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. And so when the binding takes place, there is a change in surface to surface interaction between these two adjacent polypeptide chains. Notice the distance here and the distance here basically changes. And this changes the electrostatic interaction between these two polypeptide chains.