No abstract

Calcium phosphates are the main minerals in human bone, enamel, atherosclerosis, and dental calculus. Amorphous precursors may play a key role in biomineralization. We studied the formation and transformation of calcium phosphate particles of amorphous phase by stopped-flow spectrophotometry, simultaneous measurements of particle size and solution pH, and high-resolution transmission electron microscopy. Ion pairs and clusters formed in the first few seconds. They then constituted initial amorphous phase containing protonated phosphates and hydrated calcium ions, which was different from that containing Ca 9 (PO 4 ) 6 . Crystalline domains developed at multiple sites inside the primary particles of the amorphous phase. With the consuming of interdomain constituents, these particles partially collapsed, liberating crystallites and inducing rapid precipitation. This study sheds new light on the understanding of crystallization in amorphous phase, as well as the induction period in precipitation kinetics.

Amorphous calcium phosphate often forms as a precursor phase in a solution at sufficiently high supersaturation and pH, and then transforms to the thermodynamically stable hydroxyapatite. The chemical composition, structure, and property of the amorphous phase are dependent on the structure of its composing clusters. Based on the results from the measurements of in situ Ca K-edge X-ray near-edge structure and ex situ X-ray diffraction, as well as the concomitant pH change in the reaction process, here we propose an improved model for the structure of “Posner’s cluster” and identify the three types of reactions that lead to the formation of amorphous calcium phosphate and its subsequent transition to crystalline hydroxyapatite.