Several solution properties of complexes formed between the trivalent lanthanide ions (LnIII) and the macrocyclic ligand DOTP8-, including stability constants, protonation equilibria, and interactions of the LnDOTP5- complexes with alkali metal ions, have been examined by spectrophotometry, potentiometry, osmometry, and 1H, 31P, and 23Na NMR spectroscopy. Spectrophotometric competition experiments between DOTP and arsenazo III for complexation with the LnIII ions at pH 4 indicate that the thermodynamic stability constants (log KML) of LnDOTP5- range from 27.6 to 29.6 from LaIII to LuIII. The value for LaDOTP5- obtained by colorimetry (27.6) was supported by a competition experiment between DOTP and EDTA monitored by 1H NMR (27.1) and by a potentiometric competition titration between DTPA and DOTP (27.4). Potentiometric titrations of several LnDOTP5- complexes indicated that four protonation steps occur between pH 10 and 2; the protonation constants determined by potentiometry were consistent with 31P shift titrations of the LnDOTP5- complexes. Dissection of the 31P shifts of the heavy LnDOTP5- complexes (Tb → Tm) into contact and pseudocontact contributions showed that the latter dominated at all pH values. The smaller 31P shifts observed at lower pH for TmDOTP5- were partially due to relaxation of the chelate structure which occurred upon protonation. The 31P shifts of other LnDOTP5- complexes (Ln = Pr, Nd, Eu) showed a different pH-dependent behavior, with a change in chemical shift direction occurring after two protonation steps. This behavior was traced to a pH-dependent alteration of the contact shift at the phosphorus nuclei as these complexes were protonated. 23Na NMR studies of the interactions of TmDOTP5- with alkali and ammonium cations showed that Et4N+ and Me4N+ did not compete effectively with Na+ for the binding sites on TmDOTP5-, while K+ and NH4+ competed more effectively and Cs+ and Li+ less effectively. A 23Na shift of more than 400 ppm was observed at low Na+/TmDOTP5- ratios and high pH, indicating that Na+ was bound near the 4-fold symmetry axis of TmDOTP5- under these conditions. Osmolality measurements of chelate samples containing various amounts of Na+ indicated that at high Na+/TmDOTP5- ratios at least three Na+ ions were bound to TmDOTP5-. These ions showed a significantly smaller 23Na-bound shift, indicating they must bind to the chelate at sites further away from the 4-fold symmetry axis. Fully bound 23Na shifts and relaxation rate enhancements and binding constants for all NaxHyTmDOTP species were obtained by fitting the observed 23Na shift and relaxation data and the osmometric data, using a spreadsheet approach. This model successfully explained the 23Na shift and osmolality observed for the commercial reagent Na4HTmDOTP·3NaOAc (at 80 mM at pH 7.4).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry