Calcium-induced aggregation of bovine caseins: effect of phosphate and citrate

The formation of colloidal particles by Ca²⁺ precipitation of whole caseinates in the presence of phosphate (Pi), citrate (Cit), or both of the anions in concentrations found to be effective in previous works was followed comparing the colloidal particle size and the ionic and proteic composition of the precipitates obtained. Ca²⁺ was incorporated to the precipitate and colloidal particles in a different way than Pi, differences which were related to the presence of Pi and/or Cit in the media. A sequential salting-out process due to progressive Ca²⁺ binding to at least two kinds of sites was observed. The precipitation curves were fitted, and the affinity constants and binding site numbers were calculated with a modification of the Farrell’s equation based on the concept of Wyman’s linked functions. Precipitates obtained at low total Ca²⁺ concentrations in different conditions varied their casein composition. Colloidal particles appeared at the beginning of the second salting-out step, in different amount, and in average size according to the presence or absence of Pi and/or Cit in the media. Consideration of these differences showed that Cit favored the formation of bigger colloidal particles, acting especially in the first steps of the casein aggregation and conditioning the mechanism of this process.     

Binding Study of the [Ru(NH3)5pz]2+ Complex to Bile Anion Aggregates through Kinetic Measurements

The electron transfer reaction between [Ru(NH₃)₅pz]²⁺ and [Co(C₂O₄)₃]^3? was studied in the presence of monomers and aggregates of bile salts (sodium deoxycholate, sodium taurodeoxycholate, and sodium glycocholate) at 298.2 ± 0.1 K. The results show a decreasing rate constant with the successive addition of bile salts. To rationalize the trends of the reaction rate on the [bile salts], two models were used. One of them takes into account the aggregation feature by considering a stepwise self‐association between monomers, whereas the other assumes the formation of a critical micellar concentration. Binding constants between [Ru(NH₃)₅pz]²⁺ species and deoxycholate or taurodeoxycholate aggregates were higher than that for glycocholate aggregates. These results are consistent with the way in which the monomers are added to form the bile anion aggregates.     

Observation of salt-induced β-lactoglobulin aggregation using sedimentation field-flow fractionation

Sedimentation field-flow fractionation (SdFFF) was applied in order to characterize particle sizes of β-lactoglobulin aggregates induced by Ca²⁺ or Zn²⁺. Aggregation induced by Zn²⁺ was faster than that induced by Ca²⁺. Effects of Zn²⁺ and β-lactoglobulin concentrations, as well as contact time, on the aggregation of β-lactoglobulin were examined. All factors exhibited a combined effect on the size of aggregates, whereby larger aggregates were obtained at increased concentrations of Zn²⁺ and β-lactoglobulin. At fixed concentrations of 2% (w/v) β-lactoglobulin and 10 mM Zn²⁺, the particle size of the aggregates increased from 0.19 μm (at 15 min) to 0.38 μm (at 2880 min). Further, a hyphenated technique of SdFFF and inductively coupled plasma–optical emission spectrometry (ICP–OES) was used to examine whether intermolecular ionic bridges take part in salt-induced β-lactoglobulin aggregation. With SdFFF–ICP–OES, protein–cation–protein cross-linkages were observed for β-lactoglobulin aggregation induced by Zn²⁺, but not for that induced by Ca²⁺.      

