The Carbonate And Sodium Environments In Precipitated And Biomimetic Calcium Hydroxy-Carbonate Apatite Contrasted With Bone Mineral: Structural Insights From Solid-State Nmr

Bone mineral consists of calcium hydroxy-carbonate apatite (HCA) that incorporates other minor cation substituents, primarily Na+ (0.5-0.8 wt %). We examine the carbonate species in various HCA specimens with variable CO32- contents (4-10 wt %), encompassing phases prepared by precipitation and a biomimetic specimen formed from a bioactive glass inside a simulated body fluid as well as bone tissue from beagle dog. Using magic-angle spinning (MAS) nuclear magnetic resonance (NMR) along with infrared spectroscopy experiments, we identified and quantified carbonate anions replacing either hydroxyl ("A-type" CO32-) or phosphate ("B-type" CO32-) anions in the HCA lattice, along with the carbonate species present in the amorphous surface layer present at all synthetic and biogenic nanocrystalline HCA particles. Advanced C-13-based NMR experimentation enabled the selective detection of the minor (CO32-)-C-13 population of intact bone monoliths, whose C-13 NMR signals are otherwise swamped by those from collagen, unless chemically invasive deproteination procedures are invoked. The CO32- species present in 4 week- and 8 month-old bone of the alveolar process from beagle dog revealed mainly B-type lattice sites and carbonates present in the amorphous surface layer. No tissue aging effects were observed in the local CO32- environments. Likewise, NMR revealed very similar Na-23(+) parameters in HCA, regardless of its synthetic or biogenic origin or degree of structural order. A combination of interatomic-distance-sensitive MAS NMR experiments allowed the identification of the local environments of the various carbonate, phosphate, hydroxyl, and water species present in both the interior and the surface layer of the synthetic HCA particles. We highlight the similarities/differences in the chemical speciation and the spatial distribution of CO32- anions present in carbonate-bearing amorphous calcium phosphate (ACP) relative to the ACP-like surface layer of HCA and discuss the C-13 NMR peak assignments of the up to four coexisting CO32- populations in the synthetic/biogenic HCA phases.