The final 2F_o-F_c electron density maps of the E2P–bufalin and E2P–digoxin, respectively, complexes (contoured at 1.0σ level). The maps are represented by gray mesh. (D) Structural alignment of the E2P–bufalin, E2P–digoxin, and E2P–ouabain complexes performed on the segments αM7–10 showing a high degree of overall structural similarity. (E) The CTS-binding site visualized from the extracellular site based on the same alignment as above. The alignment reveals similar hydrophobic interactions between the α-surface of the CTS core and αM4–6. In contrast, different interactions are formed between the substituents at the β-surface of the CTS core and αM1–2, leading to minor CTS-induced rearrangements. (F) The CTS-binding site visualized from αM1–2. αM4 overlays well for Mg²⁺-bound complexes of E2P–digoxin and E2P–ouabain as well as the E2P–bufalin complex, despite potassium bound in the cation-binding sites.     

Crystal structure of the high-affinity Na+,K+-ATPase–ouabain complex with Mg2+ bound in the cation binding site

The Na⁺,K⁺-ATPase maintains electrochemical gradients for Na⁺ and K⁺ that are critical for animal cells. Cardiotonic steroids (CTSs), widely used in the clinic and recently assigned a role as endogenous regulators of intracellular processes, are highly specific inhibitors of the Na⁺,K⁺-ATPase. Here we describe a crystal structure of the phosphorylated pig kidney Na⁺,K⁺-ATPase in complex with the CTS representative ouabain, extending to 3.4 Å resolution. The structure provides key details on CTS binding, revealing an extensive hydrogen bonding network formed by the β-surface of the steroid core of ouabain and the side chains of αM1, αM2, and αM6. Furthermore, the structure reveals that cation transport site II is occupied by Mg²⁺, and crystallographic studies indicate that Rb⁺ and Mn²⁺, but not Na⁺, bind to this site. Comparison with the low-affinity [K₂]E2–MgF_x–ouabain structure [Ogawa et al. (2009) Proc Natl Acad Sci USA 106(33):13742–13747) shows that the CTS binding pocket of [Mg]E2P allows deep ouabain binding with possible long-range interactions between its polarized five-membered lactone ring and the Mg²⁺. K⁺ binding at the same site unwinds a turn of αM4, dragging residues Ile318–Val325 toward the cation site and thereby hindering deep ouabain binding. Thus, the structural data establish a basis for the interpretation of the biochemical evidence pointing at direct K⁺–Mg²⁺ competition and explain the well-known antagonistic effect of K⁺ on CTS binding.     

K+-dependent paradoxical membrane depolarization and Na+ overload, major and reversible contributors to weakness by ion channel leaks

Normal resting potential (P1) of myofibers follows the Nernst equation, exhibiting about −85 mV at a normal extracellular K⁺ concentration ([K⁺]_o) of 4 mM. Hyperpolarization occurs with decreased [K⁺]_o, although at [K⁺]_o < 1.0 mM, myofibers paradoxically depolarize to a second stable potential of −60 mV (P2). In rat myofiber bundles, P2 also was found at more physiological [K⁺]_o and was associated with inexcitability. To increase the relative frequency of P2 to 50%, [K⁺]_o needed to be lowered to 1.5 mM. In the presence of the ionophore gramicidin, [K⁺]_o reduction to only 2.5 mM yielded the same effect. Acetazolamide normalized this increased frequency of P2 fibers. The findings mimic hypokalemic periodic paralysis (HypoPP), a channelopathy characterized by hypokalemia-induced weakness. Of myofibers from 7 HypoPP patients, up to 25% were in P2 at a [K⁺]_o of 4 mM, in accordance with their permanent weakness, and up to 99% were in P2 at a [K⁺]_o of 1.5 mM, in accordance with their paralytic attacks. Of 36 HypoPP patients, 25 had permanent weakness and myoplasmic intracellular Na⁺ ([Na⁺]_i) overload (up to 24 mM) as shown by in vivo ²³Na-MRI. Acetazolamide normalized [Na⁺]_i and increased muscle strength. HypoPP myofibers showed a nonselective cation leak of 12–19.5 μS/cm², which may explain the Na⁺ overload. The leak sensitizes myofibers to reduced serum K⁺, and the resulting membrane depolarization causes the weakness. We postulate that the principle of paradoxical depolarization and loss of function upon [K⁺]_o reduction may apply to other tissues, such as heart or brain, when they become leaky (e.g., because of ischemia).     

