Calcium Orthophosphates: Crystallization and Dissolution
Calcium Orthophosphates: Crystallization and Dissolution
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DOI：
10.1002/chin.200905233
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发表时间：
2009-02
期刊：
影响因子：
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通讯作者：
Lijun Wang;G. H. Nancollas
中科院分区：
文献类型：
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作者：
Lijun Wang;G. H. Nancollas
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、溶解和相稳定的物理机制非常感兴趣。根据定义，所有的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正磷基(PO43-)在结构上不同于间位(PO3-)、焦磷(P2O74-)和聚(PO3)n-。在这篇综述中，将只讨论正磷酸钙。许多正磷酸钙的化学组成包括氢，或者以酸性正磷酸盐阴离子的形式，如HPO42-或HPO4-，和/或结合水，如二水磷酸氢钙(CaHPO4·2H2O)。1大多数正磷酸钙少量溶于水，但都溶于酸；钙与磷的摩尔比(Ca/P)和溶解度是区分相的重要参数(表1)与表2总结的结晶学数据。通常，钙/磷比越低，磷酸钙相越酸性和可溶。2现在普遍认为，许多磷酸钙的结晶涉及亚稳前体相的形成，这些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，体内存在的小肽、蛋白质以及除钙和磷之外的无机添加剂对结晶有相当大的影响，使得预测可能形成的相变变得困难。3磷灰石矿物形成的研究因可能形成多个钙磷酸盐相而变得复杂。最难溶解的羟基磷灰石(HAP)优先在中性或碱性条件下形成。在酸性较强的溶液中，经常会遇到刷石(DCPD)和磷酸八钙(OCP)等相。即使在理想的HAP沉淀条件下，沉淀物通常也是非化学计量比的，这表明形成了缺钙的磷灰石。DCPD和OCP都被认为是磷灰石形成的可能前体。这可能是由于DCPD和/或OCP的初始析出，然后转变为更多的磷灰化相。虽然DCPD和OCP经常在体外结晶过程中被检测到，但体内的骨形成研究很少显示这些酸性磷酸钙相的存在。在后一种情况下，情况更为复杂，因为存在大量的离子和分子，它们可以并入晶格或吸附在微晶表面。在生物磷灰石中，DCPD和OCP通常只有在病理性钙化过程中才能检测到，那里的pH值往往相对较低。在正常的活体钙化中，没有发现这些相，这表明有其他前体参与，或者形成了初始的无定形磷酸钙相(ACP)，然后转变为磷灰石。
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.