Gear up for a pH shift: a responsive iron(II) 2-amino-6-picolyl-appendent macrocyclic paraCEST agent that protonates at a pendent group

Two high-spin Fe(II) and Co(II) complexes of 1,4,7,10-tetraazacyclododecane (CYCLEN) appended with four 2-amino-6-picolyl groups, denoted as [Fe(TAPC)] 2+ and [Co(TAPC)] 2+ , are reported. These complexes demonstrate C 2 -symmetrical geometry from coordination of two pendents, and they are present in a single diastereomeric form in aqueous solution as shown by 1 H NMR spectroscopy, and by a single-crystal X-ray structure for the Co(II) complex. A highly-shifted, but low intensity CEST ( C hemical E xchange S aturation Transfer) signal from NH groups is observed at −118 ppm for [Co(TAPC)] 2+ at pH 6.0 and 37 °C. A higher intensity CEST peak is observed for [Fe(TAPC)] 2+ which demonstrates a pH-dependent frequency shift from −72 to −79 ppm at pH 7.7 to pH 4.8, respectively, at 37 °C. This shift in the CEST peak correlates with the protonation of the unbound 2-amino-6-picolyl pendents, as suggested by UV-vis and 1 H NMR spectroscopy studies at different pH values. Phantom imaging demonstrates the challenges and feasibility of using the [Fe(TAPC)] 2+ agent on a low field MRI scanner. The [Fe(TAPC)] 2+ complex is the first transition metal-based paraCEST agent that produces a pH-induced CEST frequency change towards the development of probes for concentration independent imaging of pH. The first Fe(II) paraCEST agent that demonstrates a pH-dependent CEST frequency shift undergoes protonation of ancillary 2-amino-6-picolyl pendent groups.