Abstract
Hydroxyapatite (HA), the main mineral phase of mammalian tooth enamel and bone, originates in body fluids from amorphous calcium phosphate (ACP). ACP presents short-range order in the form of small domains with size of 0.9 nm and chemical formula Ca9(PO4)6, known as Posner's clusters. In this study, the aggregation and clustering of calcium and phosphate ions in water has been investigated by means of shell-model molecular dynamics simulations. Calcium phosphate aggregates form in solution with compositions and Ca coordination that are similar to those found in Posner's cluster, but the stoichiometry of these species is dependent on the ionic composition of the solution: calcium-deficient clusters in solutions with low Ca : P ratio; cluster containing protonated phosphate groups in neutral solutions; sodium ions partially substituting calcium in solutions containing a mixture of sodium and calcium ions. These Posner-like clusters can be connected by phosphate groups, which act as a bridge between their central calcium ions. The simulations of the aggregation in solution of calcium phosphate clusters are an unbiased and unequivocal validation of Posner's model, and reveal for the first time the structure and composition of the species that form during the early stages of ACP nucleation at a scale still inaccessible to experiment.
Highlights
Owing to the importance of calcium phosphate as a biomaterial, several experimental[4,5,6] and theoretical[7] studies have focussed on its nucleation and growth
Experimental studies have shown that the aggregation of calcium and phosphate ions results in the formation of cluster and in ACP (Ca)-deficient clusters, which incorporate other calcium ions from the solution, thereby increasing the Ca : P ratio, releasing protons, and lowering the pH of the solution.[4,35]
In the original Posner’s cluster, the Ca : P ratio is 1.5 but such a large amount of calcium is not available in the solutions I and Ia, and Posner-like clusters made of six phosphate and nine cations were not observed during the simulations
Summary
Owing to the importance of calcium phosphate as a biomaterial, several experimental[4,5,6] and theoretical[7] studies have focussed on its nucleation and growth. Using X-ray experiments, Betts and Posner discovered in the 1970s that, despite its lack of long-range order, ACP is characterised by spherical domains of approximately 1 nm in size, with a Ca : P molar ratio of 1.5 and chemical formula Ca9(PO4)[6]. These clusters, which were later named Posner’s clusters, aggregate randomly in solution and form spherical particles, with water molecules filling the intercluster space.[8].
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