Physicochemical characterization of the dimeric lanthanide complexes [en{Ln(DO3A)(H₂O)}₂] and [pi{Ln(DTTA)(H ₂O)}₂]^2-: A variable-temperature ¹⁷O NMR study

The Gd(III) complexes of the two dimeric ligands [en(DO3A)₂] {N,N′-bis[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecan-10-yl- methylcarbonyl]-N,N′-ethylenediamme} and [pi(DTTA)₂] ^8- [bisdiethylene-triaminepentaacetic acid (trans-1,2- cyclohexanediamine)] were synthesized and characterized. The ¹⁷O NMR chemical shift of H₂O induced by [en{Dy(DO3A)}₂] and [pi{Dy(DTTA)}₂]^2- at pH 6.80 proved the presence of 2.1 and 2.2 inner-sphere water molecules, respectively. Water proton spin-lattice relaxation rates for [en{Gd(DO3A)(H₂O)}₂] and [pi{Gd(DTTA)(H₂O)}₂]^2- at 37.0 ± 0.1°C and 20 MHz are 3.60 ± 0.05 and 5.25 ± 0.05 mM ^-1 s^-1 per Gd, respectively. The EPR transverse electronic relaxation rate and ¹⁷O NMR transverse relaxation time for the exchange lifetime of the coordinated H₂O molecule and the ²H NMR longitudinal relaxation rate of the deuterated diamagnetic lanthanum complex for the rotational correlation time were thoroughly investigated, and the results were compared with those reported previously for other lanthanide(III) complexes. The exchange lifetimes for [en{Gd(DO3A)(H ₂O)}₂] (769 ± 10 ns) and [pi{Gd(DTTA)(H ₂O)}₂]^2- (910 ± 10 ns) are significantly higher than those of [Gd(DOTA)(H₂O)]^- (243 ns) and [Gd(DTPA)(H₂O)]^2- (303 ns) complexes. The rotational correlation times for [en{Gd(DO3A)(H₂O)}₂] (150 ± 11 ps) and [pi{Gd(DTTA)(H₂O)}₂]^2- (130 ± 12 ps) are slightly greater than those of [Gd(DOTA)(H₂O)] ^- (77 ps) and [Gd(DTPA)(H₂O)]^2- (58 ps) complexes. The marked increase in relaxivity (r₁) of [en{Gd(DO3A)(H₂O)}₂] and [pi{Gd(DTTA)(H ₂O)}₂]^2- result mainly from their longer rotational correlation time and higher molecular weight.