ARE THE PROPERTIES OF BIOAPATITE DIFFERENT IN BONE WITH DIFFERENT PERCENT MINERALIZATION?

Nanocrystalline carbonated (±hydroxyl) apatite is the main mineral component in bone, dentin, enamel, conodonts, and some brachiopod shells. Just as geological apatite, bioapatite expresses a wide range of compositions. These, in turn, affect bioapatite's physical-chemical characteristics and therefore its biological functionality. Biomineralized materials also have an organic component, dominantly type I collagen in the case of bone tissue. In the present study, we compare the CO₃^2- concentration, OH^- concentration, and degree of crystallinity (crystallite size) of apatite in bone tissues having two extremes of mineral concentration: "typical bone" (mouse femur with about 65 wt.% mineral) and hypermineralized bone (rostrum/"beak" of a toothed whale, Mesoplodon densirostris, with about 96 wt.% mineral). Zioupos, et al. (2000) concluded from the well-resolved, narrow XRD bands obtained on powdered rostrum that this was not only the most highly mineralized of bone tissues, but that it also comprised crystallites that were substantially larger and much more coherently oriented than those in typical bone. Our Raman microprobe analyses of rostrum samples, in contrast, show the well-recognized pairing of high [CO₃^2-] and low degree of crystallinity (inferred from peak width). Raman spectroscopy suggests that, despite the vast difference in the mineral:collagen ratio of mouse bone and whale rostrum the [OH^-], [CO₃^2-], and crystallite size of their bioapatite are indistinguishable. The contradiction in crystallite size as inferred from the XRD compared to the Raman data appears to reflect the different length-scales of structural analysis inherent to each technique. To reconcile the analytical contradiction, we are investigating 1) whether extended arrays of well-aligned nanocrystallites may diffract coherently and thus give the impression of a larger crystallite size, and 2) whether Raman spectroscopy may instead sense the inherent disorder within individual crystallites. This study illustrates the analytical challenge in characterizing nanocrystalline bioapatite and the usefulness of applying multiple techniques. It also emphasizes the need to isolate and understand how the measurable XRD and Raman spectral parameters reflect the dimensions, shape, and alignment of crystallites.