The salting-out effect and phase separation in aqueous solutions of tri-sodium citrate and 1-butyl-3-methylimidazolium bromide

The aim of this work is to obtain further evidence about the salting-out effect produced by the addition of tri-sodium citrate to aqueous solutions of water miscible ionic liquid 1-butyl-3-methylimidazolium bromide ([C₄mim][Br]) by evaluating the effect of tri-sodium citrate on the thermodynamic properties of aqueous solutions of this ionic liquid. Experimental measurements of density and sound velocity at different temperatures ranging from (288.15 to 308.15) K, the refractive index at 308.15 K and the liquid–liquid phase diagram at different temperatures ranging from (288.15 to 338.15) K for aqueous solutions containing 1-butyl-3-methylimidazolium bromide ([C₄mim][Br]) and tri-sodium citrate (Na₃Cit) are taken. The apparent molar volume of transfer of [C₄mim][Br] from water to aqueous solutions of Na₃Cit have positive values and it increases by increasing salt molality. Although at high IL molality, the apparent molar isentropic compressibility shows similar behaviour with that of the apparent molar volume. However at low concentrations of IL, the apparent molar isentropic compressibility of transfer of [C₄mim][Br] from water to aqueous solutions of Na₃Cit have negative values. The effects of temperature and the addition of Na₃Cit and [C₄mim][Br] on the liquid–liquid phase diagram of the investigated system have been studied. It was found that an increase in temperature caused the expansion of the one-phase region. The presence of Na₃Cit triggers a salting-out effect, leading to significant upward shifts of the liquid–liquid de-mixing temperatures of the system. The effect of temperature on the phase-forming ability in the system investigated has been studied based on a salting-out coefficient obtained from fitting the binodal values to a Setschenow-type equation for each temperature. Based on cloud point values, the energetics of the clouding process have been estimated and it was found that both of entropy and enthalpy are the driving forces for biphasic formation.     

枸橼酸二乙酯、枸橼酸钠和膦甲酸钠对高钙诱导小鼠血管平滑肌细胞钙化的抑制

在高钙及不同浓度枸橼酸二乙酯（Et₂Cit）、枸橼酸钠（Na₃Cit）和膦甲酸钠（PFA）存在下培养小鼠血管平滑肌细胞（MOVAS）14 d，分别通过茜素红染色、免疫荧光和annexin V染色检测细胞的分化、凋亡和细胞钙沉积量，研究了Et₂Cit、Na₃Cit和PFA对高钙诱导MOVAS细胞钙化的抑制效果及可能的作用机制。结果表明，Et₂Cit、Na₃Cit和PFA均能减少高钙诱导的MOVAS钙化，减少细胞外钙化斑块和钙沉积量。这些抑制剂均能抑制平滑肌细胞向成骨样细胞表型转化，导致向成骨细胞转化的标记物碱性磷酸酶（ALP）活性降低。抑制效果均存在浓度依赖性。当抑制剂浓度相同时，其抑制效果从大到小为：PFA > Na₃Cit > Et₂Cit。低浓度的Et₂Cit和Na₃Cit可通过减少细胞凋亡来抑制钙化，但高浓度的Et₂Cit、Na₃Cit和PFA自身具有毒性，这增加了细胞的凋亡。作为血液抗凝剂的Et₂Cit和Na₃Cit可以有效地抑制MOVAS的钙化。     

