H. Füredi-Milhofer

Adsorption of human serum albumin on precipitated hydroxyapatite

Abstract The adsorption of human serum albumin on precipitated stoichiometric hydroxyapatite Ca10(PO4)6(OH)2, /CaHA/ was followed in a wide pH range. Adsorption was irreversible toward dilution; the isotherms were not of the Langmuir type, but of a stepwise nature. The amount of HSA adsorbed was found to be dependent on the pH and the ionic strength. It decreased with increasing pH, thus indicating the effect of charge repulsion. This effect could be counteracted by increasing the ionic strength, i.e., the concentration of the neutral electrolyte, potassium nitrate. Partial desorption could be effected by increasing the pH of the solution. Adsorption of human serum albumin decreased with increasing temperature.

Precipitation of calcium oxalates from high ionic strength solutions: I. Kinetics of spontaneous precipitation of calcium oxalate trihydrate

Abstract A method for analysing kinetic precipitation experiments in terms of the dominant rate controlling processes is presented. The method has been applied to the precipitation of calcium oxalate trihydrate from 0.3 mol dm −3 sodium chloride solutions. The experiments have been carried out at constant temperature (25°C) in the range of total initial reactant concentrations 5.5×10 −4 ≤[Ca] init =[C 2 O 4 ] init ≤1.6×10 −3 . The precipitation was followed solution concentration measurement (calcium selectrode) and particle number and size analysis (Coulter counter). The precipitation was initiated by heterogeneous nucleation. On the rate versus supersaturation curves three parts could be distinguished in which heterogeneous nucleation, crystal growth, or growth and aggregation were dominant. The curves are described by the equation: (dβ/dt)β −2 3 =KN 1 3 t (c t −c s ) n , where α is the fraction precipiated, N t is the number of particles, c t − c s is the supersaturation, K is a constant and n is the order of the reaction. For dominant crystal growth, n =3.6±0.3 and log (K′/dm 12 mol −4 s −1 )= log KN s 1 3 =9.81±0.62 . Fu rther analysis shows a linear dependence of the linear growth rate on 1/ ln ( c t / c s ), which indicates that the rate controlling process is surface diffusion or integration of the growth unit into the crystal lattice.

Precipitation of calcium phosphates from electrolyte solutions. V. The influence of citrate ions.

SummaryThe influence of citrate ions on the precipitation of crystalline apatitic precipitates with low Ca/P molar ratios [octacalcium phosphate (OCP) and calcium-deficient apatites (DA) (system A)] and of the intercrystalline mixtures of calcium hydrogen phosphate dihydrate (DCPD) and DA (system B) was investigated. Samples were prepared by direct mixing of calcium chloride solutions (A, 6·10−3 mol dm−3; B, 1·10−1 mol dm−3) and sodium phosphate solutions (A, 6·10−3 mol dm−3; B, 2·10−2 mol dm−3) containing citrate (0–2·10−3 mol dm−3) and preadjusted to pH 7.4.In the presence of citrate ions: (a) crystal growth of OCP and DA was slowed down; (b) habit modification of DCPD crystals occurred; and (c) equilibration in intercrystalline mixtures of DCPD and DAs was slowed down. All phenomena were caused by surface adsorption of negatively charged ions, most probably CaC6H5O7-, which is the prevalent calcium citrate species under the given experimental conditions. Habit modification of DCPD was induced by preferential adsorption at the (001) crystal plane.

Influence of gelatin on the precipitation of amorphous calcium phosphate

Abstract The influence of gelatin on the formation, stability and transformation of amorphous calcium phosphate, ACP, at 298 K and initial pH 7.4 was investigated. Precipitation of ACP was brought about from equimolar solutions (3×10 −3 mol dm −3 ) of calcium chloride and sodium phosphate while the concentration of gelatin varied from 0.0001 to 1.12 g dm −3 . The interaction between gelatin and ACP was characterized by particle electrophoretic mobility, pH and turbidity measurements. The ACP electrophoretic mobility during the precipitation in the presence of gelatin indicated gelatin adsorption onto ACP as the dominating interaction mode. By adsorbing on the surface of primary particles gelatin slows down or even prevents ACP aggregation thus stabilizing its colloidal state. Besides, the presence of gelatin promotes the transformation of ACP into a crystalline phase.

Temperature-programmed dehydration of hydroxyapatite

Abstract The temperature-programmed dehydration of several near-stoichiometric hydroxyapatites from aqueous medium has been investigated over the range from 20 to 700°C at constant heating rate. In most of this work the sample was subjected to a vacuum of the order of 10−6 to 10−7 Torr and the effluent water vapor was detected by a mass spectrometer. We shall designate this as the MTA procedure. We find two major peaks in the MTA spectra for all these samples from aqueous medium. The first, at 90–100°C, is attributed to desorption of reversibly adsorbed water on the external surfaces; and the second, at 225–260°C, we attribute to the irreversible removal of water from ultrafine pores in the solid. The MTA technique has been employed to demonstrate that hydroxyapatite in the hydrated state undergoes H2O2O exchange when soaked in heavy water. This applies to both the 90–100°C peaks and the 225–260°C peaks. It is suggested that the considerable variation in the peak temperatures observed over the higher range from 221 to 257°C may be attributed to a difference in pore diameters, and that we must attain a higher temperature to empty a narrower pore.

