Equilibrium studies on the Gd~(3+), Cu~(2+) and Zn ~(2+) complexes of BOPTA, DTPA and DTPA-BMA ligands: Kinetics of metal-exchange reactions of [Gd(BOPTA)]~(2-)

摘要

The stability and protonation, constants of the complexes formed between the ligand BOPTA [H_5BOPTA: 4-carboxy5, 8,11-tris(carboxymethyl)-1- phenyl-2-oxa-5,8,11-triazatridecan-13-oic acidj and Gd~(3+), Cu ~(2+), and Zn~(2+) have been determined by pH potentiometry (Gd~(3+), Zn~(2+)) and spectrophotometry (Cu~(2+)) at 25 °C and at constant ionic strength maintained by 0.1.5 M NaCl. For comparison, the stability and protonation constants of the complexes of Gd ~(3+), Cu~(2+), and Zn~(2+) formed with DTPA and DTPA-BMA (H_5DTPA: diethylenetriamine-N, N, N', N", N"-pentaacetic acid, H_3DTPA-BMA: 2, 2'[(carboxy:methyl)im.ino] bis[et.hylenebis(methylcarbamoylmethyl)imino]diacetic acid} have also been determined under similar conditions. The stability constants (log K _(ML)) of the complexes of BOPTA and DTPA are very similar, but in 0.1.5 M. NaCl the protonation constants of the ligands (log K_1~H) and. the log K_(ML)., values are lower than those obtained, in 0.1M KCl or Me_4NCl by 0.3-0.9 log If units. The order of selectivity of the ligands for Gd~(3+) over Zn1 is BOPTA > DTPA > DTPABMA. The complex [Cu(DTPA-BMA)].....deprotonates in the pH range 7-1.0 with the dissociation of an amide NH group and the coordination of the amide N atom. The kinetics of the exchange/transmetallation reactions between the complex [Gd(BOPTA)]~(2-) and the metal ions Cu~(2+), Zn~(2+), and Eu~(3+) have been studied by spectrophotometry (Cu~(2+) Eu~(3+)) and relaxometry (Zn~(2+)) in the pH range 3.3-6 at: 25 °C in 0.1.5 M NaCl. The reactions with Cu~(2+) and Zn~(2+) occur predominantly with direct attack of the metal ions on the [Gd(BOPTA)] ~(2-) complex. The kinetic activity of Eu~(3+) is lower, and in the exchange reactions with Eu~(3+), the proton-assisted dissociation of [Gd(BOPTA)]~(2-)(which is followed, by fast reaction between the free ligand and Eu~(3+)) could be also investigated. The rate constants, characterizing the proton-assisted dissociation of [Gd(BOPTA)]~(2-) and the exchange reactions occurring with the direct attack of Cu~(2+), Zn~(2+), and Eu~(3+) on the complex, are lower by about 30-90% than the rate constants obtained earlier for similar transmetallation reactions of [Gd(DTPA)]~(2-). The half-time for the dissociation of [Gd(BOPTA)J~(2-) at pH 7.4 and at 25 °C in the presence of 1×10~(-5)M Zn~(2+) and 1 × 108M Cu2+is 1.69 h.