Influence of the Type of Cement on the Action of the Admixture Containing Aluminum Powder

The study of the effect of cement type on the action of an admixture increasing the volume of concrete (containing aluminum powder), used in amounts of 0.5–1.5% of cement mass, was presented. The tests were carried out on cement mortars with Portland (CEM I) and ground granulated blast-furnace slag cement (CEM III). The following tests were carried out for the tested mortars: the air content in fresh mortars, compressive strength, flexural strength, increase in mortar volume, bulk density, pore structure evaluation (by the computer image analysis method) and changes in the concentration of OH⁻ ions during the hydration of used cements. Differences in the action of the tested admixture depending on the cement used were found. To induce the expansion of CEM III mortars, a smaller amount of admixture is required than in the case of CEM I cement. Using the admixture in amounts above 1% of the cement mass causes cracks of mortars with CEM III cement due to slow hydrogen evolution, which occurs after mortar plasticity is lost. The use of an aluminum-containing admixture reduces the strength properties of the cement mortars, the effect being stronger in the case of CEM III cement. The influence of the sample molding time on the admixture action was also found.     

Electrochemical Energy Storage Properties of High-Porosity Foamed Cement

Foamed porous cement materials were fabricated with H₂O₂ as foaming agent. The effect of H₂O₂ dosage on the multifunctional performance is analyzed. The result shows that the obtained specimen with 0.6% H₂O₂ of the ordinary Portland cement mass (PC0.6) has appropriate porosity, leading to outstanding multifunctional property. The ionic conductivity is 29.07 mS cm⁻¹ and the compressive strength is 19.6 MPa. Furthermore, the electrochemical energy storage performance is studied in novel ways. The PC0.6 also shows the highest areal capacitance of 178.28 mF cm⁻² and remarkable cycle stability with 90.67% of initial capacitance after 2000 cycles at a current density of 0.1 mA cm⁻². The superior electrochemical energy storage property may be attributed to the high porosity of foamed cement, which enlarges the contact area with the electrode and provides a rich ion transport channel. This report on cement–matrix materials is of great significance for large scale civil engineering application.     

Importance of trivalency and the eg1 configuration in the photocatalytic oxidation of water by Mn and Co oxides

Prompted by the early results on the catalytic activity of LiMn₂O₄ and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn₂O₃, LaMnO₃, and MgMn₂O₄ are found to be very good catalysts with turnover frequencies of 5 × 10⁻⁴ s⁻¹, 4.8 × 10⁻⁴ s⁻¹, and 0.8 ×10⁻⁴ s⁻¹, respectively. Among the cobalt oxides, Li₂Co₂O₄ and LaCoO₃—especially the latter—exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10⁻⁴ s⁻¹ and 1.4 × 10⁻³ s⁻¹, respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co³⁺ in the Co oxides is in the intermediate t_2g⁵e_g¹ state whereas Mn³⁺ is in the t_2g³e_g¹ state. The presence of the e_g¹ electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.     

Durability of Construction and Demolition Waste-Bearing Ternary Eco-Cements

In recent years, the development of ternary cements has become a priority research line for obtaining cements with a lower carbon footprint, with the goal to contribute to achieve climate neutrality by 2050. This study compared ordinary Portland cement (OPC) durability to the performance of ternary cements bearing OPC plus 7% of a 2:1 binary blend of either calcareous (Hc) or siliceous (Hs) concrete waste fines and shatterproof glass. Durability was measured further to the existing legislation for testing concrete water absorption, effective porosity, pressurized water absorption and resistance to chlorides and CO₂. The experimental findings showed that the 7% blended mortars performed better than the reference cement in terms of total and effective porosity, but they absorbed more pressurized water. They also exhibited lower CO₂ resistance, particularly in the calcareous blend, likely due to its higher porosity. Including the binary blend of CDW enhanced chloride resistance with diffusion coefficients of 2.9 × 10⁻¹¹ m² s⁻¹ (calcareous fines-glass, 7%Hc-G) and 1.5 × 10⁻¹¹ m² s⁻¹ (siliceous fines-glass, 7%Hs-G) compared to the reference cement’s 4.3 × 10⁻¹¹ m² s⁻¹. The siliceous fines-glass blend out-performed the calcareous blend in all the durability tests. As the mortars with and without CDW (construction and demolition waste) performed to similar standards overall, the former were deemed viable for the manufacture of future eco-efficient cements.     

