Titanium(IV) and Vitamin C: Aqueous Complexes of a Bioactive Form of Ti(IV)
Titanium(IV) and Vitamin C: Aqueous Complexes of a Bioactive Form of Ti(IV)
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DOI：
10.1021/ic301545m
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发表时间：
2012-10-15
影响因子：
4.6
通讯作者：
Valentine, Ann M.
中科院分区：
文献类型：
--
作者：
Buettner, Katherine M.;Collins, Joseph M.;Valentine, Ann M.
抗坏血酸是使Ti（IV）在水溶液中稳定的生物相关配体之一。本文报道了一系列pH依赖的L-抗坏血酸钛（IV）配合物。在光谱电位法的指导下，研究了该体系中水溶液相互作用的重要方面，包括配体结合模式，pH依赖的金属-配体化学计量，以及金属离子促进的水解和氢氧化物结合的重要性。所有金属离子-配体-质子络合物的稳定常数确定的模型优化和非线性最小二乘拟合的结合光谱电位滴定数据的日志β（MLH）的值在模型中的过程。一个物种形成图是从模型中描述的稳定常数集生成的。在pH 3-10范围内，水相形态的特征在于以下络合物作为添加的碱的函数的顺序出现：Ti（asc）（2）（0）-> Ti（asc）（3）（2-）-> Ti（asc）（2）（OH）（2）（2-）-> Ti（asc）（OH）（4）（2-）。这些物种分别在pH < 3、pH 4-5、pH类似于8和pH > 10下占主导地位，最小对数稳定常数（β值）为25.70、36.91、16.43和-6.91。从电喷雾质谱，金属配体结合实验，和动力学测量的结果支持的形态，其特征在于通过双齿螯合的抗坏血酸二价阴离子的钛（IV）离子通过质子置换，和pH值依赖的金属配体结合基序的配体添加，然后由金属离子促进水解，逐步配体解离，并伴随着结合的氢氧根离子。此外，报道了抗坏血酸钛与柠檬酸盐的配体交换动力学，以更好地理解抗坏血酸钛在生物学相关条件下的可能命运。
Ascorbic acid is among the biorelevant ligands that render Ti(IV) stable in aqueous solution. A series of pH-dependent titanium(IV) coordination complexes of L-ascorbic acid is described. Directed by spectropotentiometric methods, important aspects of the aqueous interactions in this system are investigated, including ligand binding mode, pH-dependent metal-ligand stoichiometry, and the importance of metal ion-promoted hydrolysis and the binding of hydroxide. Stability constants are determined for all metal ion-ligand-proton complexes by a process of model optimization and nonlinear least-squares fitting of the combined spectropotentiometric titration data to the log beta(MLH) values in the model. A speciation diagram is generated from the set of stability constants described in the model. In the range pH 3-10, the aqueous speciation is characterized by the sequential appearance of the following complexes as a function of added base: Ti(asc)(2)(0) -> Ti(asc)(3)(2-) -> Ti(asc)(2)(OH)(2)(2-) -> Ti(asc)(OH)(4)(2-). These species dominate the speciation at pH < 3, pH 4-5, pH similar to 8, and pH > 10, respectively, with minimum log stability constants (beta values) of 25.70, 36.91, 16.43, and -6.91. Results from electrospray mass spectrometry, metal-ligand binding experiments, and kinetics measurements support the speciation, which is characterized by bidentate chelation of the ascorbate dianion to the titanium(IV) ion via proton displacement, and a pH-dependent metal ligand binding motif of ligand addition followed by metal ion-promoted hydrolysis, stepwise ligand dissociation, and the concomitant binding of hydroxide ion. Additionally, the kinetics of ligand exchange of titanium ascorbate with citrate are reported to understand better the possible fate of titanium ascorbate under biologically relevant conditions.