Chemical Mechanisms of pH-Dependent Relaxivities for a Series of Structurally Related Mn(II) Cyclen Derivatives
<p>Complexes of Mn(II) have received attention recently as potential new contrast agents for magnetic resonance imaging. Specifically, contrast agents that respond to local pH changes are among those of current interest. The synthesis and characterization of three Mn(II) complexes of structurally related cyclen-based ligands with amide, carboxylate, or phosphonate side arms, which may coordinate to the metal ion are reported. The pH dependencies of the<sup> 1</sup>H relaxivities of the complexes are significantly different for each complex. On the basis of these data, potentiometric titrations, pH-dependent solution IR spectroscopy and <sup>17</sup>O transverse relaxation measurements, a model of the solution structures of the complexes is proposed. The model also rationalizes the pH-dependent <sup>1</sup>H relaxivities in terms of the chemical exchange mechanisms contributing to relaxation and provides effective values for the relaxivities of each protonation state of each species.</p> <p>The chemical mechanisms of<sup> 1</sup>H relaxivities were investigated extensively for each complex. The phosphonates appear to enhance the overall stability and may provide sites for enhanced hydrogen bonding to the bulk water and prototropic exchange.</p>