Spectroscopic Analysis of Ligand Binding to Lanthanide−Macrocycle Platforms

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• 作者：James P. Kirby ; Morgan L. Cable ; Dana J. Levine ; Harry B. Gray ; Adrian Ponce
• 刊名：Analytical Chemistry
• 出版年：2008
• 出版时间：August 1, 2008
• 年：2008
• 卷：80
• 期：15
• 页码：5750-5754
• 全文大小：167K
• 年卷期：v.80,no.15(August 1, 2008)
• ISSN：1520-6882

文摘

A high-affinity, binary Eu³⁺ receptor site consisting of 1,4,7,10-tetraazacyclododecane-1,7-diacetate (DO2A) was constructed with the goal of improving the detection of dipicolinic acid (DPA), a major component of bacterial spores. Ternary Eu(DO2A)(DPA)⁻ complex solutions (1.0 µM crystallographically characterized TBA·Eu(DO2A)(DPA)) were titrated with EuCl₃ (1.0 nM−1.0 mM); increased Eu³⁺ concentration resulted in a shift in equilibrium population from Eu(DO2A)(DPA)⁻ to Eu(DO2A)⁺ and Eu(DPA)⁺, which was monitored via the ligand field sensitive ⁵D₀ → ⁷F₃ transition (λ_em = 670−700 nm) using luminescence spectroscopy. A best fit of luminescence intensity titration data to a two-state thermodynamic model yielded the competition equilibrium constant (K_c), which in conjunction with independent measurement of the Eu(DPA)⁺ formation constant (K_a) allowed calculation of the ternary complex formation constant (K_a′). With this binding affinity by competition (BAC) assay, we determined that K_a′ = 10^8.21 M⁻¹, which is ∼1 order of magnitude greater than the formation of Eu(DPA)⁺. In general, the BAC assay can be employed to determine ligand binding constants of systems where the lanthanide platform (usually a binary complex) is stable and the ligand bound versus unbound states can be spectroscopically distinguished.