Question

#1

Consider the iron sulfur rubredoxins having a single iron center in their natural biological environment. Which of the following statements is incorrect.

The iron atom is usually found in a nearly tetrahedral geometry surrounded by sulfurs.

Iron(III) rubredoxins act as reducing agents during electron transfer reactions.

The iron(III) rubredoxins exhibit a larger paramagnetism than their corresponding iron (II) rubredoxins.

Rubredoxins are only capable of gaining or loosing a single electron.

Iron(II) and iron(III) rubredoxins are found as high-spin iron complexes.

#2

Which of the following formal oxidation states are present under normal biological conditions for the most oxidized form of a Fe4S4 ferredoxin center?

Fe(III)2Fe(II)2

Fe(III)3Fe(II)1

Fe(III)1Fe(II)3

Fe(III)4

Fe(II)4

Answer 1

Rubredoxins perform one-electron transfer processes. The central iron atom changes between the +2 and +3 oxidation states. In both oxidation states, the metal remains high spin, which helps to minimize structural changes.

The structue of iron in rubredoxins is

So it is tetrahedral.

Since the iron is in high spin state it will have unpaired electrons and so will be paramagnetic.

So the only statement among those given which is not true is

Iron(III) rubredoxins act as reducing agents during electron transfer reactions.

Because if Iron(III) rubredoxins act as reducing agents, the other compound gets reduced and so Iron(III) rubredoxins should get oxidised Iron(III) rubredoxins is already having iron in 3+ state it can not go to 2+ state which means it will get reduced so the other species should get oxidised so it acts as an oxidising agent.

2)

The [Fe₄S₄] ferredoxins may be further subdivided into low-potential (bacterial-type) and high-potential (HiPIP) ferredoxins.

Low- and high-potential ferredoxins are related by the following redox scheme:

The formal oxidation numbers of the iron ions can be [2Fe³⁺, 2Fe²⁺] or [1Fe³⁺, 3Fe²⁺] in low-potential ferredoxins. The oxidation numbers of the iron ions in high-potential ferredoxins can be [3Fe³⁺, 1Fe²⁺] or [2Fe³⁺, 2Fe²⁺].

So the correct answer is

Fe(III)3Fe(II)1