Impacts of Initial Ca/P on Amorphous Calcium Phosphate

Amorphous calcium phosphate (ACP) is a metastable phase in the crystallization pathway of calcium phosphates. Understanding its chemical and structural properties could provide critical insights into biomineralization mechanisms. Of particular interest is the impact of the initial Ca/P on the structure of ACP. Further, the influence of Ca/P on the size, precipitate chemistry, and transformation of ACP is crucial to understanding the metastability of amorphous phosphates. In situ pair distribution function (PDF) analysis results 5 min after mixing show that the Ca-P bonding geometry varies from predominantly monodentate, to mixed monodentate and bidentate, to predominantly bidentate as Ca/P increases from 0.2 to 5.0. This relationship is consistent across a pH range of 6-11. Samples with only monodentate Ca-P geometries transform directly to hydroxylapatite, while samples some or all bidentate geometries form brushite. Regardless of the initial ratio, the Ca/P of precipitates is close to 1.0. In situ small-angle X-ray scattering shows particle size increases with increasing Ca/P. This is the first evidence of structural variations in ACP and is directly linked to system chemistry. Synthesis of ACP with monodentate Ca-P geometries is a promising method to control the crystallization pathway to form hydroxylapatite at circumneutral pH without stabilizing ions.

Automated summary

Understanding the chemical and structural properties of amorphous calcium phosphate (ACP) is crucial for gaining insights into biomineralization mechanisms. The influence of the initial Ca/P ratio on the structure and transformation of ACP is significant, with structural variations directly linked to system chemistry. A promising method to control the crystallization pathway to form hydroxylapatite at circumneutral pH without stabilizing ions is the synthesis of ACP with monodentate Ca-P geometries.