Amorphous calcium phosphate (ACP) and brushite are known precursors to the formation of hydroxylapatite, an important biomineral. 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 are the solution to examining the structure and transformation of ACP and brushite as a function of synthesis conditions. We have developed a new method for in situ analyses of the structure of ACP and brushite shortly after mixing, without chemical stabilization. We used this method to examine how initial Ca/P affects the structure and transformation of ACP. Our results identify the first structural differences in types of ACP, controlled by the Ca/P. At ratio 0.2 the Ca – P bonding geometry is primarily monodentate, ratio 5.0 is primarily bidentate, and there is a mix of monodentate and bidentate 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 it transformed first to brushite and then hydroxylapatite. This is a promising mechanism for direct ACP to hydroxylapatite conversion at a biologically relevant pH. The new in situ method described in this work will be useful for studying complex crystallization pathways and associated metastable intermediates in other chemical systems.

Impacts of initial Ca/P on the formation and transformation of amorphous calcium phosphate

ACS Spring Meeting 2021

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