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. So we see that if we're talking about myoglobin or hemoglobin, both of these proteins contain a heme group that is responsible for binding that diatomic oxygen. And it's the iron atom, the metal atom at the center of that heme group that is actually responsible for directly binding onto that diatomic oxygen. Now, the other side of that iron atom is actually bound onto the amino acid found in that protein.