Aggregation and fusion of phospholipid vesicles

Membrane fusion and aggregation of phospholipid vesicles are reviewed and discussed. The fusion process is viewed as consisting of several stages: aggregation and close apposition of the particles, destabilization, and finally, merging of the bilayers. A procedure is presented which yields both the rate constant of the dimerization (C₁₁) and the rate constant for fusion of the dimers (f₁₁), which is a direct measure of the probability that two apposed vesicles will fuse. Experimental methods used in the study of membrane fusion are reviewed, primarily with respect to their capacity to monitor the kinetics of vesicle fusion. A few kinetic studies on the mixing of aqueous contents, leakage and increase in size of fusing vesicles are presented in detail.The range of C₁₁ values for Ca²⁺-induced aggregation and fusion of small unilamellar vesicles (SUV, ～ 125 Å radius) composed of phosphatidylserine (PS) is 10⁶ to 5 × 10⁷ M^-1 in the presence of Ca²⁺ concentrations from 1.15 to 2 mM, respectively. For larger PS vesicles (LUV, ～ 500 Å radius) C₁₁ = 6.5 × 10⁷ M^-1s^-1 in the presence of 5 mM Ca²⁺. These values are in good agreement with theoretical calculations based on van der Waals and electrostatic interactions, in which binding of cations is explicitly taken into account. The rate constants of fusion, f₁₁, are 5 s^-1 for PS SUV and 0.08 s^-1 for LUV in the presence of 2 mM and 5 mM Ca²⁺, respectively. The significance of these fusion rate constants to the duration of the fusion event is discussed.Factors affecting fusion such as cations, temperature, membrane composition vesicle concentration and size are reviewed and analyzed. Di- or tri-valent cations induce fusion of acidic phospholipid vesicles (except for phosphatidylinositol) in either pure or mixed form. Among the neutral phospholipids, phosphatidylcholine (PC) inhibits but phosphatidylethanolamine (PE) sustains or enhances the fusion capacity of acidic phospholipid vesicles. Monovalent cations induce reversible aggregation of negatively charged vesicles, but they inhibit the fusion induced by divalent cations such as Ca²⁺ or Mg²⁺. Fusion of neutral phospholipid vesicles, and it occurs the cation-induced fusion of acidic phospholipid vesicles, and it occurs only at temperatures below the gel to liquid crystalline phase transition temperature Tc. This is in contrast to the acidic phospholipid vesicle fusion which is greatly enhanced when the temperature is above the Tc of the phospholipid.     

Extraction of Ce(III), Gd(III) and Yb(III) from Citrate Medium by High Molecular Weight Amines

Abstract

High molecular weight amines have been used for the extraction of citrate complexes of Ce(III), Gd(III) and Yb(III). The effect of different variables on extraction has been studied. The citrate species extracted in the organic phase have been proposed as [(RNH₃ ⁺)₃] [M(Cit)₂]^3-.     

Thermal aggregation of bovine serum albumin studied by asymmetrical flow field-flow fractionation

The use of asymmetrical flow field-flow fractionation (AsFlFFF) in the study of heat-induced aggregation of proteins is demonstrated with bovine serum albumin (BSA) as a model analyte. The hydrodynamic diameter (d_h), the molar mass of heat-induced aggregates, and the radius of gyration (R_g) were calculated in order to get more detailed understanding of the conformational changes of BSA upon heating. The hydrodynamic diameter of native BSA at ambient temperature was ∼7 nm. The particle size was relatively stable up to 60 °C; above 63 °C, however, BSA underwent aggregation (growth of hydrodynamic diameter). The hydrodynamic diameters of the aggregated particles, heated to 80 °C, ranged from 15 to 149 nm depending on the BSA concentration, duration of incubation, and the ionic strength of the solvent. Heating of BSA in the presence of sodium dodecyl sulfate (1.7 or 17 mM) did not lead to aggregation. The heat-induced aggregates were characterized in terms of their molar mass and particle size together with their respective distributions with a hyphenated technique consisting of an asymmetrical field-flow fractionation device and a multi-angle light scattering detector and a UV-detector. The carrier solution comprised 8.5 mM phosphate and 150 mM sodium chloride at pH 7.4. The weight-average molar mass (M_w) of native BSA at ambient temperature is 6.6 × 10⁴ g mol⁻¹. Incubation of solutions with BSA concentrations of 1.0 and 2.5 mg mL⁻¹ at 80 °C for 1 h resulted in aggregates with M_w 1.2 × 10⁶ and 1.9 × 10⁶ g mol⁻¹, respectively. The average radius of gyration and the average hydrodynamic radius of the heat-induced aggregate samples were calculated and compared to the values obtained from the size distributions measured by AsFlFFF. For comparison static light scattering measurements were carried out and the corresponding average molar mass distributions of solutions with BSA concentrations of 1.0 and 2.5 mg mL⁻¹ at 80 °C for 1 h gave aggregates with M_w 1.7 × 10⁶ and 3.5 × 10⁶ g mol⁻¹, respectively.     

