1H and17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups

Article abstract of DOI:10.1002/mrc.2316

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by ¹H and ¹⁷O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the ¹⁷O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of themonophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as anMRI probe. The rate of exchange of thewatermolecule, ²⁹⁸k_ex = 4.2 × 10⁸_s ^-1, is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields. Copyright