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(it is assumed this is hydroxide) binds to the pendant Fe atom and the Ni atom is
coordinated by three sulfido ligands from the [3Fe-4S] core with a distorted
T-shaped coordination geometry. The structure of CO
2
-bound CODH II
Ch
obtained
by incubating CODH II
Ch
with NaHCO
3
at -600 mV (Figure
3b
) reveals further that
the C-atom from CO
2
binds to the Ni-atom at a distance of 1.96
, completing a
distorted square-planar geometry, and one of the O-atoms of the bound CO
2
binds
to the pendant Fe at a distance of 2.05
Å
.
These results suggest a mechanism in which CO oxidation involves nucleophilic
attack on CO-bound Ni by a OH
ligand that is coordinated to the pendant Fe,
thereby yielding the Ni-(CO(OH)) intermediate that is detected. In the reverse
reaction, the pendant Fe atom abstracts an O-atom from the C-coordinated CO
2
via
a proton-coupled two-electron process, and leaves CO bound to Ni with OH
on the
pendant Fe. This picture of the mechanism is included in Scheme
1
[
4
]. Spectroscopic
data add further structural definition to this description, while PFE data discussed
below show how the reactions with substrates and inhibitors depend on potential.
From potentiometric redox titrations, it is known that the inactive and EPR-silent
state, C
ox
, is reduced by one electron to give the active state C
red1
, which exhibits a
characteristic EPR signal,
g
av
~1.82 (
g
Å
2.01, 1.80, 1.65) [
24
]. Based on the
structural evidence described above, CO binds to the Ni atom in the C
red1
state to
start the catalytic cycle for CO oxidation, the eventual result of which is that CO
2
is
released leaving the two-electron reduced state C
red2
. The C-cluster is re-reduced
back to C
red1
by two one-electron transfers, via an EPR-silent state known as C
int
.
It is very significant that C
red2
displays an EPR signal with
g
av
~1.86 (
g
¼
¼
1.97, 1.86,
1.75) [
24
].
Based on earlier M¨ssbauer spectroscopy data, Lindahl suggested that C
ox
should be assigned as [Ni
2+
Fe
p
3+
]:[3Fe-4S]
1
(Fe
p
¼
pendant Fe site) and C
red1
should be assigned as [Ni
2+
Fe
p
2+
]:[3Fe-4S]
1
[
24
]. The fact that C
red1
and C
red2
differ by two electrons but both show an EPR spectrum with
g
av
<
2 suggests that
the underlying electronic structure of the [3Fe-4S] core fragment is unchanged
between the two states. At least one electron must therefore be transferred at the Ni
subsite, since it is unclear how a two-electron change could be accommodated at
the pendant Fe that is also coordinated by sulfide, histidine-N, and cysteine-S.
The question then arises: what formal redox changes actually occur at Ni? If the
pendant Fe remains as Fe
2+
, both electrons must be transferred at the Ni subsite.
Two alternative descriptions of the Ni atom in the C
red2
state have been suggested:
Ni(0) or the isoelectronic protonated site formulated as a nickel hydrido species
Ni(II)-H [
8
,
25
]. Further light on this question stems from Ni K- or L-edge X-ray
absorption spectroscopy (XAS) studies on CODH
Rr
and CODH II
Ch
in differing
redox states [
26
-
28
]. Results from as-isolated CODH II
Ch
, CO-treated CODH II
Ch
,
dithionite-reduced CODH II
Ch
, indigo-carmine-oxidized CODH
Rr
, dithionite-
reduced CODH II
Rr
and CO-treated CODH
Rr
, all suggest that the Ni subsite in
the C-cluster in these samples is present as Ni(II) regardless of redox state.
The reduction potential for the two-electron interconversion between C
red1
and
C
red2
is approximately -520 mV according to EPR potentiometric titrations [
29
].
The reduced B-cluster exhibits the typical EPR signal of a [4Fe-4S]
1+
cluster
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