Complex formation of indium(<Emphasis Type="SmallCaps">iii</Emphasis>) with citric acid in aqueous solution

Complex formation of In³⁺ ion with citric acid in an aqueous solution was studied by pH-metric titration at the molar ratio of the reactants [In³⁺] : [H₄Cit] = 1 : 1, 1 : 2, and 1 : 3 in a range of pH 2—10. The mathematical simulation of equilibria in an indium(iii)—citric acid system was performed using the CPESSP program package. The formation of 1 : 1, 1 : 2, and 1 : 3 indium(iii) citrate complexes with different degrees of nuclearity and protonation was established. The equilibrium constants of formation of the complexes were calculated. The predomination of the polynuclear indium(iii) citrate complexes at the equimolar metal to ligand ratio of was observed in almost the whole pH range studied (3—10).     

Oligonucleotide charge states in negative ionization electrospray-mass spectrometry are a function of solution ammonium ion concentration

The charge state distribution for oligonucleotides detected using negative ionization electro-spray-mass spectrometry has been studied as a function of solution counterion concentration. In the absence of added buffer, an average charge state (Z) of ?7.2 is observed for a 10 µM aqueous solution of a 14mer DNA at pH 7.0, with [M ? 8H]^8? the most abundant ion. As the solution concentration of ammonium acetate increases from 0.1 to 33 mM, Z shifts to ?3.8 with [M ? 4H]^4? the most abundant charge state. The shift in most abundant charge state from [M ? 7H]^7? to [M ? 4H]^4? occurs abruptly between 1.0 and 10 mM NH₄OAc. Above 100 mM NH₄OAc, the value of Z plateaus at ?3.1, with [M ? 3H]^3? the most abundant charge state. The addition of 1–50 mM glycine to the analyte solution does not alter Z, suggesting that the changes in charge state observed by using ammonium acetate result from a solution equilibration of cations around the DNA strand, rather than nonspecific gas-phase proton transfers during the electrospray process. The fraction of neutralized phosphate groups reaches a maximum of 0.79 ± 0.03 independent of length and sequence.     

Direct Quantitative Analysis of a 20 kDa PEGylated Human Calcitonin Gene Peptide Antagonist in Cynomolgus Monkey Serum Using In-Source CID and UPLC-MS/MS

PEGylation is a successful strategy to improve the pharmacokinetic and pharmaceutical properties of therapeutic peptides. However, quantitative analysis of PEGylated peptides in biomatrix by LC-MS/MS poses significant analytical challenge due to the polydispersity of the polyethylene glycol (PEG), and the multiple charge states observed for both the peptide and PEG moieties. In this report, a novel LC-MS/MS method for direct quantitative analysis of 20 kDa PEGylated CGRP[Cit, Cit] in cynomolgus monkey serum is presented. CGRP[Cit, Cit] is an investigational human calcitonin gene peptide receptor antagonist with amino acid sequence Ac -WVTH[Cit]LAGLLS[Cit]SGGVVRKNFVPT DVGPFAF-NH ₂ . In-source collision-induced dissociation (in-source CID) of 20 kDa PEGylated peptide was used to generate CGRP[Cit, Cit] fragment ions, among which the most abundant b ₈⁺ ion was selected and measured as a surrogate for the 20 kDa PEGylated peptide. A solid phase extraction (SPE) method was used to extract the PEGylated peptides from the biomatrix prior to the UPLC-MS/MS analysis. This method achieved a lower limit of quantitation (LLOQ) of 5.00 ng/mL with a serum sample volume of 100 μL, and was linear over the calibration range of 5.00 to 500 ng/mL in cynomolgus monkey serum. Intraday and interday accuracy and precision from QC samples were within ±15%. This method was successfully applied to a pharmacokinetic study of the 20 kDa PEGylated CGRP[Cit, Cit] in cynomolgus monkeys.     

