我们提供证据证实了骨骼矿物血小板之间的柠檬酸盐阴离子桥并推测他们的存在维持了它们之间的无序区域中独立的血小板，而不是逐渐转变成更大更有序的矿物盐。为了评估这一假说，我们采用了磷酸钙矿物层间的柠檬酸盐桥模型，即磷酸八钙柠檬酸复盐（OCP-柠檬酸盐）。我们使用多核固体NMR光谱组合，粉末X-射线衍射和第一原理电子结构运算来提出这种材料的定量结构，其中柠檬酸阴离子存在于水化层中，连接了相邻磷灰石层。为了评估原生骨矿物质中这种结构的相关性，就当前所知，我们首次展示了骨骼中的17O NMR数据并将其与OCP-柠檬酸盐和其它与骨骼相关的磷酸钙矿物中的17O NMR数据进行比较。我们从这项工作中所推导出的建议骨矿物结构模型是一种在OCP-柠檬酸盐样水化层之间夹有极薄磷灰石血小板的层状结构。这种结构可以解释一些骨骼矿物质已知的结构特点：成熟骨矿物晶体的极薄板状形态，强键合水分子显著数量的存在和相对高浓度的磷酸氢盐以及矿物血小板之间无序区域的维护。
We provide evidence that citrate anions bridge between mineral platelets in bone and hypothesize that their presence acts to maintain separate platelets with disordered regions between them rather than gradual transformations into larger, more ordered blocks of mineral. To assess this hypothesis, we take as a model for a citrate bridging between layers of calcium phosphate mineral a double salt octacalcium phosphate citrate (OCP-citrate). We use a combination of multinuclear solid-state NMR spectroscopy, powder X-ray diffraction, and first principles electronic structure calculations to propose a quantitative structure for this material, in which citrate anions reside in a hydrated layer, bridging between apatitic layers. To assess the relevance of such a structure in native bone mineral, we present for the first time, to our knowledge, 17O NMR data on bone and compare them with 17O NMR data for OCP-citrate and other calcium phosphate minerals relevant to bone. The proposed structural model that we deduce from this work for bone mineral is a layered structure with thin apatitic platelets sandwiched between OCP-citrate–like hydrated layers. Such a structure can explain a number of known structural features of bone mineral: the thin, plate-like morphology of mature bone mineral crystals, the presence of significant quantities of strongly bound water molecules, and the relatively high concentration of hydrogen phosphate as well as the maintenance of a disordered region between mineral platelets.
Bone contains ∼2% wt citrate; however, its role in bone remains a much-debated question. We propose a new structure for bone mineral in which citrate in hydrated layers forms bridges between mineral platelets, which can explain a number of observations at odds with previous models. The incorporation of citrate between mineral platelets can explain the flat, plate-like morphology of bone mineral platelets and may be important in controlling the crystallinity of bone mineral, which in turn, is highly relevant to the mechanical properties of bone.