APTRA-based luminescent lanthanide complexes displaying enhanced selectivity for Mg2+

Abstract

A series of three europium(III) complexes has been created in which an APTRA moiety has been integrated into the sensitising chromophore (APTRA = o‐aminophenol‐N,N,N‐triacetate). The constitutionally isomeric complexes EuL1 and EuL2 feature the APTRA unit linked to a metal‐bound pyridine ring through an alkynyl unit, differing according to the disposition of the APTRA substituents relative to the C≡C unit (para‐N and para‐O respectively). In EuL3, the APTRA ring is directly bonded to the Eu‐coordinated pyridine (para‐O). The metal binding affinities for magnesium, calcium and zinc ions have been measured using emission and excitation spectroscopy. The pyridylalkynylaryl systems, EuL1 and EuL2, offer superior affinity and selectivity for Mg2+. The Mg2+ affinities are surprisingly very different from prior studies on structurally related systems that incorporate organic fluorophores as reporters, as opposed to the macrocyclic Eu complex moiety. A much reduced affinity for calcium and zinc ‐ possibly arising from the lower donor ability of the aryl N or O atoms arising from extended conjugation ‐ means that magnesium ion concentrations can be measured directly in serum for the first time, using such an approach. An apparent dissociation constant for magnesium binding of Kd = 2.4 mM was calculated in the serum background.