Precipitation of calcium oxalates from high ionic strength solutions III. the influence of reactant concentrations on the properties of the precipitates

The influence of the initial reactant concentrations ([Ca]=1×10-5-3×10-1 mol dm-3, [Ox]=1×10-5-1×10-1 mol dm-3) and the aging time on the composition and morphology of calcium oxalates precipitating from 0.3 mol dm-3 sodium chloride solutions at pH=6.5±0.2, 298 K was investigated. In the whole investigated concentration region only monoclinic calcium oxalate monohydrate (COM) and tetragonal dihydrate (COD, CaC2O4·(2+x)H2O, x=0.25 or 0.5) were identified. The relative amounts of these species and their morphologies were dependent on the initial supersaturation, the reactant concentration ratio and the aging time. Compact crystals, dendrites and microcrystalline aggregates have been obtained at low, medium and high supersaturations respectively. Excess calcium and/or oxalate ions inhibited recrystallization of COD into the thermodynamically stable COM. The results are summarized in the form of precipitation curves and diagrams. Thermograms and IR spectra (in the region of 4000-200 cm-1) of COM, COD and the triclinic trihydrate (COT) are compared and discussed. The position and shape of IR absorption bands are defined by the crystal structure, the number of water molecules present and the system of hydrogen bonds.

Precipitation of calcium oxalate from high ionic strength solutions VII. The influence of glutamic acid

Abstract The influence of glutamic acid on calcium oxalate precipitation was investigated. The model system was a mixture of different hydrates of spontaneously precipitating calcium oxalate, e.g. calcium oxalate monohydrate, calcium oxalate trihydrate and less than 5% of calcium oxalate dihydrate. Addition of glutamic acid into the precipitating system affected the composition of the solid phase by increasing the content of COM by about 20%. In addition in the presence of glutamic acid the rate of crystal growth was lower and substantial aggregation was observed. The latter effects were strongest in a certain narrow range of low concentrations (3–5 ppm) of glutamic acid.

Factors Influencing Nucleation From Solutions Supersaturated to Different Crystal Hydrates

Abstract The influence of the reactant concentrations, the stirring mode and some additives (serine, Ser, glutamic acid, Glu, ornithine, Orn, tryptophan, Trp, histidin, His, sodium dodecyl sulphate, SDS) on the mode of nucleation of calcium oxalate hydrates (the thermodynamically stable monoclinic monohydrate, COM, the metastable tetragonal dihydrate, COD, and triclinic trihydrate, COT) and of dodecylammonium nitrate, DDA on the formation of the hexagonal and cubic modifications of AgI has been investigated. To obtain the above information, changes in the induction times, the number of particles, the volume and composition of the precipitates have been considered. The change in the mechanism of nucleation from heterogeneous to homogeneous (achieved as a consequence of an increase in supersaturation) has a striking effect not only on the particle number and size distribution, but also on the hydration of the crystalline phase. The interfacial energies and radii of the homogeneous nuclei (as determined from the critical concentration for homogeneous nucleation) are σ(COD) = 123 ± 6 mJ m−2, r* (COD) = 0.56 nm, σ(COT) = 135 ± 6 mJ m−2 and r* (COT) = 0.54 nm respectively. For the heterogeneous nucleus σHET = 53 ± 5 mJ m−2 has been determined from induction periods. In the range of reactant concentrations where heterogeneous nucleation is dominant, the mode of agitation influenced the induction time and the type of the dominant nucleating phase. Introducing stirring promoted the formation of COT whereas in unstirred systems COM was preferentially formed. The addition of amino acids and/or surfactants also influenced the composition of the nascent precipitates. Ser, Glu, Orn promoted the formation of COM while in the presence of Trp, His and SDS the formation of COD was enhanced. DDA promoted crystallization of the cubic modification of AgI whereas without the surfactant the hexagonal form prevailed. The effect of the surfactants (SDS and DDA) was related to the respective c.m.c.

Effect of Tamm-Horsfall Protein on Calcium Oxalate Precipitation

The effect of Tamm-Horsfall protein isolated from urine of healthy subjects on calcium oxalate precipitation was studied in model systems of precipitation. The study was performed using following conditions: concentrations of calcium chloride 10 mmol/l, sodium chloride 150 mmol/l, oxalic acid 300 mumol/l; pH 6.0, and temperature 310 K. The concentration of Tamm-Horsfall protein varied between 1-10 mg/l. The kinetics of calcium oxalate precipitation was observed by measuring the number and volume of particles in the suspension, and the precipitate composition by an optic microscope. In all the studied systems, the precipitate morphology corresponded to pure calcium oxalate monohydrate. Tamm-Horsfall protein was found to inhibit the growth of calcium oxalate monohydrate crystals and stimulate their aggregation in the given experimental conditions. Both effects were enhanced by increase in the concentrations of Tamm-Horsfall protein and were most pronounced at the concentration of Tamm-Horsfall protein of 10 mg/l.

Influence of amino acids on the precipitation kinetics of calcium oxalate monohydrate

Abstract Precipitation kinetics of calcium oxalate monohydrate has been studied by monitoring the concentration variation of calcium ions in the solution with a selective electrode. The experimental data for both pure oxalate solutions and solutions containing 7 to 56 ppm of glutamic acid indicate that two-dimensional nucleation is likely to be the growth controlling mechanism. Supporting this assertion is the fact that the value of the ledge (interface) free energy, determined from the constant A i in the exponential law, agrees quite well with those obtained by other authors in similar systems. Furthermore, the same constant A i turns out to be independent of the presence of additives in the solution, their inhibiting effect being due to variations of the pre-exponential factor K i only. As in other known cases, the specific adsorption of additives at the ledges of the two-dimensional nuclei, leading to decrease of the ledge free energy, has not been evidenced.