Nanocrystals of apatitic calcium phosphate impart the organic-inorganic nanocomposite in bone with favorable mechanical properties, but the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Schmidt-Rohr and coworkers have shown by multinuclear magnetic resonance (NMR) that the nanocrystal surfaces in bone are studded with strongly bound citrate molecules and have characterized the density (0.25 nm-2) and geometry of the bound citrate with the three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.
Relevant Publication(s): PNAS, in press.