Altered Na+ transport after an intracellular α-subunit deletion reveals strict external sequential release of Na+ from the Na/K pump

The Na/K pump actively exports 3 Na⁺ in exchange for 2 K⁺ across the plasmalemma of animal cells. As in other P-type ATPases, pump function is more effective when the relative affinity for transported ions is altered as the ion binding sites alternate between opposite sides of the membrane. Deletion of the five C-terminal residues from the α-subunit diminishes internal Na⁺ (Na_i⁺) affinity ≈25-fold [Morth et al. (2007) Nature 450:1043–1049]. Because external Na⁺ (Na_o⁺) binding is voltage-dependent, we studied the reactions involving this process by using two-electrode and inside-out patch voltage clamp in normal and truncated (ΔKESYY) Xenopus-α1 pumps expressed in oocytes. We observed that ΔKESYY (i) decreased both Na_o⁺ and Na_i⁺ apparent affinities in the absence of K_o⁺, and (ii) did not affect apparent Na_o⁺ affinity at high K_o⁺. These results support a model of strict sequential external release of Na⁺ ions, where the Na⁺-exclusive site releases Na⁺ before the sites shared with K⁺ and the ΔKESYY deletion only reduces Na_o⁺ affinity at the shared sites. Moreover, at nonsaturating K_o⁺, ΔKESYY induced an inward flow of Na⁺ through Na/K pumps at negative potentials. Guanidinium⁺ can also permeate truncated pumps, whereas N-methyl-D-glucamine cannot. Because guanidinium_o⁺ can also traverse normal Na/K pumps in the absence of both Na_o⁺ and K_o⁺ and can also inhibit Na/K pump currents in a Na⁺-like voltage-dependent manner, we conclude that the normal pathway transited by the first externally released Na⁺ is large enough to accommodate guanidinium⁺.     

Intracellular Cl? as a signaling ion that potently regulates Na+/HCO3? transporters