Studies on Cement Pastes Exposed to Water and Solutions of Biological Waste

The paper presents studies on the early stages of biological corrosion of ordinary Portland cements (OPC) subjected to the reactive media from the agricultural industry. For ten months, cement pastes of CEM I type with various chemical compositions were exposed to pig slurry, and water was used as a reference. The phase composition and structure of hydrating cement pastes were characterized by X-ray diffraction (XRD), thermal analysis (DTA/TG/DTG/EGA), and infrared spectroscopy (FT-IR). The mechanical strength of the cement pastes was examined. A 10 to 16% decrease in the mechanical strength of the samples subjected to pig slurry was observed. The results indicated the presence of thaumasite (C₃S·CO₂·SO₃·15H₂O) as a biological corrosion product, likely formed by the reaction of cement components with living matter resulting from the presence of bacteria in pig slurry. Apart from thaumasite, portlandite (Ca(OH)₂)—the product of hydration—as well as ettringite (C₃A·3CaSO₄·32H₂O) were also observed. The study showed the increase in the calcium carbonate (CaCO₃) phase. The occurrence of unreacted phases of cement clinker, i.e., dicalcium silicate (C₂S) and tricalcium aluminate (C₃A), in the samples was confirmed. The presence of thaumasite phase and the exposure condition-dependent disappearance of CSH phase (calcium silicate hydrate), resulting from the hydration of the cements, were demonstrated.     

Combining Raman Spectroscopy,DFT Calculations,and Atomic Force Microscopy in the Study of Clinker Materials

Raman spectroscopy and Raman mapping analysis, combined with density functional theory calculations were applied to the problem of differentiating similar clinker materials such as alite and belite. The Portland cement clinker 217 (further: clinker) was analysed using colocalised Raman mapping and atomic force microscopy mapping, which provided both spatial and chemical information simultaneously. The main constituents found in the clinker were alite, belite, portlandite, amorphous calcium carbonate, and gypsum. Since phonon bands of alite and belite greatly overlap, and their distinction is important for the hydration process during cement setting, we provided the calculated phonon density of states for alite Ca₃SiO₅ (<M>Pc structure) and belite Ca₂SiO₄ (β P2₁/n structure) here for the first time. Both calculated phonon densities have similar distribution of phonon modes, with a gap between 560 and 810 cm⁻¹. A comparison of the calculated phonon frequencies for Ca₃SiO₅ and Ca₂SiO₄ shows that the lowest calculated phonon frequency of β-Ca₂SiO₄ lies at 102 cm⁻¹, while for <M>Pc alite the lowest phonon frequency is predicted at 27 cm⁻¹. Low frequency Raman spectroscopy could therefore be used for a clearer distinction of these two species in a clinker material.     

Effects of Sodium Hexametaphosphate Addition on the Dispersion and Hydration of Pure Calcium Aluminate Cement

The effects of sodium hexametaphosphate (SHMP) addition on the dispersion and hydration of calcium aluminate cement were investigated, and the relevant mechanisms discussed. The content of SHMP and the adsorption capacity of SHMP on the surface of cement particles were estimated using plasma adsorption spectroscopy and the residual concentration method. The rheological behavior of hydrate, ζ-potential value of cement particles, phase transformation and the microstructure of the samples were determined by coaxial cylinder rheometer, zeta probe, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that SHMP readily reacted with Ca²⁺, forming complexes [Ca₂(PO₃)₆]²⁻ ions which were subsequently adsorbed onto the surfaces of cement particles. When the content of SHMP was 0.05%, the adsorption ratio reached 99%. However, it decreased to 89% upon further increasing the addition of SHMP to 0.4%. The complexes [Ca₂(PO₃)₆]²⁻ adsorbed onto the surfaces of cement particles inhibited the concentration of Ca²⁺ and changed ζ-potential, resulting in enhanced electrostatic repulsive force between the cement particles and reduced viscosity of cement-water slurry. The experimental results indicate that the complexes [Ca₂(PO₃)₆]²⁻ covering the surfaces of cement particles led to a delayed hydration reaction, i.e., they extended the hydration time of the cement particles, and that the optimal addition of SHMP was found to be about 0.2%.     

