# Reversible Enzymatic Switching of the Oxidation State of a EuIII/II Complex Controls Relaxivity

## TL;DR

This paper shows that changing the oxidation state of a europium complex can control its ability to affect water relaxation, useful for MRI imaging.

## Contribution

A europium complex is demonstrated to reversibly switch oxidation states via enzymes, affecting MRI contrast.

## Key findings

- EuIII/II complex shows reversible redox behavior at -360 mV vs SHE.
- Reduced form has high relaxivity (3.08 mM–1 s–1), comparable to commercial MRI contrast agents.
- Oxidized form has negligible relaxivity, suggesting potential for imaging reducing environments.

## Abstract

We show that europium trisbipyridyl cryptate exhibits
an electrochemically
reversible EuIII/II redox couple at −360 mV vs the
standard hydrogen electrode (SHE) and is susceptible to reduction
or oxidation by a range of redox enzymes or biological small molecule
redox agents. Modulation of the europium oxidation state can be used
to achieve dramatic changes in the T
1 relaxation
time of bulk water in solutions containing these complexes: in the
reduced form, the complex exhibits high relaxivity (3.08 mM–1 s–1 at 499.9 MHz) comparable with that of commercial
MRI contrast media, while the oxidized form has negligible relaxivity,
illustrating the scope for application in imaging reducing environments
in biological systems.