Calcium orthophosphates: crystallization and dissolution.
Calcium orthophosphates: crystallization and dissolution.
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DOI：
10.1021/cr0782574
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发表时间：
2008-11
期刊：
影响因子：
62.1
通讯作者：
Nancollas, George H.
中科院分区：
文献类型：
--
作者：
Wang, Lijun;Nancollas, George H.
正磷酸钙是骨骼和牙齿的主要矿物成分，人们对了解其生长、溶解和相稳定性的物理机制非常感兴趣。根据定义，所有正磷酸钙都由三种主要化学元素组成：钙（氧化态+2），磷（氧化态+5）和氧（氧化态-2）。1正磷酸盐基团（PO 4 3-）在结构上不同于Meta（PO 3-）、pyro（P2 O 7 4-）和poly（PO 3）n n-。在这篇综述中，只讨论正磷酸钙。许多正磷酸钙的化学组成包括氢，作为酸性正磷酸根阴离子，如HPO 4 2-或H2 PO 4-，和/或结合的水，如磷酸二钙二水合物（CaHPO 4·2 H2O）。1大多数正磷酸钙微溶于水，但都溶于酸;钙与磷酸盐的摩尔比（Ca/P）和溶解度是区分相的重要参数（表1），晶体学数据总结在表2中。通常，Ca/P比越低，磷酸钙相的酸性和可溶性越强。2现在普遍认识到，许多磷酸钙的结晶涉及亚稳前体相的形成，该亚稳前体相随后随着沉淀反应的进行而溶解。因此，复杂的中间相可以参与结晶过程。此外，在体内存在的小肽，蛋白质，和无机添加剂以外的钙和磷酸盐有相当大的影响结晶，使其难以预测可能形成的相。3磷灰石矿物形成的研究由于可能形成几个磷酸钙相而变得复杂。最不可溶的羟基磷灰石（HAP）优先在中性或碱性条件下形成。在酸性更强的溶液中，经常会遇到透钙磷石（DCPD）和磷酸八钙（OCP）等相。即使在理想的HAP沉淀条件下，沉淀物通常是非化学计量的，这表明形成缺钙磷灰石。DCPD和OCP都被认为是磷灰石形成的可能前体。这可以通过DCPD和/或OCP的初始沉淀，随后转化为更磷灰石的相而发生。尽管在体外结晶过程中经常检测到DCPD和OCP，但在骨形成的体内研究中很少显示这些酸性磷酸钙相的存在。在后一种情况下，情况更复杂，因为存在大量的离子和分子，它们可以被结合到晶格中或吸附在微晶表面。在生物磷灰石中，DCPD和OCP通常仅在病理性钙化期间检测到，其中pH通常相对较低。在正常的体内钙化中，未发现这些相，表明涉及其他前体或形成初始无定形磷酸钙相（ACP），然后转化为磷灰石。
Calcium orthophosphates are the main mineral constituents of bones and teeth, and there is great interest in understanding the physical mechanisms that underlie their growth, dissolution, and phase stability. By definition, all calcium orthophosphates consist of three major chemical elements: calcium (oxidation state+ 2), phosphorus (oxidation state+ 5), and oxygen (oxidation state-2). 1 The orthophosphate group (PO4 3-) is structurally different from meta (PO3-), pyro (P2O7 4-), and poly (PO3) n n-. In this review, only calcium orthophosphates will be discussed. The chemical composition of many calcium orthophosphates includes hydrogen, either as an acidic orthophosphate anion such as HPO4 2-or H2PO4-, and/or incorporated water as in dicalcium phosphate dihydrate (CaHPO4· 2H2O). 1 Most calcium orthophosphates are sparingly soluble in water, but all dissolve in acids; the calcium to phosphate molar ratios (Ca/P) and the solubilities are important parameters to distinguish between the phases (Table 1) with crystallographic data summarized in Table 2. In general, the lower the Ca/P ratio, the more acidic and soluble the calcium phosphate phase. 2 It is now generally recognized that the crystallization of many calcium phosphates involves the formation of metastable precursor phases that subsequently dissolve as the precipitation reactions proceed. Thus, complex intermediate phases can participate in the crystallization process. Moreover, the in ViVo presence of small peptides, proteins, and inorganic additives other than calcium and phosphate has a considerable influence on crystallization, making it difficult to predict the possible phases that may form. 3 Studies of apatite mineral formation are complicated by the possibility of forming several calcium phosphate phases. The least soluble, hydroxyapatite (HAP), is preferentially formed under neutral or basic conditions. In more acidic solutions, phases such as brushite (DCPD) and octacalcium phosphate (OCP) are often encountered. Even under ideal HAP precipitation conditions, the precipitates are generally nonstoichiometric, suggesting the formation of calcium-deficient apatites. Both DCPD and OCP have been implicated as possible precursors to the formation of apatite. This may occur by the initial precipitation of DCPD and/or OCP followed by transformation to a more apatitic phase. Although DCPD and OCP are often detected during in Vitro crystallization, in ViVo studies of bone formation rarely show the presence of these acidic calcium phosphate phases. In the latter case, the situation is more complicated, since a large number of ions and molecules are present that can be incorporated into the crystal lattice or adsorbed at the crystallite surfaces. In biological apatite, DCPD and OCP are usually detected only during pathological calcification, where the pH is often relatively low. In normal in ViVo calcifications, these phases have not been found, suggesting the involvement of other precursors or the formation of an initial amorphous calcium phosphate phase (ACP) followed by transformation to apatite.
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