Question

Which oxidation state of iron in haemoglobin?
A) $\mathop {Fe}\nolimits^ -$
B) $\mathop {Fe}\nolimits^{2 + }$
C) $\mathop {Fe}\nolimits^{3 + }$
D) $\mathop {Fe}\nolimits^{4 + }$

Answer 1

Haemoglobin is known as a protein that is present in red blood cells or erythrocytes. It is required for the transport of oxygen through the blood to the tissues, and carbon dioxide away from the tissues back to the lungs for excretion.

Complete answer:
The oxygen carrying capacity of haemoglobin relies on the oxidation state of an atom present in the protein. Each globin unit of the haemoglobin subunit possesses an atom of iron with a heme group which can bind one atom of oxygen. The protein can therefore bind and attach four atoms of oxygen at a time. The attachment of oxygen to haemoglobin is cooperative. That is, as each ion binds, it now results easier to bind the next one as the shape of the protein alters. The more oxygenated haemoglobin is, the brighter will be the shade of red of the blood

Now let us find solution from given options:
$\mathop {Fe}\nolimits^ -$:the iron ions always donate the electrons so exhibit positive oxidation state and not negative oxidation state.
$\mathop {Fe}\nolimits^{2 + }$: The oxygen anion exists in the charged state of $\mathop O\nolimits_2^ -$that carries the negative charge. In order to attach with this, the iron cation has to correspondingly be in the oxidative state of $\mathop {Fe}\nolimits^{2 + }$ or ferrous Fe (II) iron to make the compound neutral. So correct answer is option B.
$\mathop {Fe}\nolimits^{3 + }$: Iron does exist in this oxidation state of 3 as well, but not in the case of haemoglobin
$\mathop {Fe}\nolimits^{4 + }$: Iron does not know to exist in the valency of 4+ therefore option D does not apply at all and is incorrect

Hence, the correct answer is option (B)

Note: Haemoglobin can also get attached to carbon monoxide very efficiently. If the levels of CO in the environment rise, carboxyhemoglobin is formed as a result, and oxygen cannot bind. If not reversed, this state can cause death by asphyxiation.