Progress of Structural Elucidation of Amorphous Calcium Phosphate(ACP)and Hydroxyapatite (HAp): Disorder and Surfaces as Seen by Solid State NMR

Solid-state Nuclear Magnetic Resonance (NMR) has been used for a detailed structural analysis of different amorphous calcium phosphate (ACP) and hydroxyapatite (HAp) samples. For ACP the NMR data reveal the presence of hydrogenphosphate units in much larger quantities as known previously also showing differences between the samples. Furthermore, phosphate units close to water and a third group (yet unknown) are found in the spectra. A novel proposal has been established to explain the Ca-deficiency in nanocrystalline HAp. It consists of a crystalline core of stoichiometric composition which contains only 50% of the total phosphate amount. The residual phosphate is contained in a disordered surface layer with about 1 nm thickness. Most importantly, this surface layer contains hydrogenphosphate groups to a large extend and carbonate units. A Ca/P ratio of 1.52 is obtained from the NMR data which is very close to the value of 1.51 of the chemical analysis. Thus, NMR allows the investigation of the disordered mineral-protein interface and first examples concerning bone and nacre are presented and discussed.