The enthalpies of interactions of Ca2+(aq) and C2O 4 2− (aq) ions in complexon solutions: Competition between complexation and precipitation

The thermal effects of mixing of aqueous calcium chloride with sodium citrate and ethylenedi-aminetetraacetate in the absence and presence of sodium oxalate have been measured at 25°C. The thermal effects of dilution of aqueous calcium chloride solutions were determined. The thermal effects of calcium oxalate precipitation and formation of calcium complexes with citrate and ethylenediaminetetraacetate ions were calculated. The 1% solution of sodium citrate inhibited the formation of CaC₂O₄ (s); in a 1% solution of sodium ethylenediaminetetraacetate with [Ca²⁺][C₂O ₄ ^2? ] > 10^?5, the endothermal formation of the [CaEdta]^2? complex quickly changed to exothermal precipitation. The 3 and 5% solutions of complexons showed a pronounced inhibiting effect on the formation of urinary stones even when the concentration of calcium and oxalate ions in solution exceeded the product of solubility of CaC₂O₄ by four and more orders of magnitude.     

Formation and structure of pachyman aggregates in dimethyl sulfoxide containing water

The aggregation of pachyman, β-(1 → 3)-D -glucan (M_w = 1.68 × 10⁵) from the Poria cocos mycelia, was investigated using static and dynamic laser light scattering (LLS) in dimethyl sulfoxide (DMSO) containing about 15% water, which leads to large aggregates. Both the time dependence of hydrodynamic radius and the angle dependence of the scattering intensity were used to calculate the fractal dimension (d_f) of the aggregates. The aggregation rate and average size of aggregates increase dramatically with increasing the polymer concentration from 1.7 × 10⁻⁴ g/mL to 8.6 × 10⁻⁴ g/mL, and with the decrease of the solvent quality, that is, water content from 13 to 15%. In the cases, the fractal dimensions change from 1.94 to 2.43 and from 1.92 to 2.54, respectively, suggesting that transforms of aggregation processes: a slow process called reaction-limited cluster aggregation (RLCA) to a fast process called diffusion-limited cluster aggregation (DLCA) in different polymer concentrations and water content. The fractal dimensions above 2 of the fast aggregation is larger than the 1.75 predicted for the ideal DLCA model, suggesting that the aggregation involves a restructuring process through the interchain hydrogen bonding interaction. There are no aggregates of pachyman in DMSO without water, but aggregates formed in the DMSO containing 15% water at 25°C as a compact structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3201–3207, 1999     

1,2-Dichlorobenzene affects the formation of the phosphoenzyme stage during the catalytic cycle of the Ca<Superscript>2+</Superscript>-ATPase from sarcoplasmic reticulum

Background

1,2-Dichlorobenzene (1,2-DCB) is a benzene-derived molecule with two Cl atoms that is commonly utilized in the synthesis of pesticides. 1,2-DCB can be absorbed by living creatures and its effects on naturally-occurring enzymatic systems, including the effects on Ca²⁺-ATPases, have been poorly studied. Therefore, we aimed to study the effect of 1,2-DCB on the Ca²⁺-ATPase from sarcoplasmic reticulum (SERCA), a critical regulator of intracellular Ca²⁺ concentration.

Results

Concentrations of 0.05–0.2 mM of 1,2-DCB were able to stimulate the hydrolytic activity of SERCA in a medium-containing Ca²⁺-ionophore. At higher concentrations (0.25–0.75 mM), 1,2-DCB inhibited the ATP hydrolysis to ~80 %. Moreover, ATP hydrolysis and Ca²⁺ uptake in a medium supported by K-oxalate showed that starting at 0.05 mM,1,2-DCB was able to uncouple the ratio of hydrolysis/Ca²⁺ transported. The effect of this compound on the integrity of the SR membrane loaded with Ca²⁺ remained unaffected. Finally, the analysis of phosphorylation of SERCA by [γ-³²P]ATP, starting under different conditions at 0° or 25 °C showed a reduction in the phosphoenzyme levels by 1,2-DCB, mostly at 0 °C.

