Citrate Stabilizes Bone Apatite

Apatite nanocrystals that help give bone its desirable mechanical properties unexpectedly draw support from citrate molecules, researchers at Ames Laboratory and Iowa State University report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1009219107). Bone is made of nanocrystals of carbonated apatite, a type of calcium phosphate, embedded in a collagen protein matrix. Until now, researchers haven’t known what prevents the nanocrystals from growing larger than about 3 nm. Using solid-state NMR spectroscopy, Klaus Schmidt-Rohr and coworkers found that the apatite nanocrystals are studded with strongly bound citrate molecules. Scientists have known that citrate is abundant in bone, but they thought it was involved in calcium leaching and demineralization. The NMR analysis reveals that citrate, rather than being a dissolved ­calcium- solubilizing agent, is actually an integral part of the bone nanocomposite. Citrate is too large to be incorporated in the apatite crystal lattice, so it remains on the surface. The researchers propose that citrate, which they calculate covers one-sixth of the apatite surface in bone, limits nanocrystal size by inhibiting the formation of additional phosphate layers.