High relaxivity Mn(2+)-based MRI contrast agents.

Stable Mn(2+) mono- and binuclear complexes containing pentadentate 6,6'-((methylazanediyl)bis(methylene))dipicolinic acid coordinating units give remarkably high relaxivities due to the presence of two inner-sphere water molecules. The mononuclear derivative binds human serum albumin (HSA) with an association constant of 3372 M(-1), which results in the replacement of the coordinated water molecules by donor atoms of protein residues. The dinuclear analogue also binds HSA while leaving one of the Mn(2+) centres exposed to the solvent with two coordinated water molecules. Thus, this complex shows remarkably high relaxivities upon protein binding (39.0 mM(-1)  s(-1) per Mn, at 20 MHz and 37 °C).