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Equilibrium Studies on the Gd3+, Cu2+ and Zn2+ Complexes of BOPTA, DTPA and DTPA-BMA Ligands: Kinetics of Metal-Exchange Reactions of [Gd(BOPTA)]2–

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Abstract

The stability and protonation constants of the complexes formed between the ligand BOPTA [H5BOPTA: 4-carboxy-5,8,11-tris(carboxymethyl)-1-phenyl-2-oxa-5,8,11-triazatridecan-13-oic acid] and Gd3+, Cu2+, and Zn2+ have been determined by pH potentiometry (Gd3+, Zn2+) and spectrophotometry (Cu2+) at 25 °C and at constant ionic strength maintained by 0.15 M NaCl. For comparison, the stability and protonation constants of the complexes of Gd3+, Cu2+, and Zn2+ formed with DTPA and DTPA-BMA {H5DTPA: diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid, H3DTPA-BMA: 2,2′-[(carboxymethyl)imino]bis[ethylenebis(methylcarbamoylmethyl)imino]diacetic acid} have also been determined under similar conditions. The stability constants (log KML) of the complexes of BOPTA and DTPA are very similar, but in 0.15 M NaCl the protonation constants of the ligands (log KiH) and the log KML values are lower than those obtained in 0.1 M KCl or Me4NCl by 0.3–0.9 log K units. The order of selectivity of the ligands for Gd3+ over Zn2+ is BOPTA > DTPA > DTPA-BMA. The complex [Cu(DTPA-BMA)] deprotonates in the pH range 7–10 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 Cu2+, Zn2+, and Eu3+ have been studied by spectrophotometry (Cu2+, Eu3+) and relaxometry (Zn2+) in the pH range 3.3–6 at 25 °C in 0.15 M NaCl. The reactions with Cu2+ and Zn2+ occur predominantly with direct attack of the metal ions on the [Gd(BOPTA)]2– complex. The kinetic activity of Eu3+ is lower, and in the exchange reactions with Eu3+, the proton-assisted dissociation of [Gd(BOPTA)]2– (which is followed by fast reaction between the free ligand and Eu3+) 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 Cu2+, Zn2+, and Eu3+ 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)]2– at pH 7.4 and at 25 °C in the presence of 1 × 10–5M Zn2+ and 1 ×10–6M Cu2+ is 169 h.

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