External Sulfate Attack on Cementitious Binders: Limitations and Effects of Sample Geometry on the Quantification of Expansion Stress

The hollow cylinder method was used to estimate the expansion stress that can occur in concrete due to the crystallisation pressure caused by the formation of ettringite and/or gypsum during external sulphate attack. Hardened cement paste hollow cylinders prepared with Portland cement were mounted in stress cells and exposed to sodium sulphate solutions with two different concentrations (3.0 g L SO₄²⁻ and 30.0 g L SO₄²⁻). Microstructural analysis and finite element modelling was used to evaluate the experimental observations. The expansion stress calculation was verified for a range of diameter/length ratios (0.43–0.60). Thermodynamically predicted maximum expansion stresses are larger than expansion stresses observed in experiments because the latter are affected by the sample geometry, degree of restraint, pore size distribution and relaxation processes. The results indicate that differences in self-constraint at the concave inner and convex outer surfaces of the hollow cylinder lead to an asymmetric expansion stress when ettringite is formed. This leads to macroscopic longitudinal cracks and ultimately failure. Heavy structural components made of concrete are likely to support larger maximum expansion stresses than observed by the hollow cylinder method due to their self-constraint.     

Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy

We have studied the hydrophobic water/octadecyltrichlorosilane (OTS) interface by using the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS), and we obtained detailed structural information of the interface at the molecular level. Excess ions emerging at the interface were detected by changes of the surface vibrational spectrum induced by the surface field created by the excess ions. Both hydronium (H₃O⁺) and hydroxide (OH⁻) ions were found to adsorb at the interface, and so did other negative ions such as Cl⁻. By varying the ion concentrations in the bulk water, their adsorption isotherms were measured. It was seen that among the three, OH⁻ has the highest adsorption energy, and H₃O⁺ has the lowest; OH⁻ also has the highest saturation coverage, and Cl⁻ has the lowest. The result shows that even the neat water/OTS interface is not neutral, but charged with OH⁻ ions. The result also explains the surprising observation that the isoelectric point appeared at ∼3.0 when HCl was used to decrease the pH starting from neat water.     

Br?nsted basicity of the air–water interface

Differences in the extent of protonation of functional groups lying on either side of water–hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol–gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H₂O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air–water interface predict that such event is hindered by a significant kinetic barrier unless OH⁻ ions are present therein. Thus, by detecting RCOO⁻ we demonstrate the presence of OH⁻ on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me)₃N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH⁻ densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH⁻ gradients in the interfacial region. This fact could account for the weak OH⁻ signals detected by surface-specific spectroscopies.     

Mitigation of Corrosion Initiated by Cl− and SO42−-ions in Blast Furnace Cement Concrete Mixed with Sea Water

The use of blast furnace cement is an effective way to meet the requirements of sustainable development. However, CEM III/C is characterized by slow strength gain. The problem can be worse for plasticized reinforced blast furnace cement concretes mixed with sea water in view of shorter durability. The mitigation of corrosion in plasticized blast furnace cement concretes mixed with sea water can be provided through a composition of minor additional constituents, with percentage by mass of the main constituents: alkali metal compounds, 2…3; calcium aluminate cement, 1; clinoptilolite, 1. The alkali metal compounds are known to activate hydraulic properties of ground granulated blast furnace slag. A calcium aluminate cement promotes the accelerated chemical binding of Cl⁻ and SO₄²⁻-ions with the formation of Kuzel’s salt. A clinoptilolite occludes these aggressive ions. The positive effects of the mentioned minor additional constituents in the blast furnace cement were supported by the increased early strength gain and the higher structural density, as well as by a good state of steel reinforcement, in the plasticized concretes mixed with sea water.     

