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Published August 8, 2008 | Published
Journal Article Open

Spectroscopic Analysis of Ligand Binding to Lanthanide−Macrocycle Platforms

Abstract

A high-affinity, binary Eu^(3+) 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 EuCl_3 (1.0 nM−1.0 mM); increased Eu^(3+) 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 ^5D_0 → ^7F_3 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.

Additional Information

© 2008 American Chemical Society. Received for review January 22, 2008. Accepted April 18, 2008. Publication Date (Web): June 26, 2008. The authors thank Larry Henling and Mike Day for assistance. M.L.C. acknowledges support from the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program. D.J.L. was supported by the Amgen Fellowship Program. Work at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration was supported by NASA's Astrobiology and Planetary Protection Programs and Department of Homeland Security's Chemical and Biological Research & Development Program. Work at the Beckman Institute was supported by NSF and the Arnold and Mabel Beckman Foundation.

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August 19, 2023
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