Cl⁻ is a major anion in mammalian cells involved in transport processes that determines the intracellular activity of many ions and plasma membrane potential. Surprisingly, a role of intracellular Cl⁻ (Cl⁻_in) as a signaling ion has not been previously evaluated. Here we report that Cl⁻_in functions as a regulator of cellular Na⁺ and HCO₃⁻ concentrations and transepithelial transport through modulating the activity of several electrogenic Na⁺-HCO₃⁻ transporters. We describe the molecular mechanism(s) of this regulation by physiological Cl⁻_in concentrations highlighting the role of GXXXP motifs in Cl⁻ sensing. Regulation of the ubiquitous Na⁺-HCO3⁻ co-transport (NBC)e1-B is mediated by two GXXXP-containing sites; regulation of NBCe2-C is dependent on a single GXXXP motif; and regulation of NBCe1-A depends on a cryptic GXXXP motif. In the basal state NBCe1-B is inhibited by high Cl⁻_in interacting at a low affinity GXXXP-containing site. IP₃ receptor binding protein released with IP₃ (IRBIT) activation of NBCe1-B unmasks a second high affinity Cl⁻_in interacting GXXXP-dependent site. By contrast, NBCe2-C, which does not interact with IRBIT, has a single high affinity N-terminal GXXP-containing Cl⁻_in interacting site. NBCe1-A is unaffected by Cl⁻_in between 5 and 140 mM. However, deletion of NBCe1-A residues 29–41 unmasks a cryptic GXXXP-containing site homologous with the NBCe1-B low affinity site that is involved in inhibition of NBCe1-A by Cl⁻_in. These findings reveal a cellular Cl⁻_in sensing mechanism that plays an important role in the regulation of Na⁺ and HCO₃⁻ transport, with critical implications for the role of Cl⁻ in cellular ion homeostasis and epithelial fluid and electrolyte secretion.Cl⁻ and HCO₃⁻ are the two major intracellular anions in mammalian cells. Specific transporters, channels, and the membrane potential tightly regulate their extracellular and intracellular concentrations. In turn, Cl⁻ and HCO₃⁻ regulate the concentration of other ions, including Na⁺, K⁺, and SO₄²⁻, either directly or indirectly. Known ubiquitous Cl⁻- and HCO₃⁻-coupled transporters include the NaCl cotransporters NCCs, the KCl cotransporters KCCs, the Na⁺/K⁺/2Cl⁻ cotransporter NKCC1 (1), the SLC26 Cl⁻/HCO₃⁻ exchangers and channels (2, 3), and members of the SLC4 exchangers and cotransporters family (4). The intracellular Cl⁻ (Cl⁻_in) concentration is also regulated by the ClCs (5) and Anoctamines Cl⁻ channels (6). Cl⁻ plays a role in a wide variety of cellular transport functions, including regulation of the membrane potential (6), cell volume (2), systemic and cellular acid–base balance (4), and transepithelial fluid and electrolyte secretion (7). In addition, Cl⁻ was reported to regulate transient receptor potential (TRP) channels (8), receptors assembly and function (9–11), activation of Neutrophil β2 Integrins (12), and the cell cycle (13).Like Cl⁻, HCO₃⁻ also has many important physiological roles, being the principal biological pH buffer (7) and an activator of the soluble adenylyl cyclase (14). In epithelia, HCO₃⁻ has a key role in tissue/cell viability. Among other fundamental roles, HCO₃⁻ drives Cl⁻ absorption and fluid secretion, stimulates mucin secretion, and controls solubilization of secreted macromolecules (7). Epithelial HCO₃⁻ secretion is fueled by the cellular Na⁺ gradient, which provides the driving force for HCO₃⁻ entry across the basolateral membrane mediated by the Na⁺-HCO₃⁻ cotransport NBCe1-B. HCO₃⁻ then exits the luminal membrane by the coordinated and coupled functions of the Cl⁻ channel cystic fibrosis transmembrane conductance regulator (CFTR) and the electrogenic Cl⁻/HCO₃⁻ exchanger slc26a6 (3, 7, 15). In the kidney, NBCe1-A mediates basolateral HCO₃⁻ extrusion (16, 17). In secretory ducts the basolateral NBCe1-B–mediated HCO₃⁻ influx is coupled to apical HCO₃⁻ secretion and Cl⁻ absorption via CFTR and slc26a6 (7). Cl⁻_in is reduced along the ducts as luminal Cl⁻ is reduced and luminal HCO₃⁻ is increased (18, 19).Members of the SLC4 superfamily of HCO₃⁻ cotransporters are key transporters involved in cellular HCO₃⁻ and Cl⁻ homeostasis (4, 7). The family consists of several subfamilies, including the electrogenic NBCe1 and NBCe2, electroneutral NBCn1 and NBCn2, Cl⁻-coupled anion exchangers AEs, and the Na⁺-dependent Cl⁻/HCO₃⁻ exchanger NDCBE (4). In a wide variety of tissues, NBCe1-B mediates the electrogenic transport of 1Na⁺ and 2HCO₃⁻ ions (likely Na⁺-CO₃²⁻) (4, 17) and functions as the main epithelial HCO₃⁻ entry mechanism in the basolateral membrane of polarized cells (7). Cell-specific electrogenic NBC transporters include NBCe1-A, which is expressed in the basolateral membrane of the renal proximal tubule (4, 17), and NBCe2-C, which is found in the choroid plexus (20). IRBIT, which regulates NBCe1-B (21–23) and NBCn1-A (24), binds to the IP₃ binding domain of the IP₃ receptors (IP₃Rs) (25). IRBIT is released from the IP₃Rs upon an increase in cellular IP₃ (26), becoming available for regulation of NBCe1-B (21–23), NBCn1-A (24), CFTR (22, 23), and slc26a6 (27), thereby coordinating the activation of these transporters and epithelial fluid and HCO₃⁻ secretion (27).Cl⁻_in has not been previously considered as a signaling ion. Rather, Cl⁻ has heretofore mainly been viewed as an ion that is transported by various channels, coupled to Na⁺, K⁺, or HCO₃⁻ transport, and that plays a role regulating the plasma membrane potential (1, 7). Importantly, several studies have provided clues that Cl⁻_in may have regulatory and perhaps signaling functions. High Cl⁻_in was reported to inhibit the activity of the epithelial Na⁺ channel ENaC (28), the permeability of CFTR to HCO₃⁻ (29), TRPM7 activity (8), and perhaps the activity of the Na⁺/H⁺ exchanger NHE1 (30). Muscarinic stimulation of salivary gland acinar cells resulted in reduction in Cl⁻_in that is required for Na⁺ influx by the Na⁺/H⁺ exchanger and the Na⁺/K⁺/2Cl⁻ cotransporter (31).In the present study we asked whether Cl⁻_in functions as a signaling ion that modulates cellular Na⁺ and HCO₃⁻ concentrations through regulation of electrogenic NBC transporters. Our data indicate that Cl⁻_in regulates the function of NBCe1-B, NBCe2-C, and NBCe1-A via a cryptic Cl⁻ sites. In the basal state NBCe1-B is inhibited by Cl⁻_in interacting with a low affinity Cl⁻ site, whereas NBCe1-A is resistant to Cl⁻_in. By contrast, IRBIT-activated NBCe1-B is inhibited by low and high concentrations of Cl⁻_in due to interaction with high and low affinity Cl⁻_in motifs that depend on GXXXP motifs. Mutation of the G and P or of His in GXHXP in the autoinhibitory domain of NBCe1-B eliminated inhibition by low Cl⁻_in, whereas sparing inhibition of NBCe1-B by high Cl⁻_in. Mutation of a second GXXXP motif was required to eliminate inhibition by low affinity Cl⁻_in site. NBCe2-C is inhibited by a single high affinity GXXXP motif-dependent Cl⁻_in interacting site. Remarkably, deletion of the first 48 residues of NBCe1-A or of residues 29–41 uncovered inhibition of NBCe1-A by Cl⁻_in that was mediated by a GXXXP motif-dependent site homologous with the second GXXXP motif of NBCe1-B. These findings reveal a Cl⁻_in sensing mechanism that modulates the activity of NBCe1-B, NBCe2-C, and NBCe1-A. In transporting epithelia, such as the pancreatic and salivary ducts and the choroid plexus, we predict that a reduction in Cl⁻_in will dramatically increase transepithelial HCO₃⁻ transport and fluid secretion.     

