Investigations of Complex Precipitation Systems

A survey of methods available for investigations of complex precipitation systems under dynamic conditions and under conditions of stable and metastable equilibrium is represented. Examples given pertain to the calcium and magnesium phosphate and calcium oxalate precipitation systems. Solubility curves as well as precipitation curves and diagrams yield information on the influence of reactant concentrations (and/or concentrations of other solution constituents) on the properties of precipitates. Characteristic parts of precipitation diagrams are the precipitation boundary (boundary between metastable and unstable solutions) and the boundary between the concentration regions within which heterogeneous and homogeneous nucleation respectively prevail. At equilibrium the precipitation boundary enables calculations of solubility and complex stability constants, otherwise it yields information on the kinetics of mononuclear crystal growth. From the heterogeneous/homogeneous nucleation boundary the critical supersaturation for homogeneous nucleation and the interfacial energy and critical radius of the respective homogeneous nucleus may be determined. Kinetic experiments give information on the rates and mechanisms of the rate determining precipitation processes involved. It has been shown that in the concentration region of heterogeneous nucleation crystal growth and subsequent (or simultaneous) aggregation are rate determinant, whereas in the homogeneous nucleation region aggregation of particles is dominant in all stages of precipitate formation and the formation of colloids (hydrophobic precipitates) and highly hydrated precursors (hydrophilic precipitates) may be expected. Examples of kinetic curves pertaining to the heterogeneous and homogeneous nucleation region respectively are presented

Kinetics of Precipitation and Cryst~l Growth of Dicalcium Phosphate Dihydrate

The kinetics of the precipitation and crystal growth of dicalcium phosphate dihydrate (DCPD) was followed in 0.15 M sodium chloride solutions at constant pH (pH = 5.0) and temperature (25o C). Precipitations were performed from equimolar solutions of calcium chloride and sodium phosphate prepared by direct mixing of the reactants. The amount of precipitate formed (X moles per 1) was calculated from the quantity of sodium hydroxide added by a pH-stat device. The system is of physiological interest, because precipitation of DCPD may play an important role during dental caries formatio

An Apparatus for Synchronized Precipitation under Sterile Conditions

The apparatus for .synchronized precipitation devised earlier1 enables the experimenter to achieve extremely slow and controlled mixing of the reactants and to avoid local supersaturations

Nucleation of Calcium Phosphate from Solutions at Physiological pH

In a previous paper1 it was shown that when phase transition from aqueous solutions occurs at metastable equilibrium, the stoichiometry of the nucleating phase can be conveniently evaluated from the precipitation boundary2 • The method was applied to calcium phosphates nucleating in the pH range 6.3 , to 9.1 from solutions containing calcium chloride and sodium phosphate at constant total phosphate concentration 1 X 10-3 M. The calcium phosphate ratio of the nucleating phase was found to be 1.5 and the formation of an amorphous metastable precursor to hydroxyap atite was concluded. Because of its implications on biological calcification it was of interest to reexamine this result restricting the experiments to the physiological pH region (pH 7.25-7.5), but taking into account possible variations of total phosphate and total calcium concentrations. This was done as part of more extensive investigations of the precipitation of calcium phosphates from aqu eous solutions and from media containing 0.15 M sodium chlorid e

Roles of Electrostatics and Conformation in Protein-Crystal Interactions

In vitro studies have shown that the phosphoprotein osteopontin (OPN) inhibits the nucleation and growth of hydroxyapatite (HA) and other biominerals. In vivo, OPN is believed to prevent the calcification of soft tissues. However, the nature of the interaction between OPN and HA is not understood. In the computational part of the present study, we used molecular dynamics simulations to predict the adsorption of 19 peptides, each 16 amino acids long and collectively covering the entire sequence of OPN, to the {100} face of HA. This analysis showed that there is an inverse relationship between predicted strength of adsorption and peptide isoelectric point (P<0.0001). Analysis of the OPN sequence by PONDR (Predictor of Naturally Disordered Regions) indicated that OPN sequences predicted to adsorb well to HA are highly disordered. In the experimental part of the study, we synthesized phosphorylated and non-phosphorylated peptides corresponding to OPN sequences 65–80 (pSHDHMDDDDDDDDDGD) and 220–235 (pSHEpSTEQSDAIDpSAEK). In agreement with the PONDR analysis, these were shown by circular dichroism spectroscopy to be largely disordered. A constant-composition/seeded growth assay was used to assess the HA-inhibiting potencies of the synthetic peptides. The phosphorylated versions of OPN65-80 (IC50 = 1.93 µg/ml) and OPN220-235 (IC50 = 1.48 µg/ml) are potent inhibitors of HA growth, as is the nonphosphorylated version of OPN65-80 (IC50 = 2.97 µg/ml); the nonphosphorylated version of OPN220-235 has no measurable inhibitory activity. These findings suggest that the adsorption of acidic proteins to Ca2+-rich crystal faces of biominerals is governed by electrostatics and is facilitated by conformational flexibility of the polypeptide chain

Quantitative Assessment of the Effect of Additives on Nucleation, Growth and Aggregation of Crystals

Among the physico-chemical problems associated with biological mineralization the determination of the rates and mechanisms of nuclation, crystal growth and aggregation and the assessment of the effect of potential inhibitors/ promotors thereon deserves particular attention", A typical »in vivo« situation in which these processes are initiated in supersaturated, unseeded solution is crystalluria which is considered to be the initial step in renal stone formation"