EPR spectra of [Fe(244mTC−H)2]+ (top) and [Fe(IT1−H)2]+ (bottom) in frozen DMSO/water solution (70/30%, v/v). The pH of the solution was estimated at 7.0 using colorimetric indicator strips with a sensitivity of 0.5 pH units. Experimental conditions: microwave frequency, 9.4 GHz; microwave power, 2 mW; magnetic field modulation amplitude, 0.2 mT; temperature, 77 K.