Electron transfer agents and redox couples regulate the redox potential of tissue to maintain physiological processes. However, a consequence of diseased tissue and cancerous cells is a disruption in the redox potential towards more negative values. Magnetic resonance imaging (MRI) is a non-invasive, biomedical imaging technique that produces images of soft tissue with high spatial resolution. These images can be further enhanced with the aid of contrast agents (CAs). Most MRI CAs are Gd(III) complexes, which provide contrast via a T1mechanism. Eu(II) complexes have the potential to produce T1weighted MRI contrast due to electronic similarities with Gd(III) CAs. In addition, Eu(III) complexes have shown promising results as responsive paramagnetic chemical exchange saturation transfer (PARACEST) MRI agents. This ability of the Eu(II)/Eu(III) redox couple to produce oxidation state-dependent T1and PARACEST contrast allows for the design of redox-responsive MRI CAs. The goal of this project is to better understand the influence of the ligand side-chain on the redox properties of the Eu(II)/Eu(III) couple. To date, Eu(III) complexes consisting of four acetamide-, glycinate-, and aspartate side-chains were altered by substituting one of the four side-chains with a pyridine or quinoline moiety. Each complex was analyzed by cyclic voltammetry at pH 7.5. The results showed that on average, the redox potentials of each complex registered a +67mV and +176 mV shift upon replacement of one side-arm with a pyridine and quinoline, respectively. These results were consistent across all complexes, suggesting that “softer” donor atoms that are part of highly conjugated systems are better at stabilizing the Eu(II) oxidation state upon reduction of the Eu(III) complex.

Investigation of the electrochemical properties of Eu(III) complexes as a function of ligand identity

ACS Fall 2021

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