Conclusions

The temperature-dependent decreased levels of phosphoenzyme by 1,2-DCB could be due to the acceleration of the dephosphorylation mechanism – E₂P?·?Ca₂ state to E₂ and P_i, which explains the uncoupling of the ATP hydrolysis from the Ca²⁺ transport.

     

Effects of the thermal history and concentration on the aggregation of Erwinia gum in an aqueous solution

The aggregation of Erwinia (E) gum in a 0.2 M NaCl aqueous solution was investigated by multi‐angle laser light scattering and gel permeation chromatography (GPC) combined with light scattering. The GPC chromatograms of five fractions contained two peaks; the fractions had the same elution volume but different peak areas, suggesting that aggregates and single chains coexisted in the solution at 25 °C. The apparent weight‐average molecular weights (M_w) of the aggregates and single chains for each fraction were all about 2.1 × 10⁶ and 7.8 × 10⁴, respectively. This indicates that the aggregates were composed of about 27 molecules of E gum in the concentration range used (1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ g/mL). The weight fraction of the aggregates (w_ag) increased with increasing concentration, but the aggregates still existed even in an extremely dilute solution. The fractionation process and polymer concentration hardly affected the apparent aggregation number but significantly changed w_ag. The E‐gum M_w decreased sharply with an increase in temperature. When the E‐gum solution was kept at 100 °C, w_ag decreased sharply for 20 h and leveled off after 100 h. Once the aggregates were decomposed at a higher temperature, no aggregation was observed in the solution at 25 °C, indicating that the aggregation was irreversible. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1352–1358, 2000     

Effect of imidazolium-based ionic liquids on the aggregation behavior of twin-tailed cationic gemini surfactant in aqueous solution

Micellization behavior of the twin-tailed surfactants can be modulated by the addition of various modifiers. Ionic liquids (ILs) are one of them and are documented here. The beauty of these environmentally benign neoteric molecules lies in their structural versatility. Here, we have investigated the effect of three ILs: 1-butyl-3-methylimidazolium bromide ([C₄mim][Br]), 1-hexyl-3-methylimidazolium bromide ([C₆mim][Br]), and 1-octyl-3-methylimidazolium bromide ([C₈mim][Br]) on the aggregation and surface adsorption behavior of cationic gemini surfactant, bis(hexadecyldimethyl ammonium)propane dibromide (16-3-16) through experimentally measured electrical conductivities, surface tensions, and by spectral methods (UV-vis absorbance and fluorescence measurements). The main focus of the study is to observe the effect of added ILs on the critical micelle concentration (cmc), various surface parameters, aggregation number, and size of the aggregates of gemini surfactant. The results show that the more hydrophobic ILs, that is, [C₆mim][Br] and [C₈mim][Br] behave as electrolyte at lower concentration and cosurfactant at higher concentration, whereas moderately hydrophobic IL [C₄mim][Br] acts as an electrolyte at all concentration ranges studied. The modulating effects of ILs were also compared with conventional electrolyte (NaBr) at similar conditions.     

Application of capillary electrophoresis to the uranyl/citrate system in moderately acidic solutions

The capillary electrophoresis method has been applied to the speciation study of uranium(VI) at room temperature, in 0.02M citrate buffer solutions, at pH values between 2.5 and 5.5 and at citrate/U ratios between 20 and 40. No negatively charged species have been pointed out at pH values lower than 3. For a pH value higher than 5, the electropherograms are ill-defined and the signals cannot be analyzed simply (owing to a high and rough baseline). In the pH range 3–5, up to 4 peaks can be attributed to U(VI) species. Two of them are likely due to the expected monomer [(UO₂)(Cit)]⁻and dimer [(UO₂)₂(Cit)₂]²⁻ complex species and these species are shown to be in quasi-equilibrium with two other species possessing slightly lower migration velocities, [(UO₂)H(Cit)(OH)]⁻ and [(UO₂)₂H₂(Cit)₂(OH)₂]²⁻, respectively. Speciation diagrams calculated by an exact analytical approach are proposed in order to explain the experimental results. A complete agreement between theoretical and experimental results needs to take into account kinetic and hydrolysis effects.     

