Luminescence Resonance Energy Transfer in Heterodinuclear LnIII 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 (F-7(0) --> D-5(0)) 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 Eu-2(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 Tb-2(1) and Tb-2(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 Ln(2)(1) or Ln(2)(2) both have Ln(III)-Ln(III) internuclear distances of 7.7 +/- 0.9 angstrom and the methylphosphate complexes of Ln(2)(1) and Ln(2)(2) have internuclear distances of 7.8 +/- 0.6 and 8.0 +/- 0.7 angstrom, respectively. LRET experiments also show the two lanthanide ion centers in Ln(2)(2) are 8.4 +/- 0.5 angstrom 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 angstrom.