Coordination equilibrium— a clue for fast water exchange on potential magnetic resonance imaging contrast agents?
✍ Scribed by Éva Tóth; Orla M. Ni Dhubhghaill; Ghislaine Besson; Lothar Helm; André E. Merbach
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 111 KB
- Volume
- 37
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
A temperature-dependent UV-visible spectrophotometric study on [Eu(DO3A)(H 2 O) n ] proved the presence of a hydration equilibrium (n D 1, 2), strongly shifted towards the bisaqua species [DO3A D 1,4,7tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane]. The thermodynamic parameters and the reaction volume were determined for the equilibrium [Eu
and the same results were extrapolated for the Gd(III) analogue (H °D 12.6 kJ mol 1 , S °D 25.2 J mol 1 K 1 , K Eu 298 D 7.7 and V °D 7.5 cm 3 mol 1 ). The variable-temperature 17 O NMR data on [Gd(DO3A)(H 2 O) n ] were analysed by two approaches: (i) with the Swift-Connick equations (two-site exchange) and (ii) with the Kubo-Sack formalism (three-site exchange). The comparison of the results obtained by the two different analyses show that, despite the crude approximation of treating the system as a two-site exchange problem, the Swift-Connick method gives a correct value for the water exchange rate. Based on previous observations on the relationship between inner sphere structure and water exchange rate, one can expect higher rates for complexes with a hydration equilibrium. Indeed, the water exchange on [Gd(DO3A)(H 2 O) n ] is about twice as fast as on [Gd(DOTA)(H 2 O)] (k ex 298 D 11 ð 10 6 and 4.8 ð 10 6 s 1 , respectively), although it is still much slower than that on [Gd(H 2 O) 8 ] 3C k ex 298 D 804 ð 10 6 s 1 . The limited gain in the water exchange rate is explained in terms of a rigid inner sphere structure introduced by the macrocyclic ligand which makes difficult the transition from the reactant to the transition state, and consequently, results in a slower exchange as compared to the Gd 3C aqua ion. The activation parameters of the water exchange are H 6 D ex D 33.6 kJ mol 1 and S 6 D ex D C2.4 J mol 1 K 1 , and the mechanism is proposed to be dissociatively activated.