Modulation of the aggregation behaviour and physicochemical properties of 1-dodecyl-3-methylimidazolium bromide in aqueous solutions by β-CD

The modulation of aggregation behaviour of ionic liquids (ILs) in aqueous media is one of the important research topics. In the present work, aggregation behaviour of 1-dodecyl-3-methylimidazolium bromide ([C₁₂mim]Br) modulated by beta-cyclodextrin (β-CD) has been investigated by using conductivity, volume, fluorescence, dynamic light scattering and transmission electron microscopy techniques. The results suggested that the addition of β-CD significantly affects the aggregation of [C₁₂mim]Br in aqueous solutions. For example, the apparent critical micelle concentration increases with the increase of β-CD concentration; the average micellar size reduced with the increasing concentration of β-CD, and the process for micelle formation of [C₁₂mim]Br in aqueous β-CD solution is driven by entropy at lower temperature, while driven by enthalpy at higher temperature. It is expected that findings in this study would shed light on the potential applications of IL in supramolecular chemistry.     

Critical Reappraisal of Methods for Measuring Urine Saturation with Calcium Salts

Background: Metabolic and physicochemical evaluation is recommended to manage the condition of patients with nephrolithiasis. The estimation of the saturation state (β values) is often included in the diagnostic work-up, and it is preferably performed through calculations. The free concentrations of constituent ions are estimated by considering the main ionic soluble complexes. It is contended that this approach is liable to an overestimation of β values because some complexes may be overlooked. A recent report found that β values could be significantly lowered upon the addition of new and so far neglected complexes, [Ca(PO₄)Cit]⁴⁻ and [Ca₂H₂(PO₄)₂]. The aim of this work was to assess whether these complexes can be relevant to explaining the chemistry of urine. Methods: The Ca–phosphate–citrate aqueous system was investigated by potentiometric titrations. The stability constants of the parent binary complexes [Cacit]⁻ and [CaPO₄]⁻, and the coordination tendency of PO₄³⁻ toward [Ca(cit)]⁻ to form the ternary complex, were estimated. β_CaOx and β_CaHPO4 were then calculated on 5 natural urines by chemical models, including or not including the [CaPO₄]⁻ and [Ca(PO₄)cit]⁴⁻ species. Results: Species distribution diagrams show that the [Ca(PO₄)cit]⁴⁻ species was only noticeable at pH > 8.5 and below 10% of the total calcium. β values estimated on natural urine were slightly lowered by the formation of [CaPO₄]⁻ species, whereas [Ca(PO₄)cit]⁴⁻ results were irrelevant. Conclusions: While [CaPO₄]⁻ species have an impact on saturation levels at higher pHs, the existence of ternary complex and of the dimer is rejected.     

High surface area nano-sized La0.6Ca0.4MnO3 perovskite powder prepared by low temperature pyrolysis of a modified citrate gel

Single phase nanocrystalline La_0.6Ca_0.4MnO₃ powder was synthesized by both the usual and a modified citrate gel precursor method, and the effects on the formation of homogeneous nano-sized powder with a perovskite structure were investigated. In the modified method, single phase La_0.6Ca_0.4MnO₃ powder with an average particle size of 17.2 nm was obtained when the powder was pyrolyzed at 520°C for 2 h. Its specific surface area was 40.7 m² g⁻¹, about 4-fold larger than that of powder made by the usual citrate gel method.