Abstract
Dicalcium phosphate dihydrate (DCPD) is one of the mineral phases indicated as possible precursors of biological apatites and it is widely employed in the preparation of calcium phosphate bone cements. Herein, we investigated the possibility to functionalize DCPD with aspartic acid (ASP) and poly-aspartic acid (PASP), as models of the acidic macromolecules of biomineralized tissues, and studied their influence on DCPD hydrolysis. To this aim, the synthesis of DCPD was performed in aqueous solution in the presence of increasing concentrations of PASP and ASP, whereas the hydrolysis reaction was carried out in physiological solution up to three days. The results indicate that it is possible to prepare DCPD functionalized with PASP up to a polyelectrolyte content of about 2.3 wt%. The increase of PASP content induces crystal aggregation, reduction of the yield of the reaction and of the thermal stability of the synthesized DCPD. Moreover, DCPD samples functionalized with PASP display a slower hydrolysis than pure DCPD. On the other hand, in the explored range of concentrations (up to 10 mM) ASP is not incorporated into DCPD and does not influence its crystallization nor its hydrolysis. At variance, when present in the hydrolysis solution, ASP, and even more PASP, delays the conversion into the more stable phases, octacalcium phosphate and/or hydroxyapatite. The greater influence of PASP on the synthesis and hydrolysis of DCPD can be ascribed to the cooperative action of the carboxylate groups and to its good fit with DCPD structure.
Highlights
The biomineralization processes of the hard tissues of vertebrates imply the deposition of calcium phosphate in an environment rich in acidic macromolecules
Scanning Electron Microscopy (SEM) images reported in Figure 2 show that pure Dicalcium phosphate dihydrate (DCPD) is constituted of plate like crystals with large (0k0) faces and sharp edges, whereas the crystals synthesized in the presence of poly-aspartic acid (PASP) up to PASP08 exhibit indented edges and a strong tendency to aggregate into spherulites
The crystallization of the material precipitated in the presence of 1 mM PASP is so disturbed that the presence of crystals is barely appreciable in its SEM image (Figure 2)
Summary
The biomineralization processes of the hard tissues of vertebrates imply the deposition of calcium phosphate in an environment rich in acidic macromolecules. The inorganic phase is identified as a basic calcium phosphate, which is usually described as close to the synthetic hydroxyapatite (HA). It is generally accepted that precipitation of biological apatites occurs through intermediate steps, involving transformation from less stable phosphates, including amorphous calcium phosphate (ACP), dicalcium phosphate dihydrate (DCDP), tricalcium phosphate (TCP) and octacalcium phosphate (OCP) [1,2,3]. The structure involves parallel CaP chains with interlayered lattice water molecules. Loss of structural water by heat treatment at about 180 ◦C provokes DCPD transformation into its anhydrous form, CaHPO4 (dicalcium phosphate anhydrous, DCPA) [5]. DCPD is stable at a relatively low pH, and in water it undergoes a transformation into the more stable phases, OCP and HA [6]. The presence of additives and the applied conditions can influence and mediate the phase transformation [7,8,9]
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