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

Abstract

43Ca nuclear magnetic resonance (NMR) spectroscopy has been extensively applied to the detailed study of octacalcium phosphate (OCP), Ca8(HPO4)2(PO4)4.5H2O, 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 43Ca magic angle spinning (MAS) experiments, including double-rotation (DOR) 43Ca NMR, as well as 43Ca{1H} rotational echo DOR (REDOR) and 31P{43Ca} REAPDOR NMR spectra, were recorded on a 43Ca-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 43Ca MAS NMR spectra were then recorded for the hybrid organic–inorganic derivatives, revealing changes in the 43Ca 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 43Ca NMR experiments and computational modeling for studying complex hybrid biomaterials.