A PARACEST agent responsive to inner- and outer-sphere phosphate ester interactions for MRI applications

Eu[(S-THP).sup.3+] is the first PARACEST agent that has functioned through exchange of hydroxyl protons with water protons in aqueous solution and the CEST spectrum of this complex is strongly pH dependent. The sensitivity of the alcohol proton environment to interactions with the anions has shown that the complex has potential as a responsive PARACEST MRI contrast agent.