APTRA-Based Luminescent Lanthanide Complexes Displaying Enhanced Selectivity for Mg2.

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). 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 by 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, by using such an approach. An apparent dissociation constant for magnesium binding of Kd =2.4 mm was calculated in the serum background.