Resistance of Soda Residue–Fly Ash Based Geopolymer Mortar to Acid and Sulfate Environments

The early mechanical performances of low-calcium fly ash (FFA)-based geopolymer (FFA–GEO) mortar can be enhanced by soda residue (SR). However, the resistance of SR–FFA–GEO mortar to acid or sulfate environments is unclear, owing to the various inorganic calcium salts in SR. The aim of this study was to investigate the long-term mechanical strengths of up to 360 d and evaluate the resistance of SR–FFA–GEO mortar to 5% HCl and 5% Na₂SO₄ environments through the losses in compressive strength and mass. Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and Fourier Transform Infrared Spectrometer (FTIR) experiments were conducted for the SR–FFA–GEO mortars, both before and after chemical attack, to clarify the attack mechanism. The results show that the resistances of the SR–FFA–GEO mortar with 20% SR (namely M10) to 5% HCl and 5% Na₂SO₄ environments are superior to those of cement mortar. The environmental HCl reacts with the calcites in SR to produce CaCl₂, CO₂ and H₂O to form more pores under HCl attack, and the environmental Na⁺ cations from Na₂SO₄ go into Si-O-Al network structure, to further enhance the strength of mortar under Na₂SO₄ attack. These results provide the experimental basis for the durability optimization of SR–FFA–GEO mortars.     

