Luminescence resonance energy transfer in heterodinuclear Ln^III complexes for sensing biologically relevant anions

Dinuclear lanthanide(III) complexes of the macrocycles 1,4-bis[1-4,7,10- tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane]-m-xylene (1) and 1,3-bis[1-4,7,10-tris(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane]-p-xylene (2) with Ln^III = Eu^III, Tb^III were prepared. Studies using direct excitation (⁷F₀ → ⁵D₀) europium(III) luminescence spectroscopy show that several anions including phosphate, methylphosphate, double-stranded DNA, a DNA hairpin loop, and fluoride bind to both Eu^III centers in Eu ₂(1) in solutions at pH 7.0, 0.100 M NaNO₃. Recovered luminescence lifetime data is consistent with replacement of water ligands by these anions. There are distinct changes in the relative intensities of Eu ^III emission peaks for complexes of Eu₂(1) and Eu ₂(2) with phosphate, fluoride, carbonate, and phosphate ester ligands. This suggests that the Eu^III dinuclear complexes show promise as ratiometric sensors for these anions. In contrast, the emission peak ratios for the Tb₂(1) and Tb₂(2) complex show a lower response upon binding of each anionic ligand. Luminescence resonance energy transfer (LRET) studies with mixed Eu^III/Nd^III complexes show that the phosphate complexes of Ln₂(1) or Ln₂(2) both have Ln^III-Ln^III internuclear distances of 7.7±0.9 Å and the methylphosphate complexes of Ln₂(1) and Ln ₂(2) have internuclear distances of 7.8±0.6 and 8.0±0.7 Å, respectively. LRET experiments also show the two lanthanide ion centers in Ln₂(2) are 8.4±0.5 Å apart when bound to GC rich duplex DNA. None of the other anions (carbonate, fluoride, DNA hairpins) form complexes that show measurable energy transfer between the lanthanide ions, which is consistent with intranuclear distances >8 Å. Dinuclear Eu^III or Tb^III complexes bind to a series of biologically relevant anions with concomitant loss of water ligands. Anion binding induces characteristic changes in emission peak intensities for Eu^III analogs. A few anions promote luminescence resonance energy transfer between Eu^III and Nd^III centers.