Corrosion Activity of Carbon Steel B450C and Stainless Steel SS430 Exposed to Extract Solution of a Supersulfated Cement

Carbon steel B450C and low-chromium stainless steel SS430 were exposed for 30 days to supersulfated “SS1” cement extract solution, considered as a “green” alternative for partial replacement of the Portland cement clinker. The initial pH of 12.38 dropped since the first day to 7.84, accompanied by a displacement to more negative values of the free corrosion potential (OCP) of the carbon steel up to ≈−480.74 mV, giving the formation of γ-FeOOH, α-FeOOH and Fe₂O₃, as suggested by XRD and XPS analysis. In the meantime, the OCP of the SS430 tended towards more positive values (+182.50 mV), although at lower pH, and XPS analysis revealed the presence of Cr(OH)₃ and FeO as corrosion products, as well the crystals of CaCO₃, NaCl and KCl. On both surfaces, a localized corrosion attack was observed in the vicinity of local cathodes (Cu, Mn-carbides, Cr-nitrides, among others), influenced by the presence of Cl⁻ ions in the “SS1” extract solution, originating from the pumice. Two equivalent circuits were proposed for the quantitative analysis of EIS Nyquist and Bode diagrams, whose data were correlated with the OCP values and pH change in time of the “SS1” extract solution. The thickness of the corrosion layer formed on the SS430 surface was ≈0.8 nm, while that on the B450C layer was ≈0.3 nm.     

Reduced susceptibility of mice overexpressing transforming growth factor α to dextran sodium sulphate induced colitis

Background—Transforminggrowth factor α (TGF-α) knockout mice have increased susceptibilityto dextran sodium sulphate (DSS) induced colitis.
Aim—To substantiatethe findings that TGF-α is a key mediator of colonic mucosalprotection and/or repair mechanisms by evaluating the susceptibility ofmice overexpressing TGF-α to DSS induced colitis.
Methods—TGF-αoverexpression was induced in transgenic mice by ZnSO₄administration in drinking water (TG+). Three groups were used ascontrols: one transgenic group without ZnSO₄ administration (TG−), and two non-transgenic littermate groups receivingZnSO₄ (Non-TG+) or only water (Non-TG−). Acute colitiswas induced in all groups by administration of DSS (5%, w/v) indrinking water for six days ad libitum.
Results—About 35-39%of the entire colonic mucosa was destroyed in Non-TG−, Non-TG+, andTG− animals compared with 9% in TG+ mice. The crypt damage score was18.7 (0.9), 18.2 (1.0), 18.9(0.8), and 6.8 (1.5) (means (SEM)) inNon-TG−, Non-TG+, TG−, and TG+ mice respectively. Mucin andbromodeoxyuridine staining were markedly enhanced in colons of TG+ micecompared with controls, indicating increased mucosal protection and regeneration.
Conclusions—Thesignificantly reduced susceptibility of mice overexpressing TGF-α toDSS further substantiates that endogenous TGF-α is a pivotal mediatorof protection and/or healing mechanisms in the colon.

Keywords:transforming growth factor α; epidermal growthfactor; dextran sodium sulphate; colitis; inflammatory bowel disease; transgenic mice

     

Optimization of Alumina Ceramics Corrosion Resistance in Nitric Acid

The development of ceramic materials resistance in various aggressive media combined with required mechanical properties is of considerable importance for enabling the wider application of ceramics. The corrosion resistance of ceramic materials depends on their purity and microstructure, the kind of aggressive media used and the ambient temperature. Therefore, the corrosion resistance of alumina ceramics in aqueous HNO₃ solutions of concentrations of 0.50 mol dm⁻³, 1.25 mol dm⁻³ and 2.00 mol dm⁻³ and different exposure times—up to 10 days—have been studied. The influence of temperature (25, 40 and 55 °C) was also monitored. The evaluation of Al₂O₃ ceramics corrosion resistance was based on the concentration measurements of eluted Al³⁺, Ca²⁺, Fe³⁺, Mg²⁺, Na⁺ and Si⁴⁺ ions obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES), as well as density measurements of the investigated alumina ceramics. The response surface methodology (RSM) was used for the optimization of parameters within the experimental “sample-corrosive media” area. The exposure of alumina ceramics to aqueous HNO₃ solutions was conducted according to the Box–Behnken design. After the regression functions were defined, conditions to achieve the maximum corrosion resistance of the sintered ceramics were determined by optimization within the experimental area.     

