Structure Determination of a Single Bioapatite Crystal from an Archeological Metastatic Cancer Bone - a TEM Case Study

The inorganic component of bone, bioapatite, is generally regarded as a poorly crystalline, carbonate-substituted hydroxyapatite. However, a direct structural and chemical characterization of bioapatite has not yet been achieved due to the extremely small size of its crystals  typically only a few tens of nanometers  and their intimate association with the organic matrix of bone. In this study, we examined a single crystal of bioapatite extracted from an archeological, metastatic bone cancer sample using 3D ED (3-Dimensional Electron Diffraction), which enabled structural analysis of the three-dimensional reciprocal space despite the nanometric crystal dimensions. Conventional structural techniques, such as single-crystal X-ray diffraction, require crystals several orders of magnitude larger than those found in bone, rendering them inapplicable to individual bioapatite crystallites. Powder-based methods, on the other hand, can only provide averaged structural information across billions of crystals, obscuring the intrinsic structural features of individual nanocrystals. The bioapatite single crystal was found to crystallize in space groups P6₃ and P6₃/m, with lattice parameters a = 9.414 Å; c = 6.881 Å, and showed no evidence of carbonate substitution. The measured Ca/P ratio on single crystals was 1.65 on average.