EPR spectra of Fe–MGD in the absence and presence of NO or NO−. Control, 1 mM Fe–MGD in 50 mM Tris⋅HCl buffer (pH 7.4); NO, Fe–MGD with the NO (14N) donor N-SNAP (10 μM); 15NO, with 10 μM 15N-SNAP; NO−, with 10 μM NO− donor Na2N2O3. In the presence of the trap Fe–MGD, natural abundance 14NO gives rise to the characteristic triplet spectrum whereas 15NO gives rise to a characteristic doublet. NO− did not produce any significant signal. Spectra were recorded at room temperature with a microwave frequency of 9.77 GHz by using 80 mW of microwave power and 3.9 G modulation amplitude.