title: Gear Up for a pH Shift: A Responsive Iron(II) 2-Amino-6-picolyl-Appended Macrocyclic paraCEST Agent That Protonates at a Pendent Group.
abstract: 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 by1H 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 and1H 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.
authors.affiliation: Department of Chemistry, University at Buffalo, the State University of New York , Buffalo, New York 14260, United States.
authors.forename: Pavel B
authors.initials: PB
authors.lastname: Tsitovich
authors.affiliation: Department of Chemistry, University at Buffalo, the State University of New York , Buffalo, New York 14260, United States.
authors.forename: Jordan M
authors.initials: JM
authors.lastname: Cox
authors.affiliation: Department of Cell Stress Biology, Roswell Park Cancer Institute , Buffalo, New York 14263, United States.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Spernyak
authors.affiliation: Department of Chemistry, University at Buffalo, the State University of New York , Buffalo, New York 14260, United States.
authors.forename: Janet R
authors.initials: JR
authors.lastname: Morrow