Speciation of Ferri protoporphyrin IX in Aqueous and Mixed Aqueous Solution Is Controlled by Solvent Identity, pH, and Salt Concentration
Speciation of Ferri protoporphyrin IX in Aqueous and Mixed Aqueous Solution Is Controlled by Solvent Identity, pH, and Salt Concentration
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DOI：
10.1021/ic900647y
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发表时间：
2009-08-17
影响因子：
4.6
通讯作者：
Egan, Timothy J.
中科院分区：
文献类型：
--
作者：
Asher, Constance;de Villiers, Katherine A.;Egan, Timothy J.
通过 UV-vis、H-1 NMR、磁力和扩散测量研究了铁原卟啉 IX (Fe(III)PPIX) 在水性溶剂和混合水性有机溶剂中的形态。 Fe(III)PPIX 被发现可形成单体、pi-pi 二聚体、mu-氧二聚体和 mu-氧二聚体的 pi 堆积聚集体，具体取决于浓度、pH、盐的存在、温度和溶剂特性。这凸显了 Fe(III)PPIX 在溶液中行为的复杂性。然而，mu-氧二聚体的存在或不存在显然取决于溶剂，一系列非质子溶剂（5.64 M DMSO、丙酮、DMF、THF、2,6-二甲基吡啶）在 pH 10 时均促进 mu-氧二聚体的形成。相比之下，质子溶剂（甲醇、乙醇、丙醇、乙二醇、二甘醇和甲酰胺）在相同浓度和相同条件下仅产生根据溶剂极性，pi-pi 二聚体与单体进行不同的混合。先前已证明pi-pi二聚体存在于纯水溶液中。另一方面，在水溶液中存在 4.25 M NaCl 的情况下，紫外可见光谱和扩散测量都表明在 pH 10 时存在 mu-氧二聚体的大 pi 堆积聚集体。至少在含水 DMSO 中，二聚常数的温度依赖性表明 mu-氧二聚体形成过程是吸热的，因此完全是熵驱动的。这强烈表明，mu-氧二聚体的形成是由去溶剂化驱动的，既可以充当Fe(III)PPIX的轴向水/氢氧化物配体的氢键供体和受体的溶剂，也可以阻止这种二聚体的形成，而那些不能充当氢键供体的溶剂则会促进这种二聚体的形成。这些发现允许预测不同混合溶剂系统中以及任何给定 pH、浓度和温度下的含水 DMSO 中存在的 Fe(III)PPIX 物种。
The speciation of ferriprotoporphyrin IX (Fe(III)PPIX) in aqueous and mixed aqueous-organic solvents has been investigated by UV-vis, H-1 NMR, magnetic, and diffusion measurements. Fe(III)PPIX has been found to form monomers, pi-pi dimers, mu-oxo dimers, and pi-stacked aggregates of mu-oxo dimers depending on concentration, pH, the presence of salts, temperature, and solvent identity. This highlights the complexity of the behavior of Fe(III)PPIX in solution. However, the presence or absence of the mu-oxo dimer is clearly dependent on solvent, with a series of aprotic solvents (5.64 M DMSO, acetone, DMF, THF, 2,6-lutidine) all promoting mu-oxo dimer formation at pH 10. By contrast, protic solvents (methanol, ethanol, propanol, ethylene glycol, diethylene glycol, and formamide) at the same concentration and under the same conditions give rise only to the pi-pi dimer variously mixed with monomer depending on solvent polarity. The pi-pi dimer has previously been shown to be present in purely aqueous solution. In the presence of 4.25 M NaCl in aqueous solution, on the other hand, both UV-vis spectra and diffusion measurements suggest the presence of large pi-stacked aggregates of mu-oxo dimers at pH 10. In aqueous DMSO at least, the temperature dependence of the dimerization constant shows that the process of mu-oxo dimer formation is endothermic and hence entirely entropy driven. This strongly suggests that formation of the mu-oxo dimer is driven by desolvation, with solvents that can act as both hydrogen bond donors and acceptors to the axial water/hydroxide ligand of Fe(III)PPIX preventing formation of this dimer species, while those that cannot act as hydrogen bond donors facilitate it. The findings permit prediction of the Fe(III)PPIX species present in different mixed solvent systems and in the case of aqueous DMSO at any given pH, concentration, and temperature.