Solid-State NMR Study of the Transformation of Octacalcium Phosphate to Hydroxyapatite: A Mechanistic Model for Central Dark Line Formation

For many years, octacalcium phosphate (OCP) has been postulated as the precursor phase of
biological apatite in bones and teeth. In this work, we study the molecular mechanism of OCP to
hydroxyapatite (HAp) transformation in vitro by several physical techniques, with particular emphasis on
solid-state 31P homonuclear double-quantum (DQ) NMR spectroscopy. The in vitro system is prepared by
mixing urea, sodium phosphate monobasic dehydrate, and calcium nitrate tetrahydrate at 100 °C. The
images obtained by scanning electron microscopy and transmission electron microscopy show that the
bladelike OCP crystals will transform into hexagonal rod-shaped HAp crystals as the pH of the reaction
mixture increases slowly from 4.35 to 6.69 in 12 h. Powder X-ray diffraction patterns indicate that a trace
amount of monetite was also precipitated when the pH was around 5. Together with computer-assisted
lattice 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 hard
tissues could be explained by the inherent lattice mismatch between OCP and HAp. Furthermore, the data
of the 31P{1H} cross-polarization NMR suggest that water molecules enter the hydration layers of OCP
crystals via the hydrolysis reaction HPO42- + OH- = PO43- + H2O, which also accounts for the deprotonation
of the HPO42- ions during the transformation.