Physicochemical Characterization of Four Gadolinium(III) DTPA-like Complexes

Abstract

The analysis of 17 O NMR transverse relaxation rates and EPR transverse electronic relaxation rates for aqueous solutions of the four DTPA-like (DTPA = diethylenetriamine-N,N,N,N,N-pentaacetic acid) complexes, [Gd(DTPA-PY)(H2O)] (DTPA-PY = N-(2-pyridylmethyl)), [Gd(DTPA-HP)(H2O)2] (DTPA-HP = N-(2-hydroxypropyl)), [Gd(DTPA-H1P)(H2O)2] (DTPA-H1P = N-(2-hydroxy-1-phenylethyl)) and [Gd(DTPA-H2P)(H2O)2] (DTPA-H2P =N-(2-hydroxy-2-phenylethyl)), at various temperatures allows us to understand the water exchange dynamics of these four complexes. The water-exchange lifetime (M) parameters for [Gd(DTPA-PY)(H2O)] , [Gd(DTPA-HP)(H2O)2] , [Gd(DTPA-H1P)(H2O)2] and [Gd(DTPAH2P)(H2O)2] are of 585, 98, 163, and 69 ns, respectively. Compared with [Gd(DTPA)(H2O)] 2- (M = 303 ns), the M value of [Gd(DTPA-PY)(H2O)] is slightly higher, but the other three complexes values are significantly lower than those of [Gd(DTPA)(H2O)] 2- . This difference is explained by the fact that the gadolinium(III) complexes of DTPA-HP, DTPA-H1P, and DTPA-H2P have two inner-sphere waters. The 2 H longitudinal relaxation rates of the labeled diamagnetic lanthanum complex allow the calculation of its rotational correlation time (R). The R values calculated for DTPA-PY, DTPA-HP, DTPA-H1P, and DTPA-H2P are of 127, 110, 142 and 147 ps, respectively. These four values are higher than the value of [La(DTPA)] 2- (R = 103 ps), because the rotational correlation time is related to the magnitude of its molecular weight.