Two previous reports of adduct formation between aminoxyl radicals and imidazole rings are shown in Figure 7a . These suggested potential structure 6 for an adduct between BU72 and His54 (Figure ). The stereochemistry of the bond to the modified histidine residue was dictated by the observed density. A possible intermediate aminoxyl radical is also shown; these can form from oxidation of secondary amines by ROS . proposed here, with the nickel complex and a possible aminoxyl intermediate