Short-Range Structure of Amorphous Calcium Hydrogen Phosphate

Abstract

Calcium orthophosphates (CaPs) are the hard constituents of bones and teeth, and thus of ultimate importance to humankind, while amorphous CaPs (ACPs) may play crucial roles in CaP biomineralization.Among the various ACPs with Ca/P atomic ratios between 1.0~1.5, an established structural model exists for basic ACP (Ca/P=1.5), while those of other ACPs remain unclear. Herein, the structure of amorphous calcium hydrogen phosphate (ACHP; Ca/P=1.0) obtained via aqueous routes at near-neutral pH values, without stabilizers, was studied by experiments (mainly, TEM with ED, XRD, IR and NMR spectroscopies, as well as EXAFS) and computer simulation. Our results globally show that ACHP has a distinct short-range structure, and we propose calcium hydrogen phosphate clusters (CHPCs) as its basic unit. This model is consistent with both computer simulations, and the experimental results, where CHPCs are arranged together with water molecules to build up ACHP. We demonstrate that Posner's clusters, which are conventionally accepted to be the building unit of basic ACPs, do not represent the short-range structure of ACHP, as Posner's clusters and CHPCs are structurally distinct. This finding is important not only for the determination of the structures of diverse ACPs with varying Ca/P atomic ratios, but also for fundamental understanding of a major mineral class that is central to biomineralization in vertebrates, and, thus, humans, in particular.

Structural Biology of Calcium Phosphate Nanoclusters Sequestered by Phosphoproteins

“…The empirical chemical formula Ca(PO 4 ) 0.52 (HPO 4 ) 0.22 is consistent with an ACP of Ca/P i = 1.35 [83]. In similar studies [84][85][86][87], the Ca/P i ratio of ACP has been reported to be as high as 1.5 [88] and as low as 1.18 [87], demonstrating that a range of more-or-less acidic ACP phases are possible with distinct short-range structures [89][90][91]. The variable co-ordination geometry of calcium ions in calcium salts and variable stoichiometry of CaPs are well recognized [92].…”

Introducing the crystalline phase of dicalcium phosphate monohydrate

“…H 2 O is required for the crystallization of DCPM from ACHP, as the amorphous precursor did indeed not crystallize in dry air (humidity < 5%, Methods section) for at least 1 month. The presence of too much water will lead to crystallization yielding other phases such as DCPD 38 . Properties of DCPM.…”

Diffusion-Controlled Crystallization of Calcium Phosphate in a Hydrogel toward a Homogeneous Octacalcium Phosphate/Agarose Composite

“…Further analysis using 31 P magic angle spinning (MAS) spectroscopy revealed that the chemical environment of the P atoms in acidic ACP was similar to that in DCPD. Structural similarities between the basic ACP and HAp were also reported . Furthermore, Renard et al investigated the nucleation of DCPD and HAp in real-time using in situ Raman spectroscopy .…”