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

Abstract

We show that europium trisbipyridyl cryptate exhibits an electrochemically reversible Eu^III/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 T₁ 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.