Proc. Natl Acad. Sci. http://doi.org/r6m (2014)

Credit: © 2014 NAS

The mineral phase of bone is composed of stacks of apatite platelets parallel to each other and to the neighbouring collagen fibrils, and of a highly hydrated disordered phase containing most of the mineral's phosphate ions. However, the location of the hydrated phase is unclear, and how the stack of mineral platelets is formed and maintained is unknown. Now, Melinda Duer and colleagues propose a model of bone mineral where mineral platelets are bridged by disordered citrate ions residing within inter-platelet hydrated layers. Supported by solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction and first-principles electronic-structure calculations, the model explains why the bone-mineral structure is preserved when the surrounding protein is removed and why the amount of structured water observed in bone mineral is larger than that of synthetic models of it. It also explains the presence of substantial amounts of disordered and immobile hydrogen phosphate ions in the platelet stacks, and has implications for the understanding of the mechanical properties of bone and of the changes in bone-mineral crystallinity associated with certain metabolic diseases.