A Copper(II) Macrocycle Complex for Sensing Biologically Relevant Organic Anions in a Competitive Fluorescence Assay: Oxalate Sensor or Urate Sensor?

Abstract

Fluorescence sensing of oxalate has garnered some attention in the past two decades as a result of this anion’s prominence and impact on society. Previous work on oxalate sensors and other divalent anion sensors has led to the conclusion that the sensors are selective for the anion under investigation. However, sensor selectivity is often determined by testing against a relatively small array of “guest” molecules or analytes and studies often exclude potentially interfering compounds. For example, studies on oxalate sensors have excluded compounds such as citrate and urate, which are anions in the biological matrices where oxalate is measured ( e.g ., urine, blood, and bacterial lysate). In the present study, we reassessed the selectivity of a dinuclear copper(II) macrocycle (Cu 2 L) in an eosin Y displacement assay using biologically relevant anions. Although previously reported as selective for oxalate, we found greater indicator displacement (fluorescence response) for urate and oxaloacetate and a significant response to citrate. These anions are larger than oxalate and do not appear to fit into the putative binding pocket of Cu 2 L. Consistent with previous reports, Cu 2 L did not release eosin Y in the presence of several other dicarboxylates, including adipate, glutarate, malate (except at 10 mM), fumarate, succinate, or malonate (except at 10 mM), and the monocarboxylate acetate. This was demonstrated by the failure of the anions to reverse eosin Y quenching by Cu 2 L. We also assessed, for the first time, other monocarboxylates, including butyrate, pyruvate, lactate, propionate, and formate. None of these anions were able to displace eosin Y, indicating no interaction with Cu 2 L that interfered with the eosin Y binding site. Single-crystal X-ray crystallography revealed that nonselective binding of the anions is likely partly caused by readily accessible copper(II) ions on the external surface of Cu 2 L. In addition, π–π stacking of urate with the aromatic groups of Cu 2 L cannot be ruled out as a contributor to binding. We conclude that Cu 2 L is not suitable for oxalate sensing in a biological matrix unless interfering compounds are selectively removed or masked.

Molecular Recognition of Biologically Relevant Anions with an Expanded Dinuclear Copper(II) Complex: An Efficient Sensor for Oxalate Anion in Aqueous Solution

“…found a better fit with a 1 : 2 binding model than a 1 : 1 bindng model. [38] Under the identical conditions, the association constants of 1⋅EY were determined for other anions were consistent with a 1 : 2 binding model. [59] The overall binding constants (in log K) and their corresponding free energies (À ΔG o ) are listed in Table 2.…”

“…[27][28] Using a supramolecular approach, there has been an immense interest in developing efficient chemical sensors for oxalate by colorimetric and fluorescence methods. [29][30][31][32][33][34][35][36][37][38][39][40][41] Previous studies demonstrated that the complexes of certain metal ions including zinc, [32][33] copper, [34][35][36][37][38][39] nickel, [40][41] and calcium [42] can be used as suitable chemosensors for detecting an oxalate anion in solution.…”

Analytical Methods for Oxalate Quantification: The Ubiquitous Organic Anion

“…There is substantial interest in developing indicator-displacement-based methods for detecting oxalate that do not require enzymes, liquid chromatography, or mass spectrometry. Many of these methods quantify fluorescence quenching, or, more often, gain of fluorescence upon addition of the analyte [36]. The principle of operation is shown in Fig.…”

“…A recent report by Hontz et al revisited a fluorescence-based indicator displacement method, clearly demonstrating that interference from untested compounds in biological samples may be more prevalent than is sometimes suggested [36]. Specifically, it was shown that a dinuclear copper(II)-based macrocycle with the ability to quench eosin Y fluorescence, although reported as oxalate selective, exhibited a stronger response to citrate and oxaloacetate.…”

Optical sensing of anions by macrocyclic and interlocked hosts

“…Anion binding and the concomitant release of eyosin Y was signalled by an increase in the characteristic fluorescence of the free dye. [82][83][84][85][86][87][88] In a twist on the indicator-displacement method, Zhang and co-workers synthesised a host a 2-(hydroxy)-naphthyl imino-functionalised pillar [5]arene 51 (Fig. 27), which coordinated to Fe(III) through its imine nitrogen and hydroxyl groups in H 2 O/DMSO (1 : 9, v/v).…”