Chemistry Reference
In-Depth Information
S
OX
O
2
•-
S
S
Mo 2[ Fe
2
S
2
] FAD
O
2
O
2
O
2
•-
e
-
/H
+
+
H
H
2
O
2
HOO
-
H
2
O
2
CO
2
HCO
4
-
HOOCO
2
-
e /
+
H
CO
2
/HCO
3
-
H
2
O + CO
3
•-
-
CO
3
Figure 5.3.
Schematic representation of a route proposed for carbonate radical anion
production during xO turnover in biological environments (adapted from Bonini
et al. [13] with the permission of the American Society for Biochemistry and Molecular
Biology).
In the laboratory,
CO
•−
is produced by a one-electron oxidation of the car-
bonate or bicarbonate ion. The methods include the radiolysis of solid salt
matrices and aqueous solutions, oxidation of
HCO
−
by the sulfate radical
anion, photolysis of carbonatoamine complexes, and flash photolysis of car-
bonate solutions [25-31]. generally,
CO
•−
is generated by the reactions of
•
OH
with either
CO
2−
or
HCO
−
(reactions 5.1 and 5.2):
•
OH CO
+
2
−
→
CO
•−
+
OH
−
k
=
3 0 10
.
×
8
/M/s
(
25
°
C
)
[32]
(5.1)
3
3
1
•
OH HCO
+
−
→
CO
•−
+
H O
k
=
8 5 10
.
×
6
/M/s
(
25
°
C
)
[32].
(5.2)
3
3
2
2
The reaction of
•
OH with H
2
CO
3
has also shown to produce
CO
•−
(Eq. 5.3):
•
OH H CO
+
→
CO
•−
+
H +H O
+
k
=
7 0 10
.
×
4
/M/s
(
5
°
C
)
[33].
(5.3)
2
3
3
2
3
The characteristic absorption spectrum of
CO
•−
(ε
600 nm
= 1850 ± 50 M/cm)
at different pH values is shown in Figure 5.4 [33]. A similar spectrum in the
pH range of the study suggests
HCO
•−
is acidic (Eq. 5.4):
•
+
•−
HCO
H CO
+
p
K
<
0
.
(5.4)
3
3
4
The reactions of
CO
•−
with thiocyanate, iodide, and ferrocyanide ions as a
function of pH were conducted to support
HCO
•
with a p
K
4
< 0 [34]. This
estimated value was supported by the calculated p
K
4
of −4.1 using high-level
ab initio
calculations [35]. The structure of
CO
•−
has been determined using
Search WWH ::
Custom Search