APATITE IN BONE IS NOT HYDROXYLAPATITE: THERE MUST BE A REASON

Apatite is an essential component of tooth and bone. The composition of this nanocrystalline bioapatite traditionally has been referred to as carbonated hydroxylapatite based on its several wt.% carbonate content and XRD pattern suggestive of poorly crystalline hydroxylapatite. Analyses of bone apatite by IR spectroscopy and proton NMR, however, show no evidence of OH groups (Rey, et al., 1995). We have used laser Raman microprobe spectroscopy to compare the state of hydration of geological hydroxylapatite, synthetic hydrated Ca phosphates, bioapatite in bone (unheated and heat-treated up to 1200° C), and bioapatite in tooth enamel. A Raman spectrum of apatite gives information about its degree of crystallinity, as well as its relative concentrations ([X]) of CO₃ and OH. We detect high [OH] in geological hydroxylapatite, high but very variable [OH] in synthetic hydroxylapatite, variable low to moderate [OH] in tooth enamel apatite, and extremely low [OH] in mouse bone apatite. In bone heated to 800-1000° C, however, we find very high [OH]. Overall, apatite with the highest [OH] exhibits the highest crystallinity. As [CO₃] increases and as [OH] decreases, the degree of crystallinity of the apatite decreases. Nevertheless, we find poor correlation between [OH] and [CO₃], suggesting that there is not much mutual substitution of those two components. In addition, we found it difficult to: 1) totally exclude OH from apatite synthesized aqueously under neutral-pH conditions (as in bone) even in the presence of organic additives that suppress grain growth, and 2) totally drive off OH from apatite even by heating at 1200° C for 2 hours in air. Our results show that 1) [OH] can be highly variable within the basic hydroxylapatite structure, and 2) whereas carbonated bioapatite in enamel CAN have high [OH], the carbonated bioapatite in bone contains only trace [OH]. We infer that this "hydration state" of bone apatite is biochemically imposed, and that the extremely low to non-existent [OH] in bone apatite is important to the bulk properties of the nanocrystalline mineral-collagen composite that constitutes bone.