Environmental Engineering Reference
In-Depth Information
Figure 6 The proposed reductive elimination mechanism for N
2
activation at stage M
4
. The four
reducing equivalences accumulated via the four electron transfer events are stored on the FeMoco
as two bridging hydrides. Upon N
2
binding, one molecule of H
2
is eliminated, thereby carrying
away two of the four reducing equivalences. The remaining two reducing equivalences transiently
render the FeMoco more reduced and, consequently, more reactive. The elevated reactivity then
facilitates the activation of the substrate, N
2
, and the subsequent cleavage and reduction of the N,N
triple bond. This reduction can take place either on the distant N atom first (the distal pathway) or
back and forth on both N atoms (the alternating pathway). The orange spheres represent the Fe
atoms of FeMoco.
experiment, D
2
was added to the MoFe protein variant upon N
2
turnover in hopes of
achieving the oxidative addition of D
2
on the N
2
-bound M
4
intermediate and the
subsequent formation of an N
2
-free M
4
intermediate with bound deuteride (D
)
(Figure
6
). Acetylene (C
2
H
2
), which can be reduced by FeMoco at more oxidized
states than that required for N
2
reduction, was then added to the turnover mixture to
“outcompete” N
2
for the deuterides bound to FeMoco [
81
]. The detection of
C
2
H
2
D
2
and C
2
H
3
D by GC-MS appeared to be in line with the argument that
deuteride was added on the FeMoco upon addition of D
2
and incorporated into
C
2
H
2
during substrate turnover [
81
]. Nevertheless, direct evidence is yet to be
acquired to demonstrate the formation of FeMoco-bound deuterides upon addition
of D
2
, which will provide the definitive proof for the proposed mechanism of N
2
activation via the reductive elimination of H
2
.
3.2.3 The Alternating Dinitrogen Reduction Pathway
Support for the distal N
2
reduction pathway (Scheme
2
a) came mainly from the
work on the synthetic, NH
3
-forming, mononuclear Mo complexes [
82
-
84
].
The strongest experimental support for the proposed distal pathway was the detection
of hydrazine formation upon acid- and base-quenching of the pre-steady-state turn-
over, which was interpreted as the release of the two-electron reduced hydrazido- or
bound
N-NH
2
intermediate at the M
4
stage [
4
,
28
,
64
](Scheme
2
a). Support for the
alternating reduction pathway (Scheme
2
b), on the other hand, came mainly from the
recent spectroscopic studies of a number of MoFe protein variants [
65
,
67
].
¼
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