engleski

Comparative study of amorphous phases formation and transformation in calcium phosphate and calcium carbonate precipitation systems

A number of recent studies pointed out that the amorphous calcium phosphate (ACP) or amorphous calcium carbonate (ACC) are the first step in either, invertebrate or vertebrate hard tissues formation. Therefore, the renewed interest for in vitro and in vivo investigations of mechanisms of their formation and transformation is not surprising. In spite of the fact that the calcium carbonates and calcium phosphates occur in different organisms, there are a number of similarities in mechanisms of their formation and transformation, as well as in their role in organisms. A number of researchers point out the usefulness of comparing those two systems, but this is still not fully exploited approach. In order to contribute to elucidation of a role of amorphous phases, the physical chemical properties of ACP and ACC formed in the systems at defined and comparable initial experimental conditions (supersaturation, constituent ions ratio, ionic strength and presence of relevant inorganic additives) have been studied. At that, the formation and transformation of ACP and ACC in three model precipitation systems of increased complexity were investigated: (a) system containing only constituent ions, (b) system with increased ionic strength and (c) system containing physiologically relevant co-ions. The experiments were initiated by mixing the reactant solutions at strictly controlled hydrodynamic and thermodynamic conditions, while the progress of reaction was continuously followed potentiometrically. Precipitates obtained after up to 10 minutes were isolated and analysed by means of IC, PXRD, FT-IR, TEM, EPR and TG. The results have shown that ACP is more stable and nucleated at lower relative supersaturations in comparison to ACC formation. Typically, ACC preceded the formation of either, thermodynamically stable calcite or metastable vaterite, aragonite and/or monohydrocalcite, while ACP always transformed to metastable calcium deficiente apatite. The exact transformation mechanisms were found to depend on complexity of the precipitation systems, while the magnesium ions were found to strongly stabilize both amorphous phases.

amorphous calcium phosphate (ACP) ; amorphous calcium carbonate (ACC)

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