Vibrational studies of calcium oxalate monohydrate (whewellite) and an anhydrous phase of calcium oxalate

The IR spectrum of calcium oxalate monohydrate is re-analysed, and the Raman spectrum presented for the first time. The IR and Raman spectra of an anhydrous phase of calcium oxalate are discussed in relation to the effects of dehydration and compared with the monohydrate.     

Simulation of calcium oxalate stone in vitro

Urolithiasis constitutes a serious health problem that affects a significant section of mankind. Between 3% and 14% of the population, depending on the geographical region, suffer from this illness[1]. For example, the incidence of urolithiasis in Florida in the United States of America was 15.7 in 100000 people and increased to 20.8 in 1996. Urolithiasis remains a major medical prob-lem in China, especially in Guangdong Province. A survey in 1997 in Shenzhen City, the most southern city i…     

Thermal decomposition of calcium oxalate: beyond appearances

The goal of this study is twofold: to take a fresh look at the decomposition of calcium oxalate and to warn users of thermogravimetric analysis against the hasty interpretation of results obtained. Since the pioneer work of Duval 70 years ago, the scientific community has agreed unanimously as to the decomposition of anhydrous calcium oxalate (CaC₂O₄) into calcium carbonate (CaCO₃) and CO gas, and that of the calcium carbonate into calcium oxide (CaO), and CO₂ gas. We will demonstrate how these reactions, simple in appearance, in fact result from a succession of reactive phenomena involving numerous constituents both solid (CaCO₃, free carbon) and gaseous (CO₂ and CO) produced by intermediary reactions. The mass losses evaluated in the two distinct domains correspond closely to the molar masses of CO and CO₂, respectively. The simple mathematical calculation of that mass loss has simply concealed the existence of other reactions, and, most particularly the Boudouard reaction and that of solid phases between CaCO₃ and C. It just goes to show that appearances can be deceiving.

     

Dissolved urate salts out calcium oxalate in undiluted human urine in vitro: implications for calcium oxalate stone genesis

Hyperuricosuria has long been documented as a predisposing factor to calcium oxalate (CaOx) stone pathogenesis. However, its mechanism is still without sound scientific foundation. Previously, we showed that hyperuricosuria, simulated by the addition of dissolved sodium urate, promotes the crystallization of CaOx. In the present study, we demonstrate that the urate's effect on the crystallization is attributable to its salting out CaOx from solution. Furthermore, analysis of urines revealed that their metastable limit decreased with increases in the product of the prevailing concentrations of calcium and urate: this has implications for CaOx stone genesis. We also outline anti-salting out strategies for future research for the prevention and/or treatment of CaOx calculi.     

Analysis of calcium, oxalate, and citrate interaction in idiopathic calcium urolithiasis in children

The majority of urinary stones in children are composed of calcium oxalate. To investigate the interaction between urinary calcium, oxalate, and citrate as major risk factors for calcium stones formation, their 24-h urinary excretion was determined in 30 children with urolithiasis and 15 normal healthy children. The cutoff points between children with urolithiasis and healthy children, accuracy, sensitivity, and specificity for each risk factor alone as well as for all three taken together were determined. OneR and J4.8 classifiers as parts of the larger data mining software Weka, based on machine learning algorithms, were used for the determination of the cutoff points for differentiation of the children. The decision tree based on J4.8 classifier analysis of all three risk factors together proved to be the best for differentiating stone formers from normal children. In comparison to the accuracy of the differentiation after calcium and oxalate of 80% and 75.6%, respectively, the decision tree showed an accuracy of 97.8%. Even when its stability was tested by the leave-one-out cross-validation procedure, the accuracy remained at a very acceptable percentage of 93.2% correctly classified patients. J4.8 classifier analysis gave a look inside urinary calcium, oxalate, and citrate interaction. Urinary calcium excretion was shown as the most informative in discrimination of the children with urolithiasis from healthy children. However, it was shown that oxalate and citrate excretions might influence the stone formation in a subpopulation of the stone formers. In patients with low urinary calcium, a major role in lithogenesis belongs to oxalate, in some of them alone and in others in conjunction with citrate. Decreased urinary citrate excretion in the presence of increased oxalate excretion may lead to stone formation.     

Theoretical analysis of electronic and structural properties of anhydrous calcium oxalate

The results of theoretical analysis of the electronic and crystal structural properties and bonding in relation to thermal decomposition process in anhydrous calcium oxalate are presented. The methods used in this analysis—topological analysis of electron density (Bader’s Quantum Theory of Atoms in Molecules approach) obtained from DFT calculations performed by Wien2k package (Full Potential Linearized Augmented Plane Wave Method); bond order model (Cioslowski&Mixon), applied to topological properties of the electron density; as well as Brown’s Bond Valence Model (bonds valences and strength’, and bond and crystal strains, calculated from crystal structure and bonds lengths data) are described. The analysis of the obtained results shows that these methods allow us to explain the way of thermal decomposition process of anhydrous calcium oxalate to calcium carbonate as a decomposition product, and to describe the structural transition taking place during such process (from monoclinic anhydrous CaC₂O₄ to rhombohedral calcite structure). In the light of the results of our similar calculations performed previously for other anhydrous oxalates (zinc, cadmium silver, cobalt, and mercury) the proposed theoretical approach can be considered as promising and reliable tool, which allow analyzing the properties of the structure and bonding and hence predicting the most probable way of thermal decomposition process for given crystal structure.     

