Europium(III) DOTA-tetraamide Complexes as Redox-Active MRI Sensors

PARACEST redox sensors containing the NAD(+)/NADH mimic N-methylquinolinium moiety as a redox-active functional group have been designed and synthesized. The Eu3+ complex with two quinolinium moieties was nearly completely CEST-silent in the oxidized form but was turned on upon reduction with beta-NADH. The CEST effect of the Eu3+ complex containing only one quinolinium group was much less redox-responsive but showed an unexpected sensitivity to pH in the physiologically relevant pH range.