Chemistry Reference
In-Depth Information
9
8
7
6
5
4
3
2
1
0
2
4
6
8
10
12
14
pH
Figure 4.2.
The pH dependence of the rate constants for disproportionation of
O /HO
2
−
2
(adapted from Abreu and cabelli [31] with the permission of Elsevier Inc.).
on detecting superoxide in biological processes. Examples include a fluores-
cent sensor using acridinium ion-linked porphyrin triad and single-cell
imaging techniques in skeletal muscle fibers [37, 38]. In natural waters, the
chemical reagent 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2-a]
pyrazin-3(7H)-one (McLA) has been shown to detect superoxide in picomo-
lar concentrations [39].
The spontaneous decay of superoxide in aqueous solution is pH dependent
(Fig. 4.2). The plot in Figure 4.2 has a slope of −1 at pH > 6.0 and a maximum
at pH ∼ 4.8. The second-order disproportionation process was explained by
reactions (4.10)-(4.12) [40]. The activation energies of reactions (4.10) and
(4.11) were determined as 20.5-24.7 and 8.8 kJ/mol, respectively [25]:
HO HO
•
+
•
→ +
O H O
k
= ×
1 10
6
/M/s
(4.10)
2
2
2
2
2
10
HO O
•
+
•−
(
+
H
+
)
→ +
O H O
k
= ×
1 10
8
/M/s
(4.11)
2
2
2
2
2
11
O
•−
+
O
•−
(
+
2
H
+
)
→ +
O H O
k
=
no reaction
.
(4.12)
2
2
2
2
2
12
4.1.3 Reactivity
Superoxide goes through several mechanisms depending on the nature of the
substrate [41].
O
•−
is known to react with alkyl sulfide through nucleophilic
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