Study on Concrete Deterioration in Different NaCl-Na2SO4 Solutions and the Mechanism of Cl− Diffusion

The diffusion of sulfate (SO₄²⁻) and chloride (Cl⁻) ions from rivers, salt lakes and saline soil into reinforced concrete is one of the main factors that contributes to the corrosion of steel reinforcing bars, thus reducing their mechanical properties. This work experimentally investigated the corrosion process involving various concentrations of NaCl-Na₂SO₄ leading to the coupled erosion of concrete. The appearance, weight, and mechanical properties of the concrete were measured throughout the erosion process, and the Cl⁻ and SO₄²⁻ contents in concrete were determined using Cl⁻ rapid testing and spectrophotometry, respectively. Scanning electron microscopy, energy spectrometry, X-ray diffractometry, and mercury porosimetry were also employed to analyze microstructural changes and complex mineral combinations in these samples. The results showed that with higher Na₂SO₄ concentration and longer exposure time, the mass, compressive strength, and relative dynamic elastic modulus gradually increased and large pores gradually transitioned to medium and small pores. When the Na₂SO₄ mass fraction in the salt solution was ≥10 wt%, there was a downward trend in the mechanical properties after exposure for a certain period of time. The Cl⁻ diffusion rate was thus related to Na₂SO₄ concentration. When the Na₂SO₄ mass fraction in solution was ≤5 wt% and exposure time short, SO₄²⁻ and cement hydration/corrosion products hindered Cl⁻ migration. In a concentrated Na₂SO₄ environment (≥10 wt%), the Cl⁻ diffusion rate was accelerated in the later stages of exposure. These experiments further revealed that the Cl⁻ migration rate was higher than that of SO₄²⁻.     

Association of phospholipase A with a myocardial membrane preparation containing the (Na + K)-Mg-ATPase

Membranes containing the ouabain-sensitive (Na⁺ + K⁺)-Mg²⁺-ATPase were prepared by treatment of a homogenate of canine heart muscle with deoxycholate followed by extraction with sodium iodide. Phospholipase A₁, phospholipase A₂ and lysophospholipase activities were found in this preparation of membranes. Phospholipase A₂ had a pH optimum of 8.0 and was stimulated by 1.0 mm-EDTA. Phospholipase A₁ activity had a broad pH activation from 6.5 to 8.0 and showed variable Ca²⁺ and EDTA sensitivity. Lysophospholipase activity was present only at acid pH and was stimulated by EDTA. These enzymic activities were different from those of phospholipases previously reported in cardiac microsomes, mitochondria and lysosomes. Identification of the intracellular origin of the membranes containing the (Na⁺ + K⁺)-Mg²⁺-ATPase is uncertain. However, if this preparation is enriched with sarcolemmal membranes, several roles for the phospholipases may be postulated: modulating the activity of phospholipid-dependent enzymes, such as the (Na⁺ + K⁺)-Mg²⁺-ATPase, and regulating sarcolemmal permeability.     

Bidirectional influence of sodium channel activation on NMDA receptor–dependent cerebrocortical neuron structural plasticity

Neuronal activity regulates brain development and synaptic plasticity through N-methyl-d-aspartate receptors (NMDARs) and calcium-dependent signaling pathways. Intracellular sodium ([Na⁺]_i) also exerts a regulatory influence on NMDAR channel activity, and [Na⁺]_i may, therefore, function as a signaling molecule. In an attempt to mimic the influence of neuronal activity on synaptic plasticity, we used brevetoxin-2 (PbTx-2), a voltage-gated sodium channel (VGSC) gating modifier, to manipulate [Na⁺]_i in cerebrocortical neurons. The acute application of PbTx-2 produced concentration-dependent increments in both intracellular [Na⁺] and [Ca²⁺]. Pharmacological evaluation showed that PbTx-2–induced Ca²⁺ influx primarily involved VGSC activation and NMDAR-mediated entry. Additionally, PbTx-2 robustly potentiated NMDA-induced Ca²⁺ influx. PbTx-2–exposed neurons showed enhanced neurite outgrowth, increased dendritic arbor complexity, and increased dendritic filopodia density. The appearance of spontaneous calcium oscillations, reflecting synchronous neuronal activity, was accelerated by PbTx-2 treatment. Parallel to this response, PbTx-2 increased cerebrocortical neuron synaptic density. PbTx-2 stimulation of neurite outgrowth, dendritic arborization, and synaptogenesis all exhibited bidirectional concentration–response profiles. This profile paralleled that of NMDA, which also produced bidirectional concentration–response profiles for neurite outgrowth and synaptogenesis. These data are consistent with the hypothesis that PbTx-2–enhanced neuronal plasticity involves NMDAR-dependent signaling. Our results demonstrate that PbTx-2 mimics activity-dependent neuronal structural plasticity in cerebrocortical neurons through an increase in [Na⁺]_i, up-regulation of NMDAR function, and engagement of downstream Ca²⁺-dependent signaling pathways. These data suggest that VGSC gating modifiers may represent a pharmacologic strategy to regulate neuronal plasticity through NMDAR-dependent mechanisms.     