Determination of the Influence of Hydraulic Additives on the Foaming Process and Stability of the Produced Geopolymer Foams

The research described in this article was aimed at determining the influence of hydraulic additives on the foaming process and the stability of the produced geopolymer foams. These foams can be used as insulation materials to replace the currently commonly used insulations such as expanded polystyrene or polyurethane foams. Geopolymers have low thermal conductivity, excellent fire- and heat-resistant properties, and have fairly good mechanical properties. Research on foamed materials shows that they have the highest class of fire resistance; therefore, they are most often used as insulation products in construction. Geopolymer foams were made of aluminosilicate materials (fly ash) and foaming agents (H₂O₂ and Al powder), and the stabilizers were gypsum and portland cement. Additionally, surfactants were also used. It was found that better foaming effects were obtained for H₂O₂—it is a better foaming agent for geopolymers than Al powder. When using a hydraulic additive—a stabilizer in the form of cement—lower densities and better insulation parameters were obtained than when using gypsum. Portland cement is a better stabilizer than gypsum (calcium sulfates), although the effect may change due to the addition of surfactants, for example.     

The Synergistic Inhibitions of Tungstate and Molybdate Anions on Pitting Corrosion Initiation for Q235 Carbon Steel in Chloride Solution

In this work, the synergistic inhibitions of tungstate (WO₄²⁻) and molybdate (MoO₄²⁻) anions, including role and mechanism, on the initiation of pitting corrosion (PC) for Q235 carbon steel in chloride (Cl⁻) solution were investigated with electrochemical and surface techniques. The pitting potential (E_p) of the Q235 carbon steel in WO₄²⁻ + MoO₄^2- + Cl⁻ solution was more positive than that in WO₄²⁻ + Cl⁻ or MoO₄²⁻ + Cl⁻ solution; at each E_p, both peak potential and affected region of active pitting sites in WO₄²⁻ + MoO₄²⁻ + Cl⁻ solution were smaller than those in WO₄²⁻ + Cl⁻ or MoO₄²⁻ + Cl⁻ solution. WO₄²⁻ and MoO₄²⁻ showed a synergistic role to inhibit the PC initiation of the Q235 carbon steel in Cl⁻ solution, whose mechanism was mainly attributed to the influences of two anions on passive film. Besides iron oxides and iron hydroxides, the passive film of the Q235 carbon steel formed in WO₄²⁻ + Cl⁻, MoO₄²⁻ + Cl⁻, or WO₄²⁻ + MoO₄²⁻ + Cl⁻ solution was also composed of FeWO₄ plus Fe₂(WO₄)₃, Fe₂(MoO₄)₃, or Fe₂(WO₄)₃ plus Fe₂(MoO₄)₃, respectively. The film resistance and the defect quantity for Fe₂(WO₄)₃ plus Fe₂(MoO₄)₃ film were larger and smaller than those for FeWO₄ plus Fe₂(WO₄)₃ film and Fe₂(MoO₄)₃ film, respectively; for the inhibition of PC initiation, Fe₂(WO₄)₃ plus Fe₂(MoO₄)₃ film provided better corrosion resistance to Q235 carbon steel than FeWO₄ plus Fe₂(WO₄)₃ film and Fe₂(MoO₄)₃ film did.     

Characterisation at the Bonding Zone between Fly Ash Based Geopolymer Repair Materials (GRM) and Ordinary Portland Cement Concrete (OPCC)

In recent years, research and development of geopolymers has gained significant interest in the fields of repairs and restoration. This paper investigates the application of a geopolymer as a repair material by implementation of high-calcium fly ash (FA) as a main precursor, activated by a sodium hydroxide and sodium silicate solution. Three methods of concrete substrate surface preparation were cast and patched: as-cast against ordinary Portland cement concrete (OPCC), with drilled holes, wire-brushed, and left as-cast against the OPCC grade 30. This study indicated that FA-based geopolymer repair materials (GRMs) possessed very high bonding strength at early stages and that the behavior was not affected significantly by high surface treatment roughness. In addition, the investigations using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy have revealed that the geopolymer repair material became chemically bonded to the OPC concrete substrate, due to the formation of a C–A–S–H gel. Fundamentally, the geopolymer network is composed of tetrahedral anions (SiO₄)⁴⁻ and (AlO₄)⁵⁻ sharing the oxygen, which requires positive ions such as Na⁺, K⁺, Li⁺, Ca²⁺, Na⁺, Ba²⁺, NH⁴⁺, and H₃O⁺. The availability of calcium hydroxide (Ca(OH)₂) at the surface of the OPCC substrate, which was rich in calcium ions (Ca²⁺), reacted with the geopolymer; this compensated the electron vacancies of the framework cavities at the bonding zone between the GRM and the OPCC substrate.     

