NMR of Biopolymer-Apatite Composites: Developing a Model of the Molecular Structure of the Mineral-Matrix Interface in Calcium Phosphate Biomaterials

Nanoparticulate hydroxyapatites (HAps) reproducing some of the subnanometre length scale intermolecular interactions characteristic of hard tissue have been prepared for nuclear magnetic resonance (NMR) structural elucidation. HAps were precipitated at physiological pH and temperature from dilute aqueous solutions, in the presence of acidic polysaccharides (chondroitin sulfate, dermatan sulfate, hyaluronic acid, dextran sulfate, polygalacturonic acid), and of hydrophilic poly aminoacids (poly-L-iglutamate, poly-L-asparagine, poly-L-lysine). The HAp resembles that of bone with respect to its P-31 NMR properties, broad reflections in X-ray powder diffraction, and the coexistence of an ordered crystalline core, surrounded by a less ordered surface containing water and hydrogen phosphate. C-13{P-31} rotational echo double resonance (REDOR) NMR, which probes carbon phosphorus proximities below ca. 1 nm, shows that all the HAps are molecular composites in which each biopolymer forms intimate intermolecular associations with mineral ions. REDOR effects between mineral phosphorus and ring carbons of the polysaccharides, and the side-chain terminal carboxylate and amide carbonyls of poly-L-glutamate and poly-L-asparagine, and the side chain carbons of poly-L-lysine, closely recapitulate those seen in native bone. Such model HAp-biopolymer composites will prove useful in studies of the role of biopolymers in biomineralization, and in high resolution biomineral structure elucidation.