Influence of Sintering Temperature of Kaolin,Slag, and Fly Ash Geopolymers on the Microstructure,Phase Analysis,and Electrical Conductivity

This paper clarified the microstructural element distribution and electrical conductivity changes of kaolin, fly ash, and slag geopolymer at 900 °C. The surface microstructure analysis showed the development in surface densification within the geopolymer when in contact with sintering temperature. It was found that the electrical conductivity was majorly influenced by the existence of the crystalline phase within the geopolymer sample. The highest electrical conductivity (8.3 × 10⁻⁴ Ωm⁻¹) was delivered by slag geopolymer due to the crystalline mineral of gehlenite (3Ca₂Al₂SiO₇). Using synchrotron radiation X-ray fluorescence, the high concentration Ca boundaries revealed the appearance of gehlenite crystallisation, which was believed to contribute to development of denser microstructure and electrical conductivity.     

Geographic, seasonal, and precipitation chemistry influence on the abundance and activity of biological ice nucleators in rain and snow

Biological ice nucleators (IN) function as catalysts for freezing at relatively warm temperatures (warmer than −10 °C). We examined the concentration (per volume of liquid) and nature of IN in precipitation collected from Montana and Louisiana, the Alps and Pyrenees (France), Ross Island (Antarctica), and Yukon (Canada). The temperature of detectable ice-nucleating activity for more than half of the samples was ≥ −5 °C based on immersion freezing testing. Digestion of the samples with lysozyme (i.e., to hydrolyze bacterial cell walls) led to reductions in the frequency of freezing (0–100%); heat treatment greatly reduced (95% average) or completely eliminated ice nucleation at the measured conditions in every sample. These behaviors were consistent with the activity being bacterial and/or proteinaceous in origin. Statistical analysis revealed seasonal similarities between warm-temperature ice-nucleating activities in snow samples collected over 7 months in Montana. Multiple regression was used to construct models with biogeochemical data [major ions, total organic carbon (TOC), particle, and cell concentration] that were accurate in predicting the concentration of microbial cells and biological IN in precipitation based on the concentration of TOC, Ca²⁺, and NH₄⁺, or TOC, cells, Ca²⁺, NH₄⁺, K⁺, PO₄³⁻, SO₄²⁻, Cl⁻, and HCO₃⁻. Our results indicate that biological IN are ubiquitous in precipitation and that for some geographic locations the activity and concentration of these particles is related to the season and precipitation chemistry. Thus, our research suggests that biological IN are widespread in the atmosphere and may affect meteorological processes that lead to precipitation.     

Properties of a New Insulation Material Glass Bubble in Geopolymer Concrete

This paper details analytical research results into a novel geopolymer concrete embedded with glass bubble as its thermal insulating material, fly ash as its precursor material, and a combination of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) as its alkaline activator to form a geopolymer system. The workability, density, compressive strength (per curing days), and water absorption of the sample loaded at 10% glass bubble (loading level determined to satisfy the minimum strength requirement of a load-bearing structure) were 70 mm, 2165 kg/m³, 52.58 MPa (28 days), 54.92 MPa (60 days), and 65.25 MPa (90 days), and 3.73 %, respectively. The thermal conductivity for geopolymer concrete decreased from 1.47 to 1.19 W/mK, while the thermal diffusivity decreased from 1.88 to 1.02 mm²/s due to increased specific heat from 0.96 to 1.73 MJ/m³K. The improved physicomechanical and thermal (insulating) properties resulting from embedding a glass bubble as an insulating material into geopolymer concrete resulted in a viable composite for use in the construction industry.     

