CEST spectra (11.7 T NMR) of 4 mM [Fe(AMPT)]2+ (black line), 100 mM NaCl, pH 7.0 (B1= 11 μT, 4 s, 25 ºC); 3mM [Fe(BZT)]2+ (red line), 100 mM NaCl, pH 6.3 (B1= 23 μT, 3 s, 25 ºC); and 8 mM [Co(TPT)]2+ (blue line), 100 mM NaCl, 20 mM HEPES, pH 7.0 (B1= 24 μT, 3 s, 37 ºC). Insert shows expanded CEST spectrum of [Fe(AMPT)]2+ [21, 22, 24].