Chemistry Reference
In-Depth Information
9
8
7
6
5
4
3
2
1
0
2
4
6
8
10
12
14
pH
Figure 4.3.
The pH dependence of the reaction of ferrate(VI) and superoxide ion and
ascorbate (
, superoxide;
, ascorbate).
[19, 82, 83]. The centers of cu(II), Zn(II), and Ni(II) were unreactive toward
superoxide [84, 85]. The rates of the reactions of superoxide with Fe(II,III)-
porphyrins with
O
•−
ranged from 3 × 10
5
/M/s to 2.3 × 10
9
/M/s and was depen-
dent upon the type of ligand(s) in the axial positions of the porphyrins [19].
Recently, the reaction between superoxide and the Fe(II)-porphyrin complex
has been studied in detail [82, 83]. The results demonstrated the reaction
products consisted of Fe
III
-peroxo and Fe
II
-superoxo species, which were in
equilibrium. The reactivity of
O
•−
with Mn(III)-porphyrins ranged on the
order of 10
5
-10
7
. The Mn(II)-porphyrins reacted at least two orders of magni-
tude faster. A recent study showed the formation of a [Mn
III
OO]
+
adduct as
an intermediate in the reaction of Mn(II) complexes with superoxide [86].
Below is a brief description of the catalytic conversion of superoxide radical
to oxygen and hydrogen peroxide by metalloenzymes.
4.1.4 Metalloenzymes
Superoxide dismutase (SOD) enzymes catalyze the dismutation of
O
•−
to O
2
and H
2
O
2
through sequential reduction and oxidation of the metal centers
(Eqs. 4.19-4.21):
M
(
n
+ +
1
)
+
O
•−
→
M O
n
+
+
2
(4.19)
2
M O
n
+
+
•−
(
+
2
H
+
)
→
M
(
n
+ +
1
)
+
H O
(4.20)
2
2
2
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