Spectroscopic study of the hydration equilibria and water exchange dynamics of lanthanide(III) complexes of 1,7-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO2A)

The hydration state of a series of [Ln(DO2A)(H2O)(n)](+) complexes in aqueo us solution at pH = 6.4-7.0 was studied by measuring the lanthanide-induced O-17 shifts (LIS) of water [Ln includes elements from Ce to Yb; DO2A = 1,7 -bis(carboxymethyl)-1,4,7, 10-tetraazacyclododecane]. Their contact contrib ution, obtained from Reilley plots, indicated a decrease in the inner-spher e water coordination number of the [Ln(DO2A)(H2O)(n)]+ complexes from n = 3 (Ce-Eu), to n = 2 (Tb-Yb). A temperature-dependent UV/Vis absorption study of the 578-582 nm F-7(0) --> D-5(0) transition band of [Eu(DO2A)(H2O)(n)]( +) in aqueous solution showed that this complex is present in an equilibriu m between eight- and nine-coordinate species with n = 2 and n = 3, respecti vely. The hydration equilibrium parameters (2 reversible arrow 3), K-2-3(29 8), 4.0 +/- 0.2, Delta H-2-3(0) = -12.1 +/- 1 kJ mol(-1) and Delta S-2-3(0) = -28.9 +/- 3 J mol-K-1, correspond to an average hydration number of 2.65 -2.85 in the temperature range 273-363 K. A variable temperature, multiple field O-17 NMR study combined with direct EPR measurements of the transvers e electronic relaxation rates has been used to obtain the parameters charac terizing water exchange, rotation and electronic relaxation, all influencin g the proton relaxivity of [Gd(DO2A)(H2O)(2-3)](+). The small increase in t he water exchange rate of [Gd(DO2A)(H2O)(2-3)](+) (k(ex)(298) = (10 +/- 5) x 10(6)s(-1)) relative to that of [Gd(DOTA)(H2O)](-) (4.8 x 10(6) s(-1)) is a consequence of an unfavorable interplay of charge and hydration equilibr ia. The value of tau(R)(298) = 40 +/- 1 ps is short, and the electronic rel axation rate (1/T-2e approximate to 1.2 x 10(10) s(-1)) is fast relative to [Gd(DOTA)(H2O)](-) (1.3-2.4 x 10(9) s(-1) for B = 0.34 T). These parameter s negate to some extent the expected increase in proton relaxivity of the [ Gd(DO2A)(H2O)(2-3)](+) complex.