Coordination equilibrium - a clue for fast water exchange on potential magnetic resonance imaging contrast agents?

A temperature-dependent UV-visible spectrophotometric study on [Eu(DO3A)(H2 O)(n)] proved the presence of a hydration equilibrium (n = 1,2), strongly s hifted towards the bisaqua species [DO3A = 1,4,7-tris(carboxymethyl)-1,4,7, 10-tetraazacyclododecane]. The thermodynamic parameters and the reaction vo lume were determined for the equilibrium [Eu(DO3A)(H2O)] + H2O reversible a rrow [Eu(DO3A)(H2O)(2)] and the same results were extrapolated for the Gd(I II) analogue (Delta H degrees = -12.6kJ mol(-1), Delta S degrees = -25.2J m ol(-1) K-1, K-Eu(298) = 7.7 and Delta V degrees = -7.5 cm(3) mol(-1)). The variable-temperature O-17 NMR data on [Gd(DO3A)((HO)-O-2)(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 comparis on of the results obtained by the two different analyses show that, despite the crude approximation of treating the system as a two-site exchange prob lem, the Swift-Connick method gives a correct value for the water exchange rate. Based on previous observations on the relationship between inner sphe re structure and water exchange rate, one can expect higher rates for compl exes with a hydration equilibrium. Indeed, the water exchange on [Gd(DO3A)( H20)(n)] is about twice as fast as on [Gd(DOTA)(H2O)(-) (k(ex)(298) = 11 x 10(6) and 4.8 x 10(6) s(-1), respectively), although it is still much slowe r than that on [Gd(H2O)(8)](3+) (k(ex)(298) = 804 x 10(6)s(-1)). The limite d gain in the water exchange rate is explained in terms of a rigid inner sp here structure introduced by the macrocyclic ligand which makes difficult t he transition from the reactant to the transition state, and consequently, results in a slower exchange as compared to the Gd3+ aqua ion. The activati on parameters of the water exchange are Delta H-ex(not equal) = 33.6 kJ mol (-1) and Delta S-ex(not equal) = +2.4J mol(-1) K-1, and the mechanism is pr oposed to be dissociatively activated. Copyright (C) 1999 John Wiley & Sons , Ltd.