BONE APATITE: THE SECRET IS IN THE CARBONATE

The mineral component of bone is typically, but incorrectly, referred to as "hydroxylapatite." It is actually hydroxyl-deficient, but carbonate-rich apatite. Incorporation of 6-7 wt% carbonate in bone apatite affects both the charge balance and crystal structure of the apatite. The size, shape, composition, and solubility of bone apatite have been optimized through evolution to accomplish such tasks as supporting the body, storing/releasing calcium and phosphorus, replacing damaged bone. How are these four parameters controlled biologically, and why/how can they change with age? Based on the existing literature, we postulate that the carbonate concentration in bioapatite exerts a major control on these parameters. We are testing this hypothesis through synthesis of apatites containing 1-17 wt.% CO₃ and their analysis by Raman spectroscopy and X-ray diffraction (XRD). The goal is to determine how the unit-cell parameters, grain size, degree of crystalline order, and degree of hydroxylation of the apatite change with CO₃ concentration. Samples are precipitated from pH-buffered solutions at 60 and 80 C. Raman spectra show strong correlations between several spectral parameters and CO₃ concentration. As [CO₃] increases in apatite, the ν₁ band for the P-O symmetric stretch increases in width, indicating a rise in atomic disorder; in contrast, [OH] decreases. The additional detection of calcite in some spectra indicates experiments in which apatite became saturated with respect to carbonate. XRD data on crystallites' coherence length and lattice parameters show strong temperature sensitivity. Within a single-temperature suite, XRD indicates a decrease in crystallite size as [CO₃] increases, at least up to 10 wt.%. In bone, crystallite size is important to the physical, orderly incorporation of bioapatite into the spatially fixed framework of collagen fibrils. Both the [OH] and the state of internal strain/atomic order of the bioapatite crystallites are important to the mineral's solubility, i.e., the ability of bone to be dissolved and reprecipitated during routine remodeling or emergency repair. Studies currently in progress should improve our understanding of the effect of temperature and equilibration time on carbonate incorporation in and the structural characteristics of carbonated apatites.