Environmental Engineering Reference
In-Depth Information
Figure 6 Redox states of
cluster C. Electronic states
are given together with their
spin-states and approximate
mid-point potentials. The n
in spin state of C
int
denotes
0 or an integer [
93
].
C
red2
. The operational midpoint potential of C
red2
(
E
'
-530 mV) [
87
,
90
]
coincides with the value obtained for the CO
2
/CO couple (
E
'
¼
-558 mV) [
28
].
C
red2
shows a minor shift in the EPR spectrum compared to C
red1
with
g
av
¼
¼
1.86
(
g
1
1.97,
g
2
1.87, and
g
3
1.75) [
87
,
88
]. Where the two additional electrons reside
in the C
red2
state is still under debate and involvement of Ni
0
, a hydride-bound
Ni
2+
and a Ni-Fe bond (dative metal-metal bond) have been proposed [
74
,
91
,
92
].
The half-cycle of the reaction is followedbyintramoleculartransferofone
electron to cluster B, forming a transient intermediate state devoid of any para-
magnetic signature (C
int
)[
93
]. Subsequent electron release restores cluster C in
the CO-reactive state (C
red1
).
The structure of cluster C has been deduced from the crystal structure of CODH
II
Ch
, poised to a redox potential of -320 mV with dithiothreitol [
94
]. At this
potential, a state equivalent to C
red1
was expected and the structure revealed a
[Ni4Fe4S-OH
x
] composition. Cluster C consists of a distorted [Ni3Fe4S]
heterocubane connected to an Fe(II) in exo position. The Fe(II) ion (Fe
1
) is also
known as ferrous component II (FC
II
), which is coordinated by a water or hydroxyl
ligand (OH
x
) with a distance of 2.7
to Ni (Figure
7
).
Structural insights into CO/CO
2
activation were gained from a CO
2
-bound
structure. Crystals of CODH II
Ch
were incubated in a solution containing Ti(III)-
citrate adjusted to a reduction potential of -600 mV in the presence of 45 mM
NaHCO
3
, equivalent to a concentration of solvated CO
2
of 0.45 mM at pH 8.0. The
presence of CO
2
and a potent electron donor creates turnover conditions under
which the crystals were allowed to react for several minutes before they were
shock-frozen [
94
]. The structure of the -600 mV + CO
2
state unraveled a
[NiFe
4
S
4
(CO
2
)] cluster, where a carboxylate bridges Ni and Fe
1
acting as a
ʼ
2
-
Å
2
-ligand. The carbon atom of CO
2
completes the square-planar coordination
geometry of Ni and one of the oxygens of CO
2
replaces the hydroxo/water ligand at
Fe
1
.CO
2
is in an activated state as evident from a O-C-O-bending angle of
ʷ
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