So we see that when the diatomic oxygen binds onto that fe from the top, that diatomic oxygen develops a negative charge because it is more electronegative and it pulls away those electrons, that electron, from the metal atom. And that's precisely what moves this entire residue up and what allows this metal atom to fit into the sensor portion of that protoporphy in group. Now, because the diatomic oxygen gains a negative charge, it becomes slightly less stable. And so we have to be able to somehow stabilize this diatomic oxygen inside that heme group. And in fact, what happens is we have another histidine residue that is part of the polypeptide chain inside that protein. Either myoglobin or hemoglobin that binds creates a hydrogen bond with that negative charge.