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been investigated by kinetic and spectroscopic methods [66-70]. The Fe
III
-
peroxo complex as an intermediate was also involved in a selective reduction
of
O
•−
to H
2
O
2
by superoxide reductase (SOR) [71, 72]. A recent kinetic study
proposed a thiolate-ligated hydroperoxo intermediate [Fe
III
(S
Me2
N
4
(tren)
(OOH)]
+
in the proton-dependent reduction of
O
•−
by [Fe
III
(S
Me2
N
4
(tren)]
+
[73].
The measured reactivity of
HO /O
2
• •−
with Fe
2+
and Fe
3+
as a function of pH
was used to understand the reaction mechanisms of Fenton and Fenton-like
reactions [74]. The Fenton-type reactions are presented as
2
L Fe
2
+
+
H O
→
L Fe
3
+
+
•
OH OH
+
−
(4.15)
m
2
2
m
L Fe
2
+
+
H O
→
[
L Fe O
=
]
2
+
+
H O
.
(4.16)
m
2
2
m
2
Oxidations of substrates carried out by
•
OH and ferryl, [L
m
Fe=O]
2+
, pro-
duced by reactions (4.15) and (4.16), respectively, suggested the formation of
OH radical in acidic solutions, while both the hydroxyl radical and ferryl were
responsible oxidants in neutral and basic solutions [75, 76]. Stopped-flow and
pulse radiolysis methods have thus been applied to study the chemistry of
ferryl species, which are described in detail in chapter 6. Additionally, different
pathways to high-valent iron species were also pursued by studying the reduc-
tion of stable ferrate(VI) ion (
FeO
2−
) to perferryl ion (
FeO
3−
). The perferryl
as a model was used to learn the chemistry of ferryl species with different
organic substrates [77]. The rate constants of reactions of high-valent iron
species with
HO /O
2
• •−
are provided in Table 4.1 [54, 59, 65, 78-80].
The reactivity of ferrate(VI) species with superoxide as a function of pH
has also been performed (Fig. 4.3) [78]. The values of
k
obs
for the reaction
decreased with pH resulting in a slope of
1 from pH 8-13. A plateau region
was observed between the pH range of ≈4.5-7.5, followed by a decrease in
rates below pH 4.0 (Fig. 4.3). The rate constants for the individual reactions of
ferrate(VI) species with superoxide were calculated and are provided in Table
4.1. The stoichiometries of the reactions (
[
2
Fe VI O
•−
) were found to be
1 : 1 and 1 : 2 at pH 8.2 and 10.0, respectively and reactions (4.17) and (4.18)
explain the observed stoichiometries [78]:
(
)] : [
]
Fe VI O
(
)
+
•−
2
→
Fe V O
(
)
+
2
(4.17)
Fe V O
(
)
+
•−
2
→
Fe IV O
(
)
+
2
.
(4.18)
At pH 8.2, only reaction (4.17) occurs, while both reactions (4.17) and (4.18)
participate at pH 10.0. The formed Fe(V) at pH 8.2 self-decomposes rather
than reacting further with
O
•−
. Reactions of Fe(VI) with superoxide and ascor-
bate had similar rates at neutral pH (Fig. 4.3) [78, 81].
The reactivity of superoxide with various metal-centered porphyrins has
been performed to understand the formation of metal-superoxo intermediates
in the metalloenzyme-catalyzed activation of oxygen and hydrogen peroxide
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