And this is shown in the following diagram. So the actual structure of the iron oxygen complex is resonant stabilized as shown in this diagram. One of it consists of this structure, and the other one consists of this structure here. And notice that the superoxide oxygen form has a negative charge on that oxygen and that destabilizes it. And to stabilize this structure, we see that a region of the protein, another histidine amino acid, forms a hydrogen bond with this oxygen, as shown in the following diagram. And this residue is known as the distal histidine.