Characterization of lanthanide(III) DOTP complexes: Thermodynamics, protonation, and coordination to alkali metal ions

被引:102
作者
Sherry, AD
Ren, J
Huskens, J
Brucher, E
Toth, E
Geraldes, CFCG
Castro, MMCA
Cacheris, WP
机构
[1] UNIV TEXAS,SW MED CTR,MARY NELL & RALPH B ROGERS MAGNET RESONANCE CTR,DEPT RADIOL,DALLAS,TX 75235
[2] LAJOS KOSSUTH UNIV,DEPT INORGAN & ANALYT CHEM,H-4010 DEBRECEN,HUNGARY
[3] UNIV COIMBRA,FAC SCI & TECHNOL,DEPT BIOCHEM,P-3049 COIMBRA,PORTUGAL
[4] MATTSON JACK GRP,ST LOUIS,MO 63146
关键词
D O I
10.1021/ic9600590
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
Several solution properties of complexes formed between the trivalent lanthanide ions (Ln(III)) and the macrocyclic ligand DOTP8-, including stability constants, protonation equilibria, and interactions of the LnDOTP(5-) complexes with alkali metal ions, have been examined by spectrophotometry, potentiometry, osmometry, and H-1, P-31, and Na-23 NMR spectroscopy. Spectrophotometric competition experiments between DOTP and arsenate III for complexation with the Ln(III) ions at pH 4 indicate that the thermodynamic stability constants (log K-ML) Of LnDOTP(5-) range from 27.6 to 29.6 from La-III to Lu-III. The value for LaDOTP5- obtained by colorimetry (27.6) was supported by a competition experiment between DOTP and EDTA monitored by H-1 NMR (27.1) and by a potentiometric competition titration between DTPA and DOTP (27.4). Potentiometric titrations of several LnDOTP(5-) complexes indicated that four protonation steps occur between pH 10 and 2; the protonation constants determined by potentiometry were consistent with P-31 Shift titrations of the LnDOTP(5-) complexes. Dissection of the P-31 Shifts of the heavy LnDOTP(5-) complexes (To --> Tm) into contact and pseudocontact contributions showed that the latter dominated at all pH values. The smaller P-31 shifts observed at lower pH for TmDOTP5- were partially due to relaxation of the chelate structure which occurred upon protonation. The P-31 shifts Of Other LnDOTP(5-) 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. Na-23 NMR studies of the interactions' of TmDOTP5- with alkali and ammonium cations showed that Et(4)N(+) and Me(4)N(+) 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 Na-23 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 Na-23-bound shift, indicating they must bind to the chelate at sites further away from the 4-fold symmetry axis. Fully bound Na-23 shifts and relaxation rate enhancements and binding constants for all NaxHyTmDOTP species were obtained by fitting the observed Na-23 shift and relaxation data and the osmometric data, using a spreadsheet approach. This model successfully explained the Na-23 shift and osmolality observed for the commercial reagent Na4HTmDOTP . 3NaOAc (at 80 mM at pH 7.4).
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页码:4604 / 4612
页数:9
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