Analytical investigation of calcium oxalate films on marble monuments

This paper describes an analytical approach to investigate the origin of oxalate films on marble. Calcium oxalate films were collected on buildings of historical importance in Lucca and Pisa (Italy) and characterised by optical microscopy (OM), scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy (equipped with diamond cell), and gas chromatography-mass spectrometry (GC-MS). The morphology of the films was investigated by optical and electronic microscopy. FTIR analyses highlighted the presence of calcium oxalate (both as whewellite and weddellite), gypsum, calcite, nitrates, silicates and apatite, while EDX maps showed the distribution of elements. Several samples showed traces of organic compounds, identified by GC-MS as paraffin wax, lipids of animal origin and egg. The correlation between organic material and oxalate contents suggests the origin of the films from degradation processes of past surface treatment.     

Electrophoretic behaviour of calcium oxalate monohydrate in liquid mixtures

An experimental investigation on electrophoresis of calcium oxalate monohydrate in several liquid mixtures (methanol-, ethanol-, 2-propanol-, and methanol-acetonewater) is described.The electrophoretic transport is considered from the view-point of the Thermodynamics of Irreversible Processes. Linear relations have always been found between the mass flux and the applied electric field. The -potential of the interface C.O.M./liquid mixture has been estimated by using the classial theory of electrophoretic mobility of colloidal particles. The influence of the composition of the mixture on the electrophoretic coefficient has been discussed on the basis of the variation of -potential, viscosity and dielectric constant with the molar fraction of alcohol for binary mixtures and acetone for ternary mixtures. The concentration dependence of the electrophoretic coefficient is found to be linear only in the latter case.     

Biogenic synthesis of calcium oxalate crystal by reaction of calcium ions with spinach lixivium

In this paper, we report a biogenic synthesis protocol for preparation of calcium oxalate (CaC₂O₄, CaOx) crystal at room temperature by a simple protein-mediated reaction of aqueous Ca²⁺ ions with the C₂O₄²⁻ ions spontaneously released from spinach. The aggregation of calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) with a rod-like morphology was mainly formed in the spinach root lixivium, and the proportion of COM crystal in the aggregation increased with the concentration of Ca²⁺ ions increasing, however, spindle-shaped crystal was mainly obtained in the spinach leaf lixivium and the content of COM in the product was higher than that obtained in the root lixivium with the similar concentration of Ca²⁺ ions. COM phase disappeared and only COD crystal with morphology of tetragonal bipyramidal prisms presented in the product when the leaf lixivium was replaced by the leaf broth. The biomolecules such as proteins with molecular weight of 31 kDa liberated from the spinach root are negative-charged, which played important roles for the control of CaOx crystal growth in the root lixivium corresponding to the changes of protein secondary structures after reaction with Ca²⁺ ions. This research was potentially important for unraveling the biomineralization mechanism of CaOx crystal.     

Enthalpies and heat capacities of dissolution for calcium chloride and sodium oxalate

The heats of solution at 25 and 40°C in water are measured for various concentrations of calcium chloride and sodium oxalate. The standard enthalpies of solution are determined using the Debye-Hückel second approximation. The heat capacity change upon dissolution and the partial molal heat capacities of electrolytes at infinite dilution are calculated.     

Synthesis of different polymorphic modifications of calcium oxalate by electrodeposition

Different hydrates of calcium oxalate have been electrodeposited by electrogeneration of acid at the anode from an EDTA-stabilized calcium nitrate bath containing dissolved oxalate ions. The deposition is controlled by varying the bath pH, temperature, and current density. Formation of metastable CaC₂O₄·2H₂O is favored at high current densities at ambient temperature, whereas the thermodynamically stable CaC₂O₄·H₂O is formed at elevated bath temperatures. Both the polymorphs show oriented growth with respect to the substrate normal under different deposition conditions.     

Precipitation of calcium oxalate from homogeneous solution by cation release

Calcium oxalate can be precipitated from homogeneous solution by oxidation of the calcium/EDTA complex with hydrogen peroxide in boiling solution, in the presence of oxalate ion, at pH 6-8. The method gives large crystals and enables calcium to be determined in the presence of lead, which remains complexed.     

