A ⁴³ Ca nuclear magnetic resonance perspective on octacalcium phosphate and its hybrid derivatives

⁴³ Ca nuclear magnetic resonance (NMR) spectroscopy has been extensively applied to the detailed study of octacalcium phosphate (OCP), Ca₈ (HPO₄ )₂ (PO₄ )₄ .5H₂ O, and hybrid derivatives involving intercalated metabolic acids (viz., citrate, succinate, formate, and adipate). Such phases are of importance in the development of a better understanding of bone structure. High-resolution ⁴³ Ca magic angle spinning (MAS) experiments, including double-rotation (DOR) ⁴³ Ca NMR, as well as ⁴³ Ca{¹ H} rotational echo DOR (REDOR) and ³¹ P{⁴³ Ca} REAPDOR NMR spectra, were recorded on a ⁴³ Ca-labeled OCP phase at very high magnetic field (20 T), and complemented by ab initio calculations of NMR parameters using the Gauge-Including Projector Augmented Wave-density functional theory (GIPAW-DFT) method. This enabled a partial assignment of the eight inequivalent Ca²⁺ sites of OCP. Natural-abundance ⁴³ Ca MAS NMR spectra were then recorded for the hybrid organic-inorganic derivatives, revealing changes in the ⁴³ Ca lineshape. In the case of the citrate derivative, these could be interpreted on the basis of computational models of the structure. Overall, this study highlights the advantages of combining high-resolution ⁴³ Ca NMR experiments and computational modeling for studying complex hybrid biomaterials.