Structural Role And Spatial Distribution Of Carbonate Ions In Amorphous Calcium Phosphate

The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2-14 wt %) were studied by multinuclear H-1, C-13, Na-23, and P-31 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) experiments together with infrared (IR) spectroscopy. A model for carbonate incorporation into ACP is proposed, where carbonates enter as CO32- anions, whose equal C-13 chemical shifts (delta(C) = 168.6 ppm) imply identical local CO(3)(2- )environments in the ACP structure, irrespective of its carbonate content. The bicarbonate contents were negligible, except in the CO32--richest ACP sample, where HCO3- ions accounted for 4.3% of all carbonate species. The HCO3- anions in ACP are characterized by C-13 and H-1 chemical shifts delta(C) = 162 ppm and delta(H) = 14 ppm, respectively, as deduced from C-13{H-1} heteronudear correlation (HETCOR) two-dimensional (2D) NMR experiments. Regardless of the precise carbonate content, the ACP samples contained very similar amounts of water (approximate to 15 wt %)- most of which is structure-bound (approximate to 70%) and the remaining physisorbed-along with acidic protons of HPO42- anions, which typically accounted for approximate to 20% of the phosphate speciation. The local proton and phosphate environments were probed further by heteronuclear H-1/P-31 2D NMR experiments. We also extracted the Na-23 NMR parameters of the Na+ sites present in minute amounts (0.1-1.1 wt %) in the ACP specimens, which along with their C-13/P-31/H-1 NMR counterparts of the CO32-, HCO3-, PO43-, and HPO42- moieties are discussed and contrasted with previous reports on Na/carbonate-bearing Ca phosphate phases, such as synthetic and biogenic hydroxy-carbonate apatite. The spatial distribution of the carbonate species was determined from advanced homonuclear C-13 and P-31 double-quantum together with heteronudear C-13{P-31} MAS NMR experimentation, where each technique provided independent and consistent evidence for randomly distributed CO32- moieties.