Dendritic Iron(III) Carbazole Complexes: Structural,Optical, and Magnetic Characteristics

This paper focuses on the synthesis, structural characterization, and study of the optical, magnetic, and thermal properties of novel architectures combining metal ions as magnetoactive centers and photoactive blocks formed by carbazole units. For this purpose, a series of azomethine complexes of the composition [Fe(L)₂]X (L = 3,6-bis[(3′,6′-di-tert-butyl-9-carbazol)-9-carbazol]benzoyloxy-4-salicylidene-N′-ethyl-N-ethylenediamine, X = NO₃⁻, Cl⁻, PF₆⁻) were synthesized by the reaction of metal salts with Schiff bases in a mixture of solvents. The UV–Vis absorption properties were studied in dichloromethane and rationalized via time-dependent density functional theory (DFT) calculations. Upon excitation at 350 nm, the compounds exhibited an intense dual fluorescence with two emission bands centered at ~445 and ~485 nm, which were assigned to π_carb–π* intraligand and π_carb–d_Fe ligand-to-metal charge-transfer excited states. EPR spectroscopy and SQUID magnetometry revealed solid-state partial spin crossover in some compounds, and antiferromagnetic interactions between the neighboring Fe(III) ions.     

New O3-Type Layer-Structured Na0.80[Fe0.40Co0.40Ti0.20]O2 Cathode Material for Rechargeable Sodium-Ion Batteries

Herein, we formulated a new O3-type layered Na_0.80[Fe_0.40Co_0.40Ti_0.20]O₂ (NFCTO) cathode material for sodium-ion batteries (SIBs) using a double-substitution concept of Co in the parent NaFe_0.5Co_0.5O₂, having the general formula Na_1-x[Fe_0.5–x/2Co_0.5–x/2M⁴⁺_x]O₂ (M⁴⁺ = tetravalent ions). The NFCTO electrode delivers a first discharge capacity of 108 mAhg⁻¹ with 80% discharge capacity retention after 50 cycles. Notably, the first charge–discharge profile shows asymmetric yet reversible redox reactions. Such asymmetric redox reactions and electrochemical properties of the NFCTO electrode were correlated with the phase transition behavior and charge compensation reaction using synchrotron-based in situ XRD and ex situ X-ray absorption spectroscopy. This study provides an exciting opportunity to explore the interplay between the rich chemistry of Na_1–x[Fe_0.5–x/2Co_0.5–x/2M⁴⁺_x]O₂ and sodium storage properties, which may lead to the development of new cathode materials for SIBs.     

Structures,Bonding and Sensor Properties of Some Alkaline o-Phthalatocuprates

Comprehensive study of the structure and bonding of disodium, dipotassium and diammonium di-o-phthalatocuprates(II) dihydrates has been undertaken. The crystal structure of ammonium o-phthalatocuprate has been determined. The identity of structures of phthalatocuprate chains in potassium and ammonium salts has been revealed. Vibrational spectra of all three compounds have been recorded, and the assignment of vibrational bands has been made. Force field calculations have shown a minor effect of outer-sphere cations (Na⁺, K⁺, NH₄⁺) on both intraligand (C-O) and metal–ligand bond strengths. Synthesized compounds have been tested as electrochemical sensors on D-glucose, dopamine and paracetamol. Their sensitivity to analytes varied in the order of Na⁺ > K⁺ > NH₄⁺. This effect has been explained by the more pronounced steric hindrance of copper ions in potassium and ammonium salts.     

