So because of the electronegative character of oxygen, when diatomic oxygen shown here actually binds onto this heme group, more specifically, onto the iron of that heme group, it pulls away some of that electron density from that hein group, it pulls away some of that electron density from that iron atom. And because some of the electrons move away from that iron atom, it decreases the electron density and the size of that iron atom. And now, because the iron atom is smaller, it is able to move into the plane, into the center of the plane of that protoporphy and as shown in the following diagram. So as the diatomic oxygen binds onto the iron, it pulls the iron inward into the center of the proteporphine molecule. And because the iron, which is this purple molecule here, is attached to that proximal histidine, it also pulls on that proximal histidine. And this in turn pulls on this entire alpha helix of that polypeptide chain.