Time-resolved fluorescent chirality sensing and imaging of malate in aqueous solution

Luminescent and Chiroptical Properties of 1 : 1 Eu (III) : Tetracycline Species Probed by Circularly Polarized Luminescence

This study investigates the significantly different luminescent and chiroptical properties of tetracycline (TC) when coordinated to Eu(III). The approach involves understanding the 1) speciation of TC and 2) conformation and species formed between Eu(III) and TC in a ratio of 1 : 1 in a dimethylformamide (DMF) solution and as a function of the pH value. By identifying the conformational changes of the various 1 : 1 Eu(III) : TC species, the results from this study explain information on the local microenvironment about the Eu(III) metal center. In particular, ⁵ D₀ ←⁷ F₀ Eu(III) laser excitation spectroscopy was employed to distinguish the different types of species found in solution in order to understand the interaction between Eu(III) and TC. On the other hand, circularly polarized luminescence (CPL) spectroscopy was used to understand the structural changes within the 1 : 1 Eu(III) : TC complex that could be related to the chirality of the Eu(III)-containing species. The CPL spectrum serves as a "fingerprint" to indicate the conformational changes within the 1 : 1 Eu(III) : TC complex as a result of the chiroptical signal arising from the various Eu(III) : TC species.

Citrate analysis using capillary electrophoresis and complexation with Eu3+-tetracycline

A sensitive assay for citrate was developed. Citrate was incubated with 50 μM Eu³⁺-tetracycline and the complex separated using capillary electrophoresis utilizing post-column laser-induced luminescence detection in a sheath flow cuvette. Signal was linear with citrate concentration from 10 μM to 200 nM. Injection volumes were 320 pL. For the 200 nM sample, this corresponds to the injection of 64 amoles of citrate. Separation time was < 90 s with a total run time of 5 min. As an application the method was used to analyze citrate in agricultural and medicinal products. The method was also used to develop an assay for the enzyme citrate synthase.

Spectroscopic analysis of ligand binding to lanthanide-macrocycle platforms

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 microM crystallographically characterized TBA x Eu(DO2A)(DPA)) were titrated with EuCl3 (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 (5)D0 --> (7)F3 transition (lambda(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 (Kc), which in conjunction with independent measurement of the Eu(DPA)(+) formation constant (Ka) allowed calculation of the ternary complex formation constant (Ka'). With this binding affinity by competition (BAC) assay, we determined that Ka' = 10(8.21) M(-1), which is approximately 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.