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. So this here is known as the proximal histidine. And the proximal histidine is the amino acid of that polypeptide chain that forms a bond with that fe atom that holds the heme group to that protein. And it's the distal histidine that is found on the opposite side, on the opposite plane of the protoporphy that is responsible in forming a hydrogen bond between this oxygen here of this diatomic oxygen and this nitrogen of that histidine, and this stabilizes that diatomic oxygen.