Why do red blood cells maintain Iron in the Haem group in the +2 (ferrous) oxidation state?

Question

A lot of sources tell me that RBCs contain a number of enzymes, and that these serve multiple functions from maintaining the structure and elasticity of the corpuscle wall, to preventing the oxidation of iron (ferrous) in Haemoglobin to the +3 Ferric state.But what none of them say, is what problems (if any) arise when the iron is oxidised. Hence the question, why avoid the Ferric state?I'm still a high-schooler and from what I've learnt, is that the higher the oxidation state of the cation, the higher its polarising power (Fajan's Rules) hence stronger the bond. So wouldn't iron in the Ferric state be able to bind with oxygen better, and isn't that desirable? ( Or is oxygen *release* to tissues going to be problematic? )If the Ferric state is desirable, then why do RBCs have mechanisms (the enzymes mentioned earlier) in place to *prevent* the oxidation of the Ferrous to the Ferric state?Thanks!

Answer 1

It is because the ferrous ion (Fe²⁺),with loss of two electrons only is too large to fit into the plane of protoporphyrin. Thus, a ferrous ion is often 0.4 Å away from the plane of porphyrin ring, and could deliver oxygen easily. However a ferric ion (Fe³⁺), because of the loss of one extra electron, assumes a smaller size, gets more electro-positive and attracts oxygen more strongly than ferrous ion (Fe²⁺) and fits into the protoporphyrin plane. As a result oxygen become stuck. Therefore it is necessary to keep iron in ferrous (Fe²⁺) state.