A Combination of Factors: Tuning the Affinity of Europium Receptors for Phosphate in Water

Abstract

Although recognition of hard anions by hard metal ions is primarily achieved via direct coordination, electrostatic and hydrogen-bonding interactions also play essential roles in tuning the affinity of such supramolecular receptors for their target. In the case of EuIII hydroxypyridinone-based complexes, the addition of a single charged group (−NH3 +, −CO2 –, or −SO3 –) or neutral hydrogen-bonding moiety (−OH) peripheral to the open coordination site substantially affects the affinity of the metal receptor for phosphate in water at neutral pH. A single primary ammonium increases the first association constant for phosphate in neutral water by 2 orders of magnitude over its neutral analogue. The addition of a peripheral alcohol group also increases the affinity of the receptor but to a lesser degree (21-fold). On the other hand, negatively charged complexes bearing either a carboxylate or sulfate moiety have negligible affinity for phosphate. Interestingly, the peripheral group also influences the stoichiometry of the lanthanide receptor for phosphate. While the complex bearing a −NH3 + group binds phosphate in a 1:2 ratio, those with −OH and H (control) both form 1:3 complexes. Although the positively charged EuIII-Lys-HOPO has the highest K a1 for phosphate, a greater increase in luminescence intensity (36-fold) is observed with the neutral EuIII-Ser-HOPO complex. Notably, whereas the affinity of the EuIII complexes for phosphate is substantially influenced by the presence of a single charged group or hydrogen-bond donor, their selectivity for phosphate over competing anions remains unaffected by the addition of the peripheral groups.

Advances in anion binding and sensing using luminescent lanthanide complexes

“…Reproduced from ref. 41 with permission from the American Chemical Society, copyright 2019. solution. By increasing the concentration of the Gd(III) complexes at pH 7.2 in the presence of HPO 4 2À , phosphocreatine (PCr) and ATP (8.7, 5.5 and 7.9 mM, respectively), 31…”

Advances in fluorescent and colorimetric sensors for anionic species

“…Appealingly, luminescence from this class of complex gives rise to sharp emission peaks and has a relatively long lifetime, meaning that time-gated measurements can be implemented to remove background fluorescence in complex environments. Their affinity for various analytes can be modified though changes in ligand structure or by altering the coordination number of the central metal ion [84,85]. An anion binding event can lead to detectable optical changes caused by alterations in the lanthanide-ligand coordination environment or by displacement of emission quenching water molecules, as demonstrated by a number of recent examples [85][86][87][88][89].…”

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array

“…In the presence of 5 mM AMP, a q value of 0.3-0.7 was found, indicating partial hydration, possibly reecting the weaker monodentate binding of AMP, in accordance with previous examples of phosphate binding at lanthanide centres. 18,23,25…”

“…This strategy has been utilised to develop receptors for anions including phosphate, [23][24][25] bicarbonate, 26,27 uoride, [28][29][30] and lactate. 31 Such Ln(III) complexes are kinetically stable, avoiding complications arising from metal ion dissociation upon anion binding.…”