Effects of carbonate species and chloride ions on calcium phosphate nucleation of biological apatite

Apatite is the principal inorganic component of biological hard tissues such as bone and teeth, and thus is essential to the existence of vertebrate life. Because carbonate species and chloride ions are ubiquitous in the fluids of vertebrate life forms, and these species are known to compete with phosphate species, it is critical to understand the influence of these anions on the nucleation of biological apatite. In this study, the important roles of carbonate and chloride ions in mediating inorganic calcium phosphate (CaP) phase nucleation are revealed both theoretically and experimentally. Theoretical investigations suggest that the Ca-O coordination number is an important descriptor for the nucleation of CaP and the interface interaction of the CaP-additive. Positive surface charges on the outermost Ca2+ ions of CaP clusters could attract carbonate species and Cl- ions, promoting the formation of CaP phase. In the presence of carbonate species and Cl- ions, the crystallization of hydroxyapatite (HA) from its precursor brushite (DCPD) has been observed in our experimental studies. The Cl- ion-doped B-type HA is proposed to be the most likely biological apatite based on equilibrium oxygen isotope fractionation and vibrational spectroscopic analysis. The insight gained here may help rationalize the control of biomineralization, and will guide the synthesis of anion-doped biological functional apatite materials.