Chemical Controls on the Formation of Amorphous and Crystalline Calcium Phosphates

Transformation of amorphous calcium phosphate (ACP) and brushite into hydroxylapatite, an important biomineral, has been documented. The relationships between synthesis conditions and the formation and transformation of these phases are not comprehensively understood. The metastable nature of ACP has made it historically challenging to investigate, and many analyses attempt to stabilize the phase through drying or including additional ions or proteins in the reaction. In situ investigations provide an incisive approach to examining the structure and transformation of ACP and brushite as a function of synthesis conditions. The first project develops a new method for in situ analyses of the structure of ACP and brushite shortly after reagent mixing, without chemical stabilization. This method was used in the second project to examine how the initial Ca/P affects ACP structure and transformation. Our results identify the first structural differences in types of ACP, controlled by the initial Ca/P. At ratio 0.2 the Ca – P bonding geometry is primarily monodentate, ratio 5.0 produces a coordination that is primarily bidentate, and there is a mix of monodentate and bidentate coordinates at intermediate ratios between the two. These results are independent of system pH between the examined range of 6-11. Further ex situ transformation experiments identified that at ratio 0.2, ACP transformed directly into hydroxylapatite, but at higher ratios the transformation product is brushite. This is a promising mechanism for direct ACP to hydroxylapatite conversion at a biologically relevant pH. In the final project, the statistically significant synthesis parameters (age, pH, temperature, supersaturation, and initial ion ratio) for ACP, brushite, and hydroxylapatite formation are evaluated. Binary logistic regression analysis and nonlinear continuous logistic regression analysis are applied to a dataset compiled from the literature. Equations were developed that predict the percentage of ACP and brushite that will form. The equations and significant variables seem to depend on the transformation pathway of brushite and ACP. The current analysis did not comprehensively describe hydroxylapatite formation when trying to concurrently evaluate the ACP to hydroxylapatite and brushite to hydroxylapatite pathways. Taken together, these studies provide new ways to study and interpret calcium phosphate phases as they form and transform. Experiments identified new relationships between the chemistry and structure of ACP. The new in situ experimental method and the equations we developed can be used to improve future experimental designs towards a comprehensive understanding of the calcium phosphate system.