| Abstract
| - For many years, octacalcium phosphate (OCP) has been postulated as the precursor phase ofbiological apatite in bones and teeth. In this work, we study the molecular mechanism of OCP tohydroxyapatite (HAp) transformation in vitro by several physical techniques, with particular emphasis onsolid-state 31P homonuclear double-quantum (DQ) NMR spectroscopy. The in vitro system is prepared bymixing urea, sodium phosphate monobasic dehydrate, and calcium nitrate tetrahydrate at 100 °C. Theimages obtained by scanning electron microscopy and transmission electron microscopy show that thebladelike OCP crystals will transform into hexagonal rod-shaped HAp crystals as the pH of the reactionmixture increases slowly from 4.35 to 6.69 in 12 h. Powder X-ray diffraction patterns indicate that a traceamount of monetite was also precipitated when the pH was around 5. Together with computer-assistedlattice matching, our DQ NMR data reveal that OCP crystals transform to HAp topotaxially, where the[0001̄]HAp and [21̄1̄0]HAp axes are along the same directions as the [001]OCP and [010]OCP axes, respectively.On the basis of our in vitro results, the formation of the central dark line commonly found in biological hardtissues could be explained by the inherent lattice mismatch between OCP and HAp. Furthermore, the dataof the 31P{1H} cross-polarization NMR suggest that water molecules enter the hydration layers of OCPcrystals via the hydrolysis reaction HPO42- + OH- = PO43- + H2O, which also accounts for the deprotonationof the HPO42- ions during the transformation.
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