Gear up for a pH shift: A responsive iron(II) 2-amino-6-picolyl-appended 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 C₂-symmetrical geometry from coordination of two pendents, and they are present in a single diastereomeric form in aqueous solution as shown by ¹H NMR spectroscopy and by a single-crystal X-ray structure for the Co(II) complex. A highly shifted but low-intensity CEST (chemical exchange saturation 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 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 ¹H 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 toward the development of probes for concentration-independent imaging of pH.