Lanthanide(III) ions as spectroscopic and paramagnetic probes for the elucidation of aqueous solution chemistry interactions with anions and nucleic acids

Herein we discuss a master oscillator power oscillator (MOPO)/Laser system that was built to collect spectroscopic information for elucidating the coordination environment surrounding lanthanide (III) (Ln(III)) ions bound to macrocyclic ligands and to nucleic acids in aqueous buffered solutions. This instrument has detection limits of 50 nM for direct excitation of the 7F 0 &rarr 5D0 transition of aqueous Eu(III), 1 nM for Eu(EDTA) (ethylenediaminetetraacetic acid) and 1 nM Eu(DTPA) (diethylenetriaminepentaacetic acid) at pH 6.5 and I = 0.100 M NaCl. The versatility of the instrument is demonstrated by excitation scans over a broad wavelength range in the visible for aqueous solutions of complexes of Eu(III), Dy(III), Sm(III) and Tb(III). The coordination environment of Eu(III) was studied in the presence of a variety of weakly coordinating anions, and at different pH values. Aqueous Ln(III) ions were found to exist as aggregates at near neutral pH at millimolar concentrations, but speciation was highly sensitive to different anions. Steady state excitation and time-resolved luminescence spectroscopy are consistent with the formation of inner-sphere nitrate and fluoride complexes, but outer-sphere perchlorate and chloride complexes at pH 6.5 and 5.0. The coordination environment of two dinuclear Ln(III) macrocycles created for the catalytic cleavage of ribonucleic acids (RNA) and for application as paramagnetic chemical exchange saturation transfer (PARACEST) agents were spectroscopically investigated to understand the binding modes of anions (diethyl phosphate (DEP), monophosphate (MP), fluoride, phosphate, carbonate, double stranded 5'GGCCGGCC3' (GGCC) and a DNA hairpin loop 5'GGCCTTTGGCC3' (HP1)) to the Ln(III) centers. Studies of the mixed heterodinuclear Eu(III)-Nd(III) pair used luminescence resonance energy transfer (LRET) to show that HP1, GGCC, MP, phosphate bridged between the Ln(III) centers. Fluoride and carbonate are non-bridging anions. The binding mode of DEP is unclear. Finally, Eu(III) direct excitation photoluminescence was utilized to investigate metal ion binding sites in a GAAA ribonucleic acid (RNA) hairpin loop and in similar tetraloops with either a different loop sequence or different stem. The deoxyribonucleic acid (DNA) analogs were compared in order to better understand the differences between metal ion binding to RNA versus DNA. In addition, proton and phosphorous 1D and 2D proton nuclear magnetic resonance (NMR) spectroscopy was used to generate a NMR solution structure of the GAAA tetraloop and to define the ligands of the nucleic acids which make up the metal ion binding site. Eu(III) photoluminescence and NMR spectroscopy show that the GAAA tetraloop has two Eu(III) binding sites, one in the tetraloop near the G5 base, phosphate and A6 phosphate and one in the stem near the C3 and C4 bases.