Accelerating slow excited state proton transfer

Visible light excitation of the ligand-bridged assembly [(bpy)₂Ru_a^II(L)Ru_b^II(bpy)(OH₂)⁴⁺] (bpy is 2,2′-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L^−•)Ru_b^III-OH₂ with an excited-state lifetime of 13 ± 1 ns. Near–diffusion-controlled quenching of the emission occurs with added HPO₄²⁻ and partial quenching by added acetate anion (OAc⁻) in buffered solutions with pH control. A Stern–Volmer analysis of quenching by OAc⁻ gave a quenching rate constant of k_q = 4.1 × 10⁸ M⁻¹⋅s⁻¹ and an estimated pK_a* value of ∼5 ± 1 for the [(bpy)₂Ru_a^II(L^•−)Ru_b^III(bpy)(OH₂)⁴⁺]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)₂Ru_a^II(L)Ru_b^II(bpy)(OH)³⁺] in a H₂PO₄⁻/HPO₄²⁻ buffer, back proton transfer occurs from H₂PO₄⁻ to give [(bpy)₂Ru_a^II(L)Ru_b(bpy)(OH₂)⁴⁺] with k_PT,2 = 4.4 × 10⁸ M⁻¹⋅s⁻¹. From the intercept of a plot of k_obs vs. [H₂PO₄⁻], k = 2.1 × 10⁶ s⁻¹ for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK_a values intermediate between pK_a(H₃O⁺) = −1.74 and pK_a(H₂O) = 15.7.     

Praseodymium Orthoniobate and Praseodymium Substituted Lanthanum Orthoniobate: Electrical and Structural Properties

In this paper, the structural properties and the electrical conductivity of La_1−xPr_xNbO_4+δ (x = 0.00, 0.05, 0.1, 0.15, 0.2, 0.3) and PrNbO_4+δ are presented and discussed. All synthesized samples crystallized in a monoclinic structure with similar thermal expansion coefficients. The phase transition temperature between the monoclinic and tetragonal structure increases with increasing praseodymium content from 500 °C for undoped LaNbO_4+δ to 700 °C for PrNbO_4+δ. Thermogravimetry, along with X-ray photoelectron spectroscopy, confirmed a mixed 3⁺/4⁺ oxidation state of praseodymium. All studied materials, in humid air, exhibited mixed protonic, oxygen ionic and hole conductivity. The highest total conductivity was measured in dry air at 700 °C for PrNbO_4+δ, and its value was 1.4 × 10⁻³ S/cm.     

Yeast chorismate mutase in the R state: Simulations of the active site

The isomerization of chorismate to prephenate by chorismate mutase in the biosynthetic pathway that forms Tyr and Phe involves C5—O (ether) bond cleavage and C1—C9 bond formation in a Claisen rearrangement. Development of negative charge on the ether oxygen, stabilized by Lys-168 and Glu-246, is inferred from the structure of a complex with a transition state analogue (TSA) and from the pH-rate profile of the enzyme and the E246Q mutant. These studies imply a protonated Glu-246 well above pH 7. Here, several 500-ps molecular dynamics simulations test the stability of enzyme–TSA complexes by using a solvated system with stochastic boundary conditions. The simulated systems are (i) protonated Glu-246 (stable), (ii) deprotonated Glu-246 (unstable), (iii) deprotonated Glu-246 plus one H₂O between Glu-246 and the ether oxygen (unstable), (iv) the E246Q mutant (stable), and (v) addition of OH⁻ between protonated Glu-246 and the ether oxygen. In (v), a local conformational change of Lys-168 displaced the OH⁻ into the solvent region, suggesting a possible rate-determining step that precedes the catalytic step. In a 500-ps simulation of the enzyme complexed with the reactant chorismate or the product prephenate, no water molecule remained near the oxygen of the ligand. Calculations using the linearized Poisson–Boltzmann equation show that the effective pK_a of Glu-246 is shifted from 5.8 to 8.1 as the negative charge on the ether oxygen of the TSA is changed from −0.56 electron to −0.9 electron. Altogether, these results support retention of a proton on Glu-246 to high pH and the absence of a water molecule in the catalytic steps.