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more reduced than the dithionite-reduced MoFe protein [
67
]. In the absence of any
substrate, the MoFe protein in the M
4
state exhibits a novel
S
1/2 EPR signal when
freeze-quenched under slow turnover conditions [
71
]. Annealing of this frozen
species, on the other hand, results in the relaxation to the resting state via the
successive loss of two H
2
molecules, thereby providing additional proof that
this species is an M
4
state intermediate [
71
].
¼
Figure 5 Possible structures of intermediates of N
2
reduction at stage M
4
(a), M
7
and M
8
(b). All
possible structures of intermediates were constructed based on the available spectroscopic data.
The most favored candidates of intermediates are highlighted in red boxes based on chemical
considerations. The orange spheres represent the Fe atoms of FeMoco.
Results of
1,2
H and
95
Mo ENDOR spectroscopic analyses suggest that the novel
S
1/2 EPR signal displayed by this intermediate may correspond to two M-H-M
fragments, where M represents an Fe atom (Figure
5
)[
69
,
72
]. The proposal
that the M
4
state contains two Fe atom-bridging hydrides on the FeMoco would
further imply that the four reducing equivalents are partially, if not completely,
carried by hydrides. Such a proposal [
4
,
28
,
64
,
73
] is somewhat appealing in that
(i) it offers a potential rationale for the FeMoco to be constructed with the elaborate
trigonal prismatic cage; (ii) it removes the requirement for the FeMoco to acquire
multiple redox states for the conversion of M
0
to M
4
; and (iii) the recycling
step back to M
0
can be easily understood as the protonation of the bound hydrides.
This plausible intermediate structure, depicted in Figure
5
, also forms the basis for
the hypothesis that N
2
may be reduced through a reductive elimination mechanism
(see Section
3.2.3
for details).
¼
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