Structural Role and Spatial Distribution of Carbonate Ions in Amorphous Calcium Phosphate
Ozlen F. Yasar, Wei‐Chih Liao, Baltzar Stevensson, Mattias Edén
Abstract
The local structures of a series of amorphous calcium phosphate (ACP) phases with increasing carbonate contents (2–14 wt %) were studied by multinuclear 1H, 13C, 23Na, and 31P 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 13C chemical shifts (δC = 168.6 ppm) imply identical local CO32– 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 13C and 1H chemical shifts δC = 162 ppm and δH = 14 ppm, respectively, as deduced from 13C{1H} heteronuclear correlation (HETCOR) two-dimensional (2D) NMR experiments. Regardless of the precise carbonate content, the ACP samples contained very similar amounts of water (≈15 wt %)—most of which is structure-bound (≈70%) and the remaining physisorbed—along with acidic protons of HPO42– anions, which typically accounted for ≈20% of the phosphate speciation. The local proton and phosphate environments were probed further by heteronuclear 1H/31P 2D NMR experiments. We also extracted the 23Na NMR parameters of the Na+ sites present in minute amounts (0.1–1.1 wt %) in the ACP specimens, which along with their 13C/31P/1H 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 13C and 31P double-quantum together with heteronuclear 13C{31P} MAS NMR experimentation, where each technique provided independent and consistent evidence for randomly distributed CO32– moieties.