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

Eu(S-THP)(3+) is the first PARACEST agent that functions through exchange of hydroxyl protons with water protons in aqueous solution. The CEST spectrum of this complex is strongly pH dependent and modulated by the presence of phosphate esters, as shown for diethyl phosphate and methyl phosphate, which form outer- and inner-sphere complexes, respectively, with Eu(S-THP)(3+). The sensitivity of the alcohol proton environment to interactions with these anions shows that this complex has potential as a responsive PARACEST MRI contrast agent.