Modulation of calcium oxalate crystallization by linear aspartic acid-rich peptides

Calcium oxalate monohydrate (COM) kidney stone formation is prevented in most humans by urinary crystallization inhibitors. Urinary osteopontin (OPN) is a prototype of the aspartic acid-rich proteins (AARP) that modulate biomineralization. Synthetic poly(aspartic acids) that resemble functional domains of AARPs provide surrogate molecules for exploring the role of AARPs in biomineralization. Effects of linear aspartic acid-rich peptides on COM growth kinetics and morphology were evaluated by the combination of constant composition (CC) analysis and atomic force microscopy (AFM). A spacer amino acid (either glycine or serine) was incorporated during synthesis after each group of 3 aspartic acids (DDD) in the 27-mer peptide sequences. Kinetic CC studies revealed that the DDD peptide with serine spacers (DDDS) was more than 30 times more effective in inhibiting COM crystal growth than the DDD peptide with glycine spacers (DDDG). AFM revealed changes in morphology on (010) and (-101) COM faces that were generally similar to those previously described for OPN and citrate, respectively. At comparable peptide levels, the effects of step pinning and reduced growth rate caused by DDDS were remarkably greater. In CC nucleation studies, DDDS caused a greater prolongation of induction periods than DDDG. Thus, nucleation studies link changes in interfacial energy caused by peptide adsorption to COM to the CC growth and AFM results. These studies indicate that, in addition to the number of acidic residues, the contributions of other amino acids to the conformation of DDD peptides are also important determinants of the inhibition of COM nucleation and growth.     

电石制备过程中不同含钙化合物与焦炭的反应行为研究

针对粉状原料-氧热法电石生产新工艺的需求,通过程序升温法研究了多种粉状含钙原料与粉状焦炭直接反应制备电石的过程。结果表明,升温过程中CaCO3释放的CO2、Ca(OH)2和电石渣释放的H2O对焦炭的量和质的影响很小,CaCO3、Ca(OH)2和电石渣均可直接用于电石生产,电石生成反应自1 450℃开始,1 740℃左右达到峰值。CaSO4与焦炭在920℃左右反应形成CaS,在研究的温度范围内CaS不与焦炭反应。     

Thermodynamics of the thermal decomposition of calcium oxalate monohydrate examined theoretically

Geometries and energies of isolated CaC₂O₄·H₂O, CaC₂O₄, CaCO₃, CaO, H₂O, CO and CO₂ were determined at the ab initio level using effective core potential valence basis sets of doublezeta quality, supplemented with polarization functions. The effects of electron correlation were taken into account at the second order Møller-Plesset level of theory. For CaC₂O₄·H₂O, the correlation for the basis set superposition error was also included. Common routines were employed to evaluate entropies, heat capacities, as well as enthalpies and free enthalpies of formation of all entities. The enthalphies and free enthalpies of consecutive dehydration of CaC₂O₄·H₂O, decarbonylation of CaC₂O₄ and decomposition of CaCO₃ towards CaO and CO₂ were determined on the basis of avialable data from the literature or those predicted thoretically. Assuming that upon all the above mentioned processes the system maintains equilibrium, the fractions reacted, enthalpy changes and differential dependencies of thesevs. temperature were derived and compared with experimental thermoanalytical data.     

Highly efficient preparation of aryl beta-diketo acids with tert-butyl methyl oxalate

An improved and efficient oxalylation of aryl methyl ketones was accomplished with tert-butyl methyl oxalate. This is the key step in constructing the pharmacophore of aryl beta-diketo acids, which represent a promising new class of HIV-1 integrase inhibitors. Structurally diverse aryl beta-diketo acids, including bisdiketo acids, can be prepared rapidly in impressive yields under mild conditions with this method. Advantages over conventional methods with dimethyl (or diethyl) oxalate were observed in both yield and reaction time.     

Phosphorylation of osteopontin is required for inhibition of calcium oxalate crystallization

Under near-physiological pH, temperature, and ionic strength, a kinetics constant composition (CC) method was used to examine the roles of phosphorylation of a 14 amino acid segment (DDVDDTDDSHQSDE) corresponding to potential crystal binding domains within the osteopontin (OPN) sequence. The phosphorylated 14-mer OPN peptide segment significantly inhibits both the nucleation and growth of calcium oxalate monohydrate (COM), inhibiting nucleation by markedly increasing induction times and delaying subsequent growth by at least 50% at concentrations less than 44 nM. Molecular modeling predicts that the doubly phosphorylated peptide binds much more strongly to both (-101) and (010) faces of COM. The estimated binding energies are, in part, consistent with the CC experimental observations. Circular dichroism spectroscopy indicates that phosphorylation does not result in conformational changes in the secondary peptide structure, suggesting that the local binding of negatively charged phosphate side chains to crystal faces controls growth inhibition. These in vitro results reveal that the interactions between phosphorylated peptide and COM crystal faces are predominantly electrostatic, further supporting the importance of macromolecules rich in anionic side chains in the inhibition of kidney stone formation. In addition, the phosphorylation-deficient form of this segment fails to inhibit COM crystal growth up to concentrations of 1450 nM. However, at sufficiently high concentrations, this nonphosphorylated segment promotes COM nucleation. Dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) results confirm that aggregation of the nonphosphorylated peptide segment takes place in solution above 900 nM when the aggregated peptide particles may exceed a well-defined minimum size to be effective crystallization promoters.