One-Pot Self-Assembly of Dinuclear,Tetranuclear, and H-Bonding-Directed Polynuclear Cobalt(II), Cobalt(III), and Mixed-Valence Co(II)/Co(III) Complexes of Schiff Base Ligands with Incomplete Double Cubane Core

The reaction of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) was investigated in the presence of Cobalt(II) salts, (X = ClO₄⁻, CH₃CO₂⁻, Cl⁻, NO₃⁻), sodium azide (NaN₃), and triethylamine (TEA). In one pot, the variation in Cobalt(II) salt results in the self-assembly of dinuclear, tetranuclear, and H-bonding-directed polynuclear coordination complexes of Cobalt(III), Cobalt(II), and mixed-valence Co^IICo^III: [Co₂^III(H₂L⁻¹)₂(AP⁻¹)(N₃)](ClO₄)₂ (1), [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]·4CH₃OH (2), [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ₃-OH)₂](NO₃)₂·2CH₃CH₂OH (3), and [Co₂^IICo₂^III (H₂L1²⁻)₂(THMAM⁻¹)₂](NO₃)₄ (4). In 1, two cobalt(III) ions are connected via three single atom bridges; two from deprotonated ethanolic oxygen atoms in the side arms of the ligands and one from the1-amino-2-propanol moiety forming a dinuclear unit with a very short (2.5430(11) Å) Co-Co intermetallic separation with a coordination number of 7, a rare feature for cobalt(III). In 2, two cobalt(II) ions in a dinuclear unit are bridged through phenoxide O and μ₃-1,1,1-N₃ azido bridges, and the two dinuclear units are interconnected by two μ-1,1-N₃ and two μ₃-1,1,1-N₃ azido bridges generating tetranuclear cationic [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]²⁺ units with an incomplete double cubane core, which grow into polynuclear 1D-single chains along the a-axis through H-bonding. In 3, HL²⁻ holds mixed-valent Co(II)/Co(III) ions in a dinuclear unit bridged via phenoxide O, μ-1,3-CH₃CO₂⁻, and μ₃-OH⁻ bridges, and the dinuclear units are interconnected through two deprotonated ethanolic O in the side arms of the ligands and two μ₃-OH⁻ bridges generating cationic tetranuclear [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ_3-OH)₂]²⁺ units with an incomplete double cubane core. In 4, H₂L1⁻² holds mixed-valent Co(II)/Co(III) ions in dinuclear units which dimerize through two ethanolic O (μ-RO⁻) in the side arms of the ligands and two ethanolic O (μ₃-RO⁻) of THMAM bridges producing centrosymmetric cationic tetranuclear [Co₂^IICo₂^III (H₂L1⁻²)₂(THMAM⁻¹)₂]⁴⁺ units which grow into 2D-sheets along the bc-axis through a network of H-bonding. Bulk magnetization measurements on 2 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Co(II) ions.     

Development of Lectin-Linked Immunomagnetic Separation for the Detection of Hepatitis A Virus

The accuracy and sensitivity of PCR-based methods for detection of hepatitis A virus (HAV) are dependent on the methods used to separate and concentrate the HAV from the infected cells. The pH and ionic strength affect the binding affinity of the virus to cells. In this study, we initially investigated the effects of pH (4.0–10.0) and metal ions (Fe²⁺, Co²⁺, Cu²⁺, Mg²⁺, K⁺, and Ca²⁺) on the binding of HAV to oyster digestive cells. The lowest relative binding (RB) of HAV to the cells was found at pH 4.0 and in FeSO₄ solution (64.6% and 68.1%, respectively). To develop an alternative to antibody-dependent immunomagnetic separation prior to detection of HAV using RT-PCR, the binding of HAV to five lectins, peanut agglutinin (PNA), Dolichos biflorus agglutinin (DBA), Helix pomatia agglutinin (HPA), Ulex europaeus agglutinin (UEA-1) and soybean agglutinin (SBA), was evaluated using ELISAs. SBA showed significantly higher RB to HAV than the other lectins tested. In addition, HAV could be concentrated within 30 min using SBA-linked magnetic bead separation (SMS) prior to the RT-PCR assay. Our findings demonstrate the feasibility of using SMS combined with RT-PCR to detect HAV at dilutions ranging from 10⁻¹–10⁻⁴ of a HAV stock (titer: 10⁴ TCID₅₀/mL).