Environmental Engineering Reference
In-Depth Information
NO
2
is reduced to NH
4
in a consecutive series of electron and proton transfer steps.
On the basis of the crystallographic observation of reaction intermediates and of density
functional theory (DFT) calculations, it was proposed that nitrite reduction started with a
heterolytic cleavage of the N
O bond, which is facilitated by a pronounced back-
bonding interaction of nitrite coordinated through nitrogen to the reduced [Fe(II)] but
not the oxidized [Fe(III)] active site iron (Figures
5
and
6
)[
66
-
69
].
Figure 6 Activation of nitrite at heme center 1 of cytochrome
c
nitrite reductase (NrfA). Upper
part: Back-bonding interaction in the porphyrin-nitrite complex transfers charge from the occupied
iron-derived t
2g
-like orbitals into the nitrite
* orbital. Lower part: Back-bonding interaction in
[Fe(II)(porphyrin)(Lys-NH
3
)(NO
2
)]
. The contour shows a cut through the iron d
yz
-based HOMO
that has a constructive overlap with the nitrite
ˀ
* LUMO, which indicates the back-bonding
interaction. The oxygen atoms of nitrite are below and above the plane of the paper, respectively.
The reduction of nitrite starts with a heterolytic cleavage of the N-O bond which is facilitated by a
pronounced back-bonding interaction of nitrite coordinated through nitrogen to the reduced
Fe(II) but not the oxidized Fe(III) active site iron. This step leads to the formation of an
{FeNO}
6
species according to the Enemark-Feltham notation [
66
] and a water molecule and is
further facilitated by a hydrogen bonding network that induces an electronic asymmetry in the
nitrite molecule that weakens one N-O bond and strengthens the other. Taken by permission from
[
66
]; copyright 2002 American Chemical Society.
ˀ
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