Reyes-Gasga Jose, Tehuacanero-Núñez Samuel, Sánchez-Ochoa Francisco
Departamento Materia Condensada, Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación, Cd. Universitaria, Mexico City, CDMX 04510, Mexico.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2023 Oct 1;79(Pt 5):399-407. doi: 10.1107/S2052520623006777. Epub 2023 Sep 12.
Human tooth enamel (HTE) is the hardest tissue in the human body and its structural organization shows a hierarchical composite material. At the nanometric level, HTE is composed of approximately 97% hydroxyapatite [HAP, Ca(PO)(OH)] as inorganic phase, and of 3% as organic phase and water. However, it is still controversial whether the hexagonal HAP phase crystallizes in P6/m or another space group. The observance in HTE of Ca, Mg and Na ions using X-ray characteristic energy-dispersive spectroscopy in the scanning electron microscope has been explained by substitutions in the HAP unit cell. Thus, Ca can be replaced by Na and Mg ions; the PO group can be replaced by CO ions; and the OH ions can also be replaced by CO. A unit-cell model of the hexagonal structure of HTE is not fully defined yet. In this work, density functional theory calculations are performed to study the hexagonal HAP unit cell when substitution by OH, CO, Mg and Na ions are carried out. An approach is presented to study the crystallographic unit cell of HTE by examining the changes resulting from the inclusion of these different ions in the unit cell of HAP. Enthalpies of formation and crystallographic characteristics of the electron diffraction patterns are analysed in each case. The results show an enhancement in structural stability of HAP with OH defects, atomic substitution of Mg, carbonate and interstitial Na. Simulated electron diffraction patterns of the generated structures show similar characteristics to those of human tooth enamel. Hence, the results explain the indiscernible structural changes shown in experimental X-ray diffractograms and electron diffraction patterns.
人牙釉质（HTE）是人体中最坚硬的组织，其结构组织呈现出一种分级复合材料。在纳米尺度上，HTE由约97%的羟基磷灰石[HAP，Ca(PO)(OH)]作为无机相，以及3%的有机相和水组成。然而，六方HAP相是否在P6/m或其他空间群中结晶仍存在争议。利用扫描电子显微镜中的X射线特征能量色散光谱对HTE中Ca、Mg和Na离子的观测结果已通过HAP晶胞中的取代作用得到解释。因此，Ca可被Na和Mg离子取代；PO基团可被CO离子取代；OH离子也可被CO取代。HTE六方结构的晶胞模型尚未完全确定。在这项工作中，进行了密度泛函理论计算，以研究当进行OH、CO、Mg和Na离子取代时的六方HAP晶胞。提出了一种通过研究将这些不同离子纳入HAP晶胞所产生的变化来研究HTE晶体学晶胞的方法。对每种情况下的形成焓和电子衍射图的晶体学特征进行了分析。结果表明，具有OH缺陷、Mg原子取代、碳酸盐和间隙Na时，HAP的结构稳定性增强。所生成结构的模拟电子衍射图显示出与人牙釉质相似的特征。因此，这些结果解释了实验X射线衍射图和电子衍射图中显示的难以分辨的结构变化。
