Morgan M Thomas, Sumalekshmy S, Sarwar Mysha, Beck Hillary, Crooke Stephen, Fahrni Christoph J
School of Chemistry and Biochemistry and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology , 901 Atlantic Drive, Atlanta, Georgia 30332, United States.
J Phys Chem B. 2014 Dec 11;118(49):14196-202. doi: 10.1021/jp5077406. Epub 2014 Oct 14.
Ternary complex formation with solvent molecules and other adventitious ligands may compromise the performance of metal-ion-selective fluorescent probes. As Ca(II) can accommodate more than 6 donors in the first coordination sphere, commonly used crown ether ligands are prone to ternary complex formation with this cation. The steric strain imposed by auxiliary ligands, however, may result in an ensemble of rapidly equilibrating coordination species with varying degrees of interaction between the cation and the specific donor atoms mediating the fluorescence response, thus diminishing the change in fluorescence properties upon Ca(II) binding. To explore the influence of ligand architecture on these equilibria, we tethered two structurally distinct aza-15-crown-5 ligands to pyrazoline fluorophores as reporters. Due to ultrafast photoinduced electron-transfer (PET) quenching of the fluorophore by the ligand moiety, the fluorescence decay profile directly reflects the species composition in the ground state. By adjusting the PET driving force through electronic tuning of the pyrazoline fluorophores, we were able to differentiate between species with only subtle variations in PET donor abilities. Concluding from a global analysis of the corresponding fluorescence decay profiles, the coordination species composition was indeed strongly dependent on the ligand architecture. Altogether, the combination of time-resolved fluorescence spectroscopy with selective tuning of the PET driving force represents an effective analytical tool to study dynamic coordination equilibria and thus to optimize ligand architectures for the design of high-contrast cation-responsive fluorescence switches.
与溶剂分子和其他外来配体形成三元络合物可能会影响金属离子选择性荧光探针的性能。由于Ca(II)在第一配位层中可以容纳超过6个供体，常用的冠醚配体容易与这种阳离子形成三元络合物。然而，辅助配体施加的空间应变可能导致一系列快速平衡的配位物种，阳离子与介导荧光响应的特定供体原子之间的相互作用程度各不相同，从而减少了Ca(II)结合时荧光性质的变化。为了探究配体结构对这些平衡的影响，我们将两种结构不同的氮杂-15-冠-5配体连接到吡唑啉荧光团上作为报告基团。由于配体部分对荧光团的超快光诱导电子转移（PET）猝灭，荧光衰减曲线直接反映了基态下的物种组成。通过对吡唑啉荧光团进行电子调谐来调节PET驱动力，我们能够区分PET供体能力只有细微差异的物种。从对相应荧光衰减曲线的整体分析得出结论，配位物种组成确实强烈依赖于配体结构。总之，时间分辨荧光光谱与PET驱动力的选择性调谐相结合，是研究动态配位平衡从而优化用于设计高对比度阳离子响应荧光开关的配体结构的有效分析工具。
