微量固体滴定法在磷酸钙盐复杂体系溶解度测量及生物矿化机理的研究

Calcium phosphates and related minerals are of fundamental interest to understanding dental caries, bone mineralization and many connected systems. However, so far the reported solubility for this system can not well explain many phenomena in vitro and in vivo, e.g. as previously claimed that dental enamel was protected against dissolution by nano-scale hydroxyapatite (HA) crystal, such that the adjacent solution remained undersaturated with respect to HA, typically inconsistent with the well-established thermodynamic principles, below saturation all solids tend to dissolve; if the reported phenomenon is real, the solution should have already reached saturation. In fact, the detailed knowledge of the solid phase and their true solubility are still lacking. The problem lies in the incongruent dissolution of calcium phosphates leading to phase transformations. The solubility determined by the conventional method through addition of large excess of solids has been demonstrated to be inappropriate and lack of detailed solution equilibria: all calculations have been based on simplifications, which are only crudely approximate. Thus, false equilibrium has already been used to explain many phenomena in vitro and vivo for decades. As a result, recently the absolute solid titration approach shows excellent reliability and reproducibility. Using solid titration, the true solubility isotherm of HA has been found to lie substantially lower than previously reported. In addition,contrary to wide belief, dicalcium phosphate dihydrate (DCPD) is not the most stable phase below pH 4.2, where calcium-deficient HA is less soluble. The misunderstanding here arises from the metastability of DCPD, which nucleates much more easily than HA at low pH. Such results indicate that the Ca-P system is in need of complete reappraisal. Therefore, in this proposal, the reliability of these solubility isotherms and titration curves needs to be verified, in particular the reason for the change of the solubility surface, and the true role of ions (Ca2+, PO43-, F-, CO32-) and other species (Sr2+, Mg2+, Na+) on their solubilities are still unknown. The detailed analysis of crystal structure and 3-D mapping by solid titration are therefore urgent. Then, the confusion in calcium hydroxyapatite chemistry that has persisted for decades might be resolved, and more profound understanding and explanation of the behaviour of calcium hydroxyapatite in many contexts would be obtained.

溶解度是生物矿化机理研究的基础，不仅决定着诸多化学反应的方向，还在许多生物过程中起着关键性作用。过去几十年，针对羟基磷灰石的溶解度进行了大量研究，但结果却并不能解释很多体内、外现象。我们前期的研究发现，传统的一次性添加过量固体的方法并不适用于磷酸钙盐这一复杂体系的溶解度研究。随着微量固体滴定法的发明，我们率先应用此方法在羟基磷灰石溶解度方面开展了大量的研究。本课题拟在以往研究基础上建立一个全新的针对复杂体系溶解度的测量及表征手段。为此，我们将首先应用该技术对相关物质溶解度进行反复验证，论证其可行性、可重复性及可靠性；其次，通过对溶解平衡时的固体沉淀物进行晶体学分析，绘制磷酸钙溶解度随pH 值和钙磷比变化的三维立体曲线图；最后研究不同外来离子对磷酸钙溶解度曲线图的影响，进而探讨生物矿化的机理。本研究旨在揭示过去数十年中有关磷酸钙化学的诸多谜团，并更准确和深入地理解其在不同条件下的行为机制。

采用自合成的高纯度羟基磷灰石作为滴定物，运用创新的固体滴定法重测了羟基磷灰石的体外溶解度并绘制了溶解度等温曲线，并于不同溶解曲面收集了沉淀；同时检测了烧结温度对碳酸羟基磷灰石溶解度的影响以及过量钙离子对羟基磷灰石溶解度的影响。发现新测试的溶解度曲线与先前同方法所测一致，证明了固体滴定法的可靠性和可重复性。同时发现沉淀物为低钙磷比的羟基磷灰石。此外，烧结温度会通过影响碳酸羟基磷灰石所含的碳酸根的量从而影响其溶解性能；而过量钙离子对羟基磷灰石溶解度的影响表现为在高pH区域导致其溶解度降低。