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converted to volatile H 2S with acids. The mean oxidation state in the reduced form is 2.25, in
the oxidised form 2.5, the redox potential is typically around -200 mV. (4) HiPIPs (High
Potential Iron Proteins) are identical to the [4Fe,4S] ferredoxins in as far as the core structure is
concerned. However, the mean oxidation state in the reduced form is 2.5, in the oxidised from
2.75, and the redox potential is typically around +300 mV. Along with these “classical” iron-
sulphur clusters, others are know, in which one iron centre is missing ([3Fe,4S] ferredoxins), or
where two [4Fe,4S] ferredoxins form double-cubanes, or where a fifth ligand (Ser or His) is
coordinated to one of the iron centres. The Rieske proteins have already been mentioned
above; the angle N-Fe-N is ca. 90°, i.e. there is strong distortion from tetrahedral symmetry for
this specific iron.
2-/- (His) ]1-/0
SR SR SR S SR 3-/2- SR S N
Fe Fe Fe Fe Fe
SR SR SR S SR SR S N
(His)
Rubredoxin [2Fe-2S]-Ferredoxin Rieske centre
Rieske-Zentrum

SR 3-/2- SR 2-/-
Fe S SR Fe S SR
S Fe S Fe
S Fe SR S Fe SR
Fe S Fe S
SR SR

[4Fe-4S]-Ferredoxin HiPIP
Figure 11. The iron centres of the classical (and more frequently used) iron-sulphur proteins.
SR = cysteinate(1-).

Tutorial: Oxidation and reduction

An oxidation corresponds to a removal of electrons (increase of the oxidation number),
reduction correspondingly to a transfer of electrons to a substrate (decrease of the oxidation
number). Oxidation and reduction are coupled; an example is the oxidation of ferrous to ferric
iron, coupled with the reduction of oxygen to water:
2Fe + ½O 2 + 2H → 2Fe + H 2O
2+
+
3+

In principal, all redox reactions are equilibrium reactions. The direction is determined by the

redox potentials of the two pairs of underlying electron transfer processes. Standard redox
5

0
potentials E are tabulated; standard conditions are: 298 K, 10 Pa, c = 1 mol/l:
2+
3+
0
-
Fe + e ' Fe ; E = +0.771 V
½O 2 + 2e + 2H ' H 2O; E = +1.229 V
-
0

+

c

Recalculation of the potential for real concentrations, E , is achieved with the Nernst equation:

E = E + (0.059/n)log(c Ox/c Red)
c
0

where n is the number of transferred electrons; c Ox and c Red the concentrations of the oxidised

and reduced forms, respectively. In particular, the pH dependence has to be taken into account:
0
+

-7
+
c
At pH 7, (c(H ) = 10 ), E for the pair H 2/H is -0.414 V (E = 